Blower Sizing Review
An example is provided for an 8,000 RPM Roots blower speed. That would be for a blower drive ratio at 1-to-1 on an engine spinning 8,000 RPM. It could also be for a blower overdrive ratio at 1.333-to-1 or 33.3% over for an engine spinning at 6,000 RPM.
The 6-71 Roots
The displacement of a standard diameter 6-71 Roots blower with rotor seals is 430 ci / rev. x 8,000 blower RPM = 3.44 million ci per min. / 1,728 ci per CF = 1,991 CFM w/o leakage. This high value is an indication why a couple 850 CFM carbs on even this small blower at 1-to-1 are out of breath, and an MFI hat is a big improvement in horsepower.
The 8-71 Roots
A standard diameter 8-71 Roots blower with seals is 470 ci / rev x 8,000 blower RPM = 3.76 million ci per min. / 1,728 ci per CF = 2,176 CFM w/o leakage. That is quite a bump and even more strained with conventional street carburetors
A blower with fresh seals, too tight to turn over by hand, will leak less than a blower loosened up from a few runs. After a few runs, manifold boost in blowers with conventional bottom openings will deflect rotors to reduce top seal clearance. We saw a 1 psi drop in boost in our blown Hemi racing engine subsequent to about 25 drag strip runs after a reseal. Top-rotor-to-seal clearance for new seals went from zero to about 0.010 inches after about 25 runs. That clearance went to 0.019 inches after about 50 runs. The clearances were measured with a feeler gauge between the rotor seals and case. The boost reduction was 2 psi after 50 runs from new seals.
A blower with a delta lower opening will leak less than a blower with a conventional opening. Rotor-to-case proximity is more intimate.
A blower feeding a lower boost level, such as from a lower blower overdrive, will leak less than a blower feeding a higher boost level, such as from a higher blower overdrive.
A blower feeding a larger displacement engine will have a lower boost level with less leakage.
Note: A larger engine has more friction so power is not necessarily increased from a larger Roots blown engine over that from a smaller Roots blown engine with the same blower overdrive.
A blower feed with more fuel in the hat will have a lower manifold temperature from increased manifold cooling from increased fuel vaporization. It will have a lower boost level with less leakage.
We used our jetting calculator, ProCalc, for setting up our fuel system based on the size of the blower and the amount of fuel going through the jetting. We de-rated the blower with a blower efficiency entry provision in the calculator. We use values from 98% for flesh blower seals down to 90% for blowers with more than 50 runs on the seals for our combination. For our previous racing, that worked really well for tuning. With air to fuel ratio tuning from the calculator, we could maintain spark plug color really well with the proper de-rated blower efficiency for blower wear. After setting up for air density changes, we most always ran a whisker from our drag racing bracket dial-in, right out of the trailer.