I was just wondering what it is meant by 15% underdrive and overdrive pullies on Roots Style superchargers, ie. Weiand 671, 871, etc. What pulley produces more boost, and how much more boost. Also what does the 671 and 871 mean? Air flow rating?

 

Overdrive means the blower spins faster than the crank. Underdrive means the opposite. The percentage is how much faster/slower the blower spins than the crank. For instance, if a blower is drive 15% overdrive, then at 6000 rpm, the blower is spinning 6900 rpm.

671 means that the blower flows 6 X 71 cid air per revolution. So, everytime a 671 makes one full rotation, it pumps 426 cid air. Keep in mind that a 4 cycle engine only pumps half its displacement per revolution. So, a 350 only pumps 175 cid air per revolution.

Therefore, if you try to cram 426 cid air through an engine that normally flows only 175 cid per revolution, you build boost in the intake manifold.

The 271, 471, 671, and 871 designations came from the old GMC 2-cycle diesels the blowers were originally manufactured for. The 2-cycle diesels were not crank-case scavenged like your dirt bike, chain saw, marine outboard, or weed-eater. Instead, they used a blower to pump air into the cylinders. The cylinder volumes were 71 cid each, but a 2-cycle engine pumps the full 71 cid per revolution. So, if you had a 6 cylinder GMC diesel, you needed a 671 blower to match it. They originally weren't intended to build boost, just push air into the cylinder.

Mike

 

EXCELLENT explanation. Couldn't have said it better myself.

 

Excellent job of explaining how they work Mike I know understand how they fully work and I can disprove my friends with this knowledge.

 

Yea that was a good explanation. I expected some BS about where the 6-71, 8-71 etc. etc. came from but when you got the old skool diesels I knew you knew what you were talkin about.

 

Thanks, guys.

Now a bit of trivia for you.

Most Hot Rodders know that Vortech recently bought Paxton.

Some Hot Rodders know that Paxton was originally called McCulloch, which dates all the way back to the 1930's.

Can anyone name who McCulloch bought out originally to obtain the ball-driven supercharger concept?

Mike

 

i know Francis L. Colburn had alot to do with the centrifugal supercharger

 

Great information. Unfortunately, it's wrong.

The displacement of "71-series" blowers in theoretical displacement per blower revolution is....

6-71: 411ci
8-71: 436ci
10-71: 469ci
12-71: 497ci
14-71: 522ci

The "71" designation has nothing to do with the blower's displacement. It's an old convention that simply refers to the numbers of 71ci engine cyliners the blower was typically used on. 6-71 was usually used on 6 cylinder GM diesels of 71ci per cylinder. 8-71 was typically used on 8 cylinder diesels of 71ci per cylinder. And so on. Like all terms that have been around since the "early days of hotrodding" you can't look at it "logically" and figure it out. It's just that when you said it, the old timers knew what it meant. That's all. Nothing more.

The rest of the information Mike posted is correct about under/over driving the blower, etc.

 

I was told that up to the 6-71 that all that information is correct, Nx71 = cid of displacement of the blower. I was told that for greater than a 6-71 blower that the name of the blower had nothing to do with its output, except for the fact that its larger than a 6-71. One of them "old hot rodders" told me that, I dunno how true it is.