Vacuum @ WOT

[cstmwgn]

How much is too much?
NA big block

[squirrel]

More than one inch....

[Barry Donovan]

beyond 18 is getting sucky.

[TheSilverBuick]

More than one inch....

x2, and no inches with EFI is even better! (looking at Jeff's small block with ~4300cfm worth of throttle bodies, lol)

[cstmwgn]

I thought Holley rated their carb's CFM at 1.5 inches.

Is there a formula / chart that can help relate vacuum to cfm (HP)?

Something like

5 inches - increase CFM by 15%?

[Beagle]

I think the carbs already look like that - 750 to 850 is about 13%, 850 to 950 is about 12%... I'm pretty sure it would take a small manifold or a really small carb to stack up a bunch more than an inch of vacuum @ wot ? Most aftermarket carbs are bigger already than what most engines needs at 100% v.e. unless you're putting something like a 650 on a 6000 rpm 545" mill?

I'd be wondering about tiny intake ports as much as carb on a big inch plant.

[cstmwgn]

Well, here are more specs
533 CI (4.445 x 4.300)
P51 heads
260/264 @ .050 flat tappet
Victor 460 - square flange
HP 950 (VS)

Using Holley's formula
((533 * 6,700) / 3456) * .95 = 981 CFM

2 - 3 inches of vacuum below 5,000rpm
~8 inches at 6,500!

[Beagle]

I thought an HP950 was a 750 body on an 850 base? Sounds like time for a Dominator! That's a big bastard with a ton of head on it.. I suspect the 95% VE is a bit conservative too, that is one hell of a head.

VS - do you mean Vacuum Secondary?

Can you borrow a 4500 adapter and carb from anyone? That has got to be killing off MANY numbers!!

[squirrel]

if it's a vac secondary, did you play with the secondary spring? Even so, that's a big ass motor, deserves a big carb. BBR put a dominator on his 545 Ford, seems to work well

[Aircooled]

How did you get your vacuum numbers? Engine dyno, Chassis dyno or road/strip test?

[cstmwgn]

Road test. I have a fitting on the back of my intake that I hooked up my vacuum gauge to. Then I drove it around normal for a while watching the gauge and then used second gear to test it above 5,000 rpm.

[Aircooled]

I know this is a remedial question, but it has tripped up many people in the past...

Have you and a friend visually verified that foot-to-the-floor inside equals 100% open throttle blades at the carb?

[cstmwgn]

I will double check it since it has been a while since I checked it last. Conceptually, it should not have changed but it is easy enough to verify.

[BigAL]

Look at it like this a 650cfm carb can flow 1200cfm, but you'll be pulling about 6 inches of vacuum in the manifold to do it. Holley rates their carbs at 1.5 inches of manifold vacuum, if the vacuum is high than that the carb will flow more than it is rated for, if it's lower than that it will flow less than what it is rated at. The main thing you want is the least restriction possible as that will create the most HP. With the size of your motor you should be running a 1250 dominator at the least, dual dominators would probably be better.

[cstmwgn]

I know this is a remedial question, but it has tripped up many people in the past...

Have you and a friend visually verified that foot-to-the-floor inside equals 100% open throttle blades at the carb?

I do have an update and I have to fess up - aircooled nailed it. It appears that my throttle cable is getting old (either the inner cable is stretching, the outer housing is collapsing or the nylon liner is going away) as the cable mounting bracket was in the correct spot but the blades were NOT fully open. I will have to start working on a new cable as I have no more adjustment possible with the current setup.

Anyway, I still have vacuum at WOT but it is in the 2 - 3 inch range which is not causing the power valve to close which allow the engine to pull better to the rev limiter (not being 10 jet sizes lean!).