A little help with some formulas

[haley63042]

Hi, I was wondering if I could get a little help. I am trying to figure out where my heads will make power to or stall out? The cam will have enough lift to use 238 of the cfm. I did a 9 point velocity reading in 3 places and added them up and divided by 27 and my ave. air speed is 257 fps. Seems low I guess I did that right?....So
bore x rpm x .000978474=? / 8
4.34x 5500 x .000978474=291

bore x bore x stoke x rpm x 00353=? /mcs area
4.34 x 4.34 x 3.64 x 5500 x 00353=1331.12/1.92=693.29
Anyone care to help me if its possible
Thanks,Bob

[jfholm]

Bob,
here are some of the usefull formulas from PipeMax you may want to use:

Average_CSA = Port_Volume_CC / (Port_Centerline_Length * 16.387)
To find your port length take a narrow piece of tape and put it from the top of the seat angle to where the intake manifold mates to the head. Do this for the floor and the roof and then add that length together and divide by 2 and that gives you the port centerline. Then CC your intake port and see what it holds CC wise. As an example. My intake port is 5.435" long and holds 193.5 CC.
So the first thing you do is multiply the length of 5.435" by 16.387 and that equals 89.063345. Then you would divide the volume of 193.5 CC by 89.063345 and that gives you an average CSA of 2.1726 sq inches.

Average_CSA = (Flow_CFM * 2.4) / FPS

You can also do using this formula. If I remember correcectly you take the FPS down the center of the port and average it. You should also be testing your velocity at .87 of your max lift.

[haley63042]

Ok so I will get the velocity at 3 points on the floor, down the center. and get my correct lengths. Then I will find a way to cc this runner by filling it up. Have to find someway to measure the fluid before I pour it in to see how much I used.After all that I will have the info needed to figure this out.

[1960FL]

Looked like best place to post.


Ok I have a question on calculating port velocity based on piston speed, I am working on formulas for FlowSoft and in two of my books I have two different formulas for calculating port velocity.

1) From Smiths book on intake and exhaust design that is V= Ps in FPS * ( bore ^2 / port throat dia ^2)

2) Is from Harolds book and it is V= Ps in FPS * ( bore ^2 / port CSA)


The problem is that Harolds calc yields speeds on the average 100Fps faster,

Now if I change Harolds formula to (Piston area / CSA) the numbers drop I line with Smith.

Any in sight would be greatly appreciated.

Rick

[jfholm]

In John Baechtel's Engine Math book he states the same formula PortVelocityfps=(Ps/60)*(B^2/Ap)
Where:
Ps = piston speed in feet per minute
B = bore diameter in inches
Ap = area of port in square inches

Same as you had, but then he goes on to say in a picture: To calculate port velocity, measure the port entry and exit and average the two area measurements to obtain Ap. Then plug in the mean piston speed and bore size to find the mean port velocity.

What I suggest is to use Larry Meaux's formula out of PipeMax to figure average port CSA for this formula. That formula is Average_CSA = Port_Volume_CC / (Port_Centerline_Length*16.387)

PipeMax also shows a formula for FPS = (Flow_CFM * 2.4) / Average_CSA

Now if you really wanted to get tricky you would calculate the piston speed for every degree of crank rotation. It does change as per the piston position in the cylinder and whether it is coming up going down and the rod length. I have been working on this a little myself but have not had much time. I will stay in touch with you if I can find out anything else.

[jfholm]

Here is a formula for calculating piston position in the bore relative to crank angle is:
P = S(1-Cos C)+(S*S)/L(Sin^2 of C)

Where:
S = stroke length
L = rod length
P = piston position relative to deck surface
C = crank angle relative to cylinder centerline
Cos C = cosine of angle C
Sin^2 = the sine squared of angle C

[1960FL]

Look at this chapter

Simple ratio as one would expect and Smith refers to.

http://francesa.phy.cmich.edu/people//a ... apter9.htm

Rck

[slracer]

OK Rick, If you assume that CSA in Harold's formula is PI/4*port dia^2 (a round port as it would have to be at the throat - where Smith's formula is set) then Harold would get speeds about 127% (4/PI) of Smith's. Is that what your 100fps is? When you changed the bore to piston area you added that same factor into the other half of the equation (so they cancel each other out), and the 2 results are the same.

The question now is why are they different? There is a possibility here that John alludes to in that since the piston speed is really variable, are you concerned with average speed on both equations? Perhaps, and I have NOT done the math, since piston speed goes from zero at top and bottom of the stroke, the AVERAGE speed is used in Smith's formula, but the MAX speed is used for Harold's formula. Because the actual piston speed is a sinusoidal variation, the max speed would be about that much different. Maybe one of the electrical guys can fill in that blank as AC has the same variation. Just a thought from the other side of my brain (the side that doesn't get used too much).

If this doesn't confuse the problem, I give up!

Doug

[jfholm]

I just pulled out my Port Flow Analyzer manual and here is what it says about Port Velocity as calculated in the program. "The port velocity is the average air speed flowing through a port area defined by the Average Port Diameter (or Average Port Height and Width) entered in the Head Specs Menu.

So they are using averages there also.

[slracer]

John, in your input re: Baechtel's book, he said "...mean " in regard to both piston speed and port velocity. To most of us, "mean" and "average" are the same thing. However, to a mathematician, they have entirely different meanings. I'm sure that there certain "boundary conditions" where they are equal, but with sinusoidal action, I am sure that they are different. How much, I'll have to remember how to calculate the "mean". Now, if the flow was really turbulent, would we consider it "mean & nasty"?

I just checked Wikipedia and they say the mean piston speed is just the average: "The mean piston speed is the average speed of the piston in a reciprocating engine." That settles that!

Doug

PS - John, Will I see you at Bonneville (racing dates: Aug 10-16)?
Rick, How about your cross country trip plans? I'm afraid I won't be home from Hawai'i in time for the World Finals in October so won't be there.