Venturi Flow Rates

[livingwater]

Anyone have any idea how carb companies calculate flow rates?

I took the two throttle bodies that Holley markets to GM TBI users, and calculated backwards from their rated number (CFM) to air velocity based on the area the air has to go through. It seemed like a reasonable idea, until I came up with velocity numbers which were about 30,000 ft/min apart.

My purpose for this investigation is to ensure that I am doing correct calculations for a cold air intake on my wife's Suburban. Sure, I could just throw some 4 or 6 inch tube in there, but I'd rather take the time to optimise it.

Thanks.

 

[ShaunT]

Recently, they are flow testing on a flow bench designed for wet flow. That's about all I can tell you.

Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)

 

[jimmyboost]

What were the individual velocities that you calculated for each throttle body?

 

[livingwater]

Jimmy,

For the 670 CFM throttle body (2 inch bores), I came up with a flow of 184268.6 in/min = 256 ft/sec
For the 400 CFM throttle body (1.6875 inch bores), I came up with a flow of 154492.6 in/min = 214.5 ft/sec

 

[EngJW]

Your numbers sound reasonable. I think 200-250 ft/sec is correct. I read somewhere that carbs are tested at .5 inches of mercury pressure drop, which is about what the manifold vacuum would be at full throttle.

John Woodward

 

[patprimmer]

According to SA designs book "Super Tuning And modifying Holley Carburetors" ISBN 0-931472-08-3, page 16, the carburettor industry has pretty well standardised on:-
two barrel carbs are rated at 3.0 inches of mercury
four barrel carbs are rated at 1.5 inches of mercury
As this is an American book, it might only be refering to American manufacturers.

Regards
pat

 

[EngJW]

Hi Pat,

I should have said 1.5, since I did read the book once. Bad memory.

John Woodward

 

[patprimmer]

No problem. I was lucky enough to look over at a bookshelf and spot my moth eaten old copy

Regards
pat