Blower boost number??

[sugaryuk]

Any here know how to figure out blower boost? I've tried going over a few charts and now my brain hurts . Here's my specs: 671 blower, upper pulley is #58 lower is #48- so it should be 17.2% underdriven, compression ratio is 9.2 (motor is 383 stroker). I know exact # depends on headwork, exhaust flow, cam etc ... I just want a ballpark #.

[Smiley]

That's a lot underdriven for a 383, but a 383 in a Vega doesn't need boost it needs traction so its mostly for WOW factor.
A boost gauge may be in order but my guess is 3 to 4 pounds.

[Monza Harry]

Here is a link, Sorry Ford site, with GMC Blower Ford post #8 list the different displacements http://www.429-460.com/t4519-roots-blowers-displacement 6-71 is listed with 411"^2. So 411 X 0.828 {100%- 17.2% = 82.8% or .828} = 340.308 Cu. In per rev. a 383 @ 100% equals 191.5 Cu. In's per Rev So 340.308/191.5 =1.777 X 14.7= 26.122# Absolute or - 14.7 = 11.423 #'s but that seems High compared to what most build ups show, that is also in a perfect world. I don't live there BTW. That displacement seems high to me as well. This math I know, applied to the only number I found for that blower, so I would expect 10#'s in real life if this info is correct. None of this takes reversion or heat gain into account that is math I have to look up, and it changes things quite a bit in life situations. I looked for more info and found these, Another site, different displacement [426] http://www.mini-blowers.com/gmc-blower-versions.php Wikipedia list 339 Cu. In's/rev However this "Weiand" chart puts you at about 8#'s http://www.aa1car.com/library/weiand_superchargers.pdf Harry

[spencerforhire]

The boost pressure measured is really only an indication of restriction(the amount of air you're trying to jam in there in relationship to the amount of air the engine would take in on it's own. Too many variables to calculate properly.
As a matter of fact, if you compare measured boost pressure before and after changing something major such as camshaft or cylinder heads, the better flow after will usually result in lower boost but more power.

[Smiley]

The chart says 10 pounds at 10% under on a 383 but the actual boost on the motor was only 7.5 pounds
so you have 17% under it would be a lot less than 7.5 pounds on your 383.

[sugaryuk]

The chart says 10 pounds at 10% under on a 383 but the actual boost on the motor was only 7.5 pounds
so you have 17% under it would be a lot less than 7.5 pounds on your 383.

I think you're first guess is probably more in line with what it's putting out, maybe 4-5 lbs, which is fine with me. That keeps me in the 12-13 effective compression ratio, which is where I need to keep it if I'm going to be on pump gas. I guess best way to figure it out is to get the damn motor in and run it, let the boost gauge do the math