Carburetor size for 406 ci SBC

[AirBuick]

I've got a '70 Vette with a 406 in it. The motor is fairly warmed over with heavily ported aluminum AFR heads, roller cam, headers and Edelbrock intake w/ 750 double-pump Holley.

My question is this: if the heads are ported to flow somewhere in the neighborhood of 275 cfm per cylinder (intake), shouldn't I put a dramatically larger carb on since in total the motor is capable of flowing 2200 cfm? Should I have a carb (or carbs) that flows at least half this cfm? Thanks for any ideas.

 

[ACME RACING]

Here's the formula for air capacity in an engine :

Say you want to wind out to 6000 rpm ,

Formula for street-

6000 rpm x 406 ci = 2436000, divided by 3456,

then x .85 = 600 CFM
-------------------------------

Formula for race

6000 rpm x 406 ci = 2436000, divided by 3456,

then x 1.1 = 775 CFM


Looks small, but your 750 CFM is about right. Less rpm would come out less CFM.... A TE-44 turbo is 850 CFM , but makes our little 231 CI engines feel like a 500 CI motor

 

[AirBuick]

ACME,

Thanks for the explanation. Understanding that I've got a normally aspirated situation, is there any hope to gain more horsepower by adding additional cfm in the form of a bigger carb? Or would over-carbing slow me down?

 

[tracy]

750 would be a good choice. Demon/BG make a nice piece.

 

[chevyII]

If you have the converter and rear gears to match the 750 is perfect. If the stock converter/gears maybe a 650 double.

 

[ACME RACING]

Originally posted by AirBuick
ACME,

Thanks for the explanation. Understanding that I've got a normally aspirated situation, is there any hope to gain more horsepower by adding additional cfm in the form of a bigger carb? Or would over-carbing slow me down?

A 750 CFM would be perfect, anymore like an 800 or a 900 CFM would slow you down , your engine wouldn't be able to ingest more air unless you turn 7000 rpm or more and you would lose some off idle and mid-range torque
Nitrous, a turbo or a supercharger would force more air in the cylinder to gain the horsepower you like , but that's a different equation all together .....

 

[frankg]

What size valves and what size roller cam is needed also to determine if the 750 is big enough. If it's a solid roller with 2.08" valves and ported AFR's the 750 is too small i think. We run a 950 HP on my buddies 406 SBC and it loves it. With big heads and a roller 6000 isn't enough closer to 7000 is more like it!!! That motor just gets started at like 4500-5000. Need more info to decide on the carb. Frank

 

[AirBuick]

Frank:
Now that you mention it, I do have the 2.08" valves but a hydraulic roller cam. Don't know the specs. on the cam but it's not mild. The heads are roller rockered and the motor can easily wind to the 7000 mark. I just think it feels like it's struggling for air during the higher rpm range. The exact compression ratio is not known but it measures ~185 psi in all cylinders. Maybe you can tell me if the compression ratio can be determined by amount of psi. Maybe it's worth it to try to step up to a Holley 950 with that high-rise hurricane manifold? If you get a chance let me know.

 

[77tech9]

Well everyone is on the subject, I have a 79 vette stock engine, I want to just get a new intake and carb, was thinking 750 cfm. Everyone tells me the stock quadrajet was a high cfm, but not sure what I should get.

This is just until I get enough play money for vortec heads/intake.

 

[frankg]

Well with the hyd lifters they will only handle about 6500. So i would say use the 750 and maybe jet it up some or step up to an 850. If you had a solid roller i'd say go for the 950. Make sure you have a good fuel system too, you could be running out of fuel at higher rpm's that or floating the valves with the hyd. lifters. Frank

 

[Dusty Bradford]

I vote for a 850. My camaro with a mild 355 with solid cam turning 7,000 rpm likes a 750 so I think a 850 would better suit your 406.

 

[Craig Smith]

I will chime in and say the 750 is right on the money. A larger carb may increase upper-RPM horsepower, but you aren't going to be able to spin it that fast and the larger carb will decrease air velocity, particularly in the lower RPM range where a 400 inch SBC can really shine. I'd bet a dollar that you would feel a considerable drop in acceleration with an 850 or larger carb.

Been there, done that!

 

[KENNE-BILL]

I say work with the 750, read your plugs and jet accordingly - always worked for me.

 

[musclecar neal]

A properly jetted 750 is the perfect carb for the engine if it is going to be street driven most of the time.

Do you have a friend that knows how to read plugs and properly set up a carb? Most people who think they know about carbs are lost and don't even know it. I would suggest calling Patrick James at pro systems and have him build the perfect carb for your application.

www.pro-system.com

There are without a doubt the best on the planet. With the ultimate in tech support 24 hours a day.

 

[AirBuick]

Thanks for the responses. I guess I should resist the urge to go bigger and have the air/fuel ratio checked throughout the rpm range to see if I'm rich or lean.

 

[Dave@ramcharger]

If you called me here I would not sell you anything larger than a 750.. maybe a 770 in a street avenger.

Not many motors will be over 100% VE NA .. Race or not