The holy grail of engine performance

Atty. Jerome G. Neri
The Scrutineer

TUNING an engine to get the best out of it is not an easy task. Extracting the maximum reliable power is always a challenge. A lot of amateur tuners focus too much on the air/fuel ratio as if it is the holy grail of engine tuning. Well, it is not. The air/fuel ratio is just one of many parameters to make an engine perform better. I have heard comments like “my air/fuel ratio is correct but my car is slow.” The key to a properly tuned engine is finding what is called Maximum Brake Torque or MBT.

Maximum Brake Torque (MBT) is the use of optimal ignition timing to take advantage of an internal combustion engine’s maximum power and efficiency. There is always an optimal spark timing for all operating conditions of an engine.

From the definition of MBT, one can conclude MBT is a moving target depending on the operating condition of the engine. The operating condition of the engine would mean the load and the RPM the engine is in. The optimal ignition timing for maximum power and efficiency will differ at every load and RPM point. Thus, the optimal ignition timing at 4,000 RPM at 100 percent load would be different at 7,000 RPM at 100 percent load.

The only way to find MBT is to tune the car on a Dyno. Find the highest possible torque at RPMs of interest by adjusting ignition timing while on the correct air/fuel ratio which would be around 12.5:1 for naturally aspirated engines and 11.5:1 for forced induction engines. Start with a relatively low and safe ignition timing and slowly add to it every pull on the dyno. Torque will increase as ignition timing is added until it reaches a peak, then the torque drops. The highest torque achieved is the MBT. Have several RPMs of interest while tuning. I usually start at 4,000 RPM as my first RPM of interest and every thousand RPM thereafter until redline.

There are a lot of instances where the engine that I am tuning cannot be tuned to MBT. This happens when the engine is knock limited. An engine is knock limited when there is engine knock before it can reach MBT and therefore it is only safe to tune the engine to the point before knock occurs. Engine knock, or pinging, occurs when a separate pocket of air-fuel mixture ignites after the spark has ignited the air-fuel mixture within the combustion chamber. The sound of knock is audible with the use of a knock microphone or can be seen in the datalogs an Engine Control Unit that can sense knocks.

What makes an engine knock limited? Usually too much compression for the quality of gasoline being used and/or too much boost pressure in the case of forced induction engines. The best remedy for this situation is to use a higher octane fuel to be able to tune the engine to MBT.

When I tune, I look at the torque numbers because my goal is MBT and not horsepower. The reason for this is that horsepower is a derivative of torque. The equation is horsepower = torque x RPM/5252. With this equation, torque and horsepower always are equal at 5252 RPM. That is the intersection of the torque and horsepower curve all the time. Thus, if someone shows a dyno graph where the torque and horsepower do not intersect at 5252 RPM, something is wrong and that reading is false.

Tuning the proper way has given me and my team a lot of success in racing over the years. Our engines have always been running at peak performance. We even have outperformed cars that on paper should have been faster than ours. Proper tuning is the holy grail of engine performance. Tuning on the streets can get a car running smooth and clean, but without the capability of measuring the torque, the engine will not be at MBT at the RPMs of interest and therefore never optimized. Going to a dyno facility may cost more than tuning on the streets, but it is worth the money.