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489 Lemans
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2,139 Posts
Discussion Starter · #1 · (Edited)
I went to the Holley website and used their formula for carb size. Seeing what others on this form have been using. The carb size it recommended seemed too small. It recommended a 650 cfm. I'm probably giving more info than needed. But Holley didn't hardly ask for any.

I am building a 400 sb.
.030 over bore
406 cu in
parallel decked
torque plate honed
line honed
SRP forged flat tops
SCAT forged 5.7 rods
stock cast crank turned .010 - .010
internally balanced with degreed balancer
Crane cam .500 lift int and ex, 302 duration int and ex
edelbrock 70 cc perf rpm heads
Compression 10.8 to 1
Use high octane pump gas only
38 deg total time
6500 desired rpm
edelbrock airgap dual plane intake
MSD stand alone HEI dist
2800 stall, 350 tranny custom built
car weighs 3500 with me in it
 

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140 Posts
Red65 is right , phone calls are cheep. I initially had a 650 on my 355 and I was happy till I tried a Holley 770 and was surprised at the difference on top end. I didn't notice any loss on the bottom. I only rev to 6500 but from 5500 to 6500 I could tell an obvious difference. Good luck
 

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2,384 Posts
carb size

A 750 will probably do the job real well. Other considerations are what are your peak RPM, what will be the major driving style?
How often or will this engine be raced.

A 650 will carry you well past 6000 RPM, so unless you plan to live in an RPM range above that, then that would be plenty sufficient.

When I had my 351 dynoed, it was first setup with my 600 CFM carb. Then I dropped on a buddies 750, and only gained 3 hp at peak rpm of 5500 RPM. and no changes up to 4500 rpm. The 650 would probably be better on fuel and low end response. Since you are running an automatic trans, I would also recommend going with a vacuum secondary carb whatever size carb you elect to go with.
Dropping the hammer at low rpm, acceleration could be hampered by having too much carburation with a double pumper, particularly down below 3000 rpm.

Bigger aint always better.

Spring kits are available to tailor your secondary opening to what ever rpm you want it.
 

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63 Posts
carb size

just a thought, 750cfm will be real consistent, but 850dp will
get it moving. had the same setup in an 81 malibu, ran 7.40
in 1/8 mile at 95 mph used a 850dp and it would pull the front wheels off the line, just a thought though
 

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Race it, Don't rice it!
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8,819 Posts
Annular booster are really designed for open plenums with a lot of booster signal buffing. They like a constant flow better than a pulsing flow. With a 180 intake, only four cylinders are fed from two barrals. Giving much better booster vacuum. Your mileage will improve as well as atomization properties(V.E).
 

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Technical Support Barry Grant
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Carburetor Size

Bracketeer,

You can go with either a 650 or a 750 carburetor. It's mainly a matter of what you're looking for. The smaller carburetor will give you better torque, throttle response, and acceleration, while the larger carburetor will give you better top end performance.
 
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