bigrod

what would be the best carb to run on a 347 combo...i currently have a 670 street avenger but i was reading the installation process for the victor intake and the strongly suggest a 750

tinman

This is a very complex question.
What heads, cam are being used?
What is the wieght of the car? What type transmission is being used?
If it's an automatic, what stall is the converter?
What RPM range is the engine going to be operated in?

bigrod

world windsor sr. with port work, lunati voodoo solid roller with a 640 lift..not sure of the weight...c4 trans 3 speed with a 3500 stall 3000-7000 is the rpm range

bigrod

i will run the 670 street avenger and see how it does and go from there

tinman

Sounds to me like a 750 D.P. type wouldn't be out of the question.
See if you can borrow a known 650 and a known 750 and test with each one.
Then make a decision........

scottybaccus

C.I.D. x RPM/3456 = Max CFM at 100 Volumetric Efficiency. Few engines reach 100% V.E. without forced induction.

347 x 7000/3456 = 702.8 cfm
Estimating 90% V.E., you get 632 cfm. My guess is that you will Like that 670 Street Avenger, so long as you get it tuned properly. I doubt it will be exactly right straight out of the box.

bigrod

preciate that hoping i will..i deffenently dont want it loading up on me..

tinman

That math formula is a nice piece of work but I don't think it really applies in this case.
I know of several very good running 430+C.I.D. engines (brand C) that are uaing 1250's.....
On the other hand more than a couple 306's have used 750's very successfully......

scottybaccus

The brand X 430+ means what? 431? 454? 528? You can run more cfm than you need. The formula is a way to decide what you need in order to choose a carb that will behave on a road course or on the street. If you are drag race only, you get away with running excessive carb because it passes the flow limitation to the next piece in the puzzle, intake or cylinder heads. With enough cam, you don't need the idle circuit anyway, because it won't idle below 1200 rpm. In drag racing, you snap it open and go for a run.

The whole reason to use the formula is to retain low end driveability.

On the other hand, you can turn a 306 to 8500 rpm and use every bit of 750 cfm flow. You also have to take into account the percentage of over capacity.

Sticking with 100% V.E. for simplicity, you can run 750 cfm on a 306ci at 7500rpm and miss by 12% and still be better off than that same 306ci cammed for 6500rpm having the same 750 cfm which would miss by 24%.
That's a HUGE difference in the match of flow to demand.

In short, you can't compare based on displacement alone. It's the package that counts. Fortunately, we had his cam range and displacement to work with. The lack of specifics on porting and such makes a good argument that he won't achieve 100% V.E. since a good NASCAR or pro NHRA builder struggles to get there. I'll stand by my math.