[Tech @ BG]

A 650 Vacuum Secondary carburetor would probably give you the best overall performance.

[club327]

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.

[85Sierra]

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

[stepside454]

why not keep the Q-jet, properly tuned itll give you everything you want.

[club327]

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?

[smallblocker2]

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.

[Hippie]

Originally Posted by 85Sierra

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.

[Max Keith]

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.

[85Sierra]

well i guess its time to start saving for the canfields or the Afr's
thanks for the input

[machine shop tom]

Originally Posted by 85Sierra

well i guess its time to start saving for the canfields or the Afr's
thanks for the input

Personally I think you would be happiest with a 750 vac. sec, Holley. It will pull a LOT harder than the 600 or 670.

tom

[smallblocker2]

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.

[packratwrecker]

Originally Posted by machine shop tom

Personally I think you would be happiest with a 750 vac. sec, Holley. It will pull a LOT harder than the 600 or 670.

tom

Had one on a 402 big block. Had half the power, and less than half the mileage of my worn out Q-Jet. I won't own an Ede, or a drooling Holley, until I can no longer get Q-Jets.

I'd say, stick with the Q-Jet. It will do everything you want, and probably get you decent mileage, if you keep your foot out of the ****** ports.

[lluciano77]

Originally Posted by Hippie

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.

Here the formula, and 517 CFMs are okay?

[lluciano77]

Originally Posted by Hippie

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.

But now a 750 CFM carb is okay?

If 750 is okay, that means the formula doesn't work. Didn't I just get an A S S chewing for doubting the carb formula the other day???

[Hippie]

Originally Posted by lluciano77

Here the formula, and 517 CFMs are okay?

Originally Posted by lluciano77

But now a 750 CFM carb is okay?

If 750 is okay, that means the formula doesn't work. Didn't I just get an A S S chewing for doubting the carb formula the other day???

What is your problem? I'm not the one that argued with you about the formula, we disagreed about Holley 4010's being a "new" design vs. other carbs but I put it behind me, obviously you can't. The original question here was whether or not a 600 Holley was big enough for a 400 SBC, he didn't ask if a Q-Jet would be better or if a 750 was too big, he asked if the 600 was ENOUGH. From my personal experience it is. As far as a 750 being OK a vacuum secondary carb will only flow at WOT what the motor will pull, using the dreaded "formula" his 406 will probably only flow around 517 cfm assuming a volumetric efficiency of 80% at WOT, very few street motors exceed 80% . Could it be more, yes, but with the combination he eventually listed probably not much more and realistically how much time does a street motor spend at WOT, a few seconds a month at best? A 750 Q-Jet would be better choice over a 750 square bore carb due to it's smaller primaries. A 750 square bore would kill his gas mileage and throttle response because of it's large primaries and idle circuits, if he was racing? Hell yeah go for a 750 Holley, Demon, etc. but he's not. The Q-Jet is capable of excellent throttle response, mileage, etc. and my experience with Holley spreadbores has been that they can't hold a candle to a Q-Jet for those attributes regardless of what Holley claims. That didn't come from a formula, chart or some magazine article, that comes from real world experience. Even as much as you hate "formulas" they provide a pretty good baseline to use to start making comparisons, thousands of people use them with great success. They were developed over many years by people with a lot more education and experience than just about anybody on this board. As for your "A S S" chewing the other day that didn't come from me in fact even though I felt you were out of line I felt kind of bad for you, I thought the guy over reacted. I've changed my mind.