Not true. The 350 is rated differently than the 600. A 355 cfm Holley 2 bbl has 1-3/16 venturis, a 600 4 bbl has 1-5/16 venturis. A 500 cfm 2 bbl is exactly half a 750 cfm 4 bbl.
Yeah, I have never seen a good explanation of why they rate 2-bbl carbs and 4-bbl carbs at different depression levels, neither of which has anything to do with what an actual engine provides.
That's the problem with the internet, you can't hear or see the other guy to know how he's saying what he's saying. Hope that made sense. :Handshake
I'm running a 390 Holley 4 brl. atop a performer intake and it runs smooth and has excellent throttle response.
The reason for the different pressure differential in the test to rate Holley's is simple: The 4 bbls are rated at 1.5"hg drop, the 2 bbls at 3", this is because at wide open throttle, you're pulling air thru 2 bbls vs 4. Given both have the same size throttle bore and venturis, the 2 bbl sees a stronger vaccum pull than 4 bbls. The ratings are just that, ratings given so you can compare carb sizes, they in no way tell you what any given engine actually draws thru the carb you choose. The center carb on my 331's sixpack is rated at 250 cfm, but it at WOT, before you push the pedal further to begin to open the secondary carbs, it's got about 350 cfm flowing thru it.
While I agree with the rest of what you've posted here, this cannot be done with a carb. The whole reason that fuel is drawn from the bowls is the pressure drop thru the venturis. Without it, a carb ceases to function. You can tune a larger carb to perform better than out of the box, but a smaller carb will still mix the fuel/air better than a larger carb. This is illustrated by the better throttle resonse a smaller carb provides over a larger carb.The only thing you forfeit with a smaller carb is some topend power, something that's rarely used on a street car.
As a poor example of people using too large a carb look at what size carbs the Nascar guys run. Aren't they in the 390cfm range? The dirt track guys run a lot of the 500cfm 2v Holleys around here and they are pushing some serious Hp also. If they can get all that HP out of such small carbs then I would think a mild 302 would run great on a 390 cfm carb. I'm not saying a 750 won't run god also if tuned right but I bet the average car guy that tinkers with his own stuff would have better luck getting a smaller carb tuned right. I can usually do pretty good getting my own carbs to run the way I want them to but when you start talking air bleeds and emulsion circuits I am lost. lol clint
Hmmm, I've seen where Nascar specs say they run 390 cfm carbs but a Nascar engine builder on another forum says they run 750s on the small tracks and restrictor plates under the 750s at Talladega and Daytona. I once saw a dyno test on a small block Chevy (sorry) where they compared a 390 cfm "Nascar" carb to a 650 DP. The 650 gained almost 25 hp and 20 lb ft of torque on back-to-back tests.
But on a Nascar motor, a 390 cfm carb will be flowing far more than 390 cfm at 8000 rpms. Something to think about.
I have seen a true Nascar carb and it is huge. Looks like one of the Holley's up around 1100 cfms or around there. My Brother in law is Pat Tryson. He is a Crew Chief. I don't see him very often of course but I did ask him once about this. He said the bodies of the carbs are mostly the Dominator HP carbs but most have replacable sleeves in the venturas for different tracks plus the restrictor plates. Plus they are all highly worked over to say the least. Says they are around 390-425 after all of that. Them carbs are huge though. I don't know if any of those guys would tell you exackly what they are running though. clint
there are different classes in nascar with different rules on what carbs they have to run. thats why you see 2bbl and 4bbl carb on "nascar" cars.
