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Learning Calibration Basics

At Tom Henry Racing, we've been doing custom tunes for years, but what if you want to try calibration, yourself? Some gearhead luddites whine, “Cars with computers–can't work on them. Too complicated. Too expensive. And, tuning? You gotta be kiddin' me. Only dealers or engineers can do that."

          If you understand what goes on inside your engine and you are computer-literate enough to create an Excel spreadsheet; you probably can learn to tune–or “calibrate,” as engineers say–engine control systems.

Good reading on the subject is Jeff Hartman's, How to Tune and Modify Automotive Engine Management Systems, published by Motorbooks. Additionally Greg Banish’s Designing and Tuning High-Performance Fuel Injection Systems and Engine Management: Advanced Tuning, published by CarTech are excellent choices. Finally,  Harold Bettes' and Bill Hancock's Dyno Testing and Tuning and GM Gen III Powertrain Control Systems by Mike Noonan, also, from CarTech, are good books.

There are home-study courses. If you like reading, there is A Beginner’s Guide to Tuning GM Vehicles with HP Tuners Software, offered by the “Tuning School” of Odessa, Florida. If you like watching, Greg Banish's Calibrated Success has two good training DVDs which it sells through Summit Racing. The first DVD, "GM Tuning Beginner's Guide", covers basics such as tuning for aftermarket injectors, MAF Sensors, volumetric efficiency. The Beginner's DVD focuses use of HPTuners. Calibrated Success also has "GM Tuning Advanced Guide" which covers tuning for aftermarket cams, scaling, tuning ECMs with virtual VE tables and other subjects.

The home-study books have a few confusing statements and some lax proofreading. The DVD's videography and editing were substandard in spots. Nevertheless, the training from both these courses is beneficial–particularly, the DVDs. While all this instructional material is focused on V8s, many techniques apply to calibrating V6es, too.

If you like the classroom environment, both the Tuning School and Calibrated Success offer classroom instruction. If you can afford it, such training is even better. See their web sites for more information.

All Dyno or Some Dyno

An issue which evokes strong opinions amongst calibrators is how much work must be done on a chassis dyno.

Some say the only way to tune is with a load-bearing chassis dyno and anyone doing it with road testing is a rank amateur. Others make a case for only some of the work requiring a chassis dyno with other tasks done on the test track.

Even the training material we used seemed to differ on this issue with Calibrated Success' DVD suggesting a load-bearing dyno is the only way to get the job done right whereas The Tuning School courses suggest it's practical to do some work on the road.

We used a mix of dyno runs and track testing because the cost of doing it all on a dyno was prohibitive. We believe our practical tuning needs were met, but we, also, agree that, to get a "perfect" engine controls calibration may require a lot of chassis dyno time.

Oxygen Sensor Basics

An oxygen sensor (O2S) measures the oxygen in exhaust gases. In 1976, the "lambda" or "exhaust gas oxygen" (EGO) sensor for automotive use was introduced by the Robert Bosch Corporation. Most O2Ses are “narrow-band” devices. They generate 100 to 1000 millivolts, depending on the level of exhaust oxygen, however, their output is linear only in the narrow band of voltages generated when combustion is near stoichiometric. Outside a range of 0.97-1.06-lambda, the signal is nonlinear making useful measurement when the engine is in PE impossible.

Wideband sensors, developed by Bosch in the mid-1990s, vary a reference voltage supplied by a controller. This produces a far more linear output such that, given a low level of back pressure, a "wideband" is accurate from 0.34 to 1.51 λ. A wideband is required for calibrating at wide-open throttle (WOT).  

Learning Lambda

The DVD from Calibrated Success taught us to think in "lambda," a measure of free oxygen in the exhaust identified by the greek letter "λ," rather than air:fuel ratio (AFR).

Combustion is "stoichiometric" when neither unburnt hydrocarbons (rich) nor excess oxygen (lean) are present in the exhaust. When combustion is "stoich," lambda=1 and the engine produces low exhaust emissions, high catalytic converter effectiveness and good fuel economy. Stoichiometric is 1.00 λ regardless of fuel type, whereas AFR varies. For example: stoich for pump gas is often said to be 14.68:1, but actually varies 14.55-14.8:1 depending on humidity and the gasoline blend. "Indolene Clear," an emissions test fuel used by car companies, is stoich at 14.56:1 and that value sometimes appears in GM calibrations as a reference. Oxygenated gasolines containing up to 10% ethanol, (aka "E10"), are 14.08-14.25:1. Another, E85, which is popular with racers, is 9.87:1. If you think lambda, at 1.0 λ; they're all burning stoichiometrically.

When an engine is under high load, its air: fuel ratio must be rich to prevent excessive combustion temperature and detonation. Calibrators call this "power enrichment," or "PE" and, on straight gasoline, it's about 12.5:1, air:fuel. With E10, it's about 12.2:1 and with E85 it's 8.4:1. If you're using lambda, in PE, those are all 0.84-.0.86 λ.

Thinking in Lambda makes calibration easier.

Fuel and all the Trimmings

When engine controls operate in "closed loop," they “trim” fuel flow by the amount necessary to have combustion as close to 1.0 λ as possible. If the base fuel schedule has the engine running about 1.0 λ, trim values are mid-scale, usually designated 128 or 0. If lambda is less than 1.0, the engine is rich, so the system decreases fuel flow to bring combustion back to stoichiometric and the trim numbers will be lower. If lambda >1.0, the engine is lean, so the system increases fuel flow and the trim numbers are higher.

There are two kinds of fuel trim, "short term" (STFT) and "long term" (LTFT). The first changes quickly and is not retained in memory. If the ECM detects a trend in STFT, it transfers that to LTFT, which is retained in memory, such that STFT varies from zero, less often. The total fuel trim, short-term plus long-term, is added to the base fuel cal to keep combustion stoichiometric. 

Finally

The instructional material might take a week or so to digest. After that, with HPTuners' VCM Editor, you can download a calibration from your Camaro or Firebird and start practicing the techniques you learned.

In couple of weeks, you'll be ready to "flash" the car's computer with your first calibration. Indeed, you'll have more to learn, but you'll know the basics.

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