Originally Posted by JOHN WILSON
An engine pulling 2" or more of vacumn at WOT may benefit from a larger carb. There's just no way an 825cfm carb on that motor will pull enough vacumn at WOT to close a 4.5pv. On my own 422 with a 750 BG (smaller at the venturi and throttle bore compared to the 825) it was pulling 1.8" of vacumn at 6200rpm on the engine dyno. A tad small, but not terrible.



I am trying to understand the above statement concerning the 2" vacumn at WOT and the need for a larger carb. It seems that I read this somewhere in one of the tuning books or maybe Demon manual.. Can anyone tell me where this theory orginated?

Not sure where it originated, but basically the idea is that there is always a pressure drop across the carburator at WOT. The greater the pressure drop the more the carbuator is acting like a restriction.

On restrictor plate cars the engines are run with a small carb (like 390 cfm) and there is a large pressure drop at high RPM. This pressure drop kills A LOT of power I've heard in the neigborhood of 400 HP on a SB2.2 motor.

So when you run a real small carb on a high performance engine there is a point at which the carburator becomes the restriction and power production goes flat (most say about 2"). The problem is that if you put an oversized carb on your engine to eliminate all the pressure drop and make more power, the carb can be difficult to tune and becomes basically undriveable at lower RPM.

Hope this helps.

So would you say to measure this or as a rule of thumb would be to keep the drop to a min of 2" ? I realize that when you get down lower its hard to keep fuel and air moving as you need the pressure differental. Correct? Or did I say that wrong? Whats ideal?

So would you say to measure this or as a rule of thumb would be to keep the drop to a min of 2" ? I realize that when you get down lower its hard to keep fuel and air moving as you need the pressure differental. Correct? Or did I say that wrong? Whats ideal?

What Eric said in a nutshell. 2" is just a trigger figure that is used to indicate that the carb is undersized. The smaller the carb the higher the manifold vacumn (on a given engine). Just slapping on a bigger carb to drop the vacumn to or under 1.5" (the depression where 4 barrel carbs are tested for cfm) is not a slam dunk for more power, the fuel curve has to be right too.

As an example, Rafel (67RS502) and friend Mike dyno'd Mike's mild mannered 406sbc. With a hp750 based carb the motor was pulling ~2.5" of vacumn at peak rpm. By switching to a larger hp950 the motor picked up about 15hp. Maybe Rafel or Mike will chime in with the exact details since I wasn't in the dyno cell.

Thanks,, I have several carbs,, Street Demon 650 -850 and whats on the 355 now a 750... Since going to another set of AFR 195's , questions are in my mind about increased flow above the old darts 170's... I think I will try some 3rd gear WOT pulls and monitor the vacumn guage to check.... My thinking is that if vacumn is high say above 3 or 4 " ,, I should consider the carb being too restrictive?

Yes, if you see 3-4" you need to try a bigger carb. With that said, I doubt you'll see anywhere over 2" with a 750 based carb on a 355 unless you're WAY up the rpm scale. For a "typical" 7000rpm 355 a 750dp is plenty.

Thanks to John and Eric,,, thats what I was thinking... over 6500 is not my everyday cruise.

John if youre talkin about Mikes mild 406, I dont remember how much vac. it pulled
on the dyno, but do remember it made 25hp/30tq more with the 850DP then the 750vac. carb.
Its a mild 440hp/500tq, 224/230 Comp 10:1 406.
It made more tq across the powerband. As long as an engine can handle the carb size
under a load (accelerating a car, not dyno) and pulls thru the booster well with a good signal
it should make more power with the added cfm. We run a HP950 on it now, and it works real well.
His 70 Chevelle has gone 12.0 @ 111 with the 406.

Rafel, thanks for clarifying that info. I didn't remember the part about the small carb also being VS. Side note-the 427 is done and will be dyno'd in the next day or two (maybe tomorrow), I'll let you know how it goes.

Thats cool John, cant wait to see them big #:)
Do you still plan on going back out to the track with the 422 one last time?

We are going to take my friends blown 510, 67 tubbed pro street camaro
out to the track for the 1st time, should blow the tires of pretty good...
5lbs underdriver (pump gas), then swap pulleys and be 7lbs overdriven (114)
it should make around 1000hp, were hoping for low 9s 1st time out if we
can get it to hook around the 60ft mark. The car is insane on the street:)

Also just got an email from a friend about CFE workin on some large
chambered 15deg. heads (for pump gas) He is gonna try to do a 427-434SB
with them and shoot for 700+HP on pump gas.
Here is what CFE 18deg. headed 427SB did:

427
3.875 stroke
6 inch rod
10.5 to 1 compression
950 davinci carb
cam 726/704 lift 270/277 dur. 108 lobe sep.
BFE 18 degree head 248cc
Hp 704hp @7100/7400 made peak hp at two ranges
Torque 583 @ 5900

We talked a little bit about the 15 degree head which is not done yet and he said that probably the only advantage the 15 will have over the 18 will be a little more torque and he did say that the 15 degree head will probably be about $800 more than the 18.

Guess you guys running out of Baytown... Seems everytime I go that way ,, it rains.

ZD- Baytown is where we run the most. There are some top notch 1/8 mile tracks in the area but HRP is the only game in town for 1/4mi. Yeah, it seems to like to rain around here in the cooler months when the air is good. :mad: Seems to stay pretty dry during the summer when it gets what I like to call "ignorant hot".

Rafel, those sound like some pretty big #'s for a pump gas sbc. Being that they came from Foltz makes them believable. Hopefully with 3.5 pts more compression, more cam, carb, and cnc heads evaluated/modified by a pretty sharp fella in Arlington, I'm hoping it does well. Looks like Tuesday for D-day. Yes, I'll be running the 422 once or twice more before it comes out and the car goes in for paint. I've got a 4500 Super Vic on it now and as soon as the 427 gets dyno'd I'll put the 4500 carb on it and be ready for another run.

John and Rafel,, I just got back (sidestreet) from making 4 wot pulls in 3rd gear to 6300 or little above. Watching a calibrated vacumn guage,,, after recovering from a mili second of 0 vacumn , needle leveled off at about 2 - 2.5 hg. ..Same on a hi gear pull to 6300. Guess I can say the 750 is in acceptable range?. I had a feeling these new AFR 195's would pull better than my old Darts 170's.

I have done racing in my earlier years when H/mp existed and made carb size determinations with mph and et. This is the first time I have ever applied this sizing method. I will pass it on to my grandkids. Again thanks to both. You taught an old fart a thing or two.. Guess I will put the 650 and 850 Demon out on Ebay..