supercharger guru question

new rules for our division at local track std helix limited to 6-71 at 18 over or 8-71 at 10 over which one would be more effective and reasons as to why and at these low overdrives what manufacturer is better and why?

 

6x71

6x71 will move more air...
pump up the eng comp...

 

... and the Engine RPM / Supercharger Rotor Speed....

 

Take a look at the left side of this image for all the common blower sizes. The values are stated in cubic inches per revolution of the blower.

http://www.dragracingpro.com/images/works1_i3pu.bmp

So here's the math:

6-71 = 435 cid per blower rev times 1.18 = 513.3 cid per engine rev

8-71 = 461 cid per blower rev times 1.10 = 507.1 cid per engine rev

So all else being equal, the 6-71 choice is a little better.

What max engine RPM are you going to run?

 

we would be running 7500 to 8500 rpms thanks for the help and info.

 

OK - now it get's more complicated. The standard helix roots blower volumetric efficiency (VE%) starts to fall off above 9000 blower RPM. VE% seems to fall about 2% per 1000 blower RPM increase.

6-71 math @ 18 over:
7500 engine RPM times 1.18 = 8,850 blower RPM
8500 engine RPM times 1.18 = 10,030 blower RPM

8-71 math @ 10 over:
7500 engine RPM times 1.10 = 8,250 blower RPM
8500 engine RPM times 1.10 = 9,350 blower RPM

So the 6-71 combination is going to be above 9000 blower RPM most of the time. And the 8-71 combo will be below 9000 blower RPM most of the time.

In my previous post I gave a 1.2% flow advantage to the 6-71 combo (513.3 vs 507.1). But spinning at 10,030 blower RPM that advantage would be all but wiped out. It would depend on how quickly the fall-off in VE% is for your specific blower, and exactly how much time you spend above 9000 blower RPM during the run. A simple spreadsheet could be used for these calculations based on some RacePak data.

Bottom Line: the rules makers got it about right. Both combos should be very close.

 

What about HP to turn the blower? Or is that "buried" in the VE somewhere? What is all in the calculations to get a VE?

 

Only way to get the VE% is from a blower dyno. You have to measure airflow CFM vs. blower RPM. There's lots of this kind of data around for the "big show" standard helix 14-71 blower's. The blower dyno can also measure the HP required vs. blower RPM.

For this comparison it appears that both combinations will move the same amount of air into the manifold/cylinders. Therefore the HP required will be about the same as well since the "thermal efficiency" of the 6-71 should be the same as the 8-71.

It would all come down to all the design details of one manufacturer vs another for both the VE% and the "thermal efficiency".