Quadrajet
Registered User
Joined: Feb 2011
Posts: 1,591
From: Denmark
I posted this quote, knowing the dodgy nature of the Olds FAQ, because I know people will correct me if it's wrong. Please don't make your decision based only on this.
Originally Posted by Olds FAQ
It's easy to overestimate the CFM requirements of an engine. You can calculate the cfm your engine flows using the formula everyone on this list has probably seen many times:
engine CFM = (engine CID x max rpm x volumetric efficiency)/3456
Now for a stock 455, I'll assume you won't go above 5500rpm, and stock heads with stock exhaust manifolds is probably going to keep your volumetric efficiency no higher than 85% at 5500 rpm. So your engine needs - only 615 cfm!
Here is where the universal misunderstanding begins. Usually people ignore the formula above, or use a rule-of-thumb like "use 1.5 times the airflow predicted by the formula". The fact is that 4-bbl carburetors are rated for their cfm when there is 1.5" of vaccuum sucking on them. If you use a 600cfm carb on your 455, there will be about 1.5" of pressure drop across it when your engine is at max rpm. Since atmospheric pressure is about 30 inches of pressure, you just took away 1.5" from 30", leaving the engine about 5% down on power compared to a much bigger carb. If your engine makes 300 real-world horses with those exhaust manifolds and stock heads, using the 600 cfm carburetor costs you only about 15 hp, at max rpm. Everywhere else in the rpm band the engine will be more responsive with this small carb, the car will feel better in traffic, get better gas mileage, etc. Unless the car is a drag-race only car, the 5% power drop at max rpm is really not very significant - how many minutes a day does your car spend at 10% throttle? How many at WOT? How many at WOT and 5500 rpm?
So I'd say pick any carb that flows at least 600 cfm (I believe all the true 4-bbl Quadrajets do), tune it to match your car and driving style, and you'll enjoy life with it. On the other hand, bolt on a 1050 cfm Dominator, and you'll probably be miserable with the result.
engine CFM = (engine CID x max rpm x volumetric efficiency)/3456
Now for a stock 455, I'll assume you won't go above 5500rpm, and stock heads with stock exhaust manifolds is probably going to keep your volumetric efficiency no higher than 85% at 5500 rpm. So your engine needs - only 615 cfm!
Here is where the universal misunderstanding begins. Usually people ignore the formula above, or use a rule-of-thumb like "use 1.5 times the airflow predicted by the formula". The fact is that 4-bbl carburetors are rated for their cfm when there is 1.5" of vaccuum sucking on them. If you use a 600cfm carb on your 455, there will be about 1.5" of pressure drop across it when your engine is at max rpm. Since atmospheric pressure is about 30 inches of pressure, you just took away 1.5" from 30", leaving the engine about 5% down on power compared to a much bigger carb. If your engine makes 300 real-world horses with those exhaust manifolds and stock heads, using the 600 cfm carburetor costs you only about 15 hp, at max rpm. Everywhere else in the rpm band the engine will be more responsive with this small carb, the car will feel better in traffic, get better gas mileage, etc. Unless the car is a drag-race only car, the 5% power drop at max rpm is really not very significant - how many minutes a day does your car spend at 10% throttle? How many at WOT? How many at WOT and 5500 rpm?
So I'd say pick any carb that flows at least 600 cfm (I believe all the true 4-bbl Quadrajets do), tune it to match your car and driving style, and you'll enjoy life with it. On the other hand, bolt on a 1050 cfm Dominator, and you'll probably be miserable with the result.
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