Carb Jet help

[D21]

I have a 427 bbc that is built up and bored .070 over. If calculated right that puts it at about 441 cubic inches. Im running it in a street stock and we have to run a Holley 4412 carb. Does anyone have an idea on what jets I should start with? Thanks for any help.

[cassellracing24]

A big block !!!
78's should be dead on..

[D21]

Ok, thanks for the help.

[rubbinsracin]

big block? SWEET! Must be a big a$$ track! i would have figured with a 2 barrel you would need more like something in the mid 80s like 85s or 86s. when we go to a track down the road we have to run a 4412 with our 383 and we are running 82s. are we way off?

[sj valley dave]

What people seem to forget, BBC, SBC, doesn't matter...A well built 355" SBC will need more air than a 4412 can deliver....Once you have reached the maximum amount of air that can be drawn through the carb and the jetting is correct, then if you just go bigger on the jetting because you have a BBC, you will end up rich...We had a very well built 360" SBC, Dart 215 angle milled, 14:1, Crower 308 solid cam in it and at an adjusted altitude of 1000-2000 feet we had 73's in a gauge legal Willy's 4412 and plugs were perfect and it ran 200*...

[cassellracing24]

What SJ said is true... Also as rpm's reach a certain point on any restricted carb you will begin to see very high vacuum in the intake, this begins to pull fuel from anyplace it can (accelerator pump circut,idle needle circut,discharge nozzels,ect.) So as rpm's are increased so is the A\F ratio beyond the main jet's to a point....

[crate late]

What is the maximum size jets you would typically put in a 650?

[cassellracing24]

He is referring to a Holley 4412 500 cfm 2 bbl.

[cassellracing24]

rubbinsracin:

Assuming you are talking about a gauge legal top\bottom 4412 I am very surprised that it doesn't spit\sputter & foul plug's. If it is gauge legal & you back it down to 74-76's I beet you would feel a seat of your pants HP difference & throttle response as well !!

[fast_crew]

Agreed, most 4412's are 73-76 jet, with a fairly light power valve. So that it will actually start to close on the top end to keep from over fueling as the vacuum comes up in the intake manifold caused by the restricted carb.

[Graff Spee]

I think that you guys are forgetting that you must jet the carb to the motor. Remember wide open throttle, cut off engine coast to stop, pull plugs and read them for proper coloration at base of insulator, etc etc etc. Do you guys not do it that way any longer?

[sj valley dave]

No we are not forgetting that...If you have some low powered engine that the 4412 can supply more air than the engine needs, that is great. But in a high HP deal, where the 4412 is THE restriction and you are pulling all the air that can flow through the carb, with max air flow a jet of somewhere between 72-75 will almost always get you in the ball park. Then do your plug check to fine tune depending on adjusted altitude, etc....84 jets on a gauge legal 4412 is soo fat....

[return to dirt]

i run 4cyls and on most i run 350s but on my 22r mod i run either a 500 or a 390 double pumper. when i run the 500 the jets are in the 72-76 range. it seems like you would jet the carb for the airflow and not the cubic inch.
sj dave, do you ever run up in northern calif.(like chico)?

olin

[D21]

it seems like you would jet the carb for the airflow and not the cubic inch.

I just assumed that more cubic inches meant more fuel. Is this not the case? The reason I was asking was so that I didnt start out too lean and get into trouble. I'd rather start out a little rich and work my way down from there. Just looking for a good safe starting point. Im not very smart on this topic, so I appreciate all of the help from you guys.

[sj valley dave]

In reality you do jet the carb for the air flow and volume...You want to have a specific A/F mixture...If you had a 1150 cfm dominator and had a 350" SBC and a 496" BBC, for example, you'd have big differences in the jets, as the carb can supply all the air that the engine is drawing in and the amount of air would be different, so the jetting requirements would be different as well.....Put a 350 or 500 2 bbl on the same engines and both engines can max out what the carb can pass for air, so the jetting would be basically the same...If the carb is a stock 4412 you could put in some 74's or 75's and start tuning from there...Probably end up going down a size or two, depending on your adjusted altitude...

Return to Dirt...We have one W.A.S. race scheduled for Chico this year...We hope to make it up there...we are going to Calistoga on June 11th with the DLM for sure!!

[return to dirt]

dave, hope you can make it up july 22. we don't get super lates up here very often. good luck at calistoga.

olin

[D21]

Thanks for the explination Dave. After going back and re reading everything I understand what you guys are saying and it makes sense. Thanks to everyone for the input.