[Texasbikers]

I have a 64 GAL with a mild modded 390, looks for recommendations on carb size.

Bored 030 over, Alum forged pistons, 9.5:1
Modded heads with larger cobrajet SS valves, some polishing too
Mild cam, .523 lift in/ex 292 duration, roller rockers
Edlebrock RPM Alum intake
Full length Mad Dog Headers
C6 trans, 9 inch rear end, 373 gears with traction lock
Flowmaster 40's exhaust

I want a good street hot rod, no plans for drag racing etc..
I want a good dependable driver, considering a 670 or 770 Holly Street Avenger Carb. From what I read, a 600 cfm would be too small.

Thoughts? Opinions...
Thanks

 

[JimNolan]

Texasbikers,
I'll give you my opinion for what it's worth. I like small primary jets and large secondary jets because I drive my cars. Most of my use is around town and trips. I put my car on the drag strip also. I've got two cars. One is a 390/406 in a 63 Galaxie the other is a stock 390 in a 57 Fairlane. The 57 Fairlane 500 goes through the quarter mile in 15.2 sec on street tires. That really isn't bad for a 4000 lb. car with @277 hp. The reason I'm telling you this is that same car gets over 21 mpg on the way home with a 570 street avenger. The camquest program from Comp Cams says I'd get a whopping 282 hp if I'd go to a 750 cfm carburetor. That's a increase of 5 hp. Makes me want to go out and buy one!!! Use the software programs to decide what you need. They aren't pefect but they are better than the guy down the street. 5 hp doesn't mean that much to me if I have to go from a 54 primary to a 65 just to get it. Also, look what the factory put on their big block engines for all round performance and drivability. Look what they put on a 385hp 406. That's what I use on my Galaxie engine. Jim PS If all you were wanting to do was drag race I'd say go as big a cfm as possible until it floods out and then back it off a notch.

 

[mustangboy]

I would go with the 670...It should work great with the mods you have done to the engine..

 

[arnoldtx]

Take a look at Quick Fuel HR series VS. great carbs for the price.

 

[pvan]

The standard formula is: CFM = (RPM times CID) / 3456 (NOTE: RPM = Max RPM)

CFM=(6000? x 396) / 3456

CFM=687.5

Now consider volumetric efficiency of your motor (probably in the 80%-90% range) and your get a carb size range of roughly 550 CFM - 620 CFM. There may be a little play in the numbers due to variation and quality in manufacturing (i.e. a 600 CFM carb doesn't flow 600 CFM), but this should absolutely get you in the ballpark as long as you don't use an ego multiplier.

See the following link for more information: THE CARBURETOR SHOP / Carburetor size selection criteria

http://mycarrestorationprojects.blogspot.com/

 

[rambuttt]

I'd go with a 600 because I love the throttle response a higher vacuum signal produces. But, a 750 would make a little more top end power without sacrificing too much low end. Either size will probably make you happy...as long as they are vacuum secondary carbs.

 

[puttster]

That is a pretty sweet setup you have there. If you ever move up in cam you will want the 770 I bet, or you will be leaving power at the light.

 

[jerrym3]

Is 6,000 rpm the right figure for a bone stock 390/300?

 

[rambuttt]

No...more like 5,200...if that.

 

[pvan]

Is 6,000 rpm the right figure for a bone stock 390/300?

Nope, but the motor describes in the original post is a long ways off from bone stock (i.e. cam, headers, intake, heads polished with bigger valves). He is also running a 3.73 gear. If he has a loose converter (2400-2800 or so) he should be into his power band pretty quickly, and still have crisp response off the line if erroring on the side of a slightly larger carb.

An engine is an air pump. It pulls air in through the carb and pushes it out through the exhaust. Measure the amount of oxygen coming in, mix it in a ratio of 14.7:1 with fuel and ignite it. Push it out of the cylinder and into the exhaust. Very simple. Cam size, timing, port size, etc. affect how much more air can enter the cylinder, and efficiency of the burn, so a bigger carb doesn't allow an engine to make more HP at a higher RPM unless the airflow is restricted at the carb in the first place, preventing the fuel to O2 mixture from achieving an ideal stoichiometric ratio.

Too much gas = unburned fuel. Too much air = lean conditions. There is only one "ideal" ratio and the idea is to come as close to that as possible.

A carb is an imperfect metering device that cannot perfectly account and adjust for all variables (air temp charge, atomization and mixture of charge, timing of ignition, engine load, etc.) at all RPMs. So, the equation exists as a guide to "best-fit" mean conditions. If you are running at the race track exclusively, you will tune your carb differently than if your vehicle is driven on the street all of the time, but the engine can still only move a particular amount of air without other influences such as forced induction, ram-air, nitrous (it adds O2 simulating a larger air charge), etc.

Even with vacuum secondaries, going with too large a carb can mean wasted fuel, reduced power or possibly both.

http://mycarrestorationprojects.blogspot.com/

 

[jerrym3]

No...more like 5,200...if that.

Thanks