Cut the braceing on the underside of the hood,and raise the adjustment on the hood at the rear.Going down the road no one can tell,and with the hood up who would think the back of hood isnt aligned flush.Mark

I had a holley 500cfm on my 225 in my VC valiant for a while. air cleaner was around 1" or 2" high? Had fair amount of room 3" or so. , also was using a .5" spacer and an adapter plate on the stock manifold.

Why just a 500cfm 2bbl Holley, why not the 650cfm model?

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Slant Cecil

People-remember that 500 cfm on a two barrel is not the same as 500 cfm on a four barrel-they are measured way different (I think that a 2bbl @500 cfm is half of the air flow 4bbl @500 cfm).

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50 chrysler,54 Plymouth, 64 Valiant conv 4 speed, 66 Valiant V8 wagon, 70 Challenger R/T 440+6 conv 4 speed,80 Colt, 98 Neon ACR,84 Honda V45 Magna
Taking care of 57 300C conv,48 T&C conv. Missing my 67 GTX and 36 Ply coupe

The 500 has the same bores as the front half of a Dominator 1050cfm-4 barrel...The 450 cfm Holley (like 0-1848) has front bores similar to the 350- 2barrel and 600 cfm -4 barrel (venturi size and type will determine the restriction and cfm of the carb as well as the bore diameter...)

-D.Idiot

"Sometime during the 1950’s, engineers found that a passenger engine with a four-barrel carburetor would not maintain a vacuum of 3 inches of mercury at wide-open throttle; and by some convention 1 ½ inches of mercury was chosen for rating 4-barrel carburetors. The ratings for 1-barrel and 2-barrel carburetors were left unchanged." Quote taken from the Carburetor Shop http://www.thecarburetorshop.com

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50 chrysler,54 Plymouth, 64 Valiant conv 4 speed, 66 Valiant V8 wagon, 70 Challenger R/T 440+6 conv 4 speed,80 Colt, 98 Neon ACR,84 Honda V45 Magna
Taking care of 57 300C conv,48 T&C conv. Missing my 67 GTX and 36 Ply coupe

Most Holley 4bbls have staggered venturi, like all the Carter AFB, AVS, TQ carbs, with the primaries being smaller, the secondaries larger. The primary and secondary throttle bores are normally sized the same front and rear.

Examples;
0-1850 600cfm VS, Venturi, F 1 1/4", R 1 5/16", Throttle bore, F&R 1 9/16"
0-3310 750cfm VS, Venturi, F 1 3/8", R 1 7/16", Throttle bore, F&R 1 11/16"
0-4780 800cfm DP, Venturi, F 1 3/8", R 1 7/16", Throttle bore, F&R 1 11/16"
90% or more 4bbl Holleys have staggered size F&R venturi

There are a few that have commonly sized venturi and throttle bores.
Examples;
0-8007 390cfm VS, Venturi F&R 1 1/16", Throttle bore, F&R 1 7/16"
0-4779 750cfm DP, Venturi F&R, 1 3/8", Throttle bore F&R 1 11/16"
0-4781 850cfm DP, Venturi F&R1 9/16", Throttle bore F&R 1 3/4"
0-8896 1050cfm Dom, Venturi F&R 1 11/16", Throttle F&R bore 2"

Venturi and throttle bore sizes do not dictate cfm ratings. A few other factors control true cfm, such as secondary actuation (VS or DP) and booster type.
Notice above, the 3310 VS flows 50cfm less than the 4780 DP although they have the same venturi and throttle bore size, the cfm rating reflects the slower opening secondaries.
Booster shape and design can be restrictive. The early 3310 had a higher cfm rating of 780cfm due to having dog leg boosters allowing for another 30cfm.

Although venturi and throttle bore sizes do not dictate cfm ratings, an estimate can be derived for 3.0" 2bbl cfm to 1.5" 4bbl cfm.
Get the venturi and throttle bore sizes of your Holley 2bbl. Find a Holley 4bbl with the same sizes on all four barrels, devide the 4bbl cfm in half, this should get close to a 4bbl rating for your 2bbl.
Example;
0-4412 500cfm@3.0", Venturi 1 3/8", Throttle bore 1 11/16"
0-4479 750cfm@1.5", Venturi F&R 1 3/8", Throttle bore F&R 1 11/16"
So a 0-4412 is the front half of a 0-4779 and should flow close to 375cfm if rated like a 4bbl.

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Slant Cecil

Here's a pic of a crusty 0-6425 650cfm 2bbl like I mentioned a few posts earlier. It's no longer in production. It was used on AHRA/IHRA Formula III Super Stock 2bbl classes and on some 2bbl class circle track stock cars. Formula III SS classes went away and stock car 2bbl classes are now limited to either the 500 or 350 Holleys.



It has the same 1 7/16" venturi as the 0-4412 and 04779, with a larger 1 3/4" throttle bore like the 0-4781. I estimate it would flow 450cfm rated at 1.5".
If one would like to run a 2bbl for simplicity (2 less jets to change, 1 less float to set, 1 less bowl leak to fix) but want more than 500cfm, look for a split 1050/1150/1250cfm Dominator, split meaning cut in half, 525/575/625cfm should be enough for all but a few slants.

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Slant Cecil

Edit, as you can see, the 650cfm 2bbl doesn't use restrictive boosters. There is a series of holes drilled around the smallest part of the venturi. Fuel is delivered to the airstream similar to the way a annular booster does.

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Slant Cecil