Since having the right air/fuel (A/F) ratio will be the difference between making power and grenading your engine--though modern EMSs will almost never let an engine damage itself--tuners spend a lot of time getting it just right. Too lean and the engine pings, too rich and you lose power. All tuners rely on a wide-band UEGO A/F meter for fuel tuning. Wide-band A/F meters provide rapid and precise measurement of A/F ratio by sampling the exhaust gas composition as it exits the engine.
For the naturally aspirated M5, Powerchip claims to target an A/F ratio of 12.9:1 at WOT for optimal power before detonation becomes a problem. I keep mentioning WOT because when the throttle is held open, the engine becomes loaded and the ECU will think the driver is asking for power, not economy. The ECU will kick into a power enrichment (PE) mode which uses a separate map providing a richer mixture and not bound by closed loop operation or long term adaptive corrections (see below). By performing several WOT runs with the stock software, the tuner can determine how much fuel to take out of the stock PE map, which is usually rich to prevent damage to the catalytic converters. Knowing what the stock WOT A/F is and what cells need to be changed, Powerchip will go in and change the values of the target address proportionately to get a leaner mixture to make more power. There's a lot of guessing and checking involved, but the goal is to have an A/F ratio of 12.8 to 13.0 throughout the power band at WOT.
Stoichiometry and closed loop operation
The ideal or stoichiometric A/F ratio for complete combustion of the intake charge is 14.7, meaning 14.7 parts air to 1 part fuel by mass. This is the target A/F for highway cruising or idling when power is not required, as it saves the most gas and maintains the cleanest emissions. Modern cars can maintain an A/F of 14.7 through a process called closed loop operation. When cruising, the ECU will kick into closed loop mode and maintain stoichiometric combustion by examining the exhaust gas oxygen content via an O2 sensor. If the sensor detects oxygen then the mixture is lean; if there is no oxygen then the mixture is rich. For lean mixtures, the ECU will add fuel and vice versa for rich mixtures. The end result is a cyclical or continuous loop process where the ECU constantly corrects the amount of fuel delivered to maintain 14.7 A/F. If the car consistently requires fuel to be added or taken out, the ECU eventually remembers that amount of correction and stores it as a part of its long-term memory. This way, the next time the engine goes into closed loop operation it will immediately implement that fuel correction so the process of trimming fuel to reach 14.7 takes less time. When you put your foot down, the ECU will automatically jump out of closed loop operation and richen up the mixture for more power.
Road tuning with Powerchip...
Road tuning with Powerchip using the Innovative UEGO hand held monitor and laptop tuning.
Even with the right A/F ratio, an engine won't necessarily make power unless the spark is timed sufficiently. For naturally aspirated cars, advancing ignition timing will generally help increase amount of energy harnessed from the charge expansion (more energy in the powerstroke). Too much spark advance will cause the engine to start knocking. The ECU normally controls spark timing by accessing an ignition map, which has specific timing for different loads and rpm. Even though this is programmed in, different conditions such as fuel quality and air density might cause the engine to knock prematurely. That's why the ECU constantly monitors knock via a knock sensor. Knock sensors are specially designed microphones that are tuned specifically for an engine and are sensitive to the frequencies that particular engine pings at. Should the sensor detect the onset of knock, the ECU will retard spark timing until the knocking subsides. Because the factory knock sensor is so accurate, tuners prefer to observe knock sensor activity via the ECU and the OBD-II port while they are tuning.
Powerchip tunes the part throttle and PE ignition maps to push the base values closer to the knock limit because they claim the adaptive features in the stock software will not advance the spark far enough toward the knock threshold. In the case of the E39 M5, each cylinder's spark timing can be individually adjusted in increments as small as one-thousandths of a degree. This again requires a guess and check process where tuners readjust the stock spark curve to where they think it should be and monitor for knock. If the engine is starting to knock at that point, then spark timing for that rpm and load would be adjusted appropriately. In theory this might sound simple, but knowing when and how much requires a lot of experience. This is just for the base spark map; there are actually several dozen maps that control ignition timing based on factors ranging from throttle position to coolant temperature. The M5 actually has five incremental spark maps in increasing aggressiveness.
UEGO: Some tuners will use...
UEGO: Some tuners will use the UEGO as a sniffer in exhaust to read A/F while others will mount the sensor on the down pipe before the catalytic converter to get faster and more accurate A/F readings.
Just like fuel control, the ECU has adaptive spark capabilities similar to closed loop operation, but uses the knock sensor for feedback instead of an oxygen sensor. How much the spark curve can adapt varies with the car.
When the ignition map is tuned, most tuners will go back and refine the fuel calibration further since power is affected by spark and fuel interacting with each other. A change in one parameter will require the other to be appropriately compensated.
Powerchip estimates it takes roughly two weeks to develop a basic chip upgrade for a new platform, with 90% of the tuning time on the street and 10% on the dyno.
Tuning for hardwareIn addition to basic spark maps, fuel maps, rev limiters and speed limiters, there are many other areas a tuner has to modify for alterations in hardware. Since BMWs come tuned so close to their optimum output, some tuners like Dinan, GIAC and Conforti Performance Chips prefer to do tuning for hardware-modified cars. Modifications such as supercharging or headers dramatically change the airflow characteristics, and thus the A/F ratio, of a car. In the case of supercharging, tuners have to correctly interact with the factory software so that it will recognize and accept boost where there was never meant to be any. Significant alterations like this require the stock fuel, spark, e-throttle, VANOS or VALVETRONIC and transmission maps to be almost completely remapped, not just modified. All this gets unimaginably complicated since all of these functions interact with one another. Some of the best tuners can also change the size of their maps to add resolution for finer tuning where needed. Major alterations like this also require modification of the onboard diagnostic algorithms so that the car doesn't think there's a sensor error when it sees boost. Dealing with diagnostic code operations can consume as much time as the tuning itself.
Powerchip claims the stock...
Powerchip claims the stock A/F ratio was lean due to increased exhaust flow. In this case the mixture was richened to obtain power. The A/F measurements are also lean because the exhaust gases were sampled after the cat. Dyno data courtesy of Powerchip and Evosport.
As you'll see online, there are a lot of differences in opinion. Tuners will inevitably have differing views on what needs to be tuned on a car and how. Some might target different A/F ratios, or take A/F measurements at different locations, or use different methods to compensate for added airflow. Obviously each will say they make the best product for their market segment. For the consumer, probably the best way to gauge the talent of a tuner is seeing how versatile the tuner is at modifying a given ECU to accept different go-fast parts from a basic exhaust to a complete engine build. How much an O.E. manufacturer is willing to interact with a tuner or support their products is also another excellent indicator. The really good tuners are the ones that dealers go to for help in reflashing vehicles sent in for repairs. Peak horsepower is probably the worst indicator of how good a chip is.
EVOSPORT, Powerchip's strategic...
EVOSPORT, Powerchip's strategic partner here in the US did the back to back software flashing for our test M5.
Even though this article concentrates on simple tuning methods and concepts used for a naturally aspirated car, the processes of tuning other cars follow similar fundamental concepts of fuel, spark and boost tuning using knock sensors, wide-band A/F meters, dynos and most importantly road tuning. For Porsche and Audi/VW, look up other talented tuners for like APR, GIAC, Neuspeed and REVO. When you are ready to make a chip purchase, make sure you do you do your homework and don't fall for a generic sales pitch. Remember, a good chip is a thoroughly tuned one, not one that just makes good peak power.