I want to run dual quads in my project and I am looking at e-bay. There are 2 Holley 390 CFM carbs listed. Are these to small for a 350 chevy? Edelbrock use 2 500 CFM carbs for the sbc.
What kind of intake do you have ? Certain carbs will not fit certain intakes, Carter/Edelbrock carbs are much more forgiving on an under powered motor too. To bad the dual quad thread disappeared !
I was looking at those as well for my roadster pickup. I have a 58 dual quad manifold for my sbc in the project. I wanted to use WCFB's but they have really gotten out of hand price wise. The 390 holley's will fit my manifold. I'll quit bidding rather than run your price up
I ran two edelbrock 500s on a offy intake, mostly stock-mild 350sbc years ago. And after some light tuning they worked very well on the street!
swade41 I don't have a manifold yet and what did happen to the dual quad thread? hrm2k I am not sure if I'll continue to bid unless I can be sure that the 390's will be big enough for my motor. Thanks for the offer.
Here's an Edelbrock C26 with Carters, notice how close the carbs are together Here's and Offenhauser with a pair of Carters, notice the space difference and how the carbs line up over the runners. Holley 450's did not fit this intake, they would not clear the distributor on a sbc. and with the Holleys
If you could find a Holley Pro-Dominator intake like the one here,I ran a 331 SBC with 1850's and vacuum secondaries and it worked nice.Still need to run a small head distributor like a Mallory YL or Uni-Lite.
Years ago I ran two Holley 650 2 barrel carbs with the chrome stacks on an Edelbrock STR10 crossram manifold on a roller cammed 283 drag car. The two 2 barrels were plenty of carb for the 7500+ rpm 283. Two 2 barrels would warp most peoples minds today.
Ha ha, almost any carbs will fit a tunnel ram, I was tunnel visioned into low profile dual quad for some reason. That brings up a good question, tunnel ram or low profile ?
I agree... but I would like to know if there's a 'rule of thumb' for sizing dual fours. There's the CFM calculation formula that suggest that two 390 Holleys would be more than adequate for any street motor (where you might be more interested in looks vs performance or have a RPM limit on the motor) and I would think that the smaller venturies would promote 'crisper' throttle response and better mileage. I read the other dual four thread, but that was never really addressed...
Yup! You have to know if Holleys will fit or if you need the wide & narrow Cater/Edelbrocks. Probably on that other Thread Carb King said you don't just add the CFM of the 2 carbs together. He said 2 500 CFM carbs would only result in about 700-800 CFM depending on various factors.
You need a distributor so you might not have room for Holleys unless they mount sideways. I have 2 500 edelbrocks and Offenhauser manifold on a 400 HP 350 SBC without a progressive linkage and they work great. If you buy them new you can get a pair jetted to suit the dual application.
swade41 The last picture you posted of the rear carb on a tall spacer,will that work? Would the rear carb act as a tunnel ram and some how mess with the front carb? I want a low profile manifold. The motor will be pretty much stock but for a cam. I am looking at the Offenhauser catalog on line and they have a adapter # 5880 that will space the carbs 1/2" each way " which in some cases allow installation of Holley carbs". Just wondering what " some cases" mean.
The "rule of thumb" is that there is NO rule of thumb for these questions. To try to answer in a different way: Will two 400 CFM Carter AFB number 9400 mounted on an Offenhauser low riser intake work on a basically stock street-driven 350 Chevrolet? YES! And we have a dozen or more happy customers to prove this. So, how about the 390 Holleys? PROBABLY, depending on what manifold you use, and how many do-dads you place between the carburetors and the manifold. As mentioned by several already on this thread, the longer Holleys will not fit many small block Chevrolet manifolds. CFM, as mentioned by George, is NOT an additive property. The total CFM of two carburetors, depending on what intake you use, and the number of do-dads you insert between the carbs and the manifold, is going to vary from maybe 40 percent of the total to 75 percent of the total for low rise intakes. About those do-dads: ANYTHING that changes the direction of the airflow, expecially adding corners, will lower the CFM that the carburetor(s) will supply. Example: the long ram adapters that Offenhauser used to offer to make any dual quad intake into a cross-ram looked cool, but required at least one size larger carbs to get the same performance. My suggestion: Guessing you found a pair of 390 Holleys somewhere that is a current auction. LEAVE them for someone else (no, I am not bidding on them, and won't). Do your homework, and determine the manifold you wish to use FIRST! Then determine what carburetors will fit the manifold WITHOUT any do-dads, and will be optimal for your engine and driving requirements. Then set out to acquire these. If you want to discuss, tech advice by telephone is always free, and sometimes worth as much as you paid (free) 573-392-7378 (9-4 Mon-Tues central time). Jon.
That spacer was just a block of wood to demonstrate the height needed to clear the distributor, and yes using only one spacer would indeed increase the plenum volume on only half the intake. My other Offenhauser intake has tall spacers under the Carter carbs, but never tried to mount Holley's on those. Each of these intakes have 600cfm carbs on them by the way. I also have a big block Edelbrock air gap dual quad that like the other small block will not take Holley carbs. The Carter/Edelbrock carbs really lend themselves well to a milder engine, the way they are vacuum operated only opens them up per the vacuum signal.
After all the talk about Carters and Edelbrocks I just want to ad that I do run a pair of modified 450 Holleys on my tunnel ram. I feel the Holleys have a tremendous hit at the stabbing of the throttle, something that is achievable by accelerator pump,shooter and pump cam adjustments. The Carter / Edelbrock carbs only have a pump arm adjustment that you can alter to shoot more fuel at the hit.
[/IMG] I run 2 Edelbrock 500cfm/ offy 360 intake with a tame 355 stock jetting and nitrous (100hp jets). I use progressive linkage unless I want to play around then adjust it to 1 to 1
Why did the other recent thread get the ax? It did not have tunnel rams or DZ stuff listed? I wonder these days...... Question the MAN!!
Thinking about a dual quad setup for my 32 because I stole the tri-power off it for my 36. It will be on a basically stock 1973 350 with a 268 Comp cam and stock TH350. Thinking Edelbrock Airgap manifold with 600's. 600's only because I already have one otherwise I would use the 500's. Airgap because it puts the carbs a little higher without going to a Tunnel Ram style intake filling the engine compartment a little more. Should the carb linkage be run progressive or direct? The only reason I was thinking direct was better fuel distribution front and rear but think it may be too much at one time. Any difference in driveablilty with an Airgap manifold over the low rise? I realize the fuel charge should remain cooler with the Airgap which seems like a good thing.
Yes, there is a rule of thumb. The pressure drop across each dual carb. will be 1/2 that of a single carb. The size of each dual carb should be approximately the square root of 0.50 or 70.7% of the optimum single size carb. This changes the standard single carb. formula to CID times Peak RPM / 4887 (instead of 3456). Of course this is only the "rule of thumb", and intake design and a host of other factors can change the sizing somewhat. Hope that helps. --- Steve ---
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