carbuerator gurus
Thread Starter
Le Mans Master
Joined: Mar 2001
Posts: 6,240
Likes: 1,013
From: Watkinsville, GA and Glen Cove, NY
carbuerator gurus
I have a 502 in its stock crate motor form making 440 horsepower. This motor came with a holley 750 mech. secondary carb. I plan on adding about 100 horsepower and most of the motors in the 500+ horsepower territory are using 850cfm carbs or the merlins 870 cfm modified 750 holleys with an 850 baseplate. My question is can my carb be modified for additional airflow or wii I need to purchase another carb and if it can be modified where can I have this done and how much will it cost? Thanks.
Melting Slicks
Joined: Feb 1999
Posts: 3,078
Likes: 1
Re: carbuerator gurus (69ttop502)
Yes, stock carbs can be modified for more cfm. That might be your cheapest option. To select the right carb size, use the following formula to help:
CFM = (Redline x Cubic inches x volumetric efficiency)/3456.
Most well-built street/strip engines with moderately high compression ratios can be estimated at a 90% volumetric efficiency, or 0.90. And since 502's have the same stroke as a 454, if you're running a cast crank and cast rods, you probably have about a 6000 rpm redline. Based on these assumptions:
CFM = (6000 x 502 x 0.90)/3456 = 784 cfm
So, you can go with some of the standard sizes, stay with a 750 for the throttle response, step up to a 775, or move up to an 800. But an 850 is probably going to be overcarbed. Especially being a mechanical secondary carb. With a mechanical secondary, it's better to be less carbed than more carbed.
I'd say stay with your 750, it'll be perfect, especially for the street. This is one area where bigger isn't necessarily better. If you change to a vacuum secondary carb, then go ahead and use the 850, because the engine will only use what it needs and it's a more forgiving carb. Double pumpers are not the least bit forgiving. That's why you go a little smaller with a DP generally.
CFM = (Redline x Cubic inches x volumetric efficiency)/3456.
Most well-built street/strip engines with moderately high compression ratios can be estimated at a 90% volumetric efficiency, or 0.90. And since 502's have the same stroke as a 454, if you're running a cast crank and cast rods, you probably have about a 6000 rpm redline. Based on these assumptions:
CFM = (6000 x 502 x 0.90)/3456 = 784 cfm
So, you can go with some of the standard sizes, stay with a 750 for the throttle response, step up to a 775, or move up to an 800. But an 850 is probably going to be overcarbed. Especially being a mechanical secondary carb. With a mechanical secondary, it's better to be less carbed than more carbed.
I'd say stay with your 750, it'll be perfect, especially for the street. This is one area where bigger isn't necessarily better. If you change to a vacuum secondary carb, then go ahead and use the 850, because the engine will only use what it needs and it's a more forgiving carb. Double pumpers are not the least bit forgiving. That's why you go a little smaller with a DP generally.
