Believe it for all out race type engines. But it's all about how the carb/carbs are setup.

 

What about dual plane manifolds for street/strip applications?

that is a different animal than the goal of making peak power on a dyno.

The discussion has been had in here a few times about Shift Recovery and oddly enough that the combo that made the most HP on the dyno didn't move the car down the track the quickest.

 

Not really related to this but I recently installed a Fueltech to control the ignition system and datalogging (converting to fuel injection) next year. I hooked the map sensor up to the plenum. At the hit we have around -.4 psi, after the 2nd gear shift its around -.2 psi, and around the 660 or 140 mph it goes to 0, after 150 we start to get positive pressure in the plenum.

This is a 562 with 2 Doms. From my experience I would say the intake manifold design/layout is more important then the carb size (as long as the carb size is close). Converter / Trans splits also have a major effect on the shift recovery. Its a whole system

 

That's totally related. Thanks for posting.

 

It depends on the engine combination. I've run as low as .3". On my 427 23˚ engine it lost a little power. My 461" SB2.2 doesn't care, I've run up to a 2.800 throttle blade single on it.

So to give you the explanation you have two sides to consider. First is the restriction a carb creates, it takes power to pump thru the restriction. This is called pumping losses, the lower the restriction the less power you lose. The second is the restriction creates vacuum in the intake, it is beneficial by atomizing and vaporizing the fuel better, making distribution to all cylinders more equal and allows the mixture to burn more complete, improving power. The line to find on each engine is how much you gain from added vacuum versus pumping losses. Two carbs work better by allowing better distribution with a barrel at or near each port and cutting down pumping losses. With the advent of better manifolds and carbs on a high end engine the difference is much smaller than the past. We did a 2.600 stretched carb recently that made over 1500 HP NA. 40 years ago a tunnel ram and two carbs might be worth 100 HP, I'd guess 20-30 HP on a higher end engine these days, maybe even less.

 

Mark

Is there a maximum amount of vac that you would accept seeing on a single 4 barrel motor? At what point do you feel a carb is too small and costing power?

I read some builders saying they don't want to see more than 1" while others said no more than 1.5".

 

Again it depends, some combinations won't get out of their own way without enough WOT vacuum. I would say anything over 1.5 SHOULD see improvement with a larger carb, at least on gas. If it doesn't I would investigate the intake design.

 

It should work with efi engines that see vacuum too just not sure how much so

 

overlap and reversion play a huge factor on what your reading.

 

I think it was Smokey Yunick that said if you have any vacuum at WOT, you need bigger carbs!

 

What about dual plane manifolds for street/strip applications?

This fall, we put my friend's hot rod boat motor in my Nova. It's a 0.040-over 350 with Vortec heads, HR cam, and Edelbrock Performer EPS manifold with Quick Fuel 670 vac secondary. Our Innovate DL-32 pressure/vac sensor read 0.6 in Hg with the engine off, and the minimum vac at WOT was about 1.8 in Hg just after the 2-3 shift (TH350). I'm not sure why it doesn't read zero when exposed to the atmosphere, but according to my math, that's about 1.2 in Hg actual. Next time we go to the track, we're going to have a 650 double pumper on it and see if there's any difference.

 

I don't know how much difference you'll see between the 670 and 650, here's a carb test that I've read over several times, it don't give vacuum numbers, but it was interesting.


http://www.superchevy.com/how-to/148-0403-seven-holley-carbs-test

 

Definitely an interesting article. The reason for trying the 650 is not so much that it's a slightly different size, but because it's a mechanical secondary carb. We're just playing around and trying to learn what we can with what we have on hand.

 

Do you know if the secondaries on the 670 were opening all the way and how does the jetting compare from one to the other?

 

Do you know if the secondaries on the 670 were opening all the way and how does the jetting compare from one to the other?

We don't know. We would have had to take the hood back off, and we didn't feel like doing that. We haven't gone through the 650 yet, let alone run it even just on the street, so we don't know what jetting it will want. We're hoping to get to the track again the 19th and/or the 26th; I'll post updates when we have more info.

We also have a Performer RPM intake for it. What do you guys think would be more interesting, 670 vac vs 650DP or Performer EPS vs Performer RPM?