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Posted By: BlairW Carb recommendation ? - April 01st 2017 10:52 pm
79 G20, with a 400/TH350. Currently has a 750 CFM Holley 4160, and a Weiand single plane. Cruises great but seems like the wrong combo for a big heavy van.

Want to go to a dual plane and a smaller carb. What are you guys running?
Posted By: CatFish Re: Carb recommendation ? - April 02nd 2017 3:11 am
Originally Posted by BlairW
79 G20, with a 400/TH350. Currently has a 750 CFM Holley 4160, and a Weiand single plane. Cruises great but seems like the wrong combo for a big heavy van.
Want to go to a dual plane and a smaller carb. What are you guys running?


All mine are currently EFI, but I can tell from years of past carb experience the 750 and the single plane are not the best choice. The two make their best power at much too high an RPM for efficient street use.
A dual plane will increase air flow velocity at lower RPMs and, coupled with a smaller carb, will fill the cylinder much better with an air/fuel mixture much closer to optimum at normal cruising speeds.
My 2%.
Posted By: BlairW Re: Carb recommendation ? - April 04th 2017 9:46 pm
No one runs a carburated van here?
Posted By: Reed Re: Carb recommendation ? - April 04th 2017 9:55 pm
If I were running a 400 CI engine with a dual plane intake, I would go with an Edelbrock 500 CFM AVS carburetor, number 1801.
Posted By: Reese Re: Carb recommendation ? - April 06th 2017 1:55 am
I run the factory Rochester 4 barrel. I rebuilt it myself and my van runs really well with it. The Rochester has small primaries and large secondaries the key is getting the spring tension correct on the secondary air valves. It takes two hands and the trial/error method. When dialed in, there's no "quadra-bog" and most were 650 just a few were 850 IIRC.

I'm in the process of upgrading to a Edelbrock Performer (dual plane) intake for more low end grunt. Its designed to work from idle up to about 3500 rpm if memory serves. I don't plan on winding my small block any higher than that, so a single plane would not be the best choice. Just my experience.

Posted By: Dyno_Dave Re: Carb recommendation ? - April 06th 2017 2:41 am
Nope, EFI here.

I agree with Catfish....current combo has to go. The Q-jet is always a great option, as Reese mentions.

If I were buying a new carb, I'd be interested in a new Holley Street Demon. I'm an old Carter ThermoQuad fan, and this is a TQ-inspired update on that classic design.

HOLLEY STREET DEMON

[Linked Image]
Posted By: BlairW Re: Carb recommendation ? - April 06th 2017 7:04 pm
I think i can pick up an older Edelbrock performer intake on the cheap, so thats step 1, ive been offered an old Thermoquad off a dodge truck, might give it a try if i can sort out the linkage. Failing that ill likely look for a edelbrock around 600CFM? Had one on a mopar 360 and it was great, set it and forget it. Mostly just looking for opinions on what CFM i should be looking for.
Posted By: Dyno_Dave Re: Carb recommendation ? - April 06th 2017 11:30 pm
I'd say anywhere form 600 to 750 can work nicely on a 350. Just depends on the rest of the build (heads, cam, C/R, headers, etc.)
Posted By: Reed Re: Carb recommendation ? - April 06th 2017 11:35 pm
Don't go too big on the CFM. On a street driven van with a 400 c.i. motor, a 500 CFM carb is just right. AVS is better than the Edelbrock Performer since you can adjust the timing of the air door opening on the secondaries. I also prefer Edelbrock carbs over Holleys for the simplicity and ease of adjustment.
Posted By: Dyno_Dave Re: Carb recommendation ? - April 07th 2017 11:59 pm
Opinions on carbs = Bellybuttons. Everybody has one.

I respect Reed, but disagree with his assessment on the CFM requirements of an SBC. grin cheers
Posted By: Hasan Ben Sobar Re: Carb recommendation ? - April 08th 2017 4:08 am
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.
Posted By: Reed Re: Carb recommendation ? - April 08th 2017 4:38 am
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
Posted By: Reed Re: Carb recommendation ? - April 08th 2017 4:50 am
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.
Posted By: BlairW Re: Carb recommendation ? - April 09th 2017 10:24 pm
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 ?
Posted By: BlairW Re: Carb recommendation ? - April 09th 2017 10:24 pm
Oh and thanks for the replies.
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