I may need a new carb for my 1972 base engine coupe...automatic transmission with air, if that matters. I need to know the cfm to specify....if 600 is good,is 700 better? Fuel comsumption isnt a consideration. Is this a case of bigger is better? Dont want a quadrajet....any suggestions on Holley vs. Edelbrock? Thanks. Howard

Hi Howard
the formula for calcul the CFM is
C.I.D.* R.P.M./3456
I hope this help you
excuse my english
Claude

CFM ratings don't always tell the story on a caburetor. The Quadrajet that came on your car originally probably had a CFM rating of 750-795. If you ask Edelbrock people what they suggest for your engine they will likely recommend one of their 600 CFM carbs and Holley folks would likely suggest one of their 650 models. The Q-Jet has small primaries and large secondaries which kind of skews the ratings for what a carb is capable of compared to an Edelbrock square bore.

For a base engine, since you've eliminated a Q-Jet and if you don't intend to make substantial modifcations, I'd suggest an Edelbrock 600 with electric choke. It's simpler than a Holley, won't interfere with the EGR that a Holley with side hung fuel bowl will, and will maintain its settings better. If you want pure performance, go with a Holley. You can do more with it.

The Edelbrock 600 will give you crisp low end performance, the Holley 650 will likely give better top end, so decide what rpm range is where you want the best performance.

I think going bigger than 600-650 in an unmodified engine is unnecessary and won't provide noticeable extra power, but noticeably worse fuel mileage.

I agree with Gunslinger. But why not go with a Q-jet? Properly set up, (even fresh out of the box many times they are not, you need someone who knows them well) they are excellent for the street and stock engine. The others can be better for modified or performance engines and/or operation. What is your engine, and what is your operating goal? These must be answered before making you purchase.

Ken Styer

Hi

It's all ben sayed.
If you go for a too big carb, you will loose low end torque. But this is where most of our driving is done.
You most probably don't run your engine a lot in the above 4000 RPM range, or ?
I think a 600-650 CFM should do.
I'm presently running a 650 CFM double pumper on my 427 CI BB engine and it goes up to over 6000 RPM any time.
OK, it most probably doesn't have all the HP it could make, but it's a nice driving like this.
I do have a 750 CFM for it and will try it out this summer to feel the difference. But if low end torque will suffer by installing the bigger carb, it will end up on E-bay.
I heard that the EDELBROCK carbs are bolt on and go. No adjusments required.


Gunther

the q-jet is a great carb when in good operation condition, small primaries give good low end and ecomomy and large secoundaries give good high rmp power.
whatever you choise get a spread bore like the q-jet. in other cars i have owned installing a larger carb has made it fall on its face and bog when starting out, i returned to the smaller carb and have never turned back. responsivness is more important to me than top end power.

A holley is as reliable as the edelbrock, and will give you more peformance. The holley is more adjustable than the edelbrock, and once set will remain constant, as long as it was done right. I have used both and both are good. The edelbrock does require tuning. If you just bolt it on it will work, but you won't get all the peformance it is capable of.
I have a 350 with a mild cam, headers, an edelbrock intake, with a holley 600 vac. sec. carb on it, and it does very well.

Just a suggestion, before spending your hard earned money on a different carb, check out some of the publications available that discuss carb theory. One that's been around since 1990 is "How to Build Horsepower -- Volume I" by David Vizard and is usually sold for less than $20. There's about 16 pages covering carbs. Quoting: "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 flow-operated, air-valve secondary carburetor, up to 2.3 cfm per inch displacement can be used." When I bought my '73, it had the original carb (not original engine). Changed to Edelbrock's 600 cfm with electric choke. Some minor metering rod and spring changes (pretty simple stuff, less than 5 minutes & only have to remove the air cleaner) produced +33 rear wheel hp on the dyno. After new AFR heads and a Comp Xtreme energy cam decided to go larger with a new Edelbrock AVS (air valve secondary) 800 cfm. After accounting for the improvement from the heads and cam, the 800 cfm still added another 18 hp. I've also discussed Edelbrock vs. Holley with my engine builder. His opinion is that for more race oriented applications, go with Holley. For mostly street and some strip action, Edelbrock is the way to go if for no other reason than they are easier to maintain for the "non-professional". Hope this helps.

Chevy High Performance magazine just did an article comparing holley carbs from 390 to 1000 cfm on the same 400+ HP 383. The results were surprising in that the motor ran within a half dozen hp/lbs-ft with all the carbs from 750 and up. The 650 wasn't far behind. The amazing thing is the low end didn't suffer until the 950 and 1000 cfm carbs were installed. Joe