66 Coronet - 727 auto - 4.11 gears. .30 over 400 zero deck with KB 240 Flat tops at approx. 9.5 to 1 compression, Edelbrock Performer 84cc heads with matching intake, Comp Cams Nostalgia Plus .509 valve lift hydraulic flat tappet(Intake Duration at 050 lift 247, at exhaust 254 - Advertised at Intake Duration 292, 299 at exhaust - Operating Range 3000 - 6500 RPM). Hedman 1 7/8" headers, Mallory Hyfire ignition. TCI Street Fighter Converter stall 3400 to 3600.

Mostly street driven, occasional track. Currently running Holley 950 HP. Best pass was a 12.35 with a 1.7763 60 foot.

What do you guys think should be the correct carb size? I'm wondering if the 950 is too big - it loads up a bunch on the street?

Thanks,

Bob

COMPRESSION is a little low with the alum heads ...IMO.

And that carb is waaay too big for a street car.

Doc, do you like his combo? I don't, not for street use. I'm not really sure what the OP is after...

A 600-650 cfm carb is PLENTY! IF you actually ever take it to the track and and run it in the high rpms, then switch it to a different carb in the 750-800 range.

9er ....I am not a fan of a B-engine with lower compression. This would be muchO bester with a set of cut 915 iron heads on it.

Yes ...and a 600 cfm Berri crab would be better. Hell .. when I was a Berri-Bumz ...I used a 600 Helley(1850) on my 440 in my 62. It ran 12.39 at 111.

MUCH better to uNder carb .. than OVERdo it.

TO ADD ...

I think the OP is doing REALLY WELL with the combo ...as it is.

More initial timing will help keeping it from loading up on the street....Or an annular carb...

A smaller carb IMO would help the combo a bit around the 750-800cfm range. As for the compression it is fine the Alum head/ compression ratio and been proved a myth with dyno testing.

A 750 vacuum will work real nice on the street. lighter advance springs for more idle advance would help. think I would recurve the dizzy first.
check your in gear vacuum after at idle after you give it more initial/idle advance and go 1.5 to 2 numbers under that for your power valve. if you get 7 go 5.5
maybe up your idle rpm to 950 (or 1000 to 1100 )

That cam likes more compresson than that. I'd think a 750HP carb would work well w/ your combo. What have you done to the carb? I'd think you'd want to lean it out form the factory settings. That carb is for a much bigger engine.

I assume you degreed the cam? this cam likes 104 or better yet 102???

Every motor is diff. A local racer has a 440 and after dyno and track testing his 12:1 iron headed motor liked a 1050hp best. The cfm formula came up with 880 for his combo. My 440(11.5,915 heads,.517/.533,246/252@.05,timing 22/36 all in @ 2400) came up with 780 and I run a 1000 BG race demon with no isues. I can let it idle, putt around town and when it is time to GO just mat it and no hicups just goes! It is all what your combo likes.

OK- for carb sizing use this equation.

CFM = (RPM times CID) / 3456

so-
If you think your 400 may see let' say 6,500 RPM one day then-
CFM= (6,500 X 400)/3456
which is 752 CFM assuming 100% volumetric efficency. This is the maximum CFM this engine can possibly flow. To flow more you'd need more Cubic Inches.
Since NO recipricating engine is CLOSE to 100% efficent, the standard is 80% which means 752 X .80 = 601 CFM.

This mean you need something between a 600-650 CFM rated carb.
Since they don't flow 100% either- I'd stick something like a 700 or 750 on it. The 950 is WAAAAAAYYYYY too big.

Lots of great carbs out there in the 700-750 range...

Hope this helps,
CC.

edited for the 6,500 max the cam is rated at....

OP might know more than he (& we) thinks.... 12.50's in a B body street car is WAY fast for that motor combo, and I'm sure the 950 sitting on top of it has a little to do with that. However, on the street, I think it would run much better with a 750.

Plug his info into Holley's interactive carb selection program
http://www.holley.com/applications/CarburetorSelector/CarbSelection.asp and what do know, his carb is one of the ones recommended. However, 950's are generally race calibrated which might be part of the "loads up a bunch on the street" problem.

Two suggestions and a comment:
1. Holley 750 vacuum secondary would be much crisper on the street and off the line.
2. Shorter duration cam will lower the ET dramatically. The 60' time is slow IMHO.

Now on to carb size. IF you are racing dynos then bigger is better. Lower restriction in the intake allows better cylinder filling equals more hp. For this reason the cubic inches times rpm over 3456 equation doesn't work for race cars. It will result in a 1.5" depression across the carb. I remember dyno tests where the intake depression or vacuum was measured and the hp kept increasing as they lowered it to 1/2 inchby running larger and larger carbs.

Plain and simple the reason one drives a smaller carb on the street is what happens at part throttle. If you get the primaries too large the carb signal is weak and metering goes to heck. This is the rationale behind the Q-jet and Thermoquad.

It is possible to run a larger carb on the street if you use the right booster to get decent signal. I'd say Holley tech people or a good Holley tuner might be able to help the OP keep the 950HP carb and increase drivability, too. And on the Holley HP carbs the boosters are changeable.

R.

I have no personal experience with the 950 hp but everything i've heard about them has been positive. It's really not that big anyway. It's essentially a 750 mb with radiused entry combined with a 850 bp. Plus, dual plane intakes are more tolerant of big carbs. You may need more initial timing. That cam is pretty hot for a street 400. I would say your problem has more to do with cam, ignition or carb tuning than carb cfm. Are the plugs fouling? You might try leaning out the primaries. I sure wouldn't go changing out the carb just yet. Oh, that cam would probably like more converter than what you have.

Thanks for all the feedback guys. That 12.35 was my best pass in late October at Pittsbirgh Raceway Park year before last with very cool dry temps. Since then I ran a bunch last year and it's run fairly consistently at 12.50 - 12.60 in all temps and air conditions. I thought this was a fair ET given the combo - really not that much displacement and fairly low compression. I need to find out what the comp ratio is given that I'm only guessing? It was also suggested buy some of the guys at PRP that I put another converter in - they thought a Turbo Action Model 38 would wake it up?

Primary focus for the car this year is street so I will try some of the whats been suggested to get it to behave a bit better with the current setup including carb. Should be getting back to the track more later this year and next year so all I was hoping to do I guess was get it into the 11's without a major engine revamp?