Calculating CFM used from Dyno sheet ?

General engine tech -- Drag Racing to Circle Track

Moderator: Team

Post Reply
pastry_chef
Member
Member
Posts: 139
Joined: Wed Jul 07, 2004 10:06 pm
Location: Ottawa, Canada

Re: Calculating CFM used from Dyno sheet ?

Post by pastry_chef » Mon May 14, 2018 8:55 pm

Hi Scott,
I noticed those graphs in Harold Bettes' Engine Airflow book, thought they were very interesting.
Can you provide an example please.
Mike R

maxracesoftware
Guru
Guru
Posts: 2655
Joined: Sat Feb 14, 2004 4:04 pm
Location: Abbeville, LA
Contact:

Re: Calculating CFM used from Dyno sheet ?

Post by maxracesoftware » Mon May 14, 2018 11:05 pm

swampbuggy wrote:
Mon May 14, 2018 7:29 pm
THANKS-THANKS-THANKS Larry M. You are the very best at writing information that (I) for 1 and probably (NOT) the only one can REALLY grasp . =D> If my old sweetheart was @ 107 % V.E. that tickles the S*** out of me. That motor was taken out of my 72 Chevy shortbed P.U. and went straight into a TOP quality Horton rail type dragster and went 8.20-8.30's out of the gate. Oh BTW the P.U. was a street toy, and no it was not tubbed, it was a BLAST with a Richmond 5-Speed and 3.73 posi. :lol: :lol:
"That motor was taken out of my 72 Chevy shortbed P.U. and went straight into a TOP quality Horton rail type dragster
and went 8.20-8.30's out of the gate. "


i almost didn't see your Post .

i hope the equation is useful for future Projects
Meaux Racing Heads
MaxRace Software
PipeMax and ET_Analyst for DragRacers
http://www.maxracesoftware.com
http://www.maxracesoftwares.com/forum/index.php
PipeMax v3.98   http://www.maxracesoftwares.com/forum/v ... f=14&t=249

Scotthatch
Pro
Pro
Posts: 218
Joined: Sat Apr 14, 2018 11:12 pm
Location: 7000 ft up

Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Mon May 14, 2018 11:42 pm

OK ... kinda did at the beginning of the thread
The engine that started this is 856 HP

The cam is 282 installed on a 107
1/2 of 282 is 141 plus 107 = 247

247 - 180 puts intake closing point at 67 degrees

Since we need at l/d=.1 not .050 we will cheat and just pull 20 degrees of the top so 47 degrees

If you look at the chart at 47 you go up to the cross line and back toward n that's 1.12

So 375 cfm x 1.12 = 420 cfm

420 x .257 x 8 = 863.5 HP
15238111158030.jpg
You do not have the required permissions to view the files attached to this post.

Scotthatch
Pro
Pro
Posts: 218
Joined: Sat Apr 14, 2018 11:12 pm
Location: 7000 ft up

Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Tue May 15, 2018 12:24 am

I pulled this one from clay Smith cams

Cam is a roller 233@.050 550 lift
Heads are Afr 210 so at 550 lift flow 301 cfm

Intake closing point is 40 degrees

Down and dirty pull 20 off to get to l/d=.1

Off the chart 20 degrees comes out to .9

301 cfm x .9 is 270.9

270.9 x .257 x 8= 557 HP

It made 544 but they said there was 10 more in it
15263576777160.jpg
You do not have the required permissions to view the files attached to this post.

Scotthatch
Pro
Pro
Posts: 218
Joined: Sat Apr 14, 2018 11:12 pm
Location: 7000 ft up

Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Tue May 15, 2018 7:36 am

http://tbichips.com/?page_id=112

Engine uses edelbrock etec 200 heads
So flow @ .400 is 223
Flow@ .500 is 252
Cam lift is .444 so call flow @ lift 229


Comp 12-402-4 cam 212@.050 flat tappet 114 lc
Closing point at .050 is 40 degrees

Down and dirty for flat tappet is -25 degrees so l/d=.1 is 15 degrees which charts to .88

229 cfm x .88 = 201.5

201.5 x .257 x 8 = 414 HP
15263841676571.jpg
You do not have the required permissions to view the files attached to this post.

Scotthatch
Pro
Pro
Posts: 218
Joined: Sat Apr 14, 2018 11:12 pm
Location: 7000 ft up

Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Tue May 15, 2018 8:00 am

If you are building something and you want more accurate then my down and dirty then you need to figure l/d=.1 better and what that stands for is lift/diameter of the intake valve when it = .1 that just means when valve lift is 10% of the valve dia so with a 2.02 valve it's at .202 lift and with a 2.19 valve it's .219 lift ... to find this point you would need to take rocker ratio and figure tappet lift then look at the cam profile to figure duration at that tappet lift then figure the closing point ... most of the time I find I don't have enough information on the profile to do that ..... next option is if you have the cam you can find this point as you degree it in ... that gives you better numbers to check against the dyno run to see if you got it all running correctly ...

The other thing to note is that you need cfm of the whole system and intakes are not tested as much as they should be .... if head / cam calculated HP is way high as to the dyno tested horsepower and intake is not a known value then you can be pretty sure it's a problem

With what I have covered so far you can see how tested flow and cam size works ... also you can see why ramp speed and rocker ratio and valve size will effect that HP number and why sometimes cam Co will add a few degrees to the closing point to make that cam make just a bit more power ...

Once you guys have a good handle on this part we can go over that formula and how I use it and what it tells us

User avatar
Stan Weiss
Guru
Guru
Posts: 3561
Joined: Tue Feb 20, 2007 1:31 pm
Location: Philadelphia, PA
Contact:

Re: Calculating CFM used from Dyno sheet ?

Post by Stan Weiss » Tue May 15, 2018 10:27 am

Scott,
If you get a Comp Cams Master Lobe List it will show tappet lift @ TDC which help give you a better idea about a cam (for your .1 l/d) with actually having the cam.

Also some flow software will additionally show what the l/d is on the report.

Stan

Code: Select all

In. Valve 2.55
0.25D In. 637.5 

        CFM       
Actual  Observed  L / D   
Valve   Flow      Ratio   
Lift InchIntake    Intake  
0       0         0       
200     148       0.078   
300     241       0.118   
400     335       0.157   
500     421       0.196   
600     492       0.235   
700     546       0.275   
800     567       0.314   
900     578       0.353   
1000    583       0.392   
Stan Weiss / World Wide Enterprises
Offering Performance Software Since 1987
Image
http://www.erols.com/srweiss/index.html

Scotthatch
Pro
Pro
Posts: 218
Joined: Sat Apr 14, 2018 11:12 pm
Location: 7000 ft up

Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Tue May 15, 2018 11:03 am

Stan Weiss wrote:
Tue May 15, 2018 10:27 am
Scott,
If you get a Comp Cams Master Lobe List it will show tappet lift @ TDC which help give you a better idea about a cam (for your .1 l/d) with actually having the cam.

Also some flow software will additionally show what the l/d is on the report.

Stan

Code: Select all

In. Valve 2.55
0.25D In. 637.5 

        CFM       
Actual  Observed  L / D   
Valve   Flow      Ratio   
Lift InchIntake    Intake  
0       0         0       
200     148       0.078   
300     241       0.118   
400     335       0.157   
500     421       0.196   
600     492       0.235   
700     546       0.275   
800     567       0.314   
900     578       0.353   
1000    583       0.392   
I have the lobe lists that I can get ... most of the time you kinda have to guess from the listed data as they where not ment to show this point on the lobe profiles that is why i crunched as many as I could and came up with the cheat adjustment .... the cams I use I plot out so I know for next time which means I tend to use the same ones

Most other people I have used this to help are OK if it is off by even 10 HP they want the big number not the fine tuning

DrillDawg
Expert
Expert
Posts: 872
Joined: Tue Jul 06, 2010 11:03 am

Re: Calculating CFM used from Dyno sheet ?

Post by DrillDawg » Tue May 15, 2018 1:08 pm

This formula was also in my old note book but I don't think I ever really used it.

rpm x (cuin per cyl) \ 2000(10" test presure) x 1 .67 (28" conversion)

6900 x 64.5\2000 x 1.67 = 371 cfm

Right at what he said his heads flowed.

375\411= 91% or 411\375=1.096% which way you want to look at it, VE\cfm increased around 9% from torque peak to hp peak.
BORN RIGHT THE FIRST TIME

Warp Speed
Guru
Guru
Posts: 2368
Joined: Wed Jun 25, 2008 1:46 pm
Location: NC

Re: Calculating CFM used from Dyno sheet ?

Post by Warp Speed » Tue May 15, 2018 1:26 pm

DrillDawg wrote:
Tue May 15, 2018 1:08 pm
This formula was also in my old note book but I don't think I ever really used it.

rpm x (cuin per cyl) \ 2000(10" test presure) x 1 .67 (28" conversion)

6900 x 64.5\2000 x 1.67 = 371 cfm

Right at what he said his heads flowed.

375\411= 91% or 411\375=1.096% which way you want to look at it, VE increased around 9% from torque peak to hp peak.
Maybe I misunderstood, but how does VE increase from peak tq to peak hp?

DrillDawg
Expert
Expert
Posts: 872
Joined: Tue Jul 06, 2010 11:03 am

Re: Calculating CFM used from Dyno sheet ?

Post by DrillDawg » Tue May 15, 2018 1:37 pm

I figured his hp at torque peak and with the formula I used before suggested he was using all the 375 cfm that his port flowed and as he reported the "dyno" suggested he was using 411cfm at hp peak, about 9% difference, I had added cfm while you where posting.
BORN RIGHT THE FIRST TIME

Warp Speed
Guru
Guru
Posts: 2368
Joined: Wed Jun 25, 2008 1:46 pm
Location: NC

Re: Calculating CFM used from Dyno sheet ?

Post by Warp Speed » Tue May 15, 2018 4:33 pm

DrillDawg wrote:
Tue May 15, 2018 1:37 pm
I figured his hp at torque peak and with the formula I used before suggested he was using all the 375 cfm that his port flowed and as he reported the "dyno" suggested he was using 411cfm at hp peak, about 9% difference, I had added cfm while you where posting.
True, that mass flow through an engine increases with rpm (to a point) but VE doesn't past max work/peak torque.

GARY C
Guru
Guru
Posts: 4328
Joined: Tue May 14, 2013 10:58 pm

Re: Calculating CFM used from Dyno sheet ?

Post by GARY C » Tue May 15, 2018 5:05 pm

The one thing I see with trying to use CFM alone is that if the heads, intake, carb and cam are not sized correctly the tq will not be there.
I have a dyno sheet of a 357 that made 530 hp but only 400 ft lbs. with a little over 300 CFM.

Scotthatch
Pro
Pro
Posts: 218
Joined: Sat Apr 14, 2018 11:12 pm
Location: 7000 ft up

Re: Calculating CFM used from Dyno sheet ?

Post by Scotthatch » Tue May 15, 2018 6:11 pm

I struggled with that too ... when I first figured this out I realized that I had a HP number but no rpm .... the interesting thing at this point is that you can get a number this way so that means something else sets the rpm but not the HP .... when it came to torque I new the relationship with HP but not the why ?

The one thing I should point out is that we are only setting one cam event .. intake valve closing..... there are 5 more events and I have been playing with those ... from what I have seen the intake opening event and exposure of the intake valve at peak piston velocity has a lot to do with torque .... from what I have seen so far you need 5.75 cfm per cubic inch of the cylinder at peak piston velocity to have any hope of filling the cylinder and you have to balance that with not to much overlap depending what peak rpm will be ... the higher the rpm the more overlap you can deal with ... yet another reason to keep low lift flow up

maxracesoftware
Guru
Guru
Posts: 2655
Joined: Sat Feb 14, 2004 4:04 pm
Location: Abbeville, LA
Contact:

Re: Calculating CFM used from Dyno sheet ?

Post by maxracesoftware » Tue May 15, 2018 6:22 pm

Scott ,.... the following Info is what i calculate using v4.00 :
Basic Engine + Head CFM Data :

Code: Select all

Peak TQ = 486.6 at 3800 RPM  BSFC=0.390 Fuel=130.1  333.5897436 UBHP= 1.055488086 HPCF 13.737:1 A/F
Peak HP = 414.4 at 5200 RPM  BSFC=0.447 Fuel=174.8  391.0514541 UBHP= 1.059707094 HPCF 13.910:1 A/F 
AirDensity%=94.2 at 5200 RPM

Performer RPM E-Tec 200 Cylinder Heads For S/B Chevy
    Cylinder Head Specifications:
    Combustion chamber volume: 64cc
    Intake runner volume: 200cc
    Exhaust runner volume: 80cc
    Intake valve diameter: 2.02"
    Exhaust valve diameter: 1.60"
    Valve stem diameter: 11/32"
    Valve guides: Maganese bronze
    Deck thickness: 5/8"
    Valve spring diameter: 1.46"
    Valve spring maximum lift: .575"
    Rocker stud: 3/8"
    Guideplate: Hardened steel
    Pushrod diameter: 5/16"
    Valve angle: 23°
    Exhaust port location: Stock
    Spark plug fitment: 14mm x .750 reach, gasket seat

Code: Select all

#60989 | #60985: 
Flow Numbers as tested by Edelbrock's SuperFlo SF-1020 flow bench @ 28" H2O
Valve Lift 	.100" 	.200" 	.300" 	.400" 	.500" 	.600" 	.700"
Intake 	        67 	122 	175 	223 	252 	259 	265
Exhaust 	57 	110 	153 	182 	196 	204 	207

Intake  CFM @ 0.444 Lift = 239.1281
Exhaust CFM @ 0.444 Lift = 189.6405

Code: Select all

Part Number: 12-402-4		Grind Number: 260AH-14		
Lifter Type: Hydraulic Flat Tappet	
Engine Family: 	Chevrolet 262-400 c.i. 8 cyl. 1958-1998
Description: 	Good for stock TPI 350 or TPI 305 with modified computer.
Cam Family: 	Computer Controlled

Specifications 	  	  	                Intake 	      Exhaust
RPM Range: 1500 to 5500	  Valve Lash: 	        Hyd	      Hyd
Valve Timing: 0.006	  Duration: 	        260.0         268.0	
Lobe Separation:   114.0  Duration@.050" Lift:  212.0         218.0		
Intake Centerline: 110.0  Valve Lift: 		0.444         0.444
    	  	          Lobe Lift: 		0.296         0.296

Valve Timing @ 0.006  Lift: 20/60=Intake   72/16=Exhaust  110.0 CenterLine
for 486.6 Peak TQ ... its possible you could have made 424.8 Peak instead of 414.4 HP
...v4.00 shows it could have used higher Valve Lifts to get 424.8 Peak HP


CSA + FPS + Port Volume CC Data :
You do not have the required permissions to view the files attached to this post.
Meaux Racing Heads
MaxRace Software
PipeMax and ET_Analyst for DragRacers
http://www.maxracesoftware.com
http://www.maxracesoftwares.com/forum/index.php
PipeMax v3.98   http://www.maxracesoftwares.com/forum/v ... f=14&t=249

Post Reply