engine cfm rate...

[Hemi_Express2013]

Been thinking of getting Moes ported intake manifold if and when they become available. Been doing some base calculations of present cfm usage. Looking for someone to chime in for some comparisons. My engines intake uses on average at wot 311g/s. 311x4=1244gt/s 1244x60=74640qt/min. 7464/ 29.92= 2494.65 cfm.


This formula doesn't seem right. That's way too much cfm. Any help would be appreciated.

Might help if I had tbe right unit of measurement!

Found this formula on the ls1 forum.

28316.8 cm*3/ft*3
.00129 grams/cm*3


28316x.00129 = 36.53gram/ft*3

311grams/sec x60sec/min = 18660gr/min
18660gr/min / 36.53 = 510.8 cfm.

 

[jsteinm1]

Displacement is 345ci = 0.200ft^3
At 5000RPM, the engine is displacing 345ci*5000RPM/2 (takes 2 revolutions to complete a cycle)
0.200ft^3*(5000/2)min^-1=500cfm

Thats a rough calculation, actual would be a little less since the cylinders aren't scavenged 100% each time.

 

[charonblk07]

You're right, that's way off.

The 2 calculations I use are:

CFM = ((g/s /1225)*60)*35.31 or
CFM = (CID*RPM*VE)/3456

That gives you 537 CFM with the first and with a rpm of 5800 and VE of 80% gives 468 CFM with the second.

I like the second better because it does take the Volumetric Efficiency into account and allows a variation based on rpm. Hehe, my engine moves 1189cfm

 

[Hemi_Express2013]

You're right, that's way off.

The 2 calculations I use are:

CFM = ((g/s /1225)*60)*35.31 or
CFM = (CID*RPM*VE)/3456

That gives you 537 CFM with the first and with a rpm of 5800 and VE of 80% gives 468 CFM with the second.

I like the second better because it does take the Volumetric Efficiency into account and allows a variation based on rpm. Hehe, my engine moves 1189cfm

So, if the intake volume of the 5.7 is roughly 675 cfm at 5500.
Displacement x rpm /2 and my engine is using roughly 510cfm then the VE is around
93?

Using your formula for 311g/s gives me 542.43 cfm with a VE of .98791.
But 542.43cfm is still only 80.36% of the engine capabilities of 675cfm @5500. Is this the true VE and .9879 really P-ratio?