Thread: What difference will it make in boost if I switch pulley???

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Hi Everyone! The top pulley on my blower is a 30 & the bottom one is a 28, what difference in boost will it make if I switch the top & bottom pulley? It's a 392 Hemi w/ an 8:71. I thought I'd seen a chart somewhere but I can't seem to find it..... Bill

well, what is good enough fo you? you are barely underdrive now. what is your pressure?

Kitz

It's about 7% underdriven now, swap 'em and get about 7% over. 14% increase should make a noticeable difference.

You'll go from 10 lbs to 16 lbs according to BDS charts.

.....WOW! I think I'd better leave the pulley's right where they were! {30 top/28 bottom}. Thanks Kitz! actually I haven't even driven or fired the motor yet! I just figured it's never enough so might as well switch the pulley's now! And no boost gauge, should I get one?..... Thanks R Pope! ok, now I see how the math is done! 30 divided by 28 & then 28 divided by 30.... Thanks Tech! I went to the BDS charts & wow! I'm already {with 10 pounds of boost} at the edge of not pinging! NO way 16 pounds will not ping on pump gas!!! Since I'm in the middle of getting the bottom pulley to fit I figured I'd just swap 'em now, but no, I'll go with the 28 on the bottom. Plus it fits better cuz it's smaller! Thanks again Bill

10 lb to 16 lb will boost your effective CR about 1/4. (Atmospheric pressure, 15#, plus 10#=25...AP + 16#=31. 31/25=1 6/25, which is about 1 1/4 times cylinder pressure. A lot of variables enter the picture making this inexact, but you get the idea.

Just up the nitro 5 percent

We could never do that because we were running the can anyway. And around 30% overdrive at 28* lead