So the difference between a 8" and an 8" dual booster is roughly 20% from the searches I've done. Anyone know the difference between a 9"?

Is there a formula for this stuff?


Thanks

The 8" dual will provide slightly more assist then the 11" single, so it definitely provides much more than a 9". The "formula" is the square area of the diaphragms multiplied by the vac inches, so to speak, assuming the same vac it becomes a matter of calculating the surface area of the diaphragms for comparisons sake. A bigger booster is capable of more assist at a given vac, A 9" dual works very well, 11" dual(truck size) even better, however just doesn't look or fit as well as the 8" dual. Most that I know run the 8" dual with good success.
Back to your vac issue, 7-8"'s ain't going to cut it with ANY booster.

I have an 8" dual with 12" vacuum, i can tell that it recovers way faster then my 11" single did.

I guess it's time to invest in a 8" dual.
My 13" vacuum and 7" booster ain't cutting it, that's for sure.

You got your idle vac up to 13", that should work OK with the 8" dual. Not the greatest but should work.

I have an 8" dual with 12" vacuum, i can tell that it recovers way faster then my 11" single did.


While they both provide about the same assist with good vac, the 11" does seem to fall off quicker with low vac condition

We quite selling 7" single and Dual boosters, not enough umph! Our Polished Stainless 8" dual is out most popular. And as mentianed above recovers quickly.

I got this 8" dual diaphragm stainless steel from Frank Serafine ( ^ProdigyCustoms^) above and i am very happy with my purchase and hes customer service. I still need to order hes awesome master cylinder to match.

http://i145.photobucket.com/albums/r216/skeeter55/engine005.jpg

Prodigy Customs doesnt have parts or prices online?

Here's a chart that shows brake pressures for various booster and master cylinder sizes. Maybe it will be helpful:

http://www.classicperform.com/PDFs/BoosterBrakePressureChart.pdf

-Chris