Re: Calculating CFM used from Dyno sheet ?
Posted: 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 Mark
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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.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
Don't those guys focus heavily on valve seats and top cuts as well?andyf wrote: ↑Sun May 13, 2018 2:07 pmI 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.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
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.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.
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?andyf wrote: ↑Sun May 13, 2018 2:07 pmI 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.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
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.
Getting there, I'd say velocity profile through the entire port shape. Not a number, many local numbers.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.
Larry,maxracesoftware wrote: ↑Sun May 13, 2018 1:38 pm"this formula= H.P. divided by 8 divided by .26 gives you the CFM used + or - 10 CFM."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.
.... 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
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
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
Hi John ... i thought the same thing when i read DrillDawg 's Post
in PipeMax , i'm Looping thousands of times thru many different equations .. to calculate CFM at each crankshaft degreeCalculating 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