[joes455]

I seen somewhere that one of the true methods to estimate required carb is to add 3 of the heads intake flow together .ie head flows 300 x 3 =900. So a 900cfm carb is needed.330 head would require a 990 or 1000.Whats everyones thoughts?

[Steve C.]

This might be of interest.....

Carb CFM Calculator

http://www.wallaceracing.com/calc-carb-cfm.php


Choosing the Correct Carburetor for Five Common Engine Combinations

https://www.motortrend.com/how-to/16...e-combinations


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[Steve C.]

Somewhat related.... Carburetor manufacturers' CFM ratings are a general guide, but not necessarily precise.

Example, years ago I was using a custom built Holley HP950 carburetor.
For the purpose of determining the amount of horsepower one popular calculator required for input the carb cfm. I called the fellow that built the carb and asked what should I use for input.
If memory serves me right he stated about 823 cfm. Using that figure I was able to manipulate the input in order to provide results to mimic my actual engine dyno sheets very close.

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[steve25]

If you have a combo that’s currently running then the best thing to do is to see what your pulling for vacuum at your motors peak rev’s.

In terms of ultimate peak hp if your seeing anything more then 3/4” Hg then you might try a bigger Carb.

But the Devils in the details.

Depending on the times you run your average torque and hp numbers might suffer if you go to
A bigger Carb since on a pass your motor is not spending enough time at peak rev’s for the hp allowed by a bigger Carb to help out.

Combo’s geared such that they need a 2000 to 2500 rpm wide power band to turn there best numbers will be harmed more by too big of a carb far more then too small.

[joes455]

Engine Masters just did a chevy big block that made 1040 hp with a 1050 dominator ,eng rpm was 7400.Head most likely flowed 450 +,so maybe we should add 2 intks flow instead of 3 and just add another 150.

[Dragncar]

When you look at this stuff its hard to compare the power a big Rat engine make with its higher flowing heads and shorter strokes. They always seem to make their power with less carb.
The way I understand it is Pontiacs have a longer stroke than most engines so they can take more carb than a given BBC-HP combo. (ever hear a 455 bog ?)
I don't know if I ever heard of a 1000HP Pontiac doing it with a 950 or 1050 like you hear about the large rodent engines.
Then you have the actual flows of carbs vs what is on the box.
These are wet flow numbers given to me by a LA based carb outfit.
850 cfm= 956 cfm wet
1050=1121 cfm wet
1150= 1176 cfm wet
1250= = 1306 cfm wet.
You can gain 20-30 cfm with polish work and milling throttle blade shafts.

[PAUL K]

Originally Posted by Dragncar

When you look at this stuff its hard to compare the power a big Rat engine make with its higher flowing heads and shorter strokes. They always seem to make their power with less carb.
The way I understand it is Pontiacs have a longer stroke than most engines so they can take more carb than a given BBC-HP combo. (ever hear a 455 bog ?)
I don't know if I ever heard of a 1000HP Pontiac doing it with a 950 or 1050 like you hear about the large rodent engines.
Then you have the actual flows of carbs vs what is on the box.
These are wet flow numbers given to me by a LA based carb outfit.
850 cfm= 956 cfm wet
1050=1121 cfm wet
1150= 1176 cfm wet
1250= = 1306 cfm wet.
You can gain 20-30 cfm with polish work and milling throttle blade shafts.

You sure you don't have those backwards? Usually things flow less with air and liquid compared to just air.

[Formulas]

These kind of subjects remind me of my first experience changing carbs for size reasons. ... .. Real early teens mini bike briggs & strattons cant remember the exact HP of the carbs messed with but i remember the larger carb on the smaller engine lost me off idle torque but gave a better top end.. Good times

[Dragncar]

Originally Posted by PAUL K

You sure you don't have those backwards? Usually things flow less with air and liquid compared to just air.

Numbers I have.
850 dry=1030 cfm
150 dry= 1213 cfm
1150 dry=1272cfm
1250= 1413 cfm dry

[Dragncar]

So that Wallace calculator. What is the VE % we are supposed to input ? I put in 100% with .5 Hg numbers and it came up with 1072 safe cfm.
I have a 1100 cfm 4150 carb and a 1050 so I might be close.

[johnta1]

With a well built engine and a functional scoop you could input 120%.
A street type or stock car maybe 75/80%