Does anyone know a formula to determine how boost psi changes the intake flow rate? I have heads that flow 290 cfm @ .600 lift, intake manifold that flows an average of 335 cfm and a dual 65 mm TB that flows 1350 cfm. I am wondering how the flow capacity of these three will be increased by boost, lets say 18 lbs?

 

I would be really interested in this too -- maybe Richard from WCCH can answer authoritatively on this one.

From what I understand, each atmosphere of boost increases the density of air pushed into the same space in the same time by the amount pushed at atmospheric pressures (e.g. 14.7psi of boost pushes 2x as much air as 0psi -- atmospheric pressure -- into the same space -- the cylinder). So I'm guessing that you manifold becomes a ~700cfm mani, TB = ~3000, heads = ~650.

The really cool thing to think about is the CID of the motor So NA, you have 408 cid. At 14.7 psi you have 816. At 29psi you have 1224 cid... WOOOOOOOO!

 

What size engine are we talking about here? What rpm?

 

You need to reference your engine volumetric efficiency based at intake manifold conditions.

Then figure out how much the air density changes as you apply boost. Remember, as air is boosted, it heats up, this means that the temperature in the intake manifold is just as important as the pressure.

Density = P/RT

So basically, you assume you dont get any more volume flow through the intake ports, you just get a much denser charge flowing through the intake ports.

If you measrue the volume flowrate at the inlet to the boost device, that will show a dramatic increase becuase it is at standard atmospheric density.


This is a very basic estimation.. you should get GT Power software if you want a more accurate prediction..

 

This is what I was trying to say above

 

This is exactly why I get my panties in a knot when people get hung up on volumetric flow when dealing with FI. Mass flow rate should be of more interest since it takes the density into account.

 

yes.. but you assumed manifold temperature was constant...

9 times out of ten, your Forced induction motor will have higher intake manifold temperatures. Thus twice the pressure does not mean twice the density!!

 

yes!!! since your air fuel ratio is mass based.. it would only be logical to be concerned with the air mass.. becuase you can always add more fuel!!

AFR = Air mass flow rate / fuel mass flow rate

becuase in the end.. your only concerned with the amount of oxygen molecules being combined with the fuel.. and it really sucks that 78% or so of the inducted air is nitrogen and other gases that do no good!!

 

The engine size is 408 cid. The shift rpm is 5,500. Where do I find out more information on GT power software?

 

Well that, and you said it waaay better than I did lol