Tom, during my years working with field diagnosis at the dealers, we learned to trust actual airflow measurement. We found the correlation between measured airflow and actual power was pretty good, especially on well mapped engines (it was all production stuff, of course). Stay with me, here, you'll see where I am headed a little further down...
There were two keys to our diagnostic work:
- did the peak airlow measurement (non-supercharged) calculate to the factory power curve, and then appear relational with supercharger fitted?
- for either mode, does the airflow graph (using wide open throttle from minimum useable rpm) exhibit linear change?
Bumps or sags in the power curve, accompanied by changes in dBG of intake harmonics, signals a problem in the airflow measurement at the hot wire. This was usually caused by loose or badly shaped intake components or chambers. If something creates pulsing or turbulence in the airflow, the AFM will have ranges of inaccuracy and engine performance will be non-linear throughout the powerband.
Two formulas worked well for a healthy system:
- 1lb/min airflow x 10 = HP
- 1.32-1.42 x grams/sec = HP (example- a 4-cylinder Camry idles at about 2.5-3 gm/sec or less than 5 HP)
The beauty of positive displacement blowers is that you can choose your horsepower target (based on airflow desired) and then choose an appropriate blower volume based on the maps provided. If you find a need for a little more airflow (boost) through the midrange, but need some control at high rpm WOT, the solution is a simple restrictor plate ahead of the blower intake.
We had some very satisfactory results using a little more blower, held in the best boost-to-temp ratio of Eatons maps, with the intake restricted in the upper part of the power band. That is how you can keep more compression ratio in the engine without breaking ring lands from high RPM detonation.
Restrictor plates are a great way to ease yourself into reliable power, taking small steps at a time. They take away little or nothing when the engines are below the redlne, but ramp back cylinder pressure when the intake event duration is short due to very high rpm. You also take some of the biggest load off the drive, in the blower speed ranges where surprising stuff may come apart. Changing drive ratios can be helpful, but that changes the entire operating range.
For those who wish to work in a more controlled supercharged environment, positive displacement blowers can serve us well and be a little easier on our wallets. I hope there is something useful here...I only worked on these problems after someone else had made the important choices.
There I went again...rambling on and on....
Regards, JimL