Article about Carb sizing correctly

Rocketman · Mar 3, 2015

Guys, I read this article last night that said correct carb selection was simple:

Cubic inches X max RPM divided by 3456 = CFM

Example:

440 X 5800 = 2,552,000/3456 = 738.42 cfm

Has anyone else used this formula and is it standard?

Dibbons · Mar 3, 2015

I just ran those numbers on my past combinations and it works out to be very conservative. My guess it's good for street profile motor but not a competition build.

Rocketman · Mar 3, 2015

Dibbons said:
I just ran those numbers on my past combinations and it works out to be very conservative. My guess it's good for street profile motor but not a competition build.

They used the Holley 4150 series for the example

I have a 750 cfm on the set up below and have no issues

HYRDGOON · Mar 3, 2015

I ran it too and it would make mine undersized. Came out to 900 for me and I run a QF 780 on my stroker

Rocketman · Mar 3, 2015

Here is the same story, but not where I read it:

http://www.mustangandfords.com/how-...ning-and-setup-tips-four-barrel-fundamentals/

lewtot184 · Mar 3, 2015

edelbrock had a formula they published years back but they don't show it anymore. i thought it was more realistic for a performance engine. i don't pay much attention to the holley formula.

Rocketman · Mar 3, 2015

lewtot184 said:
edelbrock had a formula they published years back but they don't show it anymore. i thought it was more realistic for a performance engine. i don't pay much attention to the holley formula.

I was looking for Edelbrocks and found this one. Very interesting:

http://www.gtsparkplugs.com/CarbCFMCalc.html

and this one:

http://www.carburetion.com/calc.asp

The first one in the thread says I have 12 cfm to spare and the rest say 50 cfm or more to spare at 5800 rpm.

texas69bee · Mar 3, 2015

I guess the best way to tell is to run what it tells you and then run what you have. See how the car reacts on the low end and the high end. If there all saying the same thing then there probably correct.

Propwash · Mar 3, 2015

Measuring the A/F ratio as well as exhaust gas temps would be the way to go, but that's obviously run on a dyno or some added on sensors/gauges. According to the initial link i'm 100CFM+ over, but my A/F and EGT's from dyno don't agree with that. I'd think that would be a pretty basic guide for a street/stock application. Adding more volume capability up top and getting creative with a cam, IMO would seem to throw a wrench in that calculation. Your last two links fall pretty close to what I have. Like others mentioned, one would have to run what they got to see it really unfold. Good luck!

Rocketman · Mar 3, 2015

Propwash said:
Measuring the A/F ratio as well as exhaust gas temps would be the way to go, but that's obviously run on a dyno or some added on sensors/gauges. According to the initial link i'm 100CFM+ over, but my A/F and EGT's from dyno don't agree with that. I'd think that would be a pretty basic guide for a street/stock application. Adding more volume capability up top and getting creative with a cam, IMO would seem to throw a wrench in that calculation. Your last two links fall pretty close to what I have. Like others mentioned, one would have to run what they got to see it really unfold. Good luck!

the last two give me 50cfm to spare

lewtot184 · Mar 3, 2015

edelbrock used to say do the cubic inches/rpm/divided by 1728 thing and then multiply by 110% for single plane intakes and 125% for dual planes. i don't pay any attention to the cfm crap formulas or ratings. ratings are just marketing numbers and formulas are CYA for the novices.

67 coronet · Mar 3, 2015

formula assumes alot of diff #'s,like volumetric efficency.it works 4 stock setups,but the #'s change very quickly with even warmed over motors.not even usefull for hot or full race applications.

747mopar · Mar 3, 2015

Every dyno test on Carbs I've seen seem to show engines like a size bigger than the formulas show? I don't think there's such a thing as an accurate carb calculator with all the variables but it will get you in the ball park.

Rocketman · Mar 3, 2015

67 coronet said:
formula assumes alot of diff #'s,like volumetric efficency.it works 4 stock setups,but the #'s change very quickly with even warmed over motors.not even usefull for hot or full race applications.

it was about street applications

- - - Updated - - -

lewtot184 said:
edelbrock used to say do the cubic inches/rpm/divided by 1728 thing and then multiply by 110% for single plane intakes and 125% for dual planes. i don't pay any attention to the cfm crap formulas or ratings. ratings are just marketing numbers and formulas are CYA for the novices.

the guy that built my motor seemed to know his stuff, I was going to buy a new 850 and he said it was overkill. He has three drag cars and one of those has a 700 cfm carb and he wins with it.

hunt2elk · Mar 4, 2015

747mopar said:
Every dyno test on Carbs I've seen seem to show engines like a size bigger than the formulas show?

That does seem to be the case if you are running at WOT.

lewtot184 · Mar 4, 2015

hunt2elk said:
That does seem to be the case if you are running at WOT.

in my opinion, this makes a good case for velocity controlled multi-carb.

Cranky · Mar 4, 2015

Track testing is what I go by and usually end up with more CFM than any calculator says and usually do the same with high performance street cars but that depends on the type of carb too. I like the big TQ's on the street.....

Article about Carb sizing correctly

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