Calculating CFM used from Dyno sheet ?

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DrillDawg
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Re: Calculating CFM used from Dyno sheet ?

Post by DrillDawg » Tue May 15, 2018 7:16 pm

RPM at peak Horsepower....

(2000/64.5)x 224cfm (375cfm converted from 28" to 10") = 6945rpm
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Re: Calculating CFM used from Dyno sheet ?

Post by maxracesoftware » Wed May 16, 2018 5:26 am

Scott , v4.00 Header Specs .... very close correlation to your Dyno Header Specs
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Re: Calculating CFM used from Dyno sheet ?

Post by swampbuggy » Wed May 16, 2018 8:51 am

Scott, on sunday the 13th at 11.15 pm you mentioned many numbers, were you talking about airspeed, i would assume so ?? Mark H.

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Re: Calculating CFM used from Dyno sheet ?

Post by pastry_chef » Wed May 16, 2018 10:00 am

swampbuggy wrote:
Wed May 16, 2018 8:51 am
Scott, on sunday the 13th at 11.15 pm you mentioned many numbers, were you talking about airspeed, i would assume so ?? Mark H.
I think that was me, yes velocity.
Over the years Larry has posted some of the best information relating to that.

Also on *'s site he used to have velocity maps of his heads.

http://x/formulassites.html

From port_measurements.doc

Measurement of Local Velocity with a Pitot Tube
This type of measurement can be done but is fraught with some problems associated with the nature of the Pitot tube itself. The Pitot tube is sensitive to yaw (angle of airstream other than parallel with Pitot tube) relative to the standard direction of the developed flowpath in a port. It is also very difficult to measure the local velocity with a Pitot tube in the vicinity of the "short side radius" (where the flowpath turns from the main stream to the area below the valve). This measurement can be done directly if using a SuperFlow FlowCom with a matching Pitot tube. The indicated local velocity can also be read with a vertical manometer (using the SuperFlow instructions supplied with the Pitot tube).
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Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Wed May 16, 2018 12:14 pm

pastry_chef wrote:
Wed May 16, 2018 10:00 am
swampbuggy wrote:
Wed May 16, 2018 8:51 am
Scott, on sunday the 13th at 11.15 pm you mentioned many numbers, were you talking about airspeed, i would assume so ?? Mark H.
I think that was me, yes velocity.
Over the years Larry has posted some of the best information relating to that.

Also on *'s site he used to have velocity maps of his heads.

http://x/formulassites.html

From port_measurements.doc

Measurement of Local Velocity with a Pitot Tube
This type of measurement can be done but is fraught with some problems associated with the nature of the Pitot tube itself. The Pitot tube is sensitive to yaw (angle of airstream other than parallel with Pitot tube) relative to the standard direction of the developed flowpath in a port. It is also very difficult to measure the local velocity with a Pitot tube in the vicinity of the "short side radius" (where the flowpath turns from the main stream to the area below the valve). This measurement can be done directly if using a SuperFlow FlowCom with a matching Pitot tube. The indicated local velocity can also be read with a vertical manometer (using the SuperFlow instructions supplied with the Pitot tube).
This goes to porting more then the math I am showing here ..

And the post above is very true ... for me with a old bench I have to be very careful that what I put in the port to measure doesn't change the reading I am trying to get some of the new benches are way better at mapping port velocity....

The only post I did in that time was effective port size ..

The idea with this is that ideally a perfect port will move a given amount of air if you take the ideal amount and the measured port flow you can figure the effective area of the port .... the effective area is what the engine will see and use in relation to the steady state math

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Re: Calculating CFM used from Dyno sheet ?

Post by swampbuggy » Wed May 16, 2018 12:56 pm

Okay we're talking about different airspeeds at different intake Port locations so my question is are we saying the air changes speed through each cycle at 300 feet per second

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Re: Calculating CFM used from Dyno sheet ?

Post by justanothermelvin » Wed May 16, 2018 3:25 pm

Why would you dyno your engine without measuring the engine ACFM?

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Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Wed May 16, 2018 4:53 pm

justanothermelvin wrote:
Wed May 16, 2018 3:25 pm
Why would you dyno your engine without measuring the engine ACFM?
Actual cubic feet per minute as measured on a dyno can be used to figure VE and other things but is useless as compared to a flowbench cfm as there is no corresponding pressure drop to equate the cfm number

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Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Wed May 16, 2018 5:07 pm

swampbuggy wrote:
Wed May 16, 2018 12:56 pm
Okay we're talking about different airspeeds at different intake Port locations so my question is are we saying the air changes speed through each cycle at 300 feet per second


OK I think is see what you are asking

First off don't think the the flowbench is ever like a running engine .... you can Jack up the pressure drop or do any sort of odd things but it will never match what happens in a running dynamic engine if you think about it in a running engine the pressure drop across the port is changing all the time as the cylinder fill and rpm change and across the runner too as the pressure in the plenum changes from this or that cylinder drawing on it and that does not even get into the reversion pulse that the ex system adds in or the reflective wave from the intake closing ..... inside a running intake system is a mess that would take a whole lot of computer and math to really model ..... what you are doing on the flow bench in simple terms is making it easier to move air and you a using a device to measure the amount in a controlled fashion to get a repeatable number

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Re: Calculating CFM used from Dyno sheet ?

Post by digger » Wed May 16, 2018 5:22 pm

Scotthatch wrote:
Wed May 16, 2018 4:53 pm
justanothermelvin wrote:
Wed May 16, 2018 3:25 pm
Why would you dyno your engine without measuring the engine ACFM?
Actual cubic feet per minute as measured on a dyno can be used to figure VE and other things but is useless as compared to a flowbench cfm as there is no corresponding pressure drop to equate the cfm number
Why do you care what the headflows once you've tested the engine? You've already got the final answer

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Re: Calculating CFM used from Dyno sheet ?

Post by swampbuggy » Wed May 16, 2018 6:36 pm

Scott what I was talking about on AirSpeed change I was talking about in a running engine nothing to do with the flow bench in a running engine at a steady seven to nine thousand RPMs is the airspeed changing in the port or is it constant and whatever that speed is would not the minimum cross-sectional area dictate how fast it is :?:

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Re: Calculating CFM used from Dyno sheet ?

Post by Stan Weiss » Wed May 16, 2018 6:59 pm

swampbuggy wrote:
Wed May 16, 2018 6:36 pm
Scott what I was talking about on AirSpeed change I was talking about in a running engine nothing to do with the flow bench in a running engine at a steady seven to nine thousand RPMs is the airspeed changing in the port or is it constant and whatever that speed is would not the minimum cross-sectional area dictate how fast it is :?:
Air speed in the intake port of a running is never a constant. At some point while the intake valve is closed the air speed will be Zero.

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Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Wed May 16, 2018 7:52 pm


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Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Wed May 16, 2018 8:06 pm

digger wrote:
Wed May 16, 2018 5:22 pm
Scotthatch wrote:
Wed May 16, 2018 4:53 pm
justanothermelvin wrote:
Wed May 16, 2018 3:25 pm
Why would you dyno your engine without measuring the engine ACFM?
Actual cubic feet per minute as measured on a dyno can be used to figure VE and other things but is useless as compared to a flowbench cfm as there is no corresponding pressure drop to equate the cfm number
Why do you care what the headflows once you've tested the engine? You've already got the final answer


The final answer as in HP ?
Knowing what an engine should put out in HP as compared to what it does put out can be lots of help I have seen engines that should be 700 plus HP that only made 600 and change because they got the intake wrong ... being able to know that this head with that camshaft should be x HP made fast work deciding it needed a different intake .... such things are why I test them now if I can

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Re: Calculating CFM used from Dyno sheet ?

Post by GARY C » Wed May 16, 2018 8:16 pm

Scotthatch wrote:
Wed May 16, 2018 7:52 pm
https://youtu.be/7Iq1B-2paCs
Here is a video inside a working combustion chamber, if you slow the video down just before the 20 second mark you get a pretty good visual of air speed and fuel automation through out lift, quite a bit happens at TDC with the piston sitting still.
https://www.youtube.com/watch?v=UvmBLqjaZxY
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THE ABOVE POST IN NO WAY REFLECTS THE VIEWS OF SPEED TALK OR IT'S MEMBER AND SHOULD BE VIEWED AS ENTERTAINMENT ONLY...Thanks, The Management!

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