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-   -   Im terribly conflicted... Carbs (http://www.hotrodders.com/forum/im-terribly-conflicted-carbs-225296.html)

chevy302builder18 10-23-2012 12:02 AM

Im terribly conflicted... Carbs
 
Ok first of all i keep hearing someones found that a smaller carb will make a small block chevy perform better. if i remember right the tail i heard was "a 570 cfm holley can suffiently offer more horsepower than that of a 650 on 350." i know this has to be correct on some circumstances, but not all engines and applications. some new imput would be wanderfull, this arguement about "every 350 small block doesnt need that many cfms" is pretty untrue or is it?

vinniekq2 10-23-2012 12:23 AM

double the cubes for an approximate carb size.heads,manifold,cam,exhaust all contributes

F-BIRD'88 10-23-2012 04:11 AM

Unless the 350 in question is a really really lame 350, a 750cfm carb will make more horsepower
than any of the other carb sizes you mentioned.. if you are building a high performance 350 and need to buy a carb
for it, buy a 750cfm carb.

bygddy 10-23-2012 04:22 AM

Quote:

Originally Posted by chevy302builder18 (Post 1602155)
Ok first of all i keep hearing someones found that a smaller carb will make a small block chevy perform better. if i remember right the tail i heard was "a 570 cfm holley can suffiently offer more horsepower than that of a 650 on 350." i know this has to be correct on some circumstances, but not all engines and applications. some new imput would be wanderfull, this arguement about "every 350 small block doesnt need that many cfms" is pretty untrue or is it?

Your confusing drivability, or perceived responsivness...with HP, the issue isn't really which makes more power, the 750 wins, almost everytime,people say all you "need" is a 570, or a 650 etc, based on a formula that Vinnie is referring to above. And its accurate, but flow is flow......if your building a performance motor, you want flow, and in this instance, for sheer numbers, bigger is better....on the dyno, the 750 will outperforme the little 570 every time......

E.Furgal 10-23-2012 05:39 AM

Quote:

Originally Posted by bygddy (Post 1602175)
Your confusing drivability, or perceived responsivness...with HP, the issue isn't really which makes more power, the 750 wins, almost everytime,people say all you "need" is a 570, or a 650 etc, based on a formula that Vinnie is referring to above. And its accurate, but flow is flow......if your building a performance motor, you want flow, and in this instance, for sheer numbers, bigger is better....on the dyno, the 750 will outperforme the little 570 every time......

untrue....

only way that is true is if you are looking at peak hp

you also left out other facts.. like factory heads flow compaired to aftermarket...
ya a 350 with heads designed to make peak power at 6500+rpm.. ya you'll need a 750
if he drives like most the 350 will be used 95% of the time from idle to 4000-4500, a 750 d/p is giving away torque.. and lots of it..
now a 750 vac 2nd thats different .

iirc a 350 would have to be 95% Volumetric Efficiency @ red line
redline being (in this exp 6500) to need a 750 cfm carb..
most mild builds that are not pro built are under 85% vol. efficience..
you'll find bottlenecks else where before the 650 carb becomes one..

E.Furgal 10-23-2012 05:41 AM

Wallace Racing - Intake CFM Requirement

bygddy 10-23-2012 09:11 AM

Quote:

Originally Posted by E.Furgal (Post 1602185)
untrue....

only way that is true is if you are looking at peak hp

you also left out other facts.. like factory heads flow compaired to aftermarket...
ya a 350 with heads designed to make peak power at 6500+rpm.. ya you'll need a 750
if he drives like most the 350 will be used 95% of the time from idle to 4000-4500, a 750 d/p is giving away torque.. and lots of it..
now a 750 vac 2nd thats different .

iirc a 350 would have to be 95% Volumetric Efficiency @ red line
redline being (in this exp 6500) to need a 750 cfm carb..
most mild builds that are not pro built are under 85% vol. efficience..
you'll find bottlenecks else where before the 650 carb becomes one..

I agree, but I wasn't discussing drivability, or pro built vs non pro built, I was simply suggesting, that the same motor, on a dyno, when tuned correctly, will make more power with a 750 vs a 570......

sedanbob 10-23-2012 09:15 AM

Thanks E.Furgal, for mentioning the difference with vacuum secondaries. I see this discussion a lot on hot rod sites, but they all seem to focus just on the mechanical secondaries. I have always thought that vacuum secondaries gives you the responsiveness of a smaller cfm carb at the midrange, but still gives the full cfm power at the upper end.

ap72 10-23-2012 09:29 AM

WHEN PROPERLY BUILT AND TUNED a smaller carb will never make more horsepower than a larger carb, not at peak, not off idle, not anywhere in between.

As carbs get larger they tend to get harder to tune though.

sedanbob 10-23-2012 09:54 AM

Another thought - The ZZ4 350 crate motor in my sedan has a warmed-up roller cam, aluminum heads, and came as a turn-key from GM with a Holley 750cfm dual-feed vacuum secondary. The new ZZ4s come with a 770cfm Holley.

techinspector1 10-23-2012 12:11 PM

My all-time favorite is a well-tuned 750 Rochester Quadrajet. The tiny little primaries give excellent throttle response and mileage, while the vacuum secondaries produce max power on a 350. Bolt it onto an Edelbrock #7104 Performer RPM Q-Jet intake manifold. Use 1/2" feed line from tank to carb, 1/2" return line carb to tank. Adjust for 5 psi fuel pressure at the carb inlet. If you have no expertise with Q-Jets, have Cliff Ruggles set it up for you according to your particular application.
http://www.cliffshighperformance.com/

E.Furgal 10-23-2012 01:05 PM

Quote:

Originally Posted by ap72 (Post 1602265)
WHEN PROPERLY BUILT AND TUNED a smaller carb will never make more horsepower than a larger carb, not at peak, not off idle, not anywhere in between.

As carbs get larger they tend to get harder to tune though.

r u ok
tell ya what you dyno a 350 with a dominator on it.. and not the baby one at 950 cfm either..
I'll make more power EVERYWHERE with a 650 d/p or a 750 vac 2nd carb

E.Furgal 10-23-2012 01:21 PM

Quote:

Originally Posted by techinspector1 (Post 1602301)
My all-time favorite is a well-tuned 750 Rochester Quadrajet. The tiny little primaries give excellent throttle response and mileage, while the vacuum secondaries produce max power on a 350. Bolt it onto an Edelbrock #7104 Performer RPM Q-Jet intake manifold. Use 1/2" feed line from tank to carb, 1/2" return line carb to tank. Adjust for 5 psi fuel pressure at the carb inlet. If you have no expertise with Q-Jets, have Cliff Ruggles set it up for you according to your particular application.
Cliffs High Performance Quadrajets :: Qjet Carburetor Rebuild Kits, Parts, Quadrajet Rebuilding, Quadrajet Parts, Bushing Kits, Carb Tuning

and a great sleeper set up..

ap72 10-23-2012 01:26 PM

Quote:

Originally Posted by E.Furgal (Post 1602320)
r u ok
tell ya what you dyno a 350 with a dominator on it.. and not the baby one at 950 cfm either..
I'll make more power EVERYWHERE with a 650 d/p or a 750 vac 2nd carb

no you won't. not if the dominator is properly built and tuned.

The properly built and tuned is the part everyone forgets about. Taking one out of the box and swapping around some jets isn't even close to properly building and tuning a carb and most people don't know what to do beyond that.

E.Furgal 10-23-2012 01:42 PM

Quote:

Originally Posted by ap72 (Post 1602335)
no you won't. not if the dominator is properly built and tuned.

The properly built and tuned is the part everyone forgets about. Taking one out of the box and swapping around some jets isn't even close to properly building and tuning a carb and most people don't know what to do beyond that.

oh, you mean rebuilt into a 690 cfm carb the size of a house...
that the fuel will never get enough signal to start out the feeds..


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