I sent a couple of emails/PM's about this and got his from Larry Meaux (aka Maxracesoftware).

Thought some may find this interesting. Here is my question to Larry followed by his response.

Larry,

I've been looking at Piston CFM demand formulas and found this one

posted by Darin Morgan and others

[Piston cfm demand] = ([CID] x [RPM] x .0009785) / [number of cylinders]]

It gives numbers that are close to peak Piston CFM Demand in Pipemax for rod ratios around 1.6. The head CFM is less.

When I calculate the actual displaced volume of a piston at peak piston velocity I get a much higher number.

Example:

Bore = 4.030"

Stroke = 3.75"

Rod = 6.0"

RPM = 7500

Peak Piston Speed = 7687.9 FPM (from PipeMax)

Bore area = 12.755573 sq.in. or .08858sq.ft.

Piston CFM = 7687.9FPM x .08858 sq.ft. = 681 CFM

How does PipeMax get a value of 351.7 CFM for Piston CFM Demand at the peak piston speed? How is a flow depression associated with the CFM? The piston speed and resulting CFM displaced should have a fixed CFM w/o any flow test pressure related.

I've been over my math and logic a few times trying to resolve this and it doesn't make sense to me. Are there fudge factors that you (or somebody) have correlated to real engine dyno/head flow data to make the math match a flow bench and dyno?

Thanks,

Rick

Larry Meaux wrote:

[Piston cfm demand] = ([CID] x [RPM] x .0009785) / [number of cylinders]]

that's the same equation i created and gave to Darin Morgan and posted it on SpeedTalk long ago.

it does not take into effects of Rod Ratios

but it comes out very close

about 25+ years ago in researching thru local College Library in books and SAE papers,

and studying Phillip Smith's book, around 105" of water depression created enough energy

to ram mixture at high enough velocity + wave tuning to create 125.8 VE potential

and that anything much over 105" WC depression began to loose out to pumping losses

and i also noticed 105" correlated quite well back to 28" Flowbench Flow CFM in my data

so i set PipeMax 1.0 DOS back then up that 105" WC would equal 126% Ve

but i made 125 VE the standard column in pipemax data

so your 681 CFM is correct

(28/105)^.5 * 681 cfm = 351.7 cfm

to this day, looking thru all the years of my Flowbench data and Dyno data

its still correct to use, the correlation is still excellent over all the engines i've dyno tested

if you input 125.0 % VE in PipeMax the Piston CFM demand column = the Cylinder CFM Column

and that should be around 105" WC which is around 125.7 % to 126 VE % potential

105" WC = around 678.5 fps

678.5 fps / speed of sound = .6078 Mach

.55 times 1116 fps SOS = 613.8 fps

613.8 fps = 85.9682 " WC

85.9682" WC = 121.13 VE % potential

basically , 132.4 % VE should be very close to max possible "trapped VE% " on earth at 14.696 psi atmosphere,

before pumping losses to create that energy overcome gains from increased VE

all your assumptions + maths were correct ! :)