Calculating CFM used from Dyno sheet ?

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Re: Calculating CFM used from Dyno sheet ?

Post by swampbuggy » Sun May 13, 2018 1:58 pm

Thanks Larry for your generous input, Mark H. Oh BTW i am going to get the Dart 9 degree heads for my engine build :D Mark

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Re: Calculating CFM used from Dyno sheet ?

Post by andyf » Sun May 13, 2018 2:07 pm

Warp Speed wrote:
Sun May 13, 2018 11:44 am
It's all about CSA, and the area vs time of the available (read affordable! Lol) valve events for a particular engine size and rpm......?
These valve events, and the achievable area, can change, and often dictate needed CSA adjustments from optimum, for a given architecture, and the resultant velocity achieved........?
IMO
Or something like that?!? Lol :?
I used to think CSA was super important but then I started to test high port heads vs. low port heads and found out that the engine cares more about the shape of the intake runner than the CSA or volume. The high port heads work a lot better even if the CSA is the same or smaller. Kasse did an interesting article a while back where he dyno tested a wedge head vs. a Hemi head. Both heads had similar flow bench numbers and CSA but the Hemi head made a ton more power.

Another example is the NHRA super stock guys who are limited to port volume but not shape. There is only so much they can do so what they tend to do is move the port up and make it straight. The CSA and volume stay the same as stock but the shape is a lot different and they make a ton more power.

I don't think any of the existing formulas capture this. Most of the formulas use CSA to determine velocity and torque peak and such but they miss the bigger picture. There would need to be some sort of effective shape formula. Total intake flow might capture some of the shape issue but I don't think it catches all of the issues.
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Re: Calculating CFM used from Dyno sheet ?

Post by GARY C » Sun May 13, 2018 2:26 pm

andyf wrote:
Sun May 13, 2018 2:07 pm
Warp Speed wrote:
Sun May 13, 2018 11:44 am
It's all about CSA, and the area vs time of the available (read affordable! Lol) valve events for a particular engine size and rpm......?
These valve events, and the achievable area, can change, and often dictate needed CSA adjustments from optimum, for a given architecture, and the resultant velocity achieved........?
IMO
Or something like that?!? Lol :?
I used to think CSA was super important but then I started to test high port heads vs. low port heads and found out that the engine cares more about the shape of the intake runner than the CSA or volume. The high port heads work a lot better even if the CSA is the same or smaller. Kasse did an interesting article a while back where he dyno tested a wedge head vs. a Hemi head. Both heads had similar flow bench numbers and CSA but the Hemi head made a ton more power.

Another example is the NHRA super stock guys who are limited to port volume but not shape. There is only so much they can do so what they tend to do is move the port up and make it straight. The CSA and volume stay the same as stock but the shape is a lot different and they make a ton more power.

I don't think any of the existing formulas capture this. Most of the formulas use CSA to determine velocity and torque peak and such but they miss the bigger picture. There would need to be some sort of effective shape formula. Total intake flow might capture some of the shape issue but I don't think it catches all of the issues.
Don't those guys focus heavily on valve seats and top cuts as well?
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Re: Calculating CFM used from Dyno sheet ?

Post by groberts101 » Sun May 13, 2018 3:27 pm

andyf wrote:
Sun May 13, 2018 2:07 pm
I used to think CSA was super important but then I started to test high port heads vs. low port heads and found out that the engine cares more about the shape of the intake runner than the CSA or volume. The high port heads work a lot better even if the CSA is the same or smaller. Kasse did an interesting article a while back where he dyno tested a wedge head vs. a Hemi head. Both heads had similar flow bench numbers and CSA but the Hemi head made a ton more power.

Another example is the NHRA super stock guys who are limited to port volume but not shape. There is only so much they can do so what they tend to do is move the port up and make it straight. The CSA and volume stay the same as stock but the shape is a lot different and they make a ton more power.

I don't think any of the existing formulas capture this. Most of the formulas use CSA to determine velocity and torque peak and such but they miss the bigger picture. There would need to be some sort of effective shape formula. Total intake flow might capture some of the shape issue but I don't think it catches all of the issues.
Excellent points. =D>

Randy and warps posts too. A better taller angled port with higher c/d can tolerate higher fps and still choke or stall less than a much larger port will.

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Re: Calculating CFM used from Dyno sheet ?

Post by hoffman900 » Sun May 13, 2018 5:17 pm

I think Warp meant shape as well, but shape is a result of the CSA or volume for any given port length. ;)
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Re: Calculating CFM used from Dyno sheet ?

Post by pastry_chef » Sun May 13, 2018 5:32 pm

Mike R

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Re: Calculating CFM used from Dyno sheet ?

Post by digger » Sun May 13, 2018 6:03 pm

The right csa with good velocity profile and it flows what it flows

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Re: Calculating CFM used from Dyno sheet ?

Post by Warp Speed » Sun May 13, 2018 7:40 pm

plovett wrote:
Sun May 13, 2018 1:19 pm
Well you can certainly have two heads with the same CSA and very different flow characteristics. So saying CSA and time is IT doesn't make sense either.

JMO.

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That is why I stated "for a given architecture"! :wink:

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Re: Calculating CFM used from Dyno sheet ?

Post by Warp Speed » Sun May 13, 2018 9:22 pm

andyf wrote:
Sun May 13, 2018 2:07 pm
Warp Speed wrote:
Sun May 13, 2018 11:44 am
It's all about CSA, and the area vs time of the available (read affordable! Lol) valve events for a particular engine size and rpm......?
These valve events, and the achievable area, can change, and often dictate needed CSA adjustments from optimum, for a given architecture, and the resultant velocity achieved........?
IMO
Or something like that?!? Lol :?
I used to think CSA was super important but then I started to test high port heads vs. low port heads and found out that the engine cares more about the shape of the intake runner than the CSA or volume. The high port heads work a lot better even if the CSA is the same or smaller. Kasse did an interesting article a while back where he dyno tested a wedge head vs. a Hemi head. Both heads had similar flow bench numbers and CSA but the Hemi head made a ton more power.

Another example is the NHRA super stock guys who are limited to port volume but not shape. There is only so much they can do so what they tend to do is move the port up and make it straight. The CSA and volume stay the same as stock but the shape is a lot different and they make a ton more power.

I don't think any of the existing formulas capture this. Most of the formulas use CSA to determine velocity and torque peak and such but they miss the bigger picture. There would need to be some sort of effective shape formula. Total intake flow might capture some of the shape issue but I don't think it catches all of the issues.
Agreed, of course the shape is super important, but the CSA still needs to be correct, be it for a good design, or a bad one, to get the most from a given layout/architecture right?

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Re: Calculating CFM used from Dyno sheet ?

Post by swampbuggy » Sun May 13, 2018 9:58 pm

Warpspeed---The CSA and or the MCSA (Minimum Cross Sectional Area) have to be designed or sized to get the air SPEED about where it is needed to be when the engine is at full song, is this correct ? Mark H. :-k

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Re: Calculating CFM used from Dyno sheet ?

Post by pastry_chef » Sun May 13, 2018 11:15 pm

swampbuggy wrote:
Sun May 13, 2018 9:58 pm
The CSA and or the MCSA (Minimum Cross Sectional Area) have to be designed or sized to get the air SPEED about where it is needed to be when the engine is at full song, is this correct ? Mark H. :-k
Getting there, I'd say velocity profile through the entire port shape. Not a number, many local numbers.
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Re: Calculating CFM used from Dyno sheet ?

Post by Stan Weiss » Mon May 14, 2018 12:15 am

maxracesoftware wrote:
Sun May 13, 2018 1:38 pm
swampbuggy wrote:
Fri May 11, 2018 11:54 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. :-k
"this formula= H.P. divided by 8 divided by .26 gives you the CFM used + or - 10 CFM."
.... this formula "as is" does sometimes get you in the "BallPark",
but also this basic Formula calculates , or can be rearranged to calculate something entirely different , but directly related ,
you would take that new value , and plug it into another equation and get something really great or fantastic :shock:

likewise .... sort of hidden in plain site on most Dyno Sheets posted on various Forums
is exactly related to your Thread Title "Calculating CFM used from Dyno sheet ?" or "Calculating Cyl Head CFM used from Dyno sheet ?"
... what i mean by "hidden in plain site" , is i've never seen anyone mention or talk about it since the Internet began .

"HP was 856 @ 6900 the TQ. was 693 @ 5900"
6900 RPM - 5900 = 1000 RPM spread between Peak HP RPM point and Peak TQ RPM

i used 12:1 CR input + i just quickly inputed various values into PipeMax v3.98... so only you know how close they are ?
i calculate it was possible to make 713.3 Peak TQ ( if you had 12:1 CR ? )
713.3 - 693.0 = 20.3 Lbs Torque missing ... and this shows up as partial reason or result from only 1000 RPM spread

one indicator for Peak TQ not quite matching up to Peak HP output ==>> is usually not enough RPM spread
i'm probably a little wrong on the exact amounts of that 20.3 TQ missing , but the missing TQ effect is the same for such a spread .

for your 375 CFM at 0.800 Valve Lift ,
you can look at PipeMax's 380.2 CFM as what the bare Cyl Head with a Radius Entry would need to Flowtest at 0.835 Valve Lift
and also look at it this way=> that the 359.0 CFM would be what you like the entire Induction Path to Flowtest at 0.835 Valve Lift
Required Intake Flow CFM @28 in. = 359.0 to 380.2 at .835 inch Valve Lift
Required Exhaust Flow CFM @28 in. = 271.6 to 294.3 at .749 inch Valve Lift
...same way with Exhaust ... 294.3 CFM might be what you see with a Flow Pipe
and 271.6 CFM would be bare exhaust Port CFM on a Flowtest

380.2 max CFM at 0.835 inch Valve Lift predicted is pretty close to your 375.0 CFM @ 0.800" Lift

you can also calculate CFM from Air/Fuel Ratio and Lbs/Fuel/Hour consumed, along with Weather data
looking at a Dyno Sheet if that info is there or available
Larry,
This is an example of both SCFM and VE being calculated.

Code: Select all

======= INPUT
;                     RPM  Torque Fuel  BSFC  A/F
;                                        lb/hr         Ratio
Acceleration = 3700 425.9 142.2 0.514 11.61
Acceleration = 3800 423.0 141.7 0.503 11.68
Acceleration = 3900 422.4 142.0 0.492 11.69
Acceleration = 4000 421.1 141.6 0.480 11.84
Acceleration = 4100 421.2 142.0 0.469 12.04
Acceleration = 4200 420.6 142.6 0.461 12.15
Acceleration = 4300 420.7 144.8 0.457 12.23
Acceleration = 4400 423.1 146.3 0.450 12.41

========= OUTPUT / Calculated

Dyno Barometric Pressure = 29.92 - Dyno Vapor Pressure = 0.45 - Dyno Air Temperature = 95.5

                                 Fuel           UnCorr  UnCorr UnCorr Correct   A/F
  RPM   Horse  Torque   BMEP    lb/hr     BSFC      HP  Torque   BMEP  Factor  Ratio    SCFM    VE%
 3700   300.0   425.9  167.8   142.20    .5140   276.7   392.7  154.8  1.0845  11.61   360.5   95.4
 3800   306.1   423.0  166.7   141.70    .5030   281.7   389.4  153.4  1.0864  11.68   361.4   93.1
 3900   313.7   422.4  166.5   142.00    .4920   288.6   388.7  153.2  1.0868  11.69   362.4   91.0
 4000   320.7   421.1  165.9   141.60    .4800   295.0   387.3  152.6  1.0872  11.84   366.1   89.6
 4100   328.8   421.2  166.0   142.00    .4690   302.8   387.8  152.8  1.0860  12.04   373.3   89.2
 4200   336.4   420.6  165.7   142.60    .4610   309.3   386.8  152.4  1.0874  12.15   378.3   88.2
 4300   344.4   420.7  165.8   144.80    .4570   316.8   387.0  152.5  1.0871  12.23   386.7   88.1
 4400   354.5   423.1  166.7   146.30    .4500   325.1   388.1  152.9  1.0903  12.41   396.4   88.2
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Re: Calculating CFM used from Dyno sheet ?

Post by DrillDawg » Mon May 14, 2018 12:36 am

Found this formula while digging through an old note book from 40 years ago.

Peak cfm demand cid x rpm x .0009785/no. of cyl.


500 x 11000 x .0009785 / 8 = 672cfm, close to the other formula,


Will anyone in the know comment on how close this cfm is to real life results?
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Re: Calculating CFM used from Dyno sheet ?

Post by John Wallace » Mon May 14, 2018 8:19 am

Try this thread:

CFM link

:)
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Re: Calculating CFM used from Dyno sheet ?

Post by maxracesoftware » Mon May 14, 2018 2:01 pm

John Wallace wrote:
Mon May 14, 2018 8:19 am
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 :lol:

... 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 !
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