Throttlebody cfm???

[Guest TT604]

Hi all, i'm new here. I dont own a Z but need some tech info & after reading some of the posts there seems to be some very knowledgable people here.

What i'm trying to find out is how many cfm does a 100mm t/b flow. Can anyone help?

 

Much appreciated, thanks.

[Kevin Shasteen]

What I've done is take all the equations presented by Corky Bell and a few other engine builders and try to put them all in one tidy equation.

 

One thing you will need is the other engine variables to make the calculations, relative to an engine. You will need additional variables; like redline rpm and displacement to name a few.

 

Which equation you use depends on which variable, CFM or TB sizing you do or dont have.

 

First you need to know the theoretical CFM needs of your engine; use this equation to obtain that number:

 

NOTE: The [.85] represents your VE% in decimal form, so if you think your theoretical VE% is higher or lower, make your changes accordingly.

 

CFM = (Displacement • Redline RPM) ÷ 3456) • .85

CFM = ((350 • 6000) ÷ 3456) • .85

CFM = (2,100,000 ÷ 3456) • .85

CFM = 607.64 • .85

CFM = 516.49

 

Once you know your CFM you can then plug that figure into this equation:

 

Gas Velocity = (((Engine CFM ÷ (3.14 • (Port Diameter ÷ 2)^2 ) • (1÷60) ÷ (1÷144))))

 

Where: Gas Velocity = Airflow Velocity in Feet Per Second

3.14 = Pie

^2 = is the only way I can convey to you this number needs to be Squared

Port Diameter = diameter of any circular port opening in INCHES! In this case the port diameter will be your TB butterfly...remember it is in inches

60 = 60 seconds per minute

144 = square feet in inches (12 inches • 12 inches = 144 square inches)

 

If you dont have the circular port opening measurement but you do have an idea of how many CFM you wish to flow at PEAK RPM, use this equation:

 

Port Opening Diameter = SQRT (CFM ÷ .7854 • (2.406) ÷ fps)

 

You can combine these two equations to make one equation:

 

SQRT((((Eng.Displ. x RPM) / 3456) x VE%) / .7854 x (2.406) / fps)

 

Where:

 

SQRT = Square Root

Eng.Displ = Total displacement of all cylinders added together in Cubic Inches

RPM = Peak RPM you will be shifting at

VE% = Volumetric Efficiency in a decimal form

fps = Feet Per Second you wish the airflow to be flowing thru the ports at Peak RPM

 

Again, keep in mind these equations are in inches. Since most throttle bodies are in mm you can then add the mm conversion factor to the end of the last equation. You still have to use the "inches" measurement when working the equation - but by adding the mm conversion factor to the equation your final answer will be in mm. This final change to the equation will look like this:

 

(SQRT((((Eng.Displ x RPM) / 3456) x VE%) / .7854 x (2.406) / fps)) x 25.4

 

Kevin,

(Yea,Still an Inliner)

[Guest TT604]

Whoa...

 

That's really great but i'm a little lost (I failed maths miserably at school).

 

What the story is; my new engine is a 604ci Dart/Chev with twin hi-flo GT45's at around 30psi & staged EFI. I'll only be turning it to 7400rpm but because of the capacity of the engine I need to find a cylinder head which will flow as close as possible to 500cfm @ 28" of vacuum.

Now, i'm getting a sheet-metal highrise built for it & I need to know if twin 80mm t/b are sufficient or I need to use twin 100mm t/b to get the desired air flow.

[Kevin Shasteen]

What is the purpose of your engine? Street weekend warrior or all out race?

 

Is your reference to 30psi the fuel injector rate or is it a reference to a turbo use?

 

The High Rise intake mnfld mfg's are really tighlip'd about their cfm flow - so, if you are going to have a pro shop create your manifold, you really need to tell them what your objectives are and they should be able to give you the hard data to meet your needs.

 

When you get into an engine that requires a custom sheet metal high rise - there is more going on inside that engine than just airflow. Airflow is important but it is not the only thing going on.

 

Kevin,

(Yea,Still an Inliner)