Demon carb CFM question

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khaley71

khaley71

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I'm looking at getting this style Demon carb to replace my thermoquad. My question is what CFM? They list a 625 and a 750. I was leaning toward the 625 at first but after doing some research it might be to small. The motors is a 440 auto trans out of a 1978 Dodge truck and is completely stock. Any help would be great.
 

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The difference seems to be in the throttle bores (eg http://www.demoncarbs.com/1904.asp) 1 3/8 vs 1 11/16 but the venturis are not published. Someone knows, but I don't.
In any event, the rating is based on air flow at wide open throttle under a specified pressure difference.

If you plan to be running this engine at high rpm (4500 - 6000) as you might on the drag strip, then the 750 should produce more power. However if the cam and exhaust and gearing are such that peak hp is at 4500 or less, then IMO the 750 will gain you nothing even at the track.

The 625 with a smaller throttle ought to produce a slightly better quality idle and off-idle and get onto the primary system quicker. This should give it an edge in efficiency and throttle response. I would e-mail or call them to see what they say.

Basically, its like a T-quad. Unless you are switching to a squarebore intake, the T-quad should be just as good.
 
No plans for a rebuild, the motor runs really well. I'm just tired of messing with the thermoquads. I've had 4 different ones on the motor. They've all been rebuilt and all 4 have nothing but problems. With all the money I've spent on them I could have just bought a new one from the start.
 
I'd call Demon (aka Holley) and pick their mind. I ran a generic 3310 750cfm Holley on a low compression 440 and it did fine.

After 4 carbs, are you sure its not something else?
 
No it's defiantly the carbs. 2 of the 4 leak gas from cracks in the
plastic bodies. One is leaking at the mixture screw. The forth one I've gone through twice no matter what I do it idles really bad.
 
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