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 Post subject: Re: cfm demand formula
 Post Posted: Sat Sep 29, 2012 9:51 am 

Joined: Sat Feb 24, 2007 6:55 pm
Posts: 847
Location: NW MO.
Rick, I've seen that formula from several sorces and yes it clearly leaves out rod ratio, but rod ratio isn't a huge factor to peak cfm demand, atleast within a given engine block configuration.

The formula is to give a general cfm target at 28". Most of us don't have a way to test at 60"+.

I think it is what the engine is asking for converted back to 28". On engine pro it gives cfm demand and the depression at different crank angles, and when I've converted what ep says back to 28" it matches pipemax.

In other words it assumes the port follows the increase in cfm vs depression increase formula.

Not sure what formula your using but it shows a very high depression.

((105.5 / 28 ) ^ .5 ) x 351 = 681.3 Did I do the math right?

I don't remember ep showing that high of depression, but I haven't looked at that combo.

Off to work on a roof now so I'll run ep later.

Slow racing is better than no racing!


 Post subject: Re: cfm demand formula
 Post Posted: Sun Sep 30, 2012 9:17 pm 
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Joined: Tue Apr 18, 2006 3:20 pm
Posts: 1457
Location: Missouri
The formula I was using is taking the max piston velocity in fpm at the chosen RPM and multiply by the piston area in sq. ft. and that will give a volume in CFM that the piston is actually displacing. It has nothing to do with any depression and I am curious how/where they (whoever uses that formula) arrived at that number for 28".

For a 383ci engine with 6.0" rod at 7500 rpm in Engine Pro I get a max piston speed of 7717. (note that PipeMax shows 7688fpm for the same combo)

Bore Diameter=4.030"/12=.33583333ft
Bore Area = 3.1416 x ((.33583333/2)^2) = .088580368 sq.ft.
7717fpm x .088580368 sq.ft. = 683.6CFM peak piston cfm


 Post subject: Re: cfm demand formula
 Post Posted: Thu Oct 04, 2012 11:15 pm 
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Joined: Tue Apr 18, 2006 3:20 pm
Posts: 1457
Location: Missouri
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.


Rick360 wrote:
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.

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?


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 ! :)

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