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Im terribly conflicted... Carbs

4K views 33 replies 9 participants last post by  chevy302builder18 
#1 ·
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?
 
#3 ·
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......
 
#4 ·
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..
 
#7 ·
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.
 
#10 · (Edited)
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/
 
#12 ·
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..
 
#15 ·
I plugged in a few numbers to that calculator
Calculate Required Induction Airflow CFM
Use to determine how large a carb or throttle body to run
Engine Displacement (cubic inches only): cubic inches
Manifold type: Dual Plane Single Plane IR (no plenum)
Volumetric Efficiency @ red line: %
Red line RPM: RPM
Required CFM: 716 to 896 cfm (calculated)
so what carb do I use? rofl?650? 900? that was using a mild 355 at 93% VEjust over 6k redlin
 
#23 ·
390 carbed roundy round cars used longer duration on the intake side.non restricted engines in the same cars made a lot more horse power. My slightly larger carb on my engine works well,even though I only use 3.5 gears and probably too big of cam in most peoples opinions
 
#28 ·
Well if you look at the first post it was a friend, and i was confused about his tail of a 570 carb on a 350. and he said somethings pertaining to cfm size and you guys went on and forgot i still have a block sitting, still in current construction of a 406. Theres been many delays on this engine build, been waiting patiently and id like to have it built by this upcoming spring. LOL if it wanst for my stupidity, i wouldnt have to get this thing line honed and line bored. As you can see im still learning each and everyday and the thirst for knowledge is instilled based off the laws and phsysics of every piece of the engine and what it does. so you can say im a big newton fan. And a guy looking to have some fun by the end of the build.:evil:
Ive also noticed something very intresting, what Techinspector has pointed out and many other dyno guys on this site, is on the kinda of engine were talking about and big block chevy,(idk if this is true for other engines like ford, dodge ect.) that at 5252RPMs peak torque meets horsepower. intriuging as it is id like to save that topic for another thread involving V.E and other things that could cotribute to this rule of horsepower and torque 5252 rpms :D
 
#29 ·
thats because hp that we think of is nothing more than a math formula..
not what that engine can do..
I'm willing to bet a 160hp engine can't pull 1/3rd what 160 horses could..
if it could you'd snap the chain(most front drive cars) or gear teeth everytime it shifted gears at wot at hp peak..
 
#33 ·
But, it's calculated based on ft lbs of torque produced while running. Dynos measure torque, tachs measure rpm. The math gets done. HP is entirely dependanton two variables. The torque being produced, and the engine speed in RPM. It's a math thing.

PatM
 
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