Question: Effect of lowering blower overdrive?? More or less power?
Bob?s answer: A trend now is gearing the blower to the racetrack. That is done with rear end gearing, tire size, shift point, and blower overdrive. Adjustment of any of those will change the speed attained by the blower at the end of the drag race or the straightaway.
One predominant blower manufacturer said their 14-71 Roots blowers work good to about 12,000 blower RPM and not well above that. So if you are spinning their blower over 12,000 RPM in your combination, slow it down to 12,000 blower RPM such as with reduced blower overdrive.? Your racer may be faster.
In addition, there is a lot going on at the inlet. The size of your inlet opening and injector scoop approach affect the mass airflow through the blower. A lot of fuel is flinging around from the rotors. Again slowing it down may help in that area also. In effect, you may slow down, stay the same, or even speed up with less blower overdrive.
I know of several racers over the last couple years who went faster when they slowed down their blowers. More info about blower overdrive in Fuel Injection Racing Secrets and 5000 Horsepower on Methanol.