Intake Air Requirements

SteveS · Post by **SteveS** » Thu Dec 22, 2005 8:58 am

http://www.slowgt.com/Calc2.htm#Intake

Would someone assist me in understanding the assumptions and math underlying the above. I believe it is derived through multiplying average CFM times four ( since fill must occur in 180 degrees as opposed to 720 ), dividing this product by 1.94 to reduce the CFM to 28" of depression and then multiplying this times 1.6 .......approximately... to allow for max piston speed/demand. Thanks.

OldSStroker · Post by **OldSStroker** » Thu Dec 22, 2005 11:55 am

The calculator seems to be much simpler than you suggest.

Displacement x rpm/2 x VE X 1/1728

Example:

355 cubic inches x 8000/2 x .97 x 1 cubic foot/1728 cubic inches = 797 cubic ft/min (CFM)

rpm is divided by 2 because intake is only every other stroke.

About all this tells you is what air the engine can eat. Depending on how VE is calculated, more air might be flowing thru the intake that the calculation suggests.

If you are using it for sizing a carb or throttle body, it's probably a place to start. In this case 900-1000+ rated carb/TB would probably work better than a 800 CFM one.

SteveS · Post by **SteveS** » Thu Dec 22, 2005 3:32 pm

Stroker, you are absolutely correct............I got tangled up in my drawers on that one. So, how would you calculate the required flow @28" to support the engine defined by the pre-set values in the java calculator or in your example above?

OldSStroker · Post by **OldSStroker** » Thu Dec 22, 2005 11:23 pm

SteveS wrote:Stroker, you are absolutely correct............I got tangled up in my drawers on that one. So, how would you calculate the required flow @28" to support the engine defined by the pre-set values in the java calculator or in your example above?

I'd tell my engine designer/builder what results I wanted, what my priorities were and how much I could afford to spend, and let him suggest a plan of action. I'd forget that calculator also.

Not what you wanted to hear, I suspect.

There is no simple answer to engine design. Even the guy who designed the LT1 intake system, as well as the C5R engine (Ron Sperry) had not only physical/durability/ cost/performance, etc. parameters to satisfy, but also bosses and bosses' bosses to appease/satisfy, I suspect. That might be the most difficult part.

There is a lot of stuff written on how much flowbench CFM is required to support XXX HP. David Vizard, among others has some good thoughts. Look at his writings.