Calculating CFM used from Dyno sheet ?

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swampbuggy
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Calculating CFM used from Dyno sheet ?

Post by swampbuggy » 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

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Re: Calculating CFM used from Dyno sheet ?

Post by Stan Weiss » Sat May 12, 2018 12:12 am

It is like a broken clock. :lol:

Also ► 693 / 8 / .26 = 333.17

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Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Sat May 12, 2018 12:55 am

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...

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Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Sat May 12, 2018 12:59 am

With torque cfm number bigger then the HP cfm the question is how big is the carb?

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Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Sat May 12, 2018 1:01 am

Stan Weiss wrote:
Sat May 12, 2018 12:12 am
It is like a broken clock. :lol:

Also ► 693 / 8 / .26 = 333.17

Stan
693 is the torque you used HP calculation....

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Re: Calculating CFM used from Dyno sheet ?

Post by swampbuggy » Sat May 12, 2018 7:06 am

Scotthatch the carb was a Barry Grant 8896, Dart single plane w/some work, older Dart 320's reworked, cam was .750 net lift 282/288 @.050 110 LSA Installed on 107 deg. I.C.L. :) Mark H. BTW 8896 was a 1050 flowing ??? I do remember with intake and carb on cyl. head the flow was not down very much, i remember appx. 20 CFM, not as much as we expected anyway.

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Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Sat May 12, 2018 10:14 am

Expected port efficiency based on the cam is 112%

Since the intake flowed 375 cfm you would expect a used flow of like 420 cfm and a peak power of 873 HP

Rough estimate on carb size needed is 1290 cfm dry flow at 20.4 inch drop

Since you are using 110% flow on the dyno numbers you are not far off expected

Being that it's a BBC I am going to guess that there is some port to port cfm variations is the 375 cfm number a average?

The motor is basically right at expected numbers was there something you felt was not up to par ? Or just checking a know valve to see how the numbers work?

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Re: Calculating CFM used from Dyno sheet ?

Post by fastblackracing » Sat May 12, 2018 11:00 am

Scotthatch wrote:
Sat May 12, 2018 10:14 am
Expected port efficiency based on the cam is 112%
Scott how are coming up with the camshaft % multiplier to hit the peak cfm with?
Thanks.

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Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Sat May 12, 2018 11:29 am

I use a formula I worked out based on the ram calculation in the superflow flowbench manual ..... the used cfm on HP is from the same manual

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Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Sat May 12, 2018 11:35 am

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Re: Calculating CFM used from Dyno sheet ?

Post by plovett » Sat May 12, 2018 11:50 am

What is the cam duration that equates to 100% of flow from the flow bench?

thanks,

paulie

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Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Sat May 12, 2018 12:39 pm

It's a bit more involved than that but I can give you a rough idea

What lobe center? Flat or roller ?

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Re: Calculating CFM used from Dyno sheet ?

Post by plovett » Sat May 12, 2018 1:08 pm

Scotthatch wrote:
Sat May 12, 2018 12:39 pm
It's a bit more involved than that but I can give you a rough idea

What lobe center? Flat or roller ?
Solid flat. 108 lobe center.

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Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Sat May 12, 2018 1:15 pm

264 duration @ .050

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Re: Calculating CFM used from Dyno sheet ?

Post by plovett » Sat May 12, 2018 3:18 pm

Scotthatch wrote:
Sat May 12, 2018 1:15 pm
264 duration @ .050
Thanks! That's interesting. I'm guessing a roller needs less duration to reach 100%? And a tighter LSA needs less duration to reach 100% also?

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