Calculating CFM used from Dyno sheet ?

[Scotthatch]

Try this thread:

CFM link

Hi John ... i thought the same thing when i read DrillDawg 's Post
... that was from an old Post of mine .

Calculating CFM used from Dyno sheet ?

Post by swampbuggy » Fri May 11, 2018 10:54 pm
A formula was posted in the Debunker thread that stated you could calculate the CFM the engine used with this formula= H.P. divided by 8 divided by .26 gives you the CFM used + or - 10 CFM.
For torque peak CFM used----use this formula=peak tq. RPM x peak tq. divided by 9000 gives you CFM used at peak tq.
My question is directed at the following. The last engine i had was ma 516" BBC, the HP was 856 @ 6900 the TQ. was 693 @ 5900.
When i run the formulas as shown above i get the following.
CFM at peak HP should be 411
CFM at peak TQ. should be 454
The fact is the intake port flow signed off at 375 CFM at .800" lift.
So is this formula not always accurate or ???? Mark H

in PipeMax , i'm Looping thousands of times thru many different equations .. to calculate CFM at each crankshaft degree
to arrive at Engine or Cylinder CFM Demand ... and this 1 line simple equation just about comes close to the same answer or result

... anyone could further fine-tune either Constants = 0.001030633 or 130 to suit their Data

CID = Cubic Inch displacement
Cylinders = the Number of Cylinders
VE% = the Volumetric Efficiency PerCent in a whole number , like 107.0 % VE
* = multiplication or like a similar sign x

an updated super simple empirical equation version is :
Intake Port CFM @28" = (CID * Peak HP RPM * 0.001030633 * VE) / (Cylinders * 130)

worked example :
Intake Port CFM @28" = (CID * Peak HP RPM * 0.001030633 * VE) / (Cylinders * 130)
Intake Port CFM @28" = (516 * 6900 * 0.001030633 *107) / (8* 130)
Intake Port CFM @28" = 377.53 CFM ..... Mark H 's CFM = 375 CFM @ 0.800" Valve Lift

107.0 VE input in PipeMax v3.98 makes 855.6 Peak HP ... Mark's Peak HP = 856.0 @ 6900
i could have nit-picked it closer to get exactly = 856.0 HP

more worked examples :
Briggs 6 HP Raptor 1 Cylinder
Intake Port CFM @28" = (12.568 * 4500* 0.001030633 *77) / (1* 130)
Intake Port CFM @28" = 34.52 CFM @ 28"

ProStock 500cid at 9800 RPM
Intake Port CFM @28" = (500 * 9800 * 0.001030633 * 125) / (8 * 130)
Intake Port CFM @28" = 606.98 CFM

Sonny's 1005.84 cid at 8000 RPM
Intake Port CFM @28" = (1005.84 * 8000 * 0.001030633 * 95) / (8 * 130)
Intake Port CFM @28" = 757.55 CFM

a 442cid SBC with Dart Pro 1's Ported to 245-246CC's 317 to 323 CFM making 816 HP at 8200 RPM
in another Thread on SpeedTalk :
Intake Port CFM @28" = (442 * 8200 * 0.001030633 * 90) / (8 * 130)
Intake Port CFM @28" = 323.26 CFM

Chris_Uratchko_466cid__CU_Marcella_SB_Chevy_ROX_1244_HP ( YellowBullet )
500 CFM Cyl Head Flow 9400 Peak HP RPM
Intake Port CFM @28" = (466.9 * 9400 * 0.001030633 *115) / (8 * 130)
Intake Port CFM @28" = 500.17 CFM

GreenLight's NHRA record setting Honda S2000 engine 7.96 ET in F/Dragster
360 CFM at 0.600 Lift on my Bench
Intake Port CFM @28" = (127.6 * 11000 * 0.001030633 * 128) / (4 * 130)
Intake Port CFM @28" = 356.09 CFM
128 VE supposedly on EnDyn's SF901 Dyno

Hoffman_32.6cid_7000RPM
32.60366191 Cubic Inches
Bore: 3.543" (90mm)
Stroke: 3.307" (84mm)
Rod Length: 5.709" (145mm)
Cylinders: 1
Compression: 10:1
Intake: 1.900" valve
Flow numbers at 28" with the entire intake tract bolted on (spigot,carburetor, velocity stack)
.100 61.6
.200 113.8
.300 172.5
.400 200
.500 208.3 <<----------
.600 216.3

Intake Port CFM @28" = (32.6* 7000 * 0.001030633 * 115) / (1 * 130)
Intake Port CFM @28" = 208.05 CFM

reduced ... even simpler easier equation version is :
Intake Port CFM @28" = (CID * Peak HP RPM * 0.000007928 * VE) / Cylinders

my SF-600 Bench is on the "conservative side" ,, around -15 CFM less ???
so someone applying my equation could adjust the CFM results to align with their Flowbench !

And how does any of that get you to camshaft size ?

[Scotthatch]

Guy's-----I love the discussion my thread has generated ! The main purpose for this thread (topic) was to try to get answers as to why the Engine produced the power it did with the head flow that it had 375 CFM. When everybody was saying that i would need 400 or more, i was excited when i returned home with that BBC in the back of my P.U. truck after the dyno session !!! Mark H.

Because the flow bench is NOT the do all, be all, end all for determining power. Many times I've lost flow on the bench and made more power and I'm not the only one saying that.

You need to learn to use the tool as a tool and not the end answer in a number.

First off as I have said before flow quality is very important ... I have had my bench since the late 1980's and have always been willing to take time and flow other people's work and flow quality is a hard thing to describe on a written forum .... and I have seen heads done by a master of quality Jimmy at PME and no matter how good I get it shows me how much I still have to go ..... and as was stated what did the intake flow? Did it need to go down to match better?

With that said anyone that looks a flow numbers has seen the formula..... cfm @28 x .257 x number of cylinders equals HP ... it was in the superflow manual when I got the bench .... and anyone that has used it knows it doesn't match what we see on the dyno ..... so the question is why? The cfm posted at the beginning of the thread is a off shoot of" why" I started by turning the formula around and seeing what cfm I should have seen on the flowbench to make that HP..... I then crunched numbers for years looking at why it did not match and I tried all of the ways you guys listed above and the bottom line is the pressure drop on the bench is not the same as the ever-changing pressure drop in a running engine .... but at some point that formula must have come from good data so I started working to find a steady state math model that would get you from what you see on a flowbench to what number you get on the dyno .... I never said that was VE .... I never said that was the real cfm that the engine used on the dyno running... it only represents the cfm used as compared to the flowbench at 28 inch drop

[maxracesoftware]

by Scotthatch » Mon May 14, 2018 2:03 pm
And how does any of that get you to camshaft size ?

same as asking ... How does this next SF equation get you to camshaft size ???
"the formula..... cfm @28 x .257 x number of cylinders equals HP"

use PipeMax v4.00 to directly calculate Cam Specs
or cycle a few Inputs in v3.98 to arrive at Cam Specs

that super easy simple empirical equation correlates Cyl Head Intake Port CFM @28" back to required CFM
needed to matchup to an Engine Dyno @ 600 RPM/SEC Test Rate
Note: some situations require you to use Flow CFM with complete Induction System Flowtested , not just bare Head test CFM
to verify correlation to this equation .

Scott ... just go back and take a 2nd look at the "worked examples"
they correlate pretty close to Flowbench CFM you would see at the your Cam's Peak Intake Lift
so you open the Intake Valve on your Flowbench like your Cam Card

you Dyno test your Engine on an Engine Dyno for Flywheel HP at 600 RPM/SEC
you then adjust the VE% input in PipeMax until PipeMax and your Engine Dyno's Peak HP and Peak TQ matchup
as close as possible...then you see what PipeMax calculates you are really accessing CFM out of your Flowbench CFM

like the 1 Cylinder Bob Hoffman example
Bob from my memory has or is using 0.500+ Valve Lift
and has Flowbench Data = 208 CFM @ 0.500 Lift
the simple empirical equation calculates it possibly requires 115.0 VE to get 208 CFM at 0.500 Lift matchup
.. once Bob Dyno tests that engine , if you matchup LbsFuel/Hour consumed + AFRatio + Weather Corr
you reverse-engineer back into VE% it must really be trapping or CFM its using
and see how close empirical equation really is

then repeat that with a few hundred various engine combinations/sizes-RPM Ranges
to get a good idea if that equation is either useful or worthless to real-world results ?

With that said anyone that looks a flow numbers has seen the formula..... cfm @28 x .257 x number of cylinders equals HP

0.257 = is just "BallPark" at best !

729.6 CFM = 1500 HP on a ProStocker ... pretty far from reality

Equation :
Intake Port CFM @28" = (CID * Peak HP RPM * 0.000007928 * VE) / Cylinders

way more real-world accuracy over a much broader range of Engine applications

[Scotthatch]

by Scotthatch » Mon May 14, 2018 2:03 pm
And how does any of that get you to camshaft size ?

use PipeMax v4.00 to directly calculate Cam Specs


you Dyno test your Engine on an Engine Dyno for Flywheel HP at 600 RPM/SEC
you then adjust the VE% input in PipeMax until PipeMax and your Engine Dyno's Peak HP and Peak TQ matchup
as close as possible...then you see what PipeMax calculates you are really accessing CFM out of your Flowbench CFM

What if you have never run the engine? How do you know VE? It looks like you can get there after the fact but when you have a short block and a set of heads and intake that you just flowed and you know what you need for the car how do you decide what camshaft to get?

[Scotthatch]

729.6 x .257 x 8 = 1500

So I think you meant it will never flow 729.6 yet it makes that power ...

So we agree it doesn't work straight across ... why?

[maxracesoftware]

by Scotthatch » Mon May 14, 2018 2:03 pm
And how does any of that get you to camshaft size ?

use PipeMax v4.00 to directly calculate Cam Specs


you Dyno test your Engine on an Engine Dyno for Flywheel HP at 600 RPM/SEC
you then adjust the VE% input in PipeMax until PipeMax and your Engine Dyno's Peak HP and Peak TQ matchup
as close as possible...then you see what PipeMax calculates you are really accessing CFM out of your Flowbench CFM

What if you have never run the engine? How do you know VE? It looks like you can get there after the fact but when you have a short block and a set of heads and intake that you just flowed and you know what you need for the car how do you decide what camshaft to get?

a Quote from one of my most favorite Movies : "My Cousin Vinny"
" you like to re-negotiate as you go along" -- Pool Hall scene

in PipeMax , you input your desired RPM you want Peak HP to occur at
then use 100.0 VE% input as 1st test
then see if that correlates to your Flowbench CFM at your Cam Card's max Intake Valve Lift
and to get an idea of the HP it would take to run your desired ET and MPH down the Dragstrip
you would run the ET HP Calculator in PipeMax
then adjust VE% input until they matchup

obviously if you use a stock hydraulic OEM Cam like 0.390/.0410 Lift in a Prostock 499cid at 9800 RPM
with a 630 CFM Head ... its never going to fully access 630 CFM .

Likewise, if you have a 1.250+ Lift Cam + a 630 CFM Head on a 499cid engine
and turn it 5000 RPM .. its never going to fully access or Trap 630 CFM

GIGO .. rules !

PipeMax will tell you how much CFM you are accessing from your Flowbench CFM

[Scotthatch]

So with yours numbers above ... just to have numbers

If the port is 630 cfm and you need 1500 HP or 115.8 % of the head flow you just input what you have and keep adjusting the VE till you hit 630 cfm? And then read what cam specs it takes .... do you just guess at cam specs to start with ? Or does it do it from scratch ? (No value)

[maxracesoftware]

So with yours numbers above ... just to have numbers

If the port is 630 cfm and you need 1500 HP or 115.8 % of the head flow you just input what you have and keep adjusting the VE till you hit 630 cfm? And then read what cam specs it takes .... do you just guess at cam specs to start with ? Or does it do it from scratch ? (No value)

i've greatly improved PipeMax v4.00 over all other previous versions

v4.00 will calculate all the Cam Specs for you
but the highest simulation accuracy or correlation , will be to use v4.00 along side an Engine Dyno

its a Catch-22 situation most times ... you will not realize the accuracy/correlation until you actually Dyno test your Engine
on a properly calibrated Engine Dyno at 600 RPM/SEC test rate .... then you compate v4.00 -vs- your Dyno test results

in v4.00 , you can override and input actual BSFC + Weather Dyno Correction Factor
and Fuel Consumed readings will perfectly matchup to your Dyno
then you can verify the Trapped VE% and CFM your Cylinder Head is really accessing

all the better Record setting Engine combinations end up accessing most all their Flowbench CFM at their Cam's Card's max Lifts

also in v4.00 , you can adjust for Valvetrain deflection or Loft effects
which will change all Cam Specs predicted

PipeMax v4.00 is a "partial" or half a real Engine simulation program .. it can run in 2 Modes ..but i still consider a "Predictor"
PipeMax v4.00 is a "Predictor style" simulation Program ... and is not a real "Engine Simulation"
v4.00 is sort of reverse of an Engine Simulation program ... v4.00 tells you what Specs you need to achieve your desired HP + RPM Range

Whats taking me so long to release v4.00, is i'm also at same time programming a real Engine Simulation version
in a new Programming Language .... so the "Predictor style" and the "Engine Simulation" styles can access same Saved Data files
and be somewhat backwards compatible between older versions

[groberts101]

lol.. nothing like tossing a little chum into shark infested waters. Now I REALLY WANT pipemax 4.0!

[Scotthatch]

Sounds like you are getting it easier to use and yet that you kinda need to know how things should be to make good use of it and it really excels at refining a cam to better values ....

It also sounds to me that we agree that to make a given HP it is a combination of cylinder head/ intake flow and camshaft

So to put it simply flowbench cfm x camshaft = HP and in your world you use pipemax program to correlate how much camshaft is needed to make the required HP

[groberts101]

Sounds like you are getting it easier to use and yet that you kinda need to know how things should be to make good use of it and it really excels at refining a cam to better values ....

It also sounds to me that we agree that to make a given HP it is a combination of cylinder head/ intake flow and camshaft

So to put it simply flowbench cfm x camshaft = HP and in your world you use pipemax program to correlate how much camshaft is needed to make the required HP

I think it goes far beyond that.. but yes.. maybe in the condensed generalized version. As others have already said many times.. flowbenches only get you so far with their raw data outputs. From there we get into pressure recovery and inertial and scavenge tuning characteristics which can easily make or break a specific number that the flowbench has generated.

As you well know.. flowbenches are static.. and running engines are considerably more dynamic which is what gives us those overachievers based on a flowbench saying they should have fallen short of what they actually produce when everything falls together just right.

[Scotthatch]

That was my post .....

First off the cfm the engine sees to make power is the flow you got on the bench and the camshaft you are running .... the cam can change the total flow .... with a small cam like say 210@ .050 you will only use like 83% of the flow you saw on a bench with say a 290 @ .050 you will use 120% of the flow from the bench...

Not disagreeing ..... I said It in my first post in this thread

[Scotthatch]

Sounds like you are getting it easier to use and yet that you kinda need to know how things should be to make good use of it and it really excels at refining a cam to better values ....

It also sounds to me that we agree that to make a given HP it is a combination of cylinder head/ intake flow and camshaft

So to put it simply flowbench cfm x camshaft = HP and in your world you use pipemax program to correlate how much camshaft is needed to make the required HP

I think it goes far beyond that.. but yes.. maybe in the condensed generalized version. As others have already said many times.. flowbenches only get you so far with their raw data outputs. From there we get into pressure recovery and inertial and scavenge tuning characteristics which can easily make or break a specific number that the flowbench has generated.

As you well know.. flowbenches are static.. and running engines are considerably more dynamic which is what gives us those overachievers based on a flowbench saying they should have fallen short of what they actually produce when everything falls together just right.

In thinking about what you said I am sure it goes beyond the flowbench and I am sure every engine I have ever built could use some refinements but as I have said before I am a 90% guy (90% of best ever will normally get the job done) ..... engines I have built are in cars unchanged for decades if it needs more normally it's not just a few HP or even 20 HP the next step up is going to need 100 or 150 more HP and that's a whole new build ..

You also have to realize that years ago we did not know all that we do today and almost no car guys used computers .... when desk top dyno came out I was still crunching numbers trying to make sense of what data I did have ..... I smashed one calculator with a hammer because it was giving me nothing that worked I felt like that old saying

15263292912610.jpg

[swampbuggy]

THANKS-THANKS-THANKS Larry M. You are the very best at writing information that (I) for 1 and probably (NOT) the only one can REALLY grasp . If my old sweetheart was @ 107 % V.E. that tickles the S*** out of me. That motor was taken out of my 72 Chevy shortbed P.U. and went straight into a TOP quality Horton rail type dragster and went 8.20-8.30's out of the gate. Oh BTW the P.U. was a street toy, and no it was not tubbed, it was a BLAST with a Richmond 5-Speed and 3.73 posi.

[Scotthatch]

But I did eventually find something that did work and the funny thing is its simple and it has been in the superflow manual for over 30 years

15261392563050.jpg

15261392733781.jpg

It's these two pages .... you can ignore the formula for the moment it the graft that makes it simple .... if you find the l/d=.1 on the camshaft and cross it on the chart it will tell you how much of the port flow you will use ...

For a 2.02 valve that would be the degree that you are at when valve lift is .202 off the seat on intake closing...

If you think about it a lot of things will effect that point but if you are just looking for an idea the down and dirty is to take the closing point of the intake at .050 and for flat tappet cam subtract 25 degrees for roller subtract 20 degrees