GM EFI Tuning for Beginners
If you’ve been around the GM EFI scene, whether it is perusing the messageboards, races, car shows, etc. then perhaps you have heard of the dreaded torque management. This is one of many safeguards that General Motors engineers have put in place to protect your Gen III or IV engine and its drivetrain from your heavy right foot. In some cases, these safeguards protect overzealous drivers from themselves. However, if you are reading Street Tech, the chances are these limits shouldn’t apply to you. We stopped by The Tuning School in Odessa, Florida to take an inside look at how to remove these electronic handicaps from a stock LSx vehicle – my 2008 Chevy Trailblazer SS – using HP Tuners software. While there are some slight differences between the E38 computer and earlier designs, most of these tables and general concepts are applicable to all Gen III/IV engines.
Before we begin, let’s talk about some basic background on GM EFI. The 1985 Corvette and the 1986 Turbo Buicks began the modern age of port fuel injection. The precision and control of port injection literally dropped a full second off the Corvette’s quarter-mile time, and almost two seconds on the Grand National (an intercooler didn’t hurt either). However, these early PROM-based computers weren’t nearly as aftermarket-friendly as the later Flash-based (OBD I & II) computers we have now become accustomed. In the modern world of EFI tuning, it is all about precision. We have injector flow data, wideband O2 sensors, full data logging, easy-to-use software, and all sorts of resources at our fingertips that have taken tuning from some sort of black art to science. If you are new to tuning, then you may be wondering: what is a “flash-based computer?” The term “flash” refers to how the computer’s calibration is modified. With a PROM-based computer, a “chip” would physically be installed on the circuit board to modify the hard-wired calibration. However, the modern Powertrain Control Module (PCM) or Engine Control Module (ECM) operates more like a regular computer. By plugging into the OBD I/II port, hardware and software such as HP Tuners can scan, download, and upload. Using the scan feature, the software monitors all of the sensor inputs going into the ECM, records it, and can play it back to review. This is extremely valuable in tuning, allowing a calibrator to see exactly what needs to be changed and under what conditions (RPM, temperature, etc). To make changes, a tuner will download the current calibration and start making changes. Once completed, the modified tune file is saved and uploaded to the ECM, rewriting the calibration. As wonderful as that sounds, once you open up the software it is immediately apparent that it isn’t as simple as adding or subtracting a little timing and a few percent of fuel. HP Tuners software is incredibly user friendly, however, there are simply too many tables to jump right in without knowing what you are doing. This is exactly why we’d recommend getting some EFI background; CarTech Books has a number of great resources. However, once you are ready to start playing around, The Tuning School is the best way to get your hands dirty under supervision. The Odessa, Florida-based school has both live seminars and at-home learning to walk you through HP Tuners software and the process of tuning a vehicle. Certificates are available for various levels of training, and excellent customer support. Bob Morreale gave us a taste of the school’s Level 1 GM Tuning course by introducing the method and demonstrating on the painfully stock Trailblazer SS. Note: the following is not meant to be a step-by-step tuning guide. It is only meant to introduce basic GM EFI tuning and what is capable on even a stock vehicle. We’d recommend training before attempting EFI tuning.
Once Bob Morreale of The Tuning School downloaded the 2008 Trailblazer SS’s stock tune to his laptop via the HP Tuners interface (hardware) and VCM Suite (software), we took a look at the Torque Management tab under the navigation for Engine. The OEM calibration wants to limit engine output (via spark advance) to protect the transmission and other components. There is a hard cap on the input and output as well as per gear. Notice that six gears are listed, even though the TBSS only has four. The TBSS’s E38 ECM is also used on the fifth-gen Camaro and C6 Corvette, which both offer a 6-speed automatic. If you wanted to convert from a 4- to a 6-speed, the framework already exists in the software to control it.
Brake Torque Limit is another cap that prevents you from making a hard launch from a standstill. If you plan on stalling the converter with a little left foot braking, you’ll want to ditch these limits.
Easily the most noticeable aspect of Torque Management is the tip-in limiting. You punch the throttle, and the timing seems lazy. But we’re inpatient and we want all 395hp right now! Here you can see exactly why this happens – the computer limits the output until near the top of the table. By modifying this table you will really feel the difference from stoplight to stoplight.
The OEM tune-up also retards timing at 86-degrees of Intake Air Temperature (IAT). The Tuning School says up to 122-degrees is safe without any change to spark advance, and from 131 to 149-degrees it needs only half as much timing retard.
Now let’s turn our attention to the Fuel tab, under Power Enrich. There are several limiting factors before the ECM lets the engine get into Wide-Open Throttle (WOT) and Power Enrich mode. Bottom left is the PE Delay, and just above that is the Enrichment Ramp In, EQ Ratio, and Hot PE Throttle/Pedal. Hot PE is set from the factory at 87% throttle.
Power Enrich EQ Ratio is essentially the commanded air/fuel ratio. By leaning out this table you can pick up some power, but fuel mileage is the major advantage. And this pig needs all the help it can get! If these values seem odd, keep in mind that they are Fuel/Air multipliers. The stoichiometric ratio of E10 is 14.13, which is divided by this value to achieve the commanded AFR.
Here are the tuned Torque Management tables. With all the Gen III/IV engines the basic concept is to max out each value. You’ll see 6,042 ft-lbs is the max for Brake Torque, Engine Torque vs. RPM vs. Gear, and ETC Tip-in as well as Trans Input and Trans Output. The Front Axle, Front Propshaft, Rear Axle, and Rear Propshaft are maxed at 96,672 ft-lbs. On the Gen V, this is radically different.
The tuned IAT Spark Advance Correction table was zeroed out up until 122-degrees, and the values from 131 to 149-degrees were cut in half using the multiply function.
The tuned PE tables reflect a target AFR of 12.0:1, and a Hot PE at 35% throttle. The end result is that the truck feels much more crisp off the line with increased MPG. Plans are in the works to start modifying this grocery-getter, so stay tuned.
The Tuning School
I have the 98 Pontiac Firebird ECU with the automatic trans code. If I locate a backup ECU that has the 6spd. manual code, can I recode it for automatic? Is it worth having a backup of this rare ECU?