I have a BB bored to 408 cu. ins. It has a weiand dual plane intake with a protronics vac. distributor and a crane powermax dual grind cam. Its a moderate cut hydraulic cam, don't have the grind specs with me.

It dynoed at the flywheel with 366 hp and 435ft. lbs. of torc. This is with a stock quadrajet carb on it.

The carb chart calls for a 670 cfm carb but a performance shop is saying probably closer to an 830cfm carb. What neighborhood carb should I be looking at for quicker responce and HP gains without running filthy rich? Holley, Edelbrock spreadbore??

Experianced help would be greatly appreciated. graemlins/beers.gif

Holley 3310 with vacuum secondaries if it's an automatic.

keep the Q-jet, they are a great carb for a nice street/strip car when tuned right (just tougher to get tuning parts for). I feel they are more tuneable that a holley, but that is my opinion.
put your hard earned bucks somewhere else

I like Qjets too but the cost to get one right or buy new can be rough. The 750 vac. Holley or Demon would be my next choice. If its just a street car, I would not turn a deal down on a 650 vac Holley.

I agree that a 750 cfm vacuum secondary carb is just about right for this motor. If you like the street manners and consistent operation of the Q jet, go with an edelbrock carb(750 of course) cause they have similar charactoristics. However, if your a tuner and like to get in there and really work the carb tuneup to get all you can out of her, a holley or demon is the ticket just because by nature, one of those carbs can be pushed further with more ease. Those carbs have more tuning options, plus there is just more aftermarket goodies you can get(if necessary) to push the carbs performance even further compared to many other carbs. Chances are though that you will have to pay more attention to the holley/demon type carbs from time to time cause they are a little more sensitive. The eddys seem to be more adaptive and streetable under different conditions, but you cant wring max performance out of them as easily as you could with one of the others...... Definitely no reason for an 830cfm. Really, you probably dont even NEED all 750cfm for your particular engine, but its a good ballpark though. And like Dragn70 said, if you found a deal on a 650-700cfm vacuum secondary carb, go for it and itll work out fine as well. That shop either needs to meet a quota on 830cfm carbs(its possible), or thats all they had on the shelf when you stopped by and they wouldnt send you elsewhere in search of a more appropriately sized carb without at least trying to sell you what they got. Or its just a young punk or a real "old schooler" type of guy....both of which often tend to have the "bigger is better" attitude, especially when it comes to camshafts, carbs, and compression and gears and all. The irony of it all is that those types often search for the biggest, most impressive and intimidating sounding stuff they can buy, yet they always want to use ported 40 year old cylinder heads cause its the way to go. Go figure.

Thanks for the imput. BTW what is the cfm on a stock quad???

stock Qjets are 750cfm and a few were 800 cfm when the secondary airdoor stop is trimmed.

the above is misleading 750 and 800 cfm models
they can flow the full number when the secondary airdoor stop is trimmed. the factory stops the secondary airdoors prematurely on some smaller engines (305,olds 307 being examples)

The entire key to using a Quadrajet is the disassemble and epoxy the plug in the fuel bowl to stop leak down. Most Q-jet repair kits use a gasket as a bandaid that usually always ends up leaking. Once set up and the primaries are jetted correctly, the secondary circuit is the quickest and easiest to enrich of any carb. I have a whole collection of metering rods with everything from progressive tapers to large steps.

Take a look at a Holley 770 avenger carb, works for me. 4150 style carb, electric choke, vac secondary with jetted plates front and back. Also has the "quick change" vac springs for tuning to your ride.

Another vote for the 3310 Holley

What is puzzling to me is that many people say a 650 to 700 CFM carb is plenty but my car has a 750 to 800 CFM rochester.... What gives???

Even the Holley carb guide says 708 CFM....for a 402BB

I wouldn't over think it.
Plenty of 396-400 CID GM's came thru with the 'ole Qjet,which is 750 cfm
Basically with vacuum secondaries,it will only give the engine what it needs.

What's the rest of your combo, rear ratio, trans, stall, etc? Do you race the car, just cruise, is it a daily driver? These things can make a lot of difference in determining what will work best. I see you have a spreadbore intake, are you planning on upgrading that any time soon?

Which demand operated carburetor introduces raw gas into the secondaries as they start to open to cover the inherent bog?

I have a 331 rear end and its an M-20 4 speed. I don't race it but want to at least look respectable when a high performance 4 banger with NOS tests my patience!!!

add another vote for the good old 3310 holley

Chevelle, just because the cfm rating is 750 does not does not mean you will get 750 from it when the air valve on a qjet or vacuum diaphragm on a holley are tuned propperly for the engine it is on. The secondarys may only open enough to flow 600 cfm 350 and 750 on a 427. Thats why a the same qjet can be used on a 283 or 454.

So what spring will give me 8oo cfm?? and how do I stop the secondary bog??

Anything without a triple tier wing looks respectable next to an import!!!

The boosters are the big difference in the 750 and 800 cfm qjet. There is a lot less bulk inside the 800. Your best bet on tuning the air valve is to go back to the stock setting wich is printed in most rebuild kits and it tells you how to do it step by step. Then you can speed up the opening rate or slow it down til you get it right.

try this. CID X RPM divided by 3.456 =if you want to run at 5000 rpm you need a 590 CFM or lets say a 600. any good quadrajet will feed this engine ats JMHO

Another point to remember is that most people over carb their engines for whet ever reason or another. The the vacuum Holley or qjet will keep this from happening.

Thanks guys!!!