Al2011

Hello everyone, I have a rebuilt stock (low compression) 72 455 block with Ga heads and will be buying a E-Brock Performer manifold & headers. A lot of people here seem to have 750 cfm carbs but when I go to Summit CFM calculator it says my car only needs a 615 cfm carb. Also says if I go too high on the CFM the car will not perform so good at low and mid RPM. Since I won't be reving past 5,500 RPM please help me (point me in the right direction) in choosing a carb cfm. Also if you can recommend me a good carb with my setup. I'm not sure what kind of cam the rebuild has since I bought it from someone else. My goal for the car is just a high torque & dependable street car. Not a everyday driver but more of a weekend car when the weather permits since I stay in Phoenix and car has no a/c. No racing but occasionally smoking the tires & fast freeway ramp take offs. Someone has a holley double pumper 750 cfm for cheap but from what i'm getting it's not good with automatic cars and streets. Another one is Holley is a 3310c 4160 for cheap also. Any help would be greatly appreciated, sorry for the long thread just trying not to leave anything out. Thanks again.....

ziff396

I myself have been through edlbrock, and holly. I now am running a custom built quadrajet and like it the best. As long as you are running vacume secondaries, the engine will only use (or draw) what it needs. It doesn't matter what size as long as it is not to small. Tuning however, is a different story. Thats why I had mine built to match my combo. Mine is an 800 cfm, but my engine is not stock.

Lee_A

The 3310 would be a decent choice, and probably the cheapest. I'm guessing you can get it for about $50-100. That is probably one of the cheaper models that has a secondary metering plate, instead of a actual metering block. For a mild motor like yours, that will probably NOT be a big deal - it just makes it so that you can't tune the secondaries.

There are conversion kits for about $50, which convert it to a secondary metering block, if you should need that later on.

Double-pumpers (when properly tuned) don't get as bad MPG's, nor have as poor low-end performance, as the magazines and keyboard-experts try to make you believe. On the recent Hot Rod Power Tour, I was getting 13+ mpg with my HP1000 double-pumper.

But vacuum secondary carbs (again, when properly tuned) don't give up as much HP as you have probably been led to believe. I chassis dyno tested a two different doublepumpers and a vacuum secondary carb on a mild 455 about a year and half ago - one DP was pretty much dead even with the VS and the other DP was about 4hp higher.

If you have a really small cam, heavy car, highway gears, and a stock torque converter, THEN the 615 MIGHT be enough. You'll probably kick up the power on the motor, eventually, and I would urge you to not go UNDER 700cfm. A 750 will NOT be too big, either, and that is probably what I'd use for your car.

Oh, on a low compression 455, which probably has a pretty mild cam, there probably is no need to even turn it past 4,500rpm. It takes a fairly big (for street use) cam to provide the air needed for a 455 to still be making power at 5,500.

Lee

507OLDS

For what you are doing,a 650 Edelbrock would do it.I've used the Holley 3310 750 on a lot of 455 street engines as well,but they were a hotter build.

joe_padavano

Most people over-carb their engines. The calculator is correct as far as the theoretical airflow needs of your engine, though going a little larger won't hurt. The calculation is very easy. Take cubic inches, multiply times max expected RPM, divide by 2 (since the intake only opens on every other stroke), and divide by 1728 to convert cubic inches to cubic feet.

350 x 5000 / 2 = 875,000 cu in per min

875,000 / 1728 = 506 cfm

oldcutlass

He has a 455 so it would be as follows:

455 x 5000/2 = 1,137,500 cu in /min

/1728 = 658.28 - He should run with a 650 - 700 cfm!

But to be realistic since his is just a driver not a track star, you can take it 1 step further and use 80% efficiency = 526.62, so he can run a 550 cfm.

Lee_A

If we want to be sticklers about all of this, we need to take into account the pressure drop at which the cfm is measured. Even Holley uses different pressures for the normal carbs compared to their HP line.

If so many motor REALLY needed such little cfm, then why in the world would GM put 750cfm Q-jet on darned near everything??

According to the magic formulas, nearly NOTHING that GM produced in the 70's would have needed a 750.

We had a saying where I used to work - "There comes a time in the project where we just have to shoot the engineers, and go with what works."

You buy a carb less than 700cfm for a 455, and eventually (if not immediately) you will regret it.

On the other hand, I had a 469" Pontiac in a '67 Firebird. Street car, DOT radials, full exhaust, pump gas..... and it ran a best of 11.27 @ 119.6mph, with just a 750 double pumper. A guy that was really good at tuning helped a friend of his buy the car from me. First thing they did, was put a bigger carb on it. They eventually tried a few different larger carbs - but it NEVER ran any quicker than it did with the 750!

507OLDS

A dual-plane intake tends to use a larger cfm intake,compared to a single plane.The 3310 mated to the Performer is a great combination.I've done a lot of 455's with these parts.Nothing wrong with the Edelbrock carbs though.They have great consistency & drivability.Whatever you can fit in your budget,new or used.If used,just make sure it's a good one.

mfgusa

I'm a street guy. I do enjoy performance and butt testing bolt-ons. Since I have no data to support anything I do, I will mearly give you my opinion of what to expect from some of these available, properly tuned Carbs.

I'm betting you're not out to post a new NHRA record with you're assumably relatively stock 455, and consider driveability a more desireable characteristic. Also tuning may be an issue due to your unrealized internals.

My motor's a bit modified, but it's got the Edlbrk Performer on it. These tests were performed w 2:56 gears and a 2800 stall conv.

Quadrajet; Easily the best light throttle respose of all the carbs. Makes your motor feel bigger than it is with its triple venturi primaries. Awesome sound when secondaries kick in. Beyond that I consider them a good baseline carb, if not a tad over complex. Fuel starvation may be an issue if you choose the wrong size and push the rpms high before 2nd.

Holley 650 square bore vac sec; Hard to get use to after experiencing those massive Q-jet secondaries. Fine performing carb, easy to work on and modify, nicer to look at, says performance, mediocre + at best in all areas.

Holley 770 square bore vac sec (Avenger); My daily driver. Great all around performer. Looks great, the motor loves it, but not as much as...

Holley 800 DP Spreadbore w/ mech sec; Actually, the low end response is poor but you get use to that. where it really shines is at WOT. That alone
keeps it active in my inventory (in conj. w/ 3:91 fun gear for special cruising). It is also the most massive carb to ever sit on my engine-and it looks HUGE. Not too many air cleaners will hide it.

Ok, there you go. I stepped in it now, didn't I?

Good luck with whatever choice you make.