Wanted to see what size carb you guys with 400's are running. I have a 600 CFM Carter Comp Series which I have to run rich in order for the car to run its best. Thinking I should probably be running more like a 750.

IMO, minimum 750cfm+. Depends alot on your build. I run an 800 with the choke plate removed(no need for a choke in Cali). Without the choke restriction they say it will pull 839cfm(somebody measured it). Of course it's got some of the other mods from Cliff's book as well so the fuel delivery is up to the extra air abilities too.

E-brock 1901 750 cfm

I think most people go for a carb that is too big. If your car is mostly stock or lightly modified 600-650 should be good.
I have a buddy with a 455 that makes 500+hp. His dyno session showed him pulling 689 cfm at 5500 rpm. He has a 950 Quickfuel.
Mine is 463 ci and a 500HP engine [dynoed] I put a 750 Holley on in place of the 800 and gained 10 HP and 10 FP. It's not just how much air will pass through but how well it supplies air and mixes the fuel at the needed ratios. Go to the Holley site and run their carb size calculator, my bet is they will suggest a carb under 700. Then again I could be wrong, wouldn't be the first time.

I've never seen a dyno that could measure cfms pulling through a carb. Blue's probably right about most people erring on the larger side than smaller. I've heard it said that a motor can only use what it can use no matter what size you put on. Every motor is built different so I'd say there is no one right and one wrong one. Just whatever works best. It would be great to be able to test several different ones to see what works best but that's pretty hard to do. In the end I guees you just have to trust someone that has supposedly done that, like the Pontiac engineers that decided a 750cfm was the right all round size for most of their applications.

With a typical divided dual plane intake only half the engine is only seeing half the carb per side. Less bad side effects of a carb that is supposedly to large

One could put on a 2000cfm carb but it wont use that much unless it's the size of a D9 cat engine. An engine will only pump so much depending on the speed, valve timming and displacment. The volumetric efficiency is higher on some engines than others and the carb will be more efficient if sized for the engine. The engine will have to run with some manifold vacuum to operate the fuel flow, too small a carb and vacuum is too high, too large and the vacuum is too low. Air flow is rummored at 1.25scfm/hp at peak torque and 1.4scfm/hp at peak power. I don't know how accurate that is. Some look at accelerator pumps, float levels, clogged filters etc. when chasing an off throttle bog, quite often it's the fact the carb is so big the mixture is just too lean when the throttle blades are opened. Then we start putting larger jets in to compensate and the cycle starts. Ends up with bad gas mileage and exhaust that stinks like unburned fuel,


The Superflow dynos monitor airflow and process it into standard day readings, SCFM air flow.

I was wrong, again, the 455 only flowed 680scfm not 689 as I stated above. [attached]

I don't think you can go wrong using a 750 Quadrajet, you may not use all of it but you can also tune it to your specific build. Pontiac engineers couldn't put a different carb on each engine they made, true there are a lot of variants but not for every change of cam, valve,exhaust, c/r, piston etc. They had to have a one size fits most carb. And they did a pretty good job of it. We can choose and buid one for our specific engine. I'm looking at one from Prosystems based on a Holley 950HP they think it will flow only 830 true cfm. That's a far cry from my buddies 455 only flowing 680scfm in his 455. I don't think mine will use 830 cfm but the carb will most likely work better than the 800 I have now.

love those torque numbers

There is much more to a carb than just the peak CFM rating. I agree with Q-jet recommendation, and most critically that it MUST be a Q-jet specifically set up for your engine and power output. Q-jets were set up by GM to flow anywhere from maybe 550 to 750 cfm plus, depending on which engine they strapped it on...and easy mods open them up near 850.

Remember Q-jets were put on early 70's Super Duty 455's putting out near 500 hp, and they were also put on Mid 1980's 5.0L smog boxes choked down to a whopping 125 hp. And everything in-between. And they ran great on every application when properly tuned. That's the beauty of a Quadrajet...they can feed a proper fuel ratio to any CFM your engine needs. Other carbs can't do that.

If you're somewhat of a DIY'er, I highly recommend getting Cliff Ruggles book, How to Rebuild and Modify Rochester Quadrajet Carburetors, read it first, and then make a decision on how you want to go. Lots of decisions to make if choosing a Quadrajet...and here's a hint...the ones from the late 70's and early 80's on the smog choked machines are among the BEST to start with and rebuild to performance applications...they much better cores than the late 60's varieties that came stock on these cars. His book explains why, and how they need to be modified in order to be opened up to the full potential of YOUR engine, on TODAY's 10% ethanol unleaded fuels...while giving excellent starting/drivability/economy at the same time.

If you actually want him to build one for you, expect to spend big bucks, and get yourself on his 1 year waiting list ASAP...

Or read his book and do it yourself. He also answers his phone and gives tips to the DIY'er if you're smart enough to order your rebuild parts from him. He was an invaluable resource who I talked to a couple times during my recent Q-jet build for my 400, and I can't think of a better way to get it done. My Q-jet was actually a Carter model, pulled from a 1977 GMC Pickup truck. It was the perfect core, and it runs incredibly well after I did all of Cliff's recommended mods and put his rebuild parts into it.

mine runs great...Holley 650 on my 400 w Edelebrock performer intake..too big is not good...and if its too big and you lean it out ,it tends to run hot.

Don't forget the Q-jets mechanical secondaries are limited by the air valves that control the secondary metering rods. That's one reason they are so versatile, they only give the motor what it needs.

Craze's correct about cfm numbers, I could drill to 3" holes in a block of wood and get great cfm flow numbers on a flow bench. Wouldn't mix much gas though.