18% UD maybe 6# boost test

I am working to transition more to the street so from 10% od 14# I had the pulleys to do this with 55tooth bottom and 67 on top so I have never been this UD on the blower so from boost charts I should get 6# but we will see and if you have done this and have info I would appreciate yours. I have a 8-71 9/1cr 446"

I am just adapting to life if I seem to change things alot so forgive me.

Are you not able to just flip the pulleys and go from 10% OD to 10% UD........ and see what that does?

Last yr I swapped the top pulley for -6% and got 12psi so not much reduction in psi and by charts it takes 5% for 1# if boost so I dropped it what I could with pulleys I have 55crank 67 blower and have a 50t for crank but felt that was way too small.
ps:the 6psi would give me an effective cr of 12/1 cr and should go with the cam ok.

By the common method: ECR = CR X (ATM + PSI)/ATM
For 9:1 CR and 6 psi: 9 X (14.696 + 6) /14.696 = 12.7:1
This does not work.

To approximate the effective compression ratio from the static compression ratio and boost, where:
ECR: effective compression ratio
CR: static (nominal) compression ratio
A: atmospheric pressure, in pounds per square inch (psi); 14.696 is approximately correct for sea level and 70° F

ECR = CR × ((B + A) ÷ A)^.74074 (.74074 exponent, the reciprocal of a sample ratio of variable heats at 1.35)

For 9:1 CR and 6 psi: 11.6:1
For 9:1 CR and 12 psi: 14.0:1

I wrote an Excel app to calculate this automatically, e-mail me for a free copy

Thank you for that info Polyspheric

One of the first roots blown Littlefield) (10:71 Hemi I built and dyno tested made 7.5 lbs. of boost with 12 % underdrive at 7300 RPM with 33 degrees of total timing on CA pump swill in 2002 or so. swapping the pulleys ended up making it have 13% O.D. and that made 12 Lbs. of boost at 6500 RPM and pick that motor up 100 HP on race gas with 25 degrees total before going into detonation
Every combination is different so I try and tune off of the spark plugs and time slips at the races

Thanks for the info. The blower is a little predictable at least I have some history with mine.

Dose anyone have an idea what compression my cam would run well with?

I did a test drive at 18%UD and I got 7 to 8 lbs of boost and on par for some boost charts.That is all the lower I can go with the pulleys I have so for now it will stay at that unless I cert my cage again,belts.I did a jet change to the intermediat cercuit but took some main jet out[2 sizes] and I need to put some main jet back in as the a/f was 14.5 to15/1 as my engine stall in the video will show and the engine was not very warm.

6 lbs sounds very pump gas happy!

I give myself to the end of the year and then think about that.