You can easily change the tune of your own LT1! For the average computer user and car enthusiast, there isn't much to it! With an initial financial investment and some time set-aside, you can get your car running great and take advantage of your mods.
I have some basics and tips I've picked up as I've tuned my own 1995 Camaro Z28. Much of it can be applied to other vehicles as well. This will be tailored to the LT1 engine, especially the '94-'95; the VE table tuning aspects will be even more helpful to '92-'93 and '96-'97 LT1s.
There are three basic items necessary to tuning your own LT1 PCM:
TunerCat is your first step, the program I recommend to read your current PCM image, change its contents, and then reupload it to your PCM. It is a great program, also known as Computer Automotive Tuning Systems (C.A.T.S.), which allows the above things to be done, all for only $69.95. You'll also need an additional PCM file: $EE for the '94-'95 LT1/L99; $DA3 for the '93 LT1 F-Body; or $DA2 for the '92-'93 Y-Body (Corvette), which will cost you an additional $19.95 from TunerCat.
Second, you'll need the appropriate cable connector to go from your car to a computer. These can be bought from AKM Electronics or ALDL Cables for a reasonable price ($60-$90). For a '94, you'll need the 12-pin connector; for a '95, you'll need the 16-pin OBDII-style connector, but it's still OBDI protocol. If you have a '93, you'll also need a memory adapter to piggyback new chips onto your existing PCM ($35), a chip burner/eraser ($85) and blank chips ($5), as their PCMs are not flashed based. For '96+, you'll likely get your ALDL cable with your tuning software (Jet's DST, HP Tuners, etc.). All together, you're looking at a starting price of at least $150, not counting the computer.
Third, get a logging program, that will allow you to see how your car is running. I suggest FreeScan, as it is, like its name implies, free! It will save the log in the common CSV format, which can be imported into spreadsheet software or viewed with a simple text editor. Another option is DataMaster, which is not free ($119), but is more user-friendly and complete in its log viewing. For '92-'93, consider WinALDL or AutoTap. For '96-'97, try Monodax's MXscan. For easy tuning of VE tables, I suggest using VEMaster, which works with '92-'95 PCMs.
Of course, you'll also need a computer to run the program! A laptop is ideal, but not necessary if you can put a desktop & monitor close to your car's parking spot. But without a laptop, you'll have no way to log your car while it is out on the road (necessary for real-life engine load and thus accurate spark timing & knock feedback).
There is no VIN or individual PCM restriction with TunerCat, so you can tune multiple cars and/or PCMs without having to pay any additional fees. However, it does not have definition file for OBDII cars ('96-'02), so you will only be able to tune '93-'95s. For those later year PCMs, you must purchase another program originally made by TunerCat and now sold by JET, the Dynamic Spectrum Tuner (DST), which is much more expensive ($400+) and is locked to a limited number of VINs. For data-logging OBDII vehicles, ECMPro Scan + enhanced GM diagnostics will work with the Jet DST kit.
It's important to be sure that your vehicle and engine are mechanically sound before you begin tuning.
If any of these things are in poor repair, they can cause the PCM to think the engine is running one way when it is not. A good tune will never be achieved if something mechanical is in need of fixing or replacing.
It's also important to understand how the PCM works, and the terminology used when tuning.
The PCM uses a table of cells to modify both spark timing and fueling. The cells are usually referenced by a combination of load/vacuum (kPa) and engine speed (RPM), with three grids or "blocks" for each.
Where the different blocks begin and end can also be modified, with the stock tune having the vacuum blocks set at 30 (32 for A4s), 50 and 80kPa, and the RPM blocks set at 900 (700 for A4s), 1200 and 2000.
Combining these, one can see how there will be 16 total blocks: below 30kpa, at 0-900RPM, 900-1200, 1200-2000, and 2000-7000RPM; then the same for 30 to 50, from 50 to 80, and from 80 to 100kPa. There is another modifier for both the engine load and speed boundaries called hysteresis. This affects how adjacent blocks of cells are combined when on the edge of a boundary (say right at 30kPa at 1200RPM).
These grids are how the fueling is trimmed, depending on O2 sensor feedback (seen in detail below under "BLMs"), so how they are set can be important to getting a car running efficiently, whether you are running a MAF sensor or in speed density off the VE tables.
When modifying the engine, especially with cams with more valve overlap, increased displacement, or ported heads, these will need to be adjusted accordingly. The modded engine will tend to spend more time in different ranges than the stock engine.
BLM stands for "block learn multiplier." As mentioned above, the PCM uses the grid of cells referenced by engine load and speed to correct for discrepancies due to air and fuel quality. The PCM will be expecting the engine to run a certain way depending on the MAF sensor calibration or the VE tables.
But because the car is not running in an ideal world, there will be correction needed for differences in air temperature, density, humidity, as well as fuel octane, quality. On top of those two big variables, there are also the changes that happen due to normal wear and tear on the engine and its sensors.
The BLM is how the PCM adjusts to correct the fueling. By default, in closed loop, it attempts to get an air:fuel ratio (AFR) of 14.7:1 (or whatever else is specified as the target in the PCM). When at this ideal, the BLM will read 128, or no correction.
If the PCM senses the combustion is too lean (not enough fuel), it will add fuel and the number will increase (> 128). If it senses the combustion is too rich (too much fuel), it will remove fuel and the number will decrease (< 128). By default it is set to modify down to 108 and up to 160, but those settings can be changed as well.
There are two sets of BLMs: long-term and short-term. These are labeled as LTerm and STerm in DataMaster, and in FreeScan as Integrator (short term) and BLM (long term). As a consistent trend in O2 sensor feedback forms while driving, the PCM will set a "permanent" modifier to accomodate the initial PCM tune being wrong; this is called the long-term modifier. Until those are changed, the PCM uses a short-term modifier which adjusts fueling on the fly depending on O2 sensor feedback. It is best to tune from the long-term BLMs, as the short-term can vary based on inconsistent variables. Both sets of BLMs are reset to 128 when power to the PCM is lost or a new tune is flashed.
Using these BLMs, one can sense whether there is a problem with something mechanical; things like exhaust leaks (the BLMs will rise, as the O2 sensors sense unmetered air thinking the combustion is running lean), or fouled spark plugs or O2 sensors (the BLMs will decrease, as the O2 sensors see less air and think the combustion is too rich). The BLM can also show a need for a change in the PCM tune; if the VE tables are set too high (low BLMs) or too low (high BLMs), or if the MAF sensor is miscalibrated. Even timing can affect combustion efficiency and thus BLMs.
At WOT, when the PCM enters PE mode, the BLMs are still used but for the most part no more feedback is recorded (unless the mixture runs very lean, then more fuel can be added). Therefore, it is important to be sure the car is running properly before entering PE mode, so that the desired WOT AFR is used and not a modified setting from the lower RPMs (seen more in detail below).
When the PCM enters PE mode can also be modified, both by RPM and throttle position; by default, it is at less than full throttle, how much so depending on RPM. For example, in the stock tune, as little as 28% throttle is necessary for PE mode, when over 4400 RPM.
When tuning an LT1 PCM, one of the first things to do is remove things that you dislike or physically removed. Then you may change constant values. Lastly, are the more complex items that involve entire tables.
Here are the basics for using C.A.T.S. OBDI Tuner (beta version of TunerCat, available by request from them once registered) or the standalone WinFlash to read and write to your '94-'95 LT1/L99 vehicle's PCM. This is tailored to the '94-'95 LT1 PCM.
With TunerCat, you can then verify whether you have one of the rare revision B calibrations by going to Constants->Fuel Parameters and seeing whether the cylinder volume and injector constants are valid.
A stock aluminum-headed LT1 should be "717.25" and "24.91," respectively. If one is zero or a strange value, go to "PCM" and choose "PCM_EEB." Then, load your stock .bin file again, and it should show valid values in those same spots.
Be certain you edit your file with the correct definition file loaded or your car will not run properly and you could damage the PCM/engine!
This process is pretty much the reverse of the above.
Here are a few pointers for using FreeScan to data-log your vehicle.
You can also click the "Engine Data" tab to see necessary feedback like BLMs, INTs, O2 voltage, MAP KPa, knock, etc. Or, view current DTCs (error codes) that your PCM is tripping.
Here are a few pointers for using DataMaster to data-log your vehicle.
Now you can open the .uni file you just created/saved, and see what your driving looked like! You'll see lines for RPM, speed, spark timing, and knock retard.
Like timing, fueling can have a big effect on both idle quality and engine temperature. Generally, a leaner fuel mixture will increase combustion temperature.
Initial fueling must be set up via the fuel injector settings, including the flow constant and the offset/lag time table. Without having those set properly at the beginning, fine-tuning via the below will never work properly.
First make sure your PCM constants are set correctly; things like:
This constant (or table in some PCMs) tells the PCM how long to pulse the injectors at various airflow/MAP points. It should be adjusted for any fuel pressure changes compared to the rating at what the fuel injectors were flowed.
For example, if you have a 30# injector that was flow-tested @ 43.5 psi (standard GM rating), but your actual fuel pressure with no vacuum is 48 psi, that means that 30# injector will flow more than 30# in your vehicle (31.51# to be exact). To correct flow rates for fuel pressure changes, divide the new/current fuel pressure (48) by the injector's rated pressure (43.5) and then take the square root and multiply that by the injector's rated pressure. You'll see that higher fuel pressures will give you a larger constant (the PCM will then pulse a shorter time), and lower pressures will give a smaller constant (the PCM will pulse a longer time).
On some PCMs that do not allow a setting of stoich AFR, where you would like a leaner or richer AFR overall, you can also use this constant to accomplish that, if you will be running in open loop only (as closed loops systems with no settable AFR will target ~14.7 AFR). To do so, you can start by dividing the desired AFR by 14.7 and multiply that by the fuel injector constant. This should only be done with a wideband O2 sensor installed, so you can verify the actual AFR that results.
This is an often overlooked setting that will have a tremendous impact on drivability, power and MPG, as it will directly affect the AFR and combustion efficiency. This setting tells the PCM how long to expect the injector to take to open once the PCM commands it to pulse. All injectors have a slight delay, usually in the hundreds of microsend (us) range.
Ideally, you want to find a factory rating for the injectors you will be using. This can be done by looking at stock tunes for OEM injectors or by finding spec sheets for aftermarket injectors. Bosch injectors are commonly used in LT1s, and most of them have substantially longer offsets than the stock Rochester LT1 injectors.
In my experience, setting the offset too low for the injector will cause overly lean running at idle and part-throttle, and overly rich running at WOT. It will also substantially kill power. Basically, the fuel will be spraying too late compared to when the spark happens, as the PCM is expecting the injector to fire sooner than it really is. The other possibility is having the offset too big/long for the injector, in which case the injector is spraying too early for the spark, which tends to cause overly rich running at idle and part-throttle. You can verify this via fuel trims from factory O2 sensors, or via a wideband O2 sensor.
If you cannot find a rating for your injectors, you can use the above to find them on your own, as the closer you get to the correct offset, the closer the true AFR will be to the tune's targeted AFR. Obviously, this method will only work if you have known good VE/MAF tables for your setup.
Once you have both the injector constant and offsets correct for your setup, you'll be a good place to start fine-tuning the fueling.
In closed loop, the main ways to affect fueling at idle are by using the VE tables (for speed density) and MAF sensor tables.
Find at what kPa and RPM your engine idles and correlate those values to the nearest cells in the VE table (for example, a stock M6 engine will be 30kPa and 800RPM).
To richen the fuel mixture (BLMs > 128), simply increase the respective VE cells.
To lean the fuel mixture (BLMs < 128), simply decrease the appropriate VE cells as mentioned above.
These values are efficiently modified using percentages (%), or multiplying the values (1.xx to richen, 0.xx to lean). Start with 1% (1.01/0.99) changes for every 2 BLMs off; so, a BLM of 138 would require a 5% increase (128 - 118 = 10 / 2 = 5), or multiplying the appropriate VE cell by 1.05. You will need to scan/log the car to be sure you haven't gone too far, or not far enough, as it's not completely linear: trial and error is needed!
To simplify and speed up the process, you can also use VEMaster for tuning VE, not only at idle but part-throttle too (see link at top of page).
The other area where VE tables are used is cranking the engine ("Crank VE Vs. %TPS vs. RPM"). For stock cars, this table won't need to be modified unless you are very fastidious; but for aftermarket cams, displacement changes, etc., it can change significantly. Since cranking doesn't take very long, it won't have a large impact on anything, but it's one more table to address.
I suggest using your corrected main VE tables to figure out the general direction you should take the crank VE tables; most often they will need decreased, as most aftermarket setups have lower VE in the low RPM/load areas.
As you can see, this table gives you the option of putting more fuel into the engine during cranking if you are using the pedal (anything over 0% TPS) while turning the key. This can help if it's extremely cold or something else is wrong with the engine or fuel. So you may wish to add more fuel (higher values) to those higher TPS% tables, as a "just in case" provision.
There are four tables for the MAF, which split up its total frequency->airflow grid.
First, find the airflow reading for your idle in your logs. Then, find the closest value(s) in the MAF sensor tables; you will sometimes have to alter two cells to affect one airflow range.
To richen your fuel mixture (BLMs > 128), increase the value(s) you found as mentioned above.
To lean the fuel mixture (BLMs < 128), simply decrease the appropriate MAF cells as mentioned above.
These values are also efficiently modified using percentages (%), or multiplying the values (1.xx to richen, 0.xx to lean). Start with 1% (1.01/0.99) changes for every 2 BLMs off; so, a BLM of 118 would require a 5% decrease (128 - 118 = 10 / 2 = 5), or multiplying the MAF airflow value(s) by 0.95. Again, trial and error will be needed to see if you have overshot a BLM of 128 with your changes.
This table alters how each cylinder is fine-tuned based off the calculated fueling needs for each bank. A number below one leans out the cylinder, and a number higher than 1 richens it up.
The ideal way to tune this table is to use an IR thermometer to measure temperature at each cylinder exit and to use the results to tune each cylinder's trim until the temp readings all closely match. It is best to start with all cylinders at 1.00, and first have the BLMs close to 128 from VE/MAF sensor table tuning.
Generally, the middle cylinders (3-6, especially 3-4) will require the least tuning, so use their temperatures as the baseline.
It has been shown through practice that a leaner fuel mixture increases combustion temperature, and therefore a richer mixture decreases temperature. You can use this logic in reverse: cylinders reading hotter than the middle ones should have their trims increased (ex. 1.01), and cooler cylinders should be lessened (ex. 0.99). Start with 0.01 increments until you see how much the temperature is affected.
However, if a cylinder is severely starved of fuel, the temperature can actually read much lower from lack of combustion. This can happen with the two front cylinders, so if there is a major difference between some, with one or more reading much lower (over a couple hundred degrees Fahrenheit), then you should consider adding fuel to those first.
Obviously, this method is best done at idle. But, there are two tables, one for idle and one for off-idle.
For off-idle, without a dyno, with the car in park you could set the throttle to about 20% (with a stick on the pedal, or someone else holding it down), and then measure; or, for tuning while the car is moving, install EGT sensors into each cylinder exit point on aftermarket headers, and either be able to log their readings or have someone in the car with you doing it.
Tuning this table can also solve BLM splits, where one side of the engine is either richer or leaner than the other side (i.e., 126 for the left, 110 for the right). Having such a condition will reduce power and MPG and could cause a dangerous lean condition for one side or cylinder depending on how bad the split is.
Now you should log the car (using FreeScan, mentioned above), while driving with all ranges of acceleration, from 800RPM up to around 2000RPM. You want all different amounts of load on the car at the different RPM ranges to get the best data possible.
The following will allow you to get the car running close to 14.7:1 air:fuel ratio (AFR) when you're not at WOT, giving the best overall gas mileage and lowest overall smog output. After logging for about 10 or 15 minutes you will have enough data to make your first changes, depending on whether you are using a MAF sensor or running in speed density mode.
If you are using speed density mode (no MAF sensor), the easiest way to begin tuning fueling and geting the BLMs back to 128 is to run the log through VEMaster. Repeat logging and running VEMaster until your long term BLMs are near 128.
There is mixed feeling on whether/how the VE tables are used when also using a MAF sensor. Some say they aren't at all; some say they are only when the car is just started (and the MAF sensor is not warmed up); some say they are used to fine-tune the AFR when the BLMs are slightly off; some say they are used at WOT.
I like to tune the VE tables regardless, in case of something happening to the MAF sensor while driving. It also would improve drivability if they are indeed used in MAF sensor mode (which I can't confirm or deny).
The easiest way to tune them if you have a MAF sensor is to simply disconnect the sensor; the fans will run and the SES will come on, but you will be in speed density mode.
The more proper way is to choose "Speed Density" in the "Switches" table in TunerCat, and run a log with that tune loaded.
Increasing the number will add fuel to the mixture (the PCM thinks the engine is more efficient so more fuel is added to get the most power) and decreasing it will lean the mixture.
Depending on engine setup, sometimes these values need to be lowered by as much as 20-30 points (as with large valve overlap cams in the lower RPM/load sections).
If you are using a MAF sensor, then find where your logs show a BLM of < 126 or > 130. Note what the airflow number is at that point, and then go to that entry in the respective MAF Sensor table in TunerCat.
If the BLM was < 128 you will want to make the MAF frequency number lower. If it was > 128 you'll want to make it higher. If the BLMs are very far off 128 (+/- 15), the quickest way to correct is to multiply the number by a %. Start with 2.5%-5%.
If adding, this will be in the format of multiplying by 1.025 (2.5%), and if subtracting, then something like 0.975 (2.5%). Continue to flash the new tunes, log the car driving again, and see how the BLMs are affected at that airflow point.
Repeat until the long terms are close to 128 (+/- 2). When you are getting closer to 128, you can begin simply adding and subtracting rather than multiplying by % to fine-tune more easily.
This mode is used when the oxygen sensors are not in use; whether at all for those who choose to run that way, or before they are warmed up.
When the PCM leaves open loop (into closed loop) can be changed depending on coolant temperature and amount of time.
You can also increase the Open Loop AFR table entries by about 3 or 4 across the board. This will make the car run leaner before going into open loop, helping increase gas mileage. If for some reason you will not be running with oxygen sensors, then this table will be very important as it will specify what AFR the PCM is commanding.
Depending on your setup, you may even need to make the table richer. Without O2 sensor feedback, it will be a matter of reading the plugs and "feeling" the car.
The AFR can make a big difference in power output at WOT just like spark timing. The factory LT1 tune has the engine running very rich at WOT (between 11.7:1 and 12.7:1), which although safer (as it can help prevent detonation by cooling the combustion temperature with more fuel), robs not only power but also fuel MPG.
Independent studies of internal combustion engines have shown that the most cylinder pressure (i.e., power) is generated with AFRs of 0.9 to 1 x lambda (varying with engine type/makeup, etc.). Lambda stands for an AFR of 14.7:1 (what the car strives for at part-throttle). 0.9 x 14.7:1 turns out to be about 13.4:1.
The tests mentioned reveal that increasing timing advance when running the engine leaner (closer to 14.7:1) can restore the power increase from a richer AFR (closer to 13.4:1). But, with day-to-day driving we cannot indiscriminately advance timing because of octane limits, or we would quickly run into detonation (knock/pinging).
Therefore, to balance making the most power with running on normal pump gasoline, the ideal AFR would be close to 13.4:1. Dynos of LT1s have shown they tend to make more power toward the richer end.
I suggest going for a WOT AFR between 12.6:1 and 13.4:1; TunerCat's help file will show you how to make the tables do this, but here is a quick summary for the two tables, %Change to AFR at WOT: 14.7 / 1.[both table values added / 100] = AFR. So, for example, if the values in both the coolant (13.7) and RPM tables' cells (0.6) add up to 14.3, you would be targeting a WOT AFR of 12.86:1 (14.7 / 1.143).
To make fine-tuning easy, I suggest zeroing the RPM table and setting your ideal base AFR with the coolant temp. table. This way, you can easily ramp the AFR down (richer) toward the torque peak of your engine using values like -1.2, -0.8, -0.4, etc., finally ending at the ideal AFR at the torque peak of your engine (stock, 2800RPM) using 0.0 in the RPM table. This is done because as engine RPM increases, there is less time for the fuel mixture to combust; because it is not ideal, having extra fuel gives more opportunity for combustion compared to lower RPMs.
You can begin another richening upwards ramp from your base power AFR in the RPM table (0.0), from the power peak (stock, 5200RPM) to the max RPM (7000) setting.
The only sure way to know which AFR will give your particular setup the most power output is to use a dynomometer and be sure your fueling is correctly calibrated in the WOT areas. With large increases in power (20hp+) you may be able to tell just from driving. But gains of 5-10hp may not be felt "in the seat of the pants." Only a dyno can tell you for sure.
There is one more step you can take to get your setup operating as close as possible to what you tune it to do.
The obvious (and safer) option to calibrate your VE and/or MAF tables in the WOT areas is to use a wide-band (WB) O2 sensor. You may either replace one or both of your stock sensors or have a new bung welded into your exhaust system.
Keep track of your AFR from the reading of the WB sensor at the different RPM points as you log the car while doing a WOT run. Then, compare your true AFR (from the WB) to what the PCM is targeting (%Change to AFR at WOT tables).
If the WB shows a richer AFR, then you will need to lower the VE/MAF tables; if it shows leaner, then raise the VE/MAF tables. Do this several times until the WB reading matches the PCM's commanded AFR at WOT.
There is also a way to calibrate your MAF and/or VE tables in the areas where you are normally WOT and therefore in PE mode, still using the stock O2 sensors.
Above, under part-throttle tuning, you saw how we use the long-term BLMs to determine whether the VE and/or MAF sensor tables are set correctly for your configuation. Through the different amounts of load put on the engine, in addition with the entire RPM range, you can get things running very nicely. The same can be done for when you are at WOT.
Normally, when at WOT, the PCM does not correct the AFR (it is in "PE" [power enrichment] mode). This means that if your VE tables or MAF sensor are calibrated wrongly at those ranges, there is nothing the PCM can do. You may specify you want an AFR of 12.8:1 at a certain RPM range, but if your MAF sensor is set wrong and detects more or less air than actually is entering your engine, the algorithms used to calculate the AFR you specified will not correct for the error (i.e., you may end up with an actual AFR of 11.8:1 if less air is entering the engine than the MAF is calibrated to detect).
The way around this is to temporarily disable the PCM's PE mode that it enters when at WOT. Go to the constants table in TunerCat and set the MAP value to enable at a value that you will never reach (like 255).
Now log for small periods at WOT in the higher RPM ranges, where the most power is produced and where tuning can make the most difference. Because PE mode is never entered, the BLMs will change depending on whether your VE/MAF tables are wrong. It will try to correct the AFR to 14.7:1, just as in part-throttle.
This is where the risk of damage enters, because of the high RPMs and load on the engine. However, it is remote, as the primary reason that AFRs are set to be richer when at WOT is for increased power, not to lessen damage to the engine. You can use this data in the same way as in tuning part-throttle.
Once you change your tables accordingly, and have gotten as close to a long-term BLM of 128 as you can, reenable PE mode by putting the constant for MAP value to enable WOT back to the former value (15 is stock).
You will then be assured that whatever AFR you specify for WOT will actually be reached, because the underlying fueling parameters will be correct for your setup.
At least one person I don't personally know has tested this method on a dyno, and I have also tested it successfully with one of my customers, so the logic is confirmed by real-world results; this gives you one more way to reach the exact tune that you desire.
"Deceleration fuel cut off" is a factory feature on many vehicles/PCMs. It does what it suggest, cuts off fuel during deceleration. This produces "engine braking," which is where the vehicle slows down solely from the engine slowing down, rather than having to use the vehicle brakes. A combination of no fuel and timing retard (some air is still entering through the IAC/bleed hole) makes the engine wind down, which when the transmission is still engaged, will brake the vehicle.
Editing these values in the tune will control where the fuel cutoff/timing retard happens when the throttle is closed and the car is moving. So, as mentioned above, when your RPMs are up and you let off the gas pedal, this table will enter play.
The way to tune these values is to log your car and determine a few things:
Tuning your idle and coast-down can make a big difference in MPG and overall throttle response. This is especially important with modified engines, but can affect stock engines too.
The first thing to change at idle is the spark timing. Increasing the advance will usually increase MPG, and can actually decrease engine temperature too. Christian Millard has a great analysis of this, which found that a big bump over stock can make a noticeable difference, especially with an aftermarket cam (notably one with a lot more valve overlap).
Increasing spark advance at idle increased fuel efficiency/MPG (by requiring less pulsewidth from the injectors, and less airflow through the MAF sensor), and also lowered engine temperature. He found 34° as the ideal spot for idle, for his LT1 with the Hotcam and a full point increase in compression ratio (11.4) over stock (10.4). I, too, have noticed a great effect when substantially increasing timing here (on my 236-242/114 cam).
The table to use is "Closed TPS Spark Advance vs. RPM." It affects not only idle, when the car is not moving, but also the timing commanded during coast down when the car is moving: for example, when you have just done a WOT run, and let off the throttle; or, when you down-shift and allow engine braking to slow down the car.
Advancing timing in non-idle closed-throttle areas, like coast-down, will also affect MPG, and help to keep the engine from stumbling or stalling in setups that have more valve overlap than stock. It will tend to make throttle response "crisper."
To fine-tune this, simply increase the values and log the car. Look for any spark retard, and back off the timing a degree at a time wherever you are getting knock. You can also look at your MAP and airflow (when using a MAF sensor) values, which will let you know how efficient the combustion is; the more efficient it is the more vacuum will be pulled (= lower kPa values) and the less airflow will be required (lower gm/s reading from MAF).
You will find changes here will also affect emission output; usually more advance leads to higher NOx and lower HC, with a usual sweet-spot found in the middle somewhere.
This is the majority portion of where the engine will be running, from idle until about half throttle, from 800 RPM to about 2600 RPM. With engine changes like cam, heads, etc., this is an especially important area for tuning, with great results achieved in drivability and fuel MPG.
In general, timing advance should decrease as load increases, and increase as RPM increases until the peak torque output is reached. Other things like the idle vacuum value can affect where the spark timing maps increase and decrease, which is why tuning is so important after substantial engine mods like an aftermarket camshaft.
For stock engines, when running 92+ octane fuel, you can begin by adding about 2° of spark advance across the two Main Spark Timing tables, to get more power/better efficiency out of your combustion. Monitor this increase with a scanner/logger and make sure you aren't getting any consistent knock retard. If you are, note in which RPM range it is, and at what MAP value, and take 1° or 2° out of the appropriate Main Spark Timing cell.
You can pretty much add advance until you feel the engine holding back or skipping, or get spark retard, and then back it off some. This will generally produce the cleanest combustion and best throttle response. However, sometimes you can have too much timing without knock (power will fall off), so the best way to fine-tune timing is on the dyno.
The other way to see whether your changes are having a good effect, beside "feeling" the car while driving, is to check your MAP readings. Generally, a more efficient combustion will produce more vacuum (lower kPa).
Therefore, if you are tuning your timing in the "cruising" area--for example, in 4th gear in an M6--you will find both the approximate MAP value and TPS % needed to keep this cruising RPM. Then, try increasing timing in that MAP/RPM cell; drive again with the new tune, at the same gear/RPM with the same TPS %, and see if your MAP has decreased. Or, see if less throttle is necessary to keep the car moving at the same speed as before the timing change. If so, then your changes were good. If you get knock, then obviously you went too far. When you back off timing to prevent knock, you can use this to find out if you've gone farther than you need (if the MAP values increase a lot at the same TPS/RPM).
Once your normal driving is fine-tuned, giving BLMs of near 128 (126-130) (or wide-band AFRs very close to what your Open Loop AFR table is targeting), with no knock retard, you can move on to tuning the power portion of your PCM tune. This is when you have "the pedal to the metal," or the throttle is fully open. The LT1 PCM calls this "power enrichment (PE) mode."
In general, timing advance should decrease as load increases, and increase as RPM increases until the peak torque output is reached. On stock LT1 engines with aluminum heads, this is about 2800 RPM. As you can see with the factory tune, timing levels off around this RPM range. If you modify your setup, with aftermarket heads, cam, etc., you will want to determine where the new torque peak is reached, and tune accordingly.
You will also want to tweak the spark advance values at the 95 and 100 MAP settings (the range of WOT). Too much advance can not only cause spark retard (and damage to your motor), but can be needless for producing the most power. However, too little advance can severely reduce the potential output of your engine as well as lower MPG.
Picturing how the engine works will reveal why too much advance can be detrimental to both power and engine longevity. As the piston travels back up the cylinder, the spark plug is fired and the air:fuel mixture is ignited. If that spark happens too soon/advanced, the explosion of the combustion will begin expanding and pushing down on the piston while it is still traveling upwards.
This will make the engine actually work against itself, with the crank pushing the piston upward while the combustion pushes it downward. Obviously this will severely hamper performance, but will also put undue strain on the crank and connecting rods.
Conversely, too little advance will generally only hamper performance. If the plug is fired too late, the piston will reach TDC and begin moving downward again before the combustion can begin expanding and putting downward pressure on the piston. Power will fall off dramatically and can even stall the engine.
Once your normal driving is fine-tuned, giving BLMs of near 128 (126-130), with no knock retard, you can move on to tuning the power portion of your PCM tune. This is when you have "the pedal to the metal," or the throttle is fully open. The LT1 PCM calls this "power enrichment (PE) mode."
In my experience, there are a few signs of having the timing set significantly wrong for your particular setup.
Too much timing advance tends to produce a bucking/skipping in throttle response, much like not having enough fuel. I'd suppose having the spark fire too early gives similar symptoms as the fuel hasn't fully sprayed and/or propagated throughout the cylinder before the spark fires in this situation. At WOT, too much timing will usually produce knock/pinging of some sort. At idle, too much can cause the engine to hunt, and will usually raise NOx and lower HC emissions, and affect exhaust smell.
Too little timing advance tends to kill MPG and power, giving a lazy feeling to throttle response. With larger cams, too little advance at idle will make the engine "chop" a lot more (less efficient combustion) and usually increase HC and decrease NOx emissions. Too little at WOT will definitely reduce power.
Transmission tuning is almost a separate undertaking, because of the many variables and wide range of driving the tables affect. But, once you understand and apply the logic, you'll have your car running even better, especially once you do engine mods or drag racing at the track.
The stock tune has these set pretty tame. They don't rev very high at part-throttle (~1400-2000), and tend to shift too late at WOT (5300) for the stock engine. Many performance-minded people like the feeling of the engine revving higher, even for everyday driving. And, once you modify the engine, it usually lowers VE at lower RPM areas, which means the shifts should be later to take advantage of the moved-up power-band.
If you change gears or tire size, which would affect your speedo, that will also affect your shift points; if it's a substantial gear change, the shift points can be broken to the point of not shifting at WOT. So, tuning shift points is an important area for performance tuning.
The quick way to start, when changing gears, is to use TunerCat's Speedo Correct Tool, which will automatically update all MPH-related shift tables based on the gear/tire change. If you aren't using TunerCat, you can just divide the gear size in your current tune (say, 3.23) by your new gear size (say, 3.73). Then multiply that value (0.866) by your shift points. For example, the stock 1->2 shift with little to no throttle is at 11mph; the new value for 3.73 gears would be 10. This will need tweaking, but it's a quick way to start.
To fine-tune, you will need a calculator that takes into account your tire size, final gearing, and individual gearing (1,2,3,OD). One good website that does this for you is: F-Body.org's Calculator.
Choose your year and vehicle, entire your tire size or diameter, and it will calculate the speed at which your car will be moving at a specific RPM in each gear. You can use those values, along with your known power-band, to modify your shift points. Obviously, the best way to tune this way is trial-and-error in your car. Only you can decide how much throttle you want to give to get a specific shift. Anything over ~20-30% TPS/throttle will be outside normal "cruising" driving-style.
The primary tables are "Normal Mode Up/Down Shift Points." For those who have installed a performance transmission switch, you can also setup a separate profile for "performance mode" (most likely, shifting later).
The important thing to remember is that for tables which have both MPH and RPM settings, such as "Kickdown Mode" (WOT), both values must be met in order for the shift to take place.
On a stock LT1 A4 car with 3.23:1 gearing, this means that if you have the Kickdown MPH table set to the stock value of 36 (equating to ~4600RPM), and leave the Kickdown RPM table at the stock value of 5350RPM, then the transmission will not shift until 5350RPM.
If, for example, you set the RPM value to 4000RPM, then the shift will happen at ~4600RPM, the speed to which the MPH value corresponds. So, keep this in mind when tuning shift points. My preference is to set the MPH values for WOT lower than you intend, and use the RPM table to set shift points for WOT ("Kickdown Mode").
Depending on your transmission build, your stall converter, your engine power, and your shift firmness, it may take anywhere from less than 100 RPM to more than 300-400 RPM for a shift to complete at WOT. This can obviously seriously affect where you should set your shift points.
You may specify "6000" in the Kickdown RPM tables, but that only means the shift will start then. It may not complete until 6200RPM, and if your rev limit is not high enough above your shift points, you may bounce into it.
Most often, this effect can be lessed by increasing line pressure in the areas of WOT (93+ TPS%), and by decreasing the same areas in the "Shift Time" tables.
The other side of transmission tuning is shift firmness. This is directly related to line pressure to the transmission; the higher the psi, the firmer the shift. The stock transmission will only firm up to a point, because of the mechanics involved. But, you can definitely firm up part-throttle shifts.
The main tables, not a surprise, are "Main Line Pressure," split between 0-64MPH and 64-128mph. They are divided by TPS %, which again brings you back to tuning via in-car feedback; only you will know what feels best to you. I suggest starting with increments of 5, whether to soften or firm the shifts.
You can also add extra pressure: when at WOT at specific RPM points (increments of 512RPM); based on TPS % in each gear; and even based on tranny temperature.
For increasing pressure at WOT, especially when upgrading your transmission, you must first increase the Max. Line Pressure Constant. It is set to 90, which is the max set in the tables too. Many set the Max. Constant to 120, and then work up the WOT areas from 90. For WOT, you will want the 100% TPS table values, and also perhaps the 93.8% ones.
Many people have warned against increasing line pressure via the PCM when you have also installed a "shift kit." Some think this creates too much pressure; others use both. Talk to your transmission builder if you have any questions about that.
This is the last "phase" of transmission tuning, the control of the torque converter clutch (TCC). Similar to a manual transmission's clutch, locking it (clutch pedal out) decreases slippage while increasing horsepower to the wheels; unlocking it increases slippage but increases torque to the wheels (via a multiplier).
Depending on the RPM and engine type, locking the TCC too early can "bog" the engine, much like letting the clutch out all the way on a manual tranny while in a high gear and low RPM (which can stall the engine). But in the appropriate RPM/load ranges, it can improve power and track times.
There are varying opinions on the mechanical effects of locking the TCC at WOT; some think it lengthens its life due to less slippage (slippage creates more heat), and others think it shortens its life because of the increased strain from being locked under high load/RPMs. Only you can decide which is best for you, with the help of your transmission builder.
The easiest way to lock the TCC at WOT is to use the tables labeled for this; use the calculator listed above to find the MPH at which your engine is near its torque peak and set the TCC to engage then; also be sure to update the "release" table to a value lower than the engage, or you will keep the TCC locked permanently.
The other side of TCC lockup is when at part-throttle, especially cruising like on the highway. Having the TCC locked will increase MPG.
There is a fairly well-known issue with OBDI LT1 PCMs ('94-'95) that can prevent shifting at WOT when using a high (2800+) RPM stall converter. It will cause the engine to hit the rev limiter, and once you let off the throttle, it will upshift.
The easiest and surest way to solve this problem is to increase Line Pressure in the high TPS% areas of those tables, and to decrease the Force Motor Current (+ and -) values in the high pressure areas (90+).
To prevent limp mode which also sometimes results from big stalls or short gears, set the "VSS Diagnostic Enable Min RPM" constant to a RPM higher than the engine will ever see.
After installing a different cam in your LT1, timing and to a degree also fueling will play an even bigger role in both drivability and power.
Most often, new camshafts have more valve overlap than stock, which often results in a power increase. However, the down side is that this means part of the incoming air & fuel is never ignited and passes right through the head along with the combusted gases.
Often, the throttle body and/or its blades will need adjusted with significant engine changes like ported heads, new cam, etc.
Generally, you will want a cold start to have IAC counts less than 160 (the max), so that your cold idle is able to hit its targeted value.
At the same time, you don't want the IAC counts ever to hit 0, which can happen with a hot engine when the blades are open too far. This will cause the idle to remain higher than is being commanded.
With many aftermarket throttle bodies, the bleed hole will need drilled through, to match the design of the stock throttle body, to get the best possible idle quality and throttle response. See these links for more information:
It involves making sure your TB has a bleed hole straight through front to back, like the stock TB, which connects to the hole on the front of the intake manifold. That hole distributes idle air evenly to each individual cylinder intake port and will give you the best quality idle and throttle response.
Most aftermarket TBs do not have the hole drilled all the way through, only to the IAC passageway, so you'll need to drill it straight through to line up with the hole in the intake, and make sure the rear of the TB is sealed around that hole so that the hole is separated from the main IAC opening on the bottom.
The factory hole size is about 9/64"; for aftermarket cams/heads, you will probably need to start with at least 11/64". Enlarge the hole until your cold-start idle quality is acceptable (and the IAC counts are below 160) when the TB blades are closed all the way without sticking, and the fully hot idle IAC counts are no lower than 10-20 or so.
Usually, when valves open sooner, the spark should fire sooner. This means more timing advance; especially at low-RPM, part-throttle areas of driving. You will also find less vacuum being pulled with such cams, making idle as well as cruising kPa higher. The timing curves need to be adjusted for this.
More efficient engines will generally require less advance, as the combustion happens quicker and more efficiently, so the spark does not need to be fired as much in advance of the piston reaching TDC. Less advance is also necessary when running lower octane fuel.
More advance is necessary with less efficient engines or range of engine operation, such as the low load and RPM sections of an engine that has a cam with a lot of valve overlap (more further below).
As mentioned in the main "Part-Throttle" timing section, you can use TPS % and MAP values in your logs to determine if your timing increases have actually had a good effect. Read that section for more.
Besides the two main timing tables, it is also a good idea to increase advance in the closed-throttle table; this is when the engine is idling, either at a complete stop or while cruising.
You will need to reorient the closed TPS table to the main timing tables, to be sure the timing is increasing as you go from no-throttle to part-throttle. Where this happens will depend on the amount of vacuum that your cam pulls, so check your data-logs and be sure your spark advance is increasing from idle to off-idle, or else you will get bogging and back-fires off-idle.
The BLM (block learn modifier) is the PCM's way of adjusting fueling in order to achieve the ideal AFR (in most cases, 14.7:1). Having them much different than 128 will cause your engine to not achieve the most power or MPG it could, and can even cause dangerous lean conditions in extreme cases.
When installing a new cam, the vacuum the engine pulls will usually be less at idle and part-throttle. The PCM has a table for adjusting where each cell of the fueling map begins and ends. For stock tunes, it is set to 30-50-80kPa, with the engine idling around 30kPa. Cams with more overlap should cause the lower limit to be set higher, starting with 35 and going as high as even 50 or so.
The way to find the ideal spot is to log your driving and see what the lowest kPa reached during cruising is, and set it a couple points below that. You can do the same for the other two settings, but I suggest not changing the high value much, because of the PCM not using BLMs for fine-tuning when at WOT.
If you want a good idea of whether your boundaries are spaced out evenly, run a "normal-driving" log of your car through VEMaster. It will give you the total amount of records per block, showing where you need to adjust both the MAP and RPM boundaries (if you have a lot more in one section than another).
Changing this table can increase your MPG as well as improve overall drivability.
As mentioned in the "Idle" section above, DFCO has an effect on engine braking and thus also MPG.
Because of the decrease in vacuum pulled with "bigger" cams, the stock settings for DFCO will never be reached and will need to be increased. Looking at the stock settings, it has the disable kPA (25) set about 5 less than idle kPa (30). But because larger cams still reach more vacuum when at part-throttle than when at idle, you cannot simply set the disable 5 below your idle kPa.
I suggest going no higher than 35kpa or so for disabling DFCO, or you may run into severe RPM dropping or even engine stalling when coming to a stop.
These days, getting better MPG (miles per gallon) out of your vehicle is even more important, with prices at the pump increasing so quickly.
Thankfully, there are several ways to substantially increase your LT1's MPG (as well as with other vehicles with similar PCMs).
There are at least two main ways to affect MPG: fueling and timing. The first is probably obvious, but many people don't realize that reduced timing can reduce combustion effeciency, which in return uses more fuel to get less torque (which moves the car).
We'll look at fueling first, since it's the easiest way to get better MPG. There are several areas and tables to be addressed, as there are several types of driving that affect MPG.
This has to do with general tuning too, but can severely impact MPG.
If your fuel pressure is set higher than stock (43.5psi on the LT1), and you have not accommodated for that in the PCM tune (by adjusting the fuel injector constant), then your car will run rich. The O2 sensors will sense this and try to correct, but the stock tune can only correct so far. Until it corrects, you'll be running rich, which decreases MPG.
Running rich, if it happens long enough, can also foul the O2 sensors, which will put you in a never-ending cycle, as the fouled sensors read less oxygen and think more fuel is needed for efficient combustion.
This can be another obvious culprit, but can sometimes be partially overlooked depending on the brand of injector.
Some aftermarket injectors are rated at a pressure other than GM's stock of 43.5psi. Even within the same brand (such as Accel, see here), some injectors flow differntly than their rating at 43.5psi.
For example, Accel's 24# injector flows 24.3# at 43.5psi (more than rated), but their 26# injector flows 25.6# at 43.5psi (less than rated). If you did not know this, and simply used their advertised flow rating in your tune with the stock fuel pressure of 43.5psi, your BLMs would be off. Obviously, the ones that flow more than advertised will make you run richer if set to the advertised number.
Do your best to find out what the injector flows at 43.5psi, and use that number. If, for some reason, you are using a different fuel pressure than the stock 43.5psi, adjust appropriately.
Many people are familiar with this table from using Bosch/Ford SVO injectors, which have a longer offset than the stock AC/Delco injectors.
In my experience, raising these values (to a point) will richen up the mixture. However, they should be set correctly for your injectors, as having them set incorrectly can cause driveability issues like bogging and backfiring. Do your best to find the correct offsets for your injectors, and your MPG should increase.
This is a good table to use to increase MPG. Open loop is the area of driving when the car is first started, and its length will depend on coolant temperature and elapsed time after startup.
By increasing the values in this table, less fuel will be used before closed loop (O2 sensor feedback) is entered. You can run up to 14.9-15.0 at idle and part-throttle MAP values when the car is fully warmed up (80*C+), without much if any impact on driveability (back-firing, etc.).
You can also increase the length of time that the PCM stays in open loop, to give you a leaner AFR for more time, before closed loop is entered and an AFR of 14.7:1 is attempted.
This was a fairly late table addition to TunerCat. Its use is not completely understood, but it must have to do with a richening of the AFR right after startup, depending on coolant temperature. It must be used at least during open loop, and perhaps also during closed loop if it is entered quickly.
The obvious way to increase MPG is to lower these values, and even zero them at fully warmed up coolant temps (80*C+). Less fuel will be added by the PCM during startup, which could be substantial if you use your vehicle for lots of short trips.
Here is the great secret to increasing MPG! A fairly late table addition to TunerCat, this one tells the PCM which voltage from the O2 sensors is the halfway point between rich and lean. Most sensors have their stoichiometric (14.7:1) point output at about 0.450V. With that value entered in this table, the PCM will think that any voltage above .450 is rich, and any voltage below .450 is lean.
The way to increase MPG is to lower this value. What you will be doing is telling the PCM that a lower voltage is the halfway point, which it will do its best to reach via the normal BLM setup. The result is that what used to be considered lean (say, .400V) will now be considered ideal, and the PCM will correct for that leaner output of the O2 sensors, thus giving an AFR of leaner than 14.7:1, which will use less fuel and increase MPG.
Because stock-type O2 sensors are "narrow-band" (they are only very accurate along a short range of voltage), you cannot go too far or you'll end up in the less accurate range of the sensor, and may get inconsistent results (a wildly swinging AFR, perhaps even dangerously lean). Exhaust temperature will also affect voltage, meaning at a different exhaust temp you may be at the same AFR but the sensor will read a different voltage.
I can't recommend any particular value because of this limitation of the sensor; but anything lower than stock should increase your MPG. It is up to your accepted level of risk of running overly lean that will determine how far you should go.
Timing makes a large difference in combustion efficiency. A more efficient combustion means less fuel is needed for the same amount of torque output, which also results in better MPG.
As mentioned above in the Idle tuning section, Christian Milliard discovered that substantially increasing idle timing reduced both airflow and pulsewidth requirements for the same RPM, and also decreased coolant temperature (probably due to less unignited fuel burning in the exhaust).
Especially with aftermarket cams and displacement, more timing advance will generally equate to more MPG. The only reliable way to tune accordingly is trial-and-error, as mentioned above for spark timing tuning. Most often, the top-end of timing that you can without any knock will give you the best combustion efficiency and, therefore, the highest possible MPG.
Now that you've done a good amount tuning, you are ready to take the car to a dyno for more intensive power increase. Raise and lower the respective timing and fueling WOT tables until you make the most power without getting any knock.
Now you have a car that is custom tuned, all by yourself!
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Thanks a million, my car is running great. I had several problems with my car that I would have never known about, but thanks to Solomon we fixed them.
I will definately use you in the future.
Thank you for the page, I'm just starting on my swap, moving a 1994 Impala SS LT1 into a 1992 V6 Camaro RS. I think it will be a lot of fun, but the electronics have me a little concerned and I was real happy to find your page.
I've purchased the TunerCat software as you suggested and am practicing on the V6 just to get the feel for the software.
Again, thanks for the page.
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Do you need a good tune, but cannot afford the down-time of mailing your PCM to me? Do you not have any interest in long-term tuning where you would need to buy your own equipment and tuning software? Now I can loan you one of my tuning kits to allow me to fine-tune your 1986-2003 PCM to match your individual setup, without you needing to send me your PCM. No more downtime, and a fully tuned vehicle!
You can now have the same tuning tips on my website in a handy, printable PDF format, for easy offline reference (over 20 full pages!).
If you decide you like the idea of tuning on your own, visit TunerCat and purchase the C.A.T.S. Tuner software to get started ($69.95). For working on your '94 or '95 LT1 PCM, you'll also need the $EE definition file ($19.95).
Then just get yourself an OBDI ALDL cable and compatible computer, and you're ready! Use my free "Tips" page or buy the PDF version for a small fee, and get started!