Say you have a 750 cfm carb sitting on the shelf, and found out the engine you have is better suited to a 500 cfm carb. Is there a way to reduce the CFM's of a carb by swapping parts? And which parts exactly?

With Holley carbs, it seems like there are dozens of different variations on certain models, like the 4160, with a wide range of CFM's available. What is the physical difference between these carburetors?

Wow, I am not a book but, I can tell you that the model number has to do with features like dual feed lines, presence of choke or not, vacuum or mechanical secondaries, and some of the linkage. I read a ton of books on Holley carbs and if you are going to be a car guy with a Holley, you should too.

You would need to swap the main body and butterflies to drop the CFMs of the carb, no worth the fussing in my opinion. Start with the one you want.

http://www.rodandcustommaga...ors_howto/index.html

There is some great math in there that should help get you close but, things like a good air filter system make a big difference. I found a lower air filter body that had a rise of about 3 inches at the entrance of the carb and a big drop off around to where the air filter fit. I used a 6 inch tall by about 14 inch paper filter with the effect, of good flow and basically a velocity stack at the carb. I prefer a thermal choke, getting heat off an exhaust manifold. (Caveman voice) Fire is hot, no more choke.

If your into Holley carbs. This site is a must for someone just learning them. http://www.bob2000.com/carb.htm

I always been a Holley guy. But I had a good teacher.

[This message has been edited by billpapps (edited 07-13-2010).]

Im an Edelbrock guy myself but................you could reduce CFM of any carb by using a restrictor plate. Check with Jegs or Summit to see if one is available off the shelf.

Restrictor plates have been around stock car racing for years. There is a problem though,with metering if you use one.

Carb's are actually delicate in terms of air flow and balance. If you have larger venturi feeding a restrictor plate, the air mixture does not occur properly due to the air moving slower past the venturi and accellerator pump nozzle. That part of the carb relies on the air speed to get proper mix.

You can do it, and it has been done often, but if was my car, I wouldn't do it to save a buck. If your engine wants 500 cfm, one of the best options on a v8 is the Holley 2 barrel, 500 cfm. It is a real torque producer. And, it is not all that pricey. I think I'd be looking at that option if I were you, and selling the double pumper sitting on the shelf.

Essentially, the size of the venturi is the single biggest difference. There are other differences as well, including the accellerator pump circuit, and power valve choice. But, if you choose a carb that feeds the appropriate amount of cfm for the engine needs, you are WAY ahead.

Arn

I agree, get the right one. Just saying if he has to use what hes got, it can work.