I am running a 600 cfm Edelbrock AVS carburetor on my 454ci big block Chevy in my Chevy C-10 truck. Is that hurting power any? Would I see a power increase going to a 750 or 800 cfm version? 

The big difference between the AVS and AVS 2 is that the AVS2 uses an annular booster (with red inserts) that dramatically improves the part-throttle response. If you are considering a new carburetor for mild street car, we heartily recommend the AVS2. (Image/Jeff Smith)

The Edelbrock AVS and now the upgraded Edelbrock AVS2 carburetors are an excellent choice for your 454 truck engine, assuming that this is a mild big block and you’re not trying to make more than about 450 hp which would be one horsepower per cubic inch. Everybody thinks that big engines need a big carburetor but that’s not always the case depending upon the application and how the engine will be used.

If you were to bolt a 750 cfm carburetor on this engine, would it make more peak horsepower? Probably—but the increase would not be dramatic. Without knowing your exact combination of parts, it’s hard to estimate specifically. My guess is that peak power might improve roughly 10 to perhaps 15 hp using a 750 cfm carb as opposed to a 600 cfm version.

Let’s assume your 454 has a mild cam, a good dual plane intake and perhaps even headers. Let’s also assume the engine makes 450 horsepower and perhaps a little more torque at around 470 lb.-ft.

Airflow Restrictions

Most people would offer many reasons why airflow would be the restricting factor. Older 454 truck engines all came with Q-jet carburetors that were rated at 750 cfm. So a larger 750 cfm carburetor would slightly increase the airflow potential. However, there are other factors involved as well. A major issue is the height of the carburetor mounting pad on the intake manifold. Generally speaking, the shorter the carb height, the more restrictive the manifold design because the air and fuel must make a sharp 90 degree turn to transition from vertical to horizontal flow.

This means that while the carburetor may be able to flow 750 cfm, but the intake manifold is so restrictive that this greater volume never makes it to the cylinder heads. Some of the original Corvette big block manifolds around 1973 and 1974 mounted the carburetor nearly on top of the intake ports which makes the transitions to horizontal flow just brutal and restrictive. These were done in order to fit this large engine under the stock hood line.

This only becomes an issue if you searching for more power. If a little more power is a part of your goal, I would first look at optimizing your existing combination. For example, if you experiment with ignition timing and the combination of the initial, mechanical, and vacuum advance curves, it’s possible to improve power and overall drivability without moving to a larger carburetor. 

The Edelbrock Carburetor Tuning Manual

Edelbrock offers a tuning manual that comes with every new carburetor that is really good at helping you balance the combination of main jets, primary metering rods, and the power valve spring.

If you don’t have a manual, you can download one from the instructions found for your part number carburetor on SummitRacing.com. Just look for the “instructions” tab and open it up and save it to your computer.

We’ve included a copy of the primary metering tuning chart below.

This is a copy of the Edelbrock primary tuning chart for the 600 cfm PN 1406 Edelbrock AVS carb. It’s really not as complicated as it looks. Consider this chart as a tuning road map that will help you fine-tune your specific combination. (Image/Edelbrock)

At first it may look confusing but it’s fairly easy to understand once you study it for a minute. The center of the grid (indicated by the large black numeral 1) is the stock tuning position as the carb comes out of the box. We’re using the grid for a 600 cfm PN 1406 carb. Each carb grid will be a little bit different but the tuning moves are all laid out the same way.

From the center point, if you move above the horizontal main line, this richens the air-fuel ratio. Each number in the upper half will represent a given combination of metering rod and primary jet size. If you want to lean the primary side air-fuel ratio, moving below the main horizontal line will achieve this. Moving left of the main vertical line puts tuning into the cruise side of the tuning equation. So the upper right hand corner of the grind is full power rich while lower left hand side will lean the cruising or part throttle jetting.

By choosing the area you want to tune, you can pick a number in that area and the number will correspond to a combination of main jetting and metering rod. The number 7 position for example, represents a slightly leaner primary jet with a slightly different metering rod that will lean out the cruise portion of the system with only a slightly leaner WOT jetting. This can be compensated for by richening the secondary jetting by one jet size. Again, it sounds complicated but by following the chart you can make minor changes that can and often help both performance and fuel mileage.

The Power Enrichment Spring

Another tuning tool is the power enrichment spring. All Edelbrock AVS and AVS2 carbs use the orange spring but you can play with spring tension supplied in the Edelbrock tuning kit to help with part throttle tuning and just going to a slightly softer spring will allow the engine to run leaner with more throttle opening. Plus, changing both the primary metering rods and/or springs is very easy as you don’t have to remove the top of the carburetor to effect the changes. We recommend trying these tuning mods before spending money on a new carburetor. You might be surprised at how much better your big-block will run with a couple of really simple changes to timing and jetting.

Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.