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cfm demand formula
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Author:  zums [ Fri Sep 21, 2012 7:55 pm ]
Post subject:  cfm demand formula

anyone know an accurate formula for figuring peak cfm demand based on cid and rpm, and can that formula be munipulated to figure min csas at different fps, eg. max fps at peak torque, or max fps at redline shift points, thanks- tom

Author:  jmarkaudio [ Sat Sep 22, 2012 1:58 pm ]
Post subject:  Re: cfm demand formula

Pipemax is one of the best, good for a lot of calculations on an engine.

http://maxracesoftware.com/pipemax36xp2.htm

Author:  rick360 [ Sat Sep 22, 2012 10:46 pm ]
Post subject:  Re: cfm demand formula

You have probably seen this formula before in some form. This one uses VE which not all use. This will give the average velocity at the min csa (or any other place in the port) at a given rpm and engine.

Velocity=([VE%]*.01*0.00353*[RPM]*[Stroke]*([Bore]^2))/[CSA]

You can rearrange it like this to give CSA for a given engine, rpm and whatever velocity you want ...

CSA = (0.00353*[VE%]*.01*[RPM]*[Stroke]*[Bore]^2)/[DesiredVelocity]

Now the real question is "what velocity do you want?"
No definite answer there that applies to everything and it depends on a lot of things.
* Where is the mcsa located?
* How straight is the port?
* How smooth are the turns in the port?
* Where is the mcsa from any turns?

I've seen a 383 with a mcsa of 2.45 and a 260º cam make peak power less than 6500 ... wanted shifted at 6600.
and ...
I've seen a 383 with a mcsa of <2.10 and a 236º cam make peak power over 6500 (dynoed) ...

The first one was a basic set of 215cc Pro1 heads that cc'd 225 with mild porting and flowed 285 and the engine made ~550hp based on et and wt.

The other was a set of large port vortecs ported and epoxied down to <210cc and worked on for many hours to make the shape as good as possible with a very small pushrod pinch (the mcsa on these) and carefully working the turns to make them work like that. It dyno'd as much as 625hp depending on other components. Using 110%VE and 2.08csa and 6600rpm makes the velocity 750fps. Well beyond what anybody says is the limit.

How sharp the turns are made affects the velocity limits of the port. More velocity has the potential for more power. When a port hits it own velocity limit determines how much power it can make. Notice how straight the ports are in an sb2, big-chief or 15º head? Straighter port = higher velocity limit = more potential power.

Rick

Author:  zums [ Sun Sep 23, 2012 10:16 am ]
Post subject:  Re: cfm demand formula

once again rick, your an ace- thanks alot- tom

Author:  zums [ Sun Sep 23, 2012 10:49 am ]
Post subject:  Re: cfm demand formula

on the lrg port vb was the pushrod wall past the pinch filled in and the commonwall opened to attempt a straighter shot or is that secret info, thanks

Author:  randy355 [ Sun Sep 23, 2012 11:23 am ]
Post subject:  Re: cfm demand formula

I don't think ve should be part of a peak cfm demand or speed calculation for a running engine.
There's no way for a piston to demand more than %100 of the area it displaces.
VE s over %100 don't happen at peak piston speed. The increase in VE above %100 happens at or after BDC, but getting the cylinders to continue to fill at BDC is because of what happens well before BDC.

I think Tom is working on a 23* vortec head.

On a 23* sbc head with a stock pushrod location, the mcsa will be at the pushrod. It will be a primary limiting factor.
Adding area by moving the pushrod wall over will add area and improve the shape. When the pushrod wall is as thin as your willing to make it you must gain area elsewhere. (roof,divider wall,floor) When all you have left to get area from is by grinding a dip in the floor or divider wall, the area gained most likely won't net the same results (rpm on the running engine) as when the pushrod wall was where area was gained.
The engine will make better use of the mcsa when it's straighter (like Rick mentioned) So on a 23* SBC head with stock pushrod area, the engine will show more rpm per inch of area, and more power per inch of area with a smaller mcsa.

I have a large port vortec head with a mcsa of a little over 2.3" by mostly raising the roof and thinning the pushrod wall and it still has decent corner radi. If I remember right Tom is building a 377" engine and a peak rpm of 8000 with these heads.

((.00353 x 7500 x 3.48 x 4.155 ^2) / ( 2.3 )) = 691

I have little dought this port can support a peak HP rpm of 7500 with a good intake and cam. 7500 is about as high as you'd want peak HP for a max rpm of 8000.

Author:  zums [ Sun Sep 23, 2012 6:41 pm ]
Post subject:  Re: cfm demand formula

thank you randy, the actual cid will be 373 and all this long cutting , filling, testing is done on cut up 062 vortecs, the actual head that will be run on the engine is a small port vortec bowtie, so its all practice since i only have one set of spvb, testing 15-20 different in and ex valves -shapes-angles-seats-port volumes in between working full time at a different job is proving to take forever, just curious, what do you do for a living, have you and rick ever thought about doing this for a living, grinding cast iron is a bitch- thanks again guys- tom

Author:  zums [ Mon Sep 24, 2012 6:47 pm ]
Post subject:  Re: cfm demand formula

randy, on your large pvb is the min csa 2.06 in the printed article or 2.3, im assuming this is the pinch because the throat would be less than89% on a 2. valve, thanks

Author:  rick360 [ Mon Sep 24, 2012 11:11 pm ]
Post subject:  Re: cfm demand formula

zums wrote:
randy, on your large pvb is the min csa 2.06 in the printed article or 2.3, im assuming this is the pinch because the throat would be less than89% on a 2. valve, thanks
The 2.06in² was at the pushrod area. The throat was quite a bit bigger than that although still small for a 2.08 valve(~87% from memory) . If you don't need it to make power at 2500 rpm then I'd make the pushrod area bigger than our EMC ports.

zums wrote:
have you and rick ever thought about doing this for a living, grinding cast iron is a bitch-
We've both done heads for people at times. I used to do quite a few for extra racing cash. Not a fun way to make a living. Now I am a technician in a large coal fired power plant. We work a lot of OT, so there isn't time to do what I want with my own racing stuff so I only help a couple people with their cars besides working on mine. When I retire I will probably work on engines and/or race cars in my shop to keep me occupied until they ship me off to a nursing home or coffin. I won't be retiring for ~10yrs or so.

Rick

Author:  zums [ Tue Sep 25, 2012 7:23 am ]
Post subject:  Re: cfm demand formula

thanks rick, when i ever get these done ill post results

Author:  randy355 [ Tue Sep 25, 2012 7:08 pm ]
Post subject:  Re: cfm demand formula

Tom, the emc mcsa was 2.06". Rick's right on the throat, it's in the %87 %88 of a 2.08" valve at the throat. Some ended up bigger than others because of the castings. The port I have with 2.3" mcsa is a large port vortec I've worked on for a different project.

Porting heads for a living????

Define living!!

Author:  zums [ Tue Sep 25, 2012 7:31 pm ]
Post subject:  Re: cfm demand formula

2.3 min csa lrg port,,,,,,,,,, what cha buildin, ive heard nothin but good about these heads, i got a good deal from a local chevy dealer last year and bought one set of each, the large ports look like they can back up some pretty healthy power #s with some work, im savin mine for a future project myself, bowls are pretty big out of the box with the smaller guide height

Author:  randy355 [ Wed Sep 26, 2012 9:30 pm ]
Post subject:  Re: cfm demand formula

Tom, the large port vortecs will be for some 4.155" bore engine. Just worked one port out close so far. Thinking maybe I should send it out to be copied, cause I don't know when I'll get time to do the others.

I see you asked for a cfm demand formula in the original post, and we've been talking calculated average speed on the running engine so far.

cfm demand = CID x RPM x .0009785 / number of cylinders

373 x 7500 x .0009785 / 8 = 342

The engines I've seen will make peak HP well past the RPM the heads meet CFM demand, even with small cams.

Author:  zums [ Thu Sep 27, 2012 7:05 am ]
Post subject:  Re: cfm demand formula

thanks randy, i coulnt find that formula in the books i have

Author:  rick360 [ Thu Sep 27, 2012 10:13 pm ]
Post subject:  Re: cfm demand formula

Randy, where did that cfm demand formula come from? The answers matche the peak cfm demand in Pipemax with a 6" rod. The peak piston velocity changes with rod length for the same engine bore, stroke and rpm so the actual piston cfm demand would change if you want peak cfm demand. (Average piston speed is the same regardless of rod length)

I did some calculations to determine the actual volume displaced per time (converted to cfm) by the piston for different bore, stroke and rpm combinations and even the average cfm demand for 180º crank angle during the intake stroke is considerably more than your formula gives. The peak demand is much higher.

For a basic 383 (4.03"bore, 3.75"stroke, 6.0"rod, 7500rpm) using my piston movement formula [avg cfm demand]=415.2cfm and [peak cfm demand]=681

Using your formula I get (382.668x7500x.0009785) / 8= 351cfm

Just wondering how this formula was derived and looking at Pipemax the answer is very close to the instantaneous cfm demand at peak piston speed. Note that Pipemax calls it Piston CFM @ 28in. Now how does the piston cfm have anything to do with a flowbench depression? Isn't the amount of volume the piston moves in a given time fixed and a cfm value without regard to depression?

hmmm ...... :-k

Rick

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