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Carburator CFM

drsantos

Active member
Joined
Jun 9, 2002
Messages
39
Location
Rehoboth, MA
Corvette
1981 Charcoal met, 4-speed
Replacing my Q-jet on stock engine 1981, what carb cfm should I use?
 
I recently replaced mine with a holley 650. It works fine right out of the box with no mods or adjustments. But I did hate giving up on the Q-jet as I still believe they are great carbs. I mainly did it to eliminate all the computer stuff including the dist. With these two mods I noticed more power right away and I was not having any problems with the original equipment.

Good luck
Steve
 
Tom,

I read somewhere that the 1980 and 1981 are closer to 795 CFM. Has anyone seen this also? It probable won't matter, since the 81 carb is controlled by the computer anyways.
 
Different brand carburetors are flow rated at different pressure drops, so the rating numbers don't compare properly from one brand to another.

If you replace the Q Jet with a Holley, the smaller Spreadbore would be the best choice. I'm not sure of the Holley rating, but I think it is 650. I wouldn't recommend a square bore since you have the Q Jet type assymetrical manifold.

Why are you changing it? The Q Jets rebuild well in the hands of someone who knows the breed. As long as the throttle shafts are not worn out, it should rebuild well.

Good luck,
 
I agree with LarryBible. Perhaps you should go with a non-computer QJet, and while you're at it, install a non-computer distributor...
 
page62 said:
I agree with LarryBible. Perhaps you should go with a non-computer QJet, and while you're at it, install a non-computer distributor...

;stupid get both non-comp qjet & non-comp HEI dist;stupid
JACK:gap
 
The Qjet is a variable flow carb. The max flow is around 750 cfm, but it is capable of flowing much less air than that, and still meter fuel correctly.

I found a lot of info on other websitet on the carb. Set up is critical, as these went on 265 ci engines as well as 454's. Same carb, different metering rods and jets. That's it.

Do what the other folks said- get an Edelbrock
 
The best way to figure the CFM required is to use the formula that Rob has provided on the Home page. I have used this for years as a way to get in the ballpark on both racing engines and street cars. This is one case where bigger is not better. I like Holley's but have used Eldebrock also and to me the Holley is a much easier carb to set up. I know others will disagree but each to their own. The flexibilty of the holley and the amount of customization parts available make it a great carb to dial in to your particular application. For street, I would recommend a vacuum secondary type carb. For racing run the double pumper. You can use the double pumper on the street but it will take some tinkering with the throttle cams, jets, and the power valve to get it on the money ( i.e., eliminating the dreaded BOG). Be sure you use a good full flow in line filter on whatever carb you choose.

:bu
 

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