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Re: Carb recommendation ?
BlairW #722225 April 07th 2017 7:59 pm
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pooh-bah
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pooh-bah
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Opinions on carbs = Bellybuttons. Everybody has one.

I respect Reed, but disagree with his assessment on the CFM requirements of an SBC. grin cheers

Re: Carb recommendation ?
BlairW #722235 April 08th 2017 12:08 am
Joined: Mar 2016
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H
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Try JUST the manifold swap first. See what happens. Forget the Thermobog. Linkage issues aside,not the right choice. Q-jet would be the way to go if you leave the Holley behind. Best guess is that a 400 SBC came with a 750 Q-jet.
As to the sizing,flow is subjective. I had a 1000+CFM Braswell on my 427 Side-oiler. And I have a 750 Q-jet waiting to go on my 318 MOPAR.

Last edited by Hasan Ben Sobar; April 08th 2017 12:10 am.

Tucson-it's a dry heat...like an oven
Re: Carb recommendation ?
BlairW #722236 April 08th 2017 12:38 am
Joined: Aug 2001
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Maniac
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I am just going on an assumed engine size and assumed max RPM of 5000 RPM. Assuming a stock motor, this carb CFM calculator ( http://www.carburetion.com/calc.asp ) says a 400 CI motor needs 491 CFM to turn 5000 RPM.

A daily driver van is not a rag car running 1/4 mile times. On the street, smaller is usually better in terms of cam profiles and carb sizing. Most of the time, a street driven engine rarely gets over 3500 RPM. For the best MPG and power it is best to go conservative on a street motor. Absolutely get the best flowing heads you can, but dual 2 1/2 inch exhaust is plenty, smaller cams are better than larger cams, and smaller carbs are better than larger ones (don't go nuts and put a 150 CFM one barrel carb on a 440, but a small four barrel is better than a large four barrel).

I used to run a 480 CFM carb on an offy dual port (yes, dual PORT, not plane) intake on the 351 in my old 84 Ford van. If I didn't have fun driving I could pull down 20-22 MPG on the highway, but I usually drove 75+ MPH and got mid teens. SIngle exhaust, I think it was 2 1/2.

But do your own research. There is lots of good info out there. Just don't believe the product marketing trying to sell high performance parts to guys drag racing cars. It is hype, and you are buying parts for a different application. You want low RPM torque, not high RPM horsepower.

Motortrend on Demad has a series called "engine masters" that has done some really useful videos about exhaust size, cam size, and carb size. Here is a great easy to understand instruction page from Holley: http://documents.holley.com/techlibrary_selecting_a_carburetor.pdf

Last edited by Reed; April 08th 2017 12:45 am.

Windows- they're what make a van worth owning!
Re: Carb recommendation ?
BlairW #722238 April 08th 2017 12:50 am
Joined: Aug 2001
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Maniac
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Quadrajets (like thermoquads) may have large total CFM ratings, but they work on smaller engines because the secondaries have a limiting mechanism that limits the amount of airflow through the secondary throttle bores to the CFM the engine can actually use. That is why you can put a 750 CFM carb on a 318. If all four bores are wide open, yeah, it will flow 750 CFM. But that hardly ever happens. The tiny primaries combined with the metered secondary airflow means that most of the time the carb actually flows much much less than the 750 CFM.

I once had a 74 Dodge van with a 360 that had been maintained at a dealership since new. It had the original thermoquad and it was working well. That van got over 20 MPG on the highway cruising at 65, all stock. When adjusted right, the Thermoquad is a fine carb that works well to maximize economy and performance.


Windows- they're what make a van worth owning!
Re: Carb recommendation ?
Reed #722256 April 09th 2017 6:24 pm
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BlairW Offline OP
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I picked up a Edelbrock Perfomer manifold, just gotta get in the right mind to tear into the van. Man they bolt a lot of crap to the manifolds on these vans. Gonna have to stick with the Holley, for now anyways, anyone have and jetting and power valve size opinions ?


1979 Nomad
2005 E350 Cutaway Class C
Re: Carb recommendation ?
BlairW #722257 April 09th 2017 6:24 pm
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Oh and thanks for the replies.


1979 Nomad
2005 E350 Cutaway Class C
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