I have a 350 in my 68 with a torker II manifold.
it has a 750cfm mechanical choke.its great at top end but slugish down low.I am switching to a edlbrock performer air-gap manifold with a pertronix distributor.
I was thinking of going down to a 600cfm.or would the 750 be o.k.

The Torker intake is really the wrong one for the street. You give up too much low end for high end that's really seen on the track. You want a good dual plane intake for the street...better low end, better idle and driveability.

The Performer Air-Gap would be a much better street intake. I would for the time being, anyway, keep the same carburetor and see if you're happy with the results. You can always change it later, but I suspect the intake change alone will be a major improvement. You may need a low profile air cleaner with the Air-Gap intake as it sits higher under the hood.

I have a standard Edelbrock Performer on my L82 with the original Q-Jet (re-jetted for a different cam) and am very content with the results.

Thanks
when i bought the vette it had a 427 hood on it and i now have a 1" spacer under the carb so i don't think there will be a clearence problem when i upgrade.

No More then 600 cfm

I agree, wrong intake, go with the dual plane, and in this case bigger is NOT better, 600 good, if you ave more motor mods possibly 650, but no more. Good luck, most of us have been down that too big of carb trail before, but you learn, and then you remember.

Dave
Mr69vett

Try the 750 before you go buy a new carb. A while back there was a good article on carb sizes, in which they started with a 600 cfm on a 350 sb, and then changed nothing but the carb, going up 650, 750, and 850. The dyno showed an increase in hp with each larger size carb.
It was not a radical engine,and the increase was not great, but it was there.
I'm about to put a 750 holley on a 355 that i'm building,using a performer RPM. I copied the spec's from a dyno built motor. If it doesn't work out I can always go back to a 650, but I'm betting that I won't have to.
You already have the 750 and it won't cost anything to try it. Tom

Tom is right that the peak hp may go up a bit, but the test does not show loss of low rpm throttle response. The slight gain on top is usually not worth the loss on the bottom. But you have it so it won't hurt to try.

Ken Styer

Agree that the single plane manifold is not the optimum choice for the street. You might want to consider the Performer EPS. Better hp and torque than the original Performer, but without potential clearance issues like the air-gap. As far as carburetor size, quoting from the Edelbrock 2003 Performance catalog, page 205:

"For maximum performance, select a carburetor that is rated higher than the engine CFM requirement. Use 110% to 130% higher on sinlge plane manifolds. For dual-plane manifolds use 120% to 150% higher."

CFM = (CID x RPM)/3456 presuming a volumetric efficiency of 1.00

for a 350 ci @ 5,500 rpm, CFM = 557
Thus correct carb size would be 670 to 835 for a dual-plane, 750 would be about right.

David Vizard, in Volume 1 of "How to Build Horsepower" (page 71) states: "When power is an important priority, a four-barrel carburetor should flow about 1.8 to 2.0 cfm per cubic inch of engine displacement. If your engine is equipped with a Rochester Quadra-Jet, Carter Thermo-Quad, or a similar flow-operated, air-valve secondary carburetor (the new Edelbrock Thunder AVS series would be an example), up to 2.3 cfm per inch of displacement can be used." According to this calculation, a 630 to 700 cfm size would be good without an air-valve secondary or up to 805 with an air-valve secondary carb.

You'll also need to make sure of your fuel pump flow and use a high-flow fuel filter. Changing to an Edelbrock fuel pump (from stock) rated at 130 gph, a performance filter (also rated at 130 gph) and an 800 cfm Edelbrock Thunder AVS carb (from a 600 cfm Performer model) boosted my hp +30 at the rear wheels on a Dyno-Jet. (1973, 355 ci, Comp Xtreme Energy cam & 1.6 roller rockers). Hope this helps your decision. Good Luck!

CFM = (CID x RPM)/3456 presuming a volumetric efficiency of 1.00

Well, that's most of the formula. The problem is that volumetric efficiency is assumed to be 100% when in reality most cars operate between 70-95% VE. Volumetric efficiency is the measure of how efficiently you are able to move air through the engine and exhaust system. A stock intake with stock manifolds and a small exhaust sytem will put you closer to 70% efficiency. A free flowing single plane intake, headers, large cam, and a shortened free flowing oversized exhaust system will put you closer to 95%. The CFM calculated by the formula above needs to be reduced by the VE in order to arrive at a more realistic CFM rating. For example, the 750 mentioned above at a 85% VE (which is ambitious for most C3's) would then become 638 CFM making a 650 more than adequate.

Also, consider the role that maximum RPM plays in the formula. Being realistic for the moment, what is the maximum RPM you will ever push your motor to? Most of us here tend not to abuse our toys and rarely if ever exceed 4500-5000 rpm's. So, the high CFM rating is only useful for the maximum RPM range that you intend to use. So, to revisit our formula again;

350ci x 5,000rpm /3456= 506cfm

506cfm x 135%(avg of 120-150%) x 85% VE = 581cfm

Even if you want to consider the 5,500 rpm example above we only end up at 639cfm.

Further, as others have eluded to, the high cfm carbs will hurt you in the lower rpm ranges. The large venturi size of the high cfm carb results in slower air flow which is less conducive to fuel atomization.

Bottom line, for a street car, the horsepower you gain from the high cfm rating is not worth the torque and throttle response you give up on the low end.

Consider too that this is from someone with a '69 C3 that was built (by the previous owner) for horsepower beyond that of the '70 LT-1. I'd trade some of the horsepower for more torque any day!!

'69 350/350 4spd built to exceed a LT-1
780 cfm Holley (replaced with a 650)
'70 LT-1 intake
higher lift cam (251 I think) than the LT-1
solid lifters
large tube long headers
2 1/2" true dual exhaust, free flow mufflers
MSD ignition
..........and buckets of 99 octane fuel!!!!

PS - Check out Holley for carb recommendations. They are very good.

Best of luck and remember, more is not always better!

I have been told by a couple of engine builders that the Toker II manifold for the small block chevy is one of the worst intake manifolds ever designed. The flat plenum did not flow well at any rpm. You can buy one at any swap meet for $40 or less. But I bought one because someone told me that it had a low profile and was one of the only ones that would fit under my Corvette hood (that was bad advise).

I purchased a Performer RPM last summer and have been very pleased. I also run a Holley 750 cfm vac.secondary and have no complaints.

I have read many articles and test comparing 600, 650, 750 & 830/850 cfm carbs. The last one I read showed an overall increase of 6 lb-ft of torque and 3 HP in favor of a 750 cfm Holley over a 650 cfm Holley on a small block Chevy. If you can feel that difference, your Vette is more sensitive than mine. The 830 cfm produced the best peak numbers, but lost some torque and HP on the low end.

My advise, if you have a 750 cfm, run it. If you have a 650 cfm, run it. If your buddy beat you in a drag race by a car length, you need to concentrate on other things beside cfms. Now carb types is a different subject.