I had an interesting email exchange with a very knowledgable and astute racer that I thought I would share. I’m happy to see he noticed the differences:
I noticed an error in the “Superchargers for IHRA” article…
“One way of looking at where all of that power came from is to look at the displacement of the blower. The 6-71 displaces a little over 400 ci per revolution (6 x 71 ci).”
…This is not correct. As mentioned in the article, the name of the supercharger comes from the diesel engine it was designed for, I.E., 6-71 = 6 cylinders, each cylinder displacing 71 cubic inches. The displacement per revolution of the supercharger can NOT be determined by following the above, as that merely leads to the displacement of the diesel engine it was originally designed for. Weiand superchargers(who produce GMC style roots superchargers obviously) says that their 6-71 moves 411 cubic inches of air per revolution, which is actually pretty close to the 426cu in that you would get doing the above math, but they say their 8-71 moves 436cu in, which is a long way from the 568cu in you would get by doing the above math.
Figured I would let you guys know, just trying to thwart off some potential disinformation(obviously unintentional).
In response, you are correct in your analysis. In racing though, things are not always as they appear if you try to extrapolate.
The 6-71 designation of the blower was derived from a 6 x 71 diesel as the article says. The blower ran at one to one engine speed. It ran at atmospheric pressure. It was not used for boost. The intake valves on the engine would open and the blower would move the air into the cylinder. It was a two cycle engine, so it displaced its size every revolution. The blower was matched to the engine size. So it displaced 6 x 71 cubic inches every revolution.
There was a 4 x 71 as well. I assume it was from a 4 x 71 diesel. And I believe the 4-71 blower size conformed the same as the 6-71.
Then the 8-71 appeared in racing. The rotors are 1 inch longer than the 6-71. I do not know if there was an 8 x 71 GMC diesel. The displacement of the 8-71 racing blower is around 450 ci. The article says that designation does not conform to the original 6-71 derivation. The article says that the rotors are simply 1 inch longer for the longer designations. Our FI Racing Secrets book provides more info on actual blower sizes for larger blowers with and without rotor seals. Those sizes were determined from the blower manufacturers for baseline blowers.
There is a variability in the actual amount of air displaced by blowers from different manufacturers and different designs. We received requests from many racers to do a book on blowers. In my initial research, I found out that the actual air pumped by the Roots blowers is dependent on many factors. Leakage is one big one. Stiffness of the parts change the leakage. Number of runs on a blower increase the leakage. Outlet opening design changes reversion in the blower and that effects leakage. The amount of manifold boost changes the leakage.
The big factor now explored by the manufacturers and racers as well is the design of the inlet. That is a big influence on the amount of air the blower displaces. One manufacturer told me they found the inlet design to be all trial an error for specific engine combinations.
That later info is all beyond the intent of the article. I think what we say in the article is all true though. You just cannot extrapolate as you indicated.
As of now, we have no further plans yet for doing a blower book.