Calculating CFM used from Dyno sheet ?

[pastry_chef]

-----14 to 1 static C.R.------9.75 to 1 Dynamic C.R.--- Mark H.

Thanks, any dyno sheet you can post?

I'm curious about BSFC at peak HP, Peak TQ.

[randy331]

I have an engine, based on CFM, the SF formula says should make peak HP at 6337 rpm, but it peaks at 7000 rpm. Why ?

Maybe we need to develope a CFM faith ratio ?? How much faith we have in each of those CFM to produce HP ?

Randy

[Scotthatch]

Such closed minded people it is truly sad that your choice on anything that is posted that you don't understand you just post negative stuff till no one wants to post .. thought speedtalk would have better .... I spent 15 years working out the math and another 10 years testing it and it works was going to share but now I have changed my mind ....

[pastry_chef]

I have an engine, based on CFM, the SF formula says should make peak HP at 6337 rpm, but it peaks at 7000 rpm. Why ?

You know why, there are other factors.
Is there a link to these engine details here?
If we tried to make a list of "hard rules" that were always true it would be extremely limited.

[Stan Weiss]

The prediction of horsepower based on airflow numbers can be applied if the test pressure is known. The results are a good estimate of the engine’s capacity to make power if everything in the system is optimized to take advantage of the airflow available. An accurate estimate of the power capacity of the engine is dependent upon having accurate flow numbers for the complete airflow system, including the cylinder head, manifold, carburetor or fuel injection system.

Part of the problem with some of the calculated numbers is most people to not know what the flow is for their full intake system.

Stan

[Warp Speed]

Such closed minded people it is truly sad that your choice on anything that is posted that you don't understand you just post negative stuff till no one wants to post .. thought speedtalk would have better .... I spent 15 years working out the math and another 10 years testing it and it works was going to share but now I have changed my mind ....

I dont see any of that happening in this thread really......?
There are always going to be differing opinions and views.
What "closed minded" stuff are you referring to?

[randy331]

I have an engine, based on CFM, the SF formula says should make peak HP at 6337 rpm, but it peaks at 7000 rpm. Why ?

You know why, there are other factors.
Is there a link to these engine details here?
If we tried to make a list of "hard rules" that were always true it would be extremely limited.

Right, cfm as measured on a flow bench doesn't seem to dictate rpm as much as CSA does, shape and how straight the intake tract is are a couple more.

No link with details to the engine I mentioned. It's the BBC I posted the header/Q16 dyno tests on.

Formulas that use cfm as the main input become less useful to you when you see more cfm make less power on a few engines.

Randy

[DrillDawg]

I think the velocity profile has a lot to do with it and sometimes working on your intake and exhaust tracts to increase your ve is a better option than hunting a cfm increase in your head's intake port.

[Warp Speed]

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

[swampbuggy]

Warpspeed---THANKS for some technical input into my thread !!! The BSFC #/HrHP was 0.40 @ peak TQ. and was the same at peak H.P. Pastrychef Mark H. will try to get help posting up dyno sheet.

[randy331]

It's all about CSA, and the area vs time of the available (read affordable! Lol)

I like that. Affordable time and area ! LOL

Lotta truth to that.

Randy

[GARY C]

From what I have seen in my minimal porting experience is that if the head is sized correctly the cfm and velocity follow suit to some degree but you can keep porting and be 20+ cc's to big and never gain a cfm and sometimes loose cfm.

[plovett]

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.

paulie

[Scotthatch]

It has to be effective cross sectional area not just the measured cross section

[maxracesoftware]

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= 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

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