GM Engine Tuning and Examples

GM PCMs used in the early TPI systems were a simple MAF based, bank-to-bank system. Early LT1 systems were speed density with sequential control with knock control. The LT1, 3800 V-6, and LS1 are primarily MAF based, sequential injection with knock control. For LT1, spark is calculated in terms of MAP, just like speed density. For LS1 applications, load is calculated as g/cyl. Engine knock is monitored by a sensor installed into the water jacket. GM knock strategy as a whole is generally good. Because of the accuracy of this system, their engineers were able to deliver production calibrations that ran the engine much closer to the knock threshold resulting in increased power and economy. When calibrating for best power on a modified engine, this knock routine can be a very useful tool. By choosing a slightly aggressive spark curve and allowing the knock circuit sufficient authority (at least 4 degrees of retard possible), the best timing can quickly be found. If the target ignition value is beyond the knock limit, the PCM should detect this knock and the scan tool should display a corresponding retard value that has been applied to alleviate the condition. Recognizing the hysteresis of knock, this whole value does not necessarily need to be removed from the target advance. If the PCM required 4 degrees of retard to eliminate the detected knock, it may only require 2 degrees adjustment to the target value to avoid initially crossing the knock threshold.

 


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This sensor often proves to be sensitive to valvetrain noise that occurs in the same frequency range as true engine knock. For the LT4, a newer, less sensitive sensor was used to reduce the chance of false knock resulting from its slightly noisier roller rockers. Some aftermarket valvetrain combinations may simply prove too noisy for the knock circuit to ignore. In these rare instances, knock can be disabled by setting maximum retard to zero and carefully calibrating the spark tables on a dynamometer. Since the knock circuit is no longer available, a few extra degrees of safety margin should be left to avoid detonation during worst case loading.

 

The GENIII (LS1, LS6, LQ9) engine family represents a large step forward in performance and technology for GM. The only things it has in common with its predecessors are bore spacing and manufacturer’s label. Although still pushrod activated, it rivals the specific output and emissions of the most advanced OHC competitors with potential for tremendous performance increases.

The GENIII (LS1, LS6, LQ9) engine family represents a large step forward in performance and technology for GM. The only things it has in common with its predecessors are bore spacing and manufacturer’s label. Although still pushrod activated, it rivals the specific output and emissions of the most advanced OHC competitors with potential for tremendous performance increases.

 

GM LT1, 3800, and LS1 applications use similar Delphi MAF sensors. These use the familiar hot wire element design, but with a frequency rather than voltage output to the PCM. The range of the factory MAF for these cars is surprisingly wide, making them flexible enough to work even in most supercharged applications. Their design allows them to be installed as either draw-through or blow-through for boosted applications with minimal concerns. The integrated honeycomb on the inlet side serves to straighten the flow in front of the metering elements. Removing this screen does almost nothing to improve flow.

The Delphi sensors also have a “wing” spanning the center of the measurement housing to provide an ideal flow past the actual metering wires. Enthusiasts often eliminate this post, mistaking it for a flow restriction. Doing so does indeed slightly reduce the pressure drop across the MAF sensor, but it also skews the output downward by 7 to 10%. The lower calculated loads both lean out fuel and advance timing, running the risk of premature knock. This lean condition must be adapted by the PCM’s closed loop fueling correction strategy before the vehicle drives properly. Even then, actual load calculations will be slightly off during all conditions unless the MAF transfer function is remapped to reflect the new actual output. If a “ported” MAF is to be used, plan on spending some time building a new transfer function before doing much other calibration work.

 

The GM MAF sensor is also a hot-wire element design. The large bore minimizes pressure drop across the element for maximum power. Output is in frequency instead of voltage like other MAFs. (Nate Tovey)

The GM MAF sensor is also a hot-wire element design. The large bore minimizes pressure drop across the element for maximum power. Output is in frequency instead of voltage like other MAFs. (Nate Tovey)

 

The ETC safety system of the C5 Corvette checks actual current MAF values against this table. If the values in the table are exceeded, a fault is triggered and throttle opening is limited. The table is shown this time in the SCT Advantage software.

The ETC safety system of the C5 Corvette checks actual current MAF values against this table. If the values in the table are exceeded, a fault is triggered and throttle opening is limited. The table is shown this time in the SCT Advantage software.

 

The Main Volumetric Efficiency table provides the GM controller with expected airflow data used to determine transient fueling. The table is shown here using the HPTuners VCM Suite 2.1.

The Main Volumetric Efficiency table provides the GM controller with expected airflow data used to determine transient fueling. The table is shown here using the HPTuners VCM Suite 2.1.

 

The GM LS1 PCMs employ a complex active intake manifold model in the form of the “base volumetric efficiency” table. Much like a pure speed density system, this table is a map of predicted engine performance based on speed and MAP. It is used to calculate transient behavior faster than would otherwise be done by reading MAF data. Serious changes to the engine’s airflow characteristic (such as a change to a longer duration camshaft) should be accommodated in this table even if the MAF curve is 100% accurate. The PCM allows for a great deal of flexibility, but idle and off-idle regions in particular can be sensitive to cam changes. Changing this table to more accurately reflect engine performance goes a long way toward smoothing out startup and low speed behavior. Changing the high load (95 to 105 kPa) portion of this map has the net effect of adjusting acceleration enrichment.

 

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Accurate mapping of this table’s data can often be accomplished by simply unplugging the MAF sensor. This triggers a MAF performance fault code and forces the PCM to default to operating based on the values in the base VE table. At this time, the table can be calibrated much the same way as a standard speed density system. It is possible to calibrate just this table and leave the MAF sensor out of the system by turning off the fault codes associated with the MAF sensor. However, better flexibility, weather, and load compensation can usually be had by restoring proper MAF function after the base VE table has been adjusted for the specific engine combination.

With the introduction of the C5 Corvette, GM introduced ETC control on the LS1. This was a pedal follower strategy with a simple safety check table. ETC performance was monitored by checking MAF input against the values in the “g/cyl for engine speed versus throttle rotation” table. If the MAF input exceeds the current value of this table, it is determined that a fault must exist with the mechanical portion of the ETC and a limp-home mode is triggered that limits engine and vehicle speed. The installation of a super-charger onto one of these engines easily exceeds the factory values for this table. To prevent prematurely triggering a limp mode, this table must be adjusted to accommodate the new higher flow rates. Great care should be taken to only increase the cells necessary. This avoids premature safety activation, and even then only by just enough for proper operation. Blindly increasing all areas of this table, or increasing the necessary areas by too much, runs the risk of failing to detect a genuine problem. Any calibrator who ignores the importance of this safety table leaves himself open to a tremendous law-suit if the vehicle surges out of control causing an accident or injury!

Aftermarket access to most GM engine control modules is readily available today. Packages such as HPTuners, CarPuting’s LS1/LT-1 Edit, C.A.T.S., and SCT advantage allow the aftermarket calibrator open access to almost all factory parameters in the PCM. These windows-based tuning packages have the ability to read the stock file from the car, allow the calibrator to edit the files using real engineering units, and reflash the modified file into the vehicle with just a laptop and adaptor cable. Most have access to the same set of GM parameters, with slightly different descriptions or chart layouts. As long as one takes the time to carefully examine the values and layout, almost any of these will get the job done. The user interface and ease of use varies from package to package, so take a look at all of the available options before spending a significant amount of money on either. Let’s take a look at a few examples.

 

Example 1:

A cold air kit, “shorty” headers, and exhaust are installed on a 1995 Camaro Z28 with a manual transmission. First, the stock file is read out from the vehicle and saved. A copy of this file is loaded into the editor and saved with a unique name. Again, we take a moment recognize what physical changes have been made to the vehicle that require calibration changes. In this case, the MAF itself remains stock, but its installation in a new air path may shift its output. The vehicle is driven at a steady speed on the dynamometer, and MAF output frequency is recorded along with fuel trims and delivered lambda. If the delivered lambda corrects back to l = 1 at cruise or idle, the fuel trims will show the necessary adjustment to the MAF curve. Typically, this free flowing inlet leads to positive fuel trims (fuel is being added to maintain the desired ratio), so the current MAF frequency is noted along with the average long-term adjustment. This same adjustment is applied to the MAF transfer function to bring the PCM’s airflow calculations back into alignment with reality. It is very important that this is done in steady state to avoid the influence of acceleration enrichment. Multiple points are taken along the MAF curve in the cruise areas of the base fuel map. A global trend is usually found in the MAF offset due to the cold air kit. This correction can be applied to the MAF transfer function and extrapolated out to maximum output.

While we have the file open in the editor, let’s take a look at a couple other items. Most customers wish to disable the Computer Assisted Gear Selection (CAGS) that forces the one to four upshift at low loads. GM implemented the strategy to force drivers to operate the vehicle in such a manner as to improve fuel economy in the city. This feature can be disabled by setting the minimum activation temperature to a very high value such as 255 degrees F. It can also be defeated by changing the maximum throttle position to enable to zero degrees. Much like the Ford applications, there are performance benefits to triggering fans earlier to improve performance for GM cars as well.

 

The MAF sensor on the GENIII engine is disabled by setting the MAF Fail Frequency to zero. This forces the computer to ignore the MAF signal and run in speed density full time.

The MAF sensor on the GENIII engine is disabled by setting the MAF Fail Frequency to zero. This forces the computer to ignore the MAF signal and run in speed density full time.

 

In order to maintain a consistent commanded air-fuel ratio during high-load testing, the Catalyst Overtemp Protection (COP) feature is disabled prior to testing.

In order to maintain a consistent commanded air-fuel ratio during high-load testing, the Catalyst Overtemp Protection (COP) feature is disabled prior to testing.

 
The GENIII PCMs use the “WOT PE vs RPM” curve to dictate an air/fuel ratio under heavy loads. The units are equivalence ratio, the inverse of lambda.

The GENIII PCMs use the “WOT PE vs RPM” curve to dictate an air/fuel ratio under heavy loads. The units are equivalence ratio, the inverse of lambda.

 
This modified table shows the results of a tuning session with our test vehicle as shown in example 2. Notice how timing does not drastically drop at high speeds and loads as before.

This modified table shows the results of a tuning session with our test vehicle as shown in example 2. Notice how timing does not drastically drop at high speeds and loads as before.

 
Aftermarket tools like the HPTuners VCM Suite perform double duty as programming and datalogging tools for almost any popular GM application. (Nate Tovey)

Aftermarket tools like the HPTuners VCM Suite perform double duty as programming and datalogging tools for almost any popular GM application. (Nate Tovey)

 

Looking at the values in the “WOT PE Equivalence Ratio,” we can see where GM has calibrated the vehicle to run rather rich to keep catalyst and exhaust gas temperatures down. This table uses Phi (j, equivalence ratio), which is the inverse of lambda, so j = 1.25 equals 25% enrichment, l = 0.75, or A/F = 11.0:1. Looking at the factory values here, it’s easy to see where significant power increases can be made. By choosing a more reasonable value such as j = 1.1, or l = 0.9, we see a better power match to this naturally aspirated engine.

 

The Chevrolet Camaro SS as delivered with 320 hp in 2002 was king of the ponycar wars. This particular example is equipped with a supercharger, bigger cam, and about double the power. (Nate Tovey)

The Chevrolet Camaro SS as delivered with 320 hp in 2002 was king of the ponycar wars. This particular example is equipped with a supercharger, bigger cam, and about double the power. (Nate Tovey)

 
The fifth-generation Corvette came with the introduction of the LS1 engine family. Output of these factory engines can be more than doubled with the right bol-tons and calibration. (Nate Tovey)

The fifth-generation Corvette came with the introduction of the LS1 engine family. Output of these factory engines can be more than doubled with the right bol-tons and calibration. (Nate Tovey)

 

Timing can be advanced in the high rpm table too. The installation of a free flowing exhaust reduces back-pressure and cylinder temperatures. This in turn buys us a little greater knock margin. Timing is increased slightly to move closer to MBT at full load. If you go too far here, you are likely to see some activity on the knock retard when datalogging at WOT. Once the vehicle can make a WOT pull to redline without any knock retard activity, the aggressiveness of the factory knock routine can be reduced. This can be done by limiting the “maximum knock versus RPM” table to roughly 4 degrees, lowering the “fast knock gain” table by about 30% above 3,000 rpm, and increasing the “knock recovery” table by about 50% above 3,000 rpm. This still allows the knock routine to remain active, but makes it less intrusive in cases of light knock.

 

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Example 2:

A new camshaft and ported cylinder heads are installed in a 2002 Corvette. The addition of the heads and cam introduces a significant departure from the factory engine performance. As a result, dynamic changes in airflow can be under-predicted by as much as 30%. Correcting this requires fundamental changes to what the PCM expects to see for airflow at a given speed-load point.

As mentioned earlier, the GM dynamic air strategy on LS1 family engines is controlled largely by the “Main VE Table.” Any time the PCM detects a change in airflow (not steady state), it references this table for its first attempt at fuel prediction. This can include the rolling or hunting idle that may accompany a large cam at stock idle speeds.

We begin by reading the stock data from the vehicle, saving, copying and renaming our working file. The next step is to set a more realistic idle speed. The LS1 can have as many as four idle speed tables to suit combinations of A/C on-off, and “park/neutral” versus “in gear.” All of the appropriate tables should be increased globally to begin.

You can always go back later and slow idle speed to get the “muscle car” idle note and shake if the customer desires later. For cams in the 230 degree (at 0.050” lift) duration range on a stock displacement engine, it would not be unusual to increase warm idle speed to about 950rpm for initial work. This should save some frustration from constant stalling during the tuning process. Remember, you can always go back and lower this after you’ve calibrated the airpath model correctly.

Fundamentally, the larger cam shifts the power band of the engine upward in speed. As such, low RPM volumetric efficiency usually drops from stock. Somewhere in the midrange there will be an inflection point where the new VE matches the stock values. Above this, the expected VE will be greater than stock.  (How else would we be making more power out of the same engine, right?) It is this change in VE versus RPM that must be entered into the “Main VE Table.” Otherwise, idle speed changes are met with slight over-fueling (and the appropriate increase in torque/speed), leading to surging. Likewise, higher RPM tip-in events would be under-fueled, leading to hesitation or bucking.

There are several ways to correct the main VE table on an LS1 PCM. Each can be equally effective as long as the calibrator uses a sound approach. The first method would be to disable the MAF sensor completely. The MAF is disabled by setting “MAF Fail Frequency” to zero and disabling error codes P0100/P0101. This forces the PCM into a failsafe mode where the air mass is calculated strictly from the main VE table like a conventional speed density system.

At this point the VE table can be calibrated in steady state, open loop on the dyno using a wideband oxygen sensor or by applying long-term fuel corrections to the values in the main VE table. Generally speaking, the VE changes resulting from a change in cam have a greater dependency upon RPM than MAP. This means that one can usually apply a correction to an entire RPM column (i.e., all load points at 2000rpm would receive the same multiplier) in the main VE table, at least to start. The more diligent one is during this process, the better the transitional behavior will be for the car.

Many DIY enthusiasts and tuners swear by leaving the car in this speed density mode. However, if the MAF is properly calibrated as well, steady state fueling accuracy will also improve. The MAF also gives the PCM the ability to respond to changes in weather conditions more immediately rather than depending upon adaptive strategies from the IAT and MAP sensors. In this case, the MAF transfer can be calibrated as before using steady state and WOT sweep dyno measurements.

The LS1 PCMs also employ a catalyst overtemp protection feature. For initial tuning, this should be disabled to make sure that target WOT lambda remains constant. The WOT target ratio is also set with the “PE versus RPM” table, using equivalence ratio as units. This table should be adjusted to a reasonable target value prior to WOT dyno testing. As before, errors between the target and delivered air/fuel ratio can be adjusted in the MAF transfer function. If catalyst temperature protection is still desired, it can be reactivated after all WOT mapping has been completed.

Spark tables also require significant changes for this example. The larger duration, and consequently overlap, of the new camshaft results in more natural EGR. This means that there will be an almost global need for increased spark advance, including idle and light cruise conditions. These changes need to be applied to the high and low octane base spark tables as well as idle spark tables for “park/neutral” and “in gear.” It is worthwhile to perform a number of checks at constant speed/load on the dyno in the midrange find out just how far off the new MBT timing is from stock. For the 230° camshaft in this example, it would not unusual to need an extra 4 to 5 degrees of advance at part load to reach the new MBT. Making this change has the added benefit of increased efficiency, fuel economy, and throttle response.

WOT spark tuning is performed as normal, with special care given to observing knock retard values when advancing spark near the new torque peak. It will likely be found that a significant amount of spark advance can be added above 5,800 rpm in this case, with continuing progressive increases until redline. Remember, the more usable engine speed/RPM that is added, the more timing will be required to offset the mechanical delay and dropping cylinder pressure. Again, the shape of the torque curve shows where timing should be added to reduce losses in cylinder pressure.

As a final check, the ETC safety strategy should be reviewed. The code 1514 “Maximum Airflow vs. Throttle Rotation and Engine Speed” table should be checked against current ariflow values. Ideally, there should be about 20% safety margin between nominal airflow values and this table. Too much clearance runs the risk of not detecting a genuine ETC fault and too little can have the driver cursing as he constantly hits a false error and the resulting limp mode.

 

Example 3:

A positive displacement super-charger is added to a 2001 Silverado Z71. Here, we pay special attention to the change in the MAF transfer function due to the supercharger’s new inlet system and set the appropriate fuel control values. Since the heads and cam have not changed, it is unlikely that there will need to be much work done to the Main VE Table. Any time the vehicle enters boost, the MAP sensor immediately jumps to the top value for the VE table. Larger fuel injectors will be the first major change to the hardware. The flow rate of the new injectors should be entered into the “Injector Flow Rate” table. This table uses kPa of vacuum as its input, so be careful when adjusting. “80 kPa” in this table really represents strong vacuum, and “0 kPa” represents atmospheric pressure in the manifold. The slope of this table confirms the decreasing flow with added manifold pressure, as shown earlier in the injector section.(Chapter 5) This table should receive a global shift equivalent to the increase in static injector flow rate. If the new injectors are 90% higher capacity than stock, a multiplier of 1.90 can be applied. If the injector off-set versus battery voltage is known, it should be entered into the “Injector Offset vs. Battery Voltage vs. KPA Vac” table. These changes should result in sufficient injector control to continue with MAF signal correction.

Appropriate target air/fuel ratio values should be chosen at this time. All off-boost areas can be set to l = 1. Catalyst overtemp protection should be turned off temporarily for tuning. WOT power enrichment should be set conservatively to approximately l = 0.78 (PE value of 1.22 in the table) for all RPMs. The trucks employ a WOT PE delay that must be turned off to allow immediate enrichment under boost. During this delay, WOT fueling remains at l = 1. The factory value for this is in excess of 5,000 rpm, which would certainly not be a good idea for a supercharged truck! This value should be reduced to a few hundred RPM above idle to avoid lean conditions and knock under boost.

 

The “PE enrichment delay RPM” is moved from 5,000 rpm downward significantly to provide instant enrichment at low speeds. This supercharger can provide boost very early, so the appropriate fueling must also be added to avoid knock.

The “PE enrichment delay RPM” is moved from 5,000 rpm downward significantly to provide instant enrichment at low speeds. This supercharger can provide boost very early, so the appropriate fueling must also be added to avoid knock.

 

Positive displacement superchargers like the Magnuson Radix system shown here are very popular for truck owners since they greatly increase available torque across a wide range. This particular system uses an Eaton Model 112 compressor. (Image courtesy of Magnuson Products)

Positive displacement superchargers like the Magnuson Radix system shown here are very popular for truck owners since they greatly increase available torque across a wide range. This particular system uses an Eaton Model 112 compressor. (Image courtesy of Magnuson Products)

 

The spark advance tables for this truck were calibrated for 87-octane fuel. When running a supercharger, hopefully we are working with a minimum of 91-octane fuel. Most of the part load tables already have the truck operating at MBT timing during cruise, so these regions will not require much adjustment, if any. Examine the spark advance values in the high-octane table above 0.8g/cyl.  This is the region where the truck begins to develop boost. The factory values are very low here to avoid engine damage (knock) during towing even though these load values are almost never reached on a stock vehicle. You may find that the stock values for 87-octane are not that far off from a conservative timing for our supercharged 91+ octane application. The torque curve dictates the shape of the spark curve as long as temperatures remain consistent. Make sure that this table still has a progressive retard of spark with increasing load. That way if the weather changes later, the new load is detected by the MAF and the appropriate spark advance can still be calculated by the PCM. This has the added benefit of adapting for pulley changes in the future without too much drama.

A key item to check on automatic transmission equipped LS1 PCMs is the “Max Engine Torque” value. Most PCMs have this set to 350 ft-lbs. This should be increased to avoid torque limiting interference with delivered spark timing near the shift points. Increasing this also results in crisper shifts, since more torque is allowed to be delivered to the transmission in the first place. There is a table labeled “Torque Reduction for Upshift” that has a similar effect. This table in used to prevent overpowering the clutches inside of the transmission. Decreasing the values in this table allows increased torque during shifts, but possibly at the cost of transmission durability. 4×4 vehicles also have axle torque limit scalars that should be increased if full torque is desired at all times.

At this point, normal MAF mapping is done as before. Steady state part load at l = 1 is performed first, and WOT sweeps follow to map the top end of the curve. Once delivered lambda equals target lambda, attention can be turned to spark advance or leaning out the target PE values. Keep in mind that the vehicle still has catalysts that must not overheat, so a little richer is better for durability. If catalyst temperatures still get too hot during WOT runs, the COT feature can be reactivated.

The added airflow of the super-charger will almost certainly extend the usable speed range of the engine. After all WOT mapping has been completed, the shift points can be reset to take advantage of the new power band. This is done with both a fixed RPM scalar for each gear and a table showing vehicle speed versus throttle position for upshifts.

 

Written by Greg Banish and Posted with Permission of CarTechBooks

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