The purpose of this article is to provide some initial exposure to OBD-II tuning and the capabilities and state of the technology as it stands today. It is by no means complete. There are several Internet articles that discuss this tuning in far more detail than can possibly be provided here.There are many aftermarket hand-held programmers that enable the installation of generic performance tuning and change some simple operating parameters (tire size, gear ratio, fan temperature, etc.). But for about the same price, you can obtain a package that enables your own custom tuning for more power and alteration of operational parameters far beyond the capabilities of the hand-held programmer. In essence, these packages provide you with the ultimate control of your vehicle's powertrain control module (PCM).Understand that you do this at your own risk. If the programming is done incorrectly you can impact emissions (perhaps violating local emissions laws), do serious damage to your engine, and possibly harm yourself and bystanders. The author consulted with a pro (Bryan Herter of PCM For Less) before attempting any of the programming.
Introduction/Overview
If your GM car/truck is a '96-and-later, it incorporates an OBD-II-based PCM, which controls the transmission as well. For engine control, the PCM takes many sensor inputs (O2, TPS, RPM, MAF, MAP, etc.) and calculates the target outputs (primarily spark advance and fuel-injector-duty cycle).We'll be using HP Tuners VCM Suite Pro for our OBD-II PCM programming. VCM Suite Pro is designed for the home enthusiast, as well as the professional tuner. HP Tuners also offers an Enhanced IO upgrade that allows you to log your own wideband oxygen sensor along with your vehicle's other OBD-II data, for example. The product is easy-to-use and has many powerful features consisting of three primary utilities: VCM Scanner (OBD-II diagnostic scanner), VCM Editor (PCM editor), and VCM Flash (to read and program your PCM). HP Tuners software allows you to change the entries of your PCM's tables to improve the performance of your vehicle. In this article we'll focus on changing your wide-open throttle (WOT) performance to avoid any changes to the vehicle's emissions during part-throttle. The HP package has many advantages. One license can program all of the same type and year vehicles (other OBD-II programming packages can only be used on one VIN), and the HP Tuner package includes an OBD-II diagnostic scanner (that can be used on other OBD-II vehicles) that includes features such as data histo-gram displays that dramatically aid with your custom tuning. The HP software runs on a 300 MHz-and-faster laptop with at least 64 MB of RAM and a serial-port connection.
The Engine and Transmission Diagnostics, System, and Speedometer Menus in VCM Editor.
Reading Your PCMEditing the PCM Parameters
VCM Editor is used to view and modify the PCM's calibration parameters. The PCM contains various switches, constants, and tables that control operation of the engine, transmission, and other aspects of the powertrain. VCM Editor is your window into the PCM. Also, read the entire help file that comes with HP Tuners because it provides a thorough tuning overview.First, open the PCM file in VCM Editor. Under the Edit menu you'll see choices for "Engine" (these are your engine's operating parameters), "Engine Diagnostics" (this is where you can change how your PCM handles engine problems), "Transmission" (these are your A4, M6, and torque man-agement parameters), "Transmission Diagnostics System" (where you set fan operating temperatures, VATS control, and A/C parameters), and "Speedometer" (where you set tire size, gear ratio, and speed limiters).
Engine and Transmission Menus in VCM Editor.
Tuning Basics
We will discuss tuning in three parts: general fuel system adjusting, power enrichment, and timing. It is preferable that you perform tuning in the order of these parts. To maximize the power increase, you'll need to use premium gas to minimize knock. Always save your progression of work so you can review it if you have to and keep notes on the changes you make.
General Fuel System Adjustment
Even though we are going to tune at WOT, we need to do some part-throttle tweaking. Your PCM's fuel system control relies on the pre-cat oxygen sensors to adjust the part-throttle air/fuel ratio to 14.7:1-15:1 for low emissions and fuel economy. The PCM adjusts the fuel-injector pulses based on the values (fuel trims) it has learned. There are two types of fuel trims: LTrim or LTFT (long-term fuel trim) and STrim or STFT (short-term fuel trim). The engine is divided in half, and there are trims for both sides of it referred to as "bank 1" and "bank 2." The sum of a particular bank's trims tells the amount of fuel the PCM is adding or subtracting. For example, if the STrim is four and the LTrim is seven, the PCM is adding 11 percent more fuel to maintain the target air/fuel ratio. The fuel trims are arranged in a table by engine rpm and manifold absolute pressure (MAP). The table is further subdivided into cells that comprise a range of rpm and MAP values. (You will most likely never operate your engine in all of the cells.) The PCM adjusts the STrims quickly to maintain the target A/F ratio during part throttle. The PCM slowly adjusts the LTrims to force the STrims to hover around zero. The first goal is to rough-in the fuel correction to get the LTrims close to zero. How close? Well, GM says +/-10 but the pros say +/-5. First, you are going to use the VCM Scanner tool. Plug the interface cable into the vehicle's OBD-II port and PC serial interface and run the VCM Scanner. Start the engine, go to Tools/VCM Control menu, and hit the button that says "Reset Fuel Trims." This forces the PCM to zero the STrims and LTrims and relearn the fuel correction. Then, go to Scan/Histogram Display and hit the "LTFT" tab. The VCM Scanner will log the various LTrim data (average of both banks) into the histogram as a function of rpm and MAP as you drive. Drive the vehicle, and you'll see this histogram filling up. The goal is to obtain a general feel for your average LTrim value. After you've determined your average LTrim value, we'll need to modify the MAF Airflow vs. Output Frequency Table (Engine/Air Flow menu) using VCM Editor. This table basically provides the calibration (g/sec as a function of the MAF output frequency) of your mass airflow (MAF) sensor. For example, if your average LTrim is eight, this means the PCM is adding 8 percent more fuel. So try scaling the entire MAF table by 1.08 and save your file as a different filename. (The Flow Rate vs. KPA table can also be used to rough in the Trims by adjusting what the PCM thinks the fuel pressure is.)After you've altered and saved the PCM program, you'll have to write the program back to the PCM using VCM Flash. Plug the interface cable into the vehicle and laptop. Put the key in the ignition and turn it to the "on" position, but don't start the engine. Make sure the doors are closed and accessories are turned off. Wait 10 to 15 seconds. Select "Write VCM" from the File menu of "VCM Flash" for standard calibration writes. Select the file you just saved to write to the PCM. Allow the laptop to write the PCM until it displays "VCM Write Completed," which will take a few minutes. Turn off the ignition and (optionally) unplug the interface cable. You'll need to repeat this procedure until you get your LTrims under control. (If this procedure doesn't work, you'll need to modify your volumetric efficiency (VE) table. That is too advanced for this article, but is discussed in more detail in the HP Tuners Help.)
The LTrim Histogram Table in VCM Scanner.
Power Enrichment
When you press the throttle to the floor, the PCM enters power enrichment or PE. The main PE table is the Power Enrich Fuel Multiplier vs. RPM (V-8 Mult. vs. RPM) in the Engine/Fuel Control/Power Enrich menu. During PE, the target A/F ratio is divided by these values. For example, if your target A/F ratio is 14.7:1 and the particular PE value is 1.23, the PE A/F ratio target is 14.7/1.23 or 11.95:1. The best tool for tuning your A/F ratio is a wideband O2 sensor. Tuning goals are 12.5:1-13:1 A/F ratio on naturally aspirated vehicles, and about 12:1 A/F ratio on supercharged vehicles. If you don't have a wide-band O2 sensor you can use the pre-cat O2 sensors. For peak efficiency, the pre-cat O2 should be in the range of 880 mV to 920 mV during full throttle. This is more of a rule of thumb since the stock O2s are not designed for full-throttle readings and are not extremely accurate at that point, but they will get you close. You'll be using the VCM Scanner to log the pre-cat O2 sensors (B1S1--bank 1 sensor 1 and B2S1--bank 2 sensor 1), engine rpm, and throttle position. After the engine has reached operating temperature, make some WOT passes. If your O2 values are higher than desired, this means you are running rich and you'll have to slightly decrease the values in the PE table to lean out. Tweak these values slowly. After you've tweaked the PE values vs. RPM, you can view your curve by hitting the 2D Line tab. You can use the Smoothing button at the Table Editor tab to iron out the data to remove any peaks or valleys. After you've tweaked the PE table and smoothed your PE curve, save the file under a new name and write it to the PCM using the steps from the previous section. You'll need to repeat this procedure until the A/F ratio is where you want it.
Here is the VCM Flash Main Screen.
Timing
Now that you have the WOT A/F ratio where you want it, move on to the WOT timing. The main timing table is the High Octane Main Spark vs. Airflow vs. RPM Open Throttle/Moving in the Engine/Spark Advance menu. There is also a low-octane table that bounds the minimum timing (low-octane) and maximum timing (high-octane). Based on your knock retard, your timing will fall in between these two tables. Some tuners actually copy the high-octane table to the low-octane table. The base timing is provided in a table based on rpm and airflow (in g/cylinder). The lower airflow values (0.24 - 0.32 g/cyl) are where your engine operates under low-rpm load. The middle airflow values (0.36 - 0.72 g/cyl) are where your engine operates under medium load. Values above 0.72 g/cyl are about where your engine operates at WOT. You'll begin tweaking the 0.76 g/cyl and larger values (basically copying the 0.76 values for the higher g/cyl rows). Then you'll use the 0.72 and 0.74 values to smooth the transition from part-throttle to WOT. Some off-road tuners add several degrees of timing below 0.76 g/cyl, which improves fuel economy and throttle response without altering emissions. However, tuning in this area may violate your local statutes. You'll be using the VCM Scanner again to log knock retard. Crank up the engine, go to the Scan/Histogram Display, and hit the "Retard" tab. The VCM Scanner will log the knock retard data into the histogram as a function of rpm and g/cyl as you drive. Also, make some WOT passes to let this table fill up for the higher values of g/cyl, as this will indicate a knock issue with your base timing. The goal is to carefully increase the timing until you get some knock retard, then back off one to two degrees of timing. Add a couple of degrees of timing to the 0.76 g/cyl row and copy this data to the higher g/cyl rows. Use the 0.72 and 0.74 g/cyl rows to smooth the transition to the 0.76 g/cyl and higher rows. Then, observe the timing in graph form by hitting the "3D Surface" tab. This surface should be smooth and not have any sharp peaks or valleys. You can use the "Smoothing" button at the "Table Editor" tab to smooth the data out to your liking. After you've added timing and smoothed the timing curve, save the file under a new name and write it to the PCM using the steps from the previous section. Repeat this procedure until you've maximized engine timing and minimized the knock retard.
Before and after dyno and air/fuel ratio results.
Transmission Tweaking
Altering the transmission data doesn't provide more power, but can provide better 0-60 times and quarter-mile performance. Most trucks benefit from a slight increase in WOT shift points. The more the truck is modified, the higher the typical shift rpm point because the engine makes its peak horsepower at a higher rpm. If you have performed a gear or tire size change and change the gear and/or tire specs in the speedometer setting, it will automatically scale the transmission tables for you. The WOT shift points are changed in the WOT Shift RPM vs. Shift Normal table and is done in rpm values for each gear. To make the downshift of the truck more aggressive, go into the WOT Shift Speed vs. Shift Normal table, which is specified in mph, and move the downshift mph values up a few mph. Since the table will command an earlier downshift, this will eliminate the lag on the downshifts. The part-throttle shift points are done by mph as well, and realized through the Shift Speed vs. TPS vs. Shift tables, and even though there is an area there for WOT throttle ranges, they are not used since the WOT tables become the default. These tables are typically only changed for part-throttle feel on heavier modified vehicles with a stouter camshaft. After setting the shift points, you can work on shift firmness. This is done through the A4 Shift Properties tables. First, you can modify the shift time ("Desired Shift Time vs. Torque Normal") to decrease it to get the transmission to complete its shifts quicker. You can also go through the other shift pressure tables and add or remove line pressure to achieve the shift feel you desire. This is one area where your subjectivity comes into play; everyone seems to have a different idea for how they want their transmission to shift. If you already have a shift kit in the transmission, you do not want to add much in the Pressure Modifiers because you can damage parts and/or seals if you aggressively combine the two. For an off-road vehicle equipped with a manual transmission, you can delete the CAGS "Skip Shift" (forced First to Fourth shift) in the transmission tables by increasing its enable temperature to one that the engine will never reach.Torque management (a set of tables) pulls out timing during shifts to prevent full power going through the transmission during the period between gears. This is done to make the upshift seem smoother and supposedly to add longevity to the transmission. The side effect of torque management is that during the shifts, you can feel the vehicle nose over-under power. Zeroing out these tables (Torque Reduction vs. Torque vs. Shift) or greatly reducing the values will provide a much better feel during shifting when combined with the quicker commanded shifts through the line-pressure tables. Most transmissions do not experience excessive wear from a dramatic torque-management reduction, however, a few people have experienced damage to the transmission. Altering the torque management presents a risk for a stock transmission. If you tow heavy loads with your vehicle, it's best to leave some torque management in the programming.
A look at the Power Enrichment Fuel Multiplier vs. RPM Table.
Test Results
The test mule was an '04 Silverado SS where the only modification was a Magnaflow muffler. We performed dyno measurements at Bryan Herter's shop. We removed the front driveshaft and parked the rear wheels on the rollers. Keep in mind that this was done on a relatively stock SS. If you plan more significant modifications (heads, cam, etc.), then you can generate proportionately more performance improvement. Our tuning gained more than 30 horsepower and nearly 40 lb-ft of torque. We especially gained significant torque down low, which provides the harder launches and better 60-foot times. With the new programming, Bryan also measured the air/fuel ratio to be about 12:1 stock and about 13:1 after the tuning.
Conclusions
Tuning the GM PCM is an excellent way to generate horsepower and get the driving feel you want from the vehicle. LS1 cars typically see 15-20 hp gains, while the truck engines can see 25-45 hp gains (depends on displacement) using premium gasoline. You can also obtain a much better shifting feel and shift points.
The Main Spark vs. Air Flow vs. RPM Open Throttle High Octane Table. | 
The Main Spark vs. Air Flow vs. RPM Open Throttle High Octane 3D Surface Plot. | 
The Transmission Shift Speed Tables. |
Transmission Shift Properties, Torque Management, and M6 CAGS Settings Tables. | 
The test mule on the dyno at Bryan's shop. | |