Well I know it's from a 4 cylinder engine with 71 cu.in per cylinder. But thats a stroke diesel and I don't know what the drive ratio is. How much does it pump per revolution?
Drive ratio was 2.1 to 1 on the inline 71's. or 2 to 1. Not sure, but the math wouldn't be that much different. Either way. Would you take the case volume minus the displacment of the rotors to figure the output? And there was relatively no boost pressure at idle (just volume) and about 7 psi at speed. Idle at about 500 engine RPM and high speed about 2100. And there was overlap of the exahust valves open while the intake ports were uncovered to scavenge the cylinder. None of this helps figure the displacement tho.
Blower displacment is reltive to over/under drive volume area of case minus displacement. The numbers by which they are frequently identified contain a code as to the original application: the engine for which it was designed, and some information as to the pumping capacity of that model. There are 3 common rotor diameters, and up to 7 case and rotor lengths in some types. The prefix is the case length (3-, 4-, etc.). The suffix indicate inches per cylinder on the original installation motor (-53, -71, -92), and determine the rotor center to center distance and case width. The actual supercharger displacement is less than the engine size; an original GMC 6-71 transfers 339” per revolution, a Weiand 6-71 replica 411”, a Weiand 8-71 replica 438”.
<TABLE class=tborder id=post3638942 cellSpacing=0 cellPadding=6 width="100%" align=center border=0><TBODY><TR vAlign=top><TD class=alt1 id=td_post_3638942 style="BORDER-RIGHT: #e5e5e5 1px solid">Correction The blower displacment is reltive to over/under drive, engine RPM, volume area of case minus displacement of rotors. The numbers by which they are frequently identified contain a code as to the original application: the engine for which it was designed, and some information as to the pumping capacity of that model. There are 3 common rotor diameters, and up to 7 case and rotor lengths in some types. The prefix is the case length (3-, 4-, etc.). The suffix indicate inches per cylinder on the original installation motor (-53, -71, -92), and determine the rotor center to center distance and case width. The actual supercharger displacement is less than the engine size; an original GMC 6-71 transfers 339” per revolution, a Weiand 6-71 replica 411”, a Weiand 8-71 replica 438”. <!-- / message --></TD></TR><TR><TD class=alt2 style="BORDER-TOP-WIDTH: 0px; BORDER-RIGHT: #e5e5e5 1px solid; BORDER-LEFT: #e5e5e5 1px solid; BORDER-BOTTOM: #e5e5e5 1px solid"> </TD><TD class=alt1 style="BORDER-TOP-WIDTH: 0px; BORDER-RIGHT: #e5e5e5 1px solid; BORDER-LEFT-WIDTH: 0px; BORDER-BOTTOM: #e5e5e5 1px solid" align=right><!-- controls --></TD></TR></TBODY></TABLE>
If it has the 5.5" dia rotors the displacement is 223 cubic inches, if they're 5.78" then it's 274 cubes. From Too Tall Ganahl's Street Supercharging book
Never knew there were 2 rotor sizes for the inlines. This is interesting, does he go into any details? Seems to me if you change the rotor size, then the drive gears need to be a different size. My 1958 GM parts book lists only the 5192234 gear set for inlines, all except 2-71. And a 80's aftermarket vendor lists only that same #. The V engines use a different gear set, so maybe those are a different size. And V-71 and V-92 blowers are the same.
Why would the gears need to be a different size? The rotor diameter needs to match the housing bore...
I think the blower on the V engine turns the opposite way from inline engines, which mght mean opposite direction for the helical gears.
Thanks for that chart!!! OK the gears are on the end of the rotor. And time the rotors. They mesh with each other. SO they need to be the same diameter of the rotor? If you spread the rotor spacing out a little further for the larger rotor, the gears need to increase size also. Unless the lobes are just deeper on the larger size (along with case dia) and the shaft spacing is the same? This must be the answer, since there is not two gear numbers. Interesting that the 2 valve version has the larger rotors, the two valve head engines were the earlier ones. Four valve heads didn't arrive till the late 60's IIRC. I'm looking at parts books to see if I can find the different rotors, this might take a while. Thanks, Squirrel, there's always something to learn here.
Has to be different gears. The formal ID is the "pitch diameter" - the total of both pitch radii (since the gears drive at 1:1, it's also a single pitch diameter) has to be the same as the rotor's center distance.
Hmmmmm.....where to start..... The two gears (left and right) have the same number of teeth. The two rotors (left and right) have the same number of lobes. Everything turns at the same speed. The way gears work, it does not matter how big they are, what matters is the number of teeth. If two meshing gears have the same number of teeth, they will turn at the same speed. Really, it does not matter how big they are, they can be small, medium sized, large, huge, or whatever..as long as they have the same number of teeth, they'll turn the right speed.
You're both right, sort of. Two gears with the same number of teeth will, connected to each other, will turn at the same speed. But the gears pitch diameter needs to be the same as the lobe center distance or the gears won't mesh.
So did we ever answer the question (because I'm curious too) - what is the displacement of the 4-71 blower? I've heard it's the same as the engine, but don't believe it's true, but cannot prove it. A few months back when I was wondering about this, I sent an email to Dyer and BDS and got nothing in reply (guess I could call). I suspect it's about 80% of the engine displacement, but I've got nothing to go on other than a hunch.
Ernie, Squirrels chart has the displacementof the blowers. 226 and 274. The engine is 284, the blower runs 2x's engine speed. 7 psi is possible @2000 rpm, depending on cam timing and intake port size. Squirrel, the pitch dia of the two different rotor sizes are the same like tunglegubbin says: "But the gears pitch diameter needs to be the same as the lobe center distance or the gears won't mesh. " I understand now Thanks. I worked with these a long time, but on the Diesels, never converted one to a car engine. And everything I worked with was pretty old, so the larger lobes would be all I've ever seen. But we also did one unit at a time, and didn't mix rotors and cases either.
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