You know, you could use a larger carb if you wanted to. However, a small carb is not always a bad way to go. You'll get snappy throttle response and you've got enough air to turn it past 5000 anyway.
Remember that the factory 396-325 horse engine had a 585 Holley from the factory. They ran very well with that carburetor. It's better to err on the small side than the large side.
The idle circuit on a 600 CFM carb may be too lean. You will definetely loose some top end at 600 CFMs. How much is hard to say. I don't think you would loose any throttle response going at least 650.
Cold hard fact, running too large a carb will hurt you more than running one that is too small. If running too small a carb was going to hurt you, then that makes it hard to explain how NASCAR engines make 700 HP with a 390 4 bbl, and a lot of saturday nite dirt trackers make lots of hi rpm hp with 500 Holley 2 bbls, with out frying the engines.
My old 383 SBC had an Erson TQ30 cam, Weiand Xcelerator single plane, Vette dual point, 1-5/8" headers, TH400 and 3.73 posi and resided in a '72 Chevelle SS. I ran a 4776 600 DP on it with the primaries leaned out 2 or 3 jet sizes. The thing would pull a house and rev to 6000 no sweat and pulled an honest to God 17 MPG on the highway. Not bad since according to conventional wisdom I had the "wrong" carb, intake, distributor, cam, torque converter and headers. I never got it to the strip but I sold the motor to a friend. Installed "as is" in his '71 Monte Carlo with a TH350 and 3.73 posi it ran 16.0's all day long through full exhaust on Sears all season radials with that same carb. The Sears tires didn't hook until about halfway through 2nd gear. Now you're probably saying "16.0's.... Big deal, that ain't fast!" No but considering what it was in and the exhaust and tires I don't think it's too darn bad. He installed Nitrous on it later on and took me for a ride, when it finally hooked in 2nd gear and the speedometer came back off the peg we were going 85 MPH. The only change he made to the carb was to rejet it. You can doubt me if you want but that's my story and I'm stickin' to it. You'll be fine with a 600.
To make an accurate carburetor recommendation we're going to need some more specific information about your combination. What it is, and how you're going to use it. I know it can be tough especially when you're young, and just getting started, but you are going to be much better off waiting a little longer and getting the proper piece, then having to purchase another one in the future. Carburetors work on air velocity, so a smaller carburetor will generally run richer on the same combination, than a larger one will. If you have to go one direction or the other the smaller carburetor is a better choice.
What kind of parts are you using? How much compression, what type of heads, and intake are you using? What is the stall speed on your torque converter?
You know, you could use a larger carb if you wanted to. However, a small carb is not always a bad way to go. You'll get snappy throttle response and you've got enough air to turn it past 5000 anyway.
Remember that the factory 396-325 horse engine had a 585 Holley from the factory. They ran very well with that carburetor. It's better to err on the small side than the large side.
You must have plugged in 100% volumetric efficiency, 80% for a street motor will probably be more like it and that's optimistic. Using 80% it comes out to a 517 cfm.
speedpro flat tops 9.7:1ish
crower , 278 462/470
performer Rpm
stock heads, 76cc, 3 angle valve job
stock stall but if i get it rebuilt i will step up to a 2400
I'd go the 750cfm route as I don't believe a it would be too big for your engine. The bottom RPM won't suffer as many are led to believe and the top end would be better off and the sound would be a lot better too.
well at the moment i have a 750 q-jet that is totally rebuilt and works perfect but in the future i would like to upgrade but im looking into the thunder avs 650 or the speed demon 650 and i cant decide
I agree. Going to a 650 from a 750QJET may be a step in the wrong direction. If you decide to go that way, then be prepared to be disapointed. Or better still, can you try a carb from one of your mates (buddies) before you commit?
I would keep the Q-jet as well. Mine gives me 21.5 mpg on the freeway, and crisp throttle response, and she pulls to 6700.
My 650 d/p didn't do any of that.
well at the moment i have a 750 q-jet that is totally rebuilt and works perfect but in the future i would like to upgrade but im looking into the thunder avs 650 or the speed demon 650 and i cant decide
Why didn't you tell us you had a Q-Jet in the first place? I wouldn't consider switching an "upgrade", get Doug Roe's book and spend some time on the Q-Jet, it probably works "OK" right now but you still have a way to go to be perfect. Once you get it "perfect" the only thing better on the street is EFI. It doesn't have the visual appeal of other carbs but it sure does work and once you've experienced one properly dialed in you'll start thinking it looks pretty good after all. Just my $.02 but from what I've seen in other posts there are quite a few experienced rodders here that feel the same way.
I tend to agree with Hippie on this. Stick with the Q-jet and get to know how it works. Other than them not being the most eye appealing carb on the market, theres not a thing wrong with them, and they have just as much potential as any other carb out there. Save your bucks for something later on that will give you more bang for the buck, than another carburetor.
If you do stick with a Q-jet, get a 75-later model. They have an adjustable part throttle feature that can be adjusted with a screwdriver while the engine's running, You just need a 1/8th" pipe thread tap and plug and a phillips screw.
Awesome fuel mileage.
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