what size carb for a 400 Pontiac?
 

My Firebird has a 400 Potniac motor/muncie 4sp combo. It has a mild cam, headers, a performer RPM manifold, and a Holley 800 double pumper carb. The problem I'm seeing is it's way too much carb for my motor. I've done the math and I keep coming up with about 600cfm. I'm planning on going to a performer intake and getting rid of the rpm. It's going to be driven on the street and maybe a track every now and then. I know I should use a vacuum secondary carb but I'm torn between the 600 and 750 e-brocks. Thoughts?

A double pumper carb is fine if it is sized right. The vac secondary carbs only work right if you have enough vac to operate them and have the engine tuned really well. I guess what I am saying is the double pumpers are a little more forgiving with regard to engine tune. Your math seems right to me for a 600 to 650 cfm carb. I personally would go with a standard performer intake and a 650 double pumper carb. The e-brock carbs are great but as far as I know there isn't any 650's in the carter/weber style. Of course all of this is dictated by your cam and heads. You said it's a mild cam, headers ...
Is there any port work on the heads? What's your driving style? (always in the gas?) Is it your daily driver/commuter? What kid of shape is your ignition system in? Getting full spark and is it timed right?
Look into a 650 DP after you make sure everything else is up to snuff and I think you'll be really happy with your cars performance.
Eric:)

A 750 Vac. sec. will work well on a 400, especially if you have a mild cam. My 406 has ported no.48's, a Crane 284H (284 dur., 228 dur @ .050, .480 lift), 750 vac. sec. on an e-performer, Hooker comps and 2 1/2" duals, TH-350 w/ 2200 conv., 3.08 posi and my 'Bird runs consistent 13.0x's.

The DP's are good for competition but will kill you on the mpg- the vac.sec. is much better suited for the street, as your engine will tell the carb when it wants more fuel and open the secondaries.

Geno

Ehh, I ran a 4500 stall with 850 DP on the street daily. It was ok for me. It's only when I went with a .630 lift and 270 280 duration @ .050 that it wasn't that much fun anymore to putt around in.

I 2nd the 650dp. Another consideration would be a Qjet modified by Cliff, do a search over on Performance Years forums. Plenty of performance, great economy.

I'm running a full race 750dp on my 383 Pontiac, but mine has all the tricks and it's running on E85, so I needed the volume to go with the increased appetite for the go juice. I don't know what I get 'exactly' for mileage, but I top it off every 100miles...the factory gas gauge works like crap even with a new sender. :)

If anything, a vac secondary carb is going to be more forgiving and easily adjusted. Adjust the air door or change the vac secondary spring to tune the opening rate. Most OOTB carbs are going to "work" ok, but to get them dialed in will take some effort which could include a wideband 02 sensor and/or dyno time.

Properly tuned, the 800 should work just fine but you may find you get better throttle response with a smaller carb.

My big debate has been between the 750 ebrock vs. the 600 ebrock. It's really confusing when you use the std carb sizing formula and get 580 cfm when the factory qjet was rated at 750cfm..............I'm running so rich right now it crazy

On the street a 750cfm looks about right according to Holley. Have you tried tuning your carb?

The qjet has a spreadbore with very small primaries, so it's like running a very small carb until you open the secondaries. A big square bore holley will have you running on alot bigger cfm/fueling at low rpms on the primaries. If you're secondaries are coming in too soon, you've just quadrupled your problem.

I like the E carbs, they work great out of the box and have good throttle response in my experience. I had two 600's on a tunnel ram and a 366 small block and it was the snappiest car I've driven on the street...go figure.

I would probably go with the 600 with your set-up....you won't be pushing it past 5800-6K anyway, so the added airflow is a moot point.

I'm pushing 500hp with my little 383 on E85 which has about 25% more fuel requirement on my motor, the 750 still feels a bit big off idle.

Carb Size
 

I have set up many carbs on Pontiac 400's and your current carb size is good for your combo.

E- carbs are cheap and will just get you from point "A" to point "B"

If it's running rich now it not because of the size it's because it is not set up correctly and the dist may need some curving.

Test your combo for carb size.....

Connect a vacuum gage to a full source and string it into the car so you or someone riding shotgun can read it while driving.

Make a substained WOT run and see how much vacuum your pulling @ WOT

Vacuum gage should go to "ZERO" if sized right.

If it's too small it will pull vacuum.
Jeff