Cross section calculation question...

OldSStroker · Post by **OldSStroker** » Thu Feb 24, 2005 1:30 pm

panic wrote:I just spent 10 minutes reading more on that page, and to be fair some of it is not only correct but quite profound, and some is complete crap.
Rather unusual for someone to be on both sides of the competence fence.

The temptation is to believe that if you are very knowledgable on one subject, you are knowledgable on every subject. Unfortunately that's rarely true......except for you and me, of course.

panic · Post by **panic** » Thu Feb 24, 2005 1:36 pm

Of course!

I wrote him a polite note with some of the mistakes corrected (i.e., bad math etc. not just differences of opinion), let's see how he takes it.

BTW: anyone sees an error on my stuff, PLEASE LMK stat.
Can't fix it until I know about it.

Duh!

cboggs · Post by **cboggs** » Thu Feb 24, 2005 4:09 pm

Ya know gang, .. while on this subject, ..

I don't know why some one hasn't written a little spread sheet or software to do these calcs.

How hard would it be to design a basic Windose graphic interface
that you could plug the H / W etc of a port to map it's area & cross sections???

As cylinder head guys we have to do this stuff every day, .. and I
SUCK at math, .. I'd love to have a little peice of software to do
the math for me.

Any one here programe or know some one who does?

Curtis

rmbuilder · Post by **rmbuilder** » Thu Feb 24, 2005 7:49 pm

These may help
http://www.efunda.com/math/areas/IndexArea.cfm
http://www.staff.vu.edu.au/mcaonline/un ... eavol.html
http://tutorial.math.lamar.edu/AllBrows ... rmulas.asp
http://mathforum.org/library/drmath/view/63777.html
http://astro.temple.edu/~dhill001/secti ... ethod.html
Bob

Rick360 · Post by **Rick360** » Thu Feb 24, 2005 8:58 pm

cboggs wrote:Ya know gang, .. while on this subject, ..

I don't know why some one hasn't written a little spread sheet or software to do these calcs.

Any one here programe or know some one who does?

Curtis

I have a couple of things I use.

A spreadsheet for calculating average velocities in a port at the throat and pushrod based on the area and flow numbers. It also has pitot dp to velocity for 2 places in a port. It's not very user friendly but it works for me. Each new flow test I have to copy the old sheet and enter new numbers. I manually calculate most intake areas to enter into the spreadsheet.

I also made a program to read a bitmap file with a shape to calculate cross section. I can cut out or draw a shape of an exhaust port exit actual size on paper, scan it into my computer. Edit the Black & white .bmp file to fill the entire area in and run my program and it tells me how many square inches that shape is. Just a couple of minutes and is as accurate as your scanner is(very). Exhaust port exits are especially hard to figure manually. You can only get close when calculating cross-sectional area of odd shapes without this.

Rick

panic · Post by **panic** » Thu Feb 24, 2005 11:59 pm

When we used a pointed stick and a wet clay tablet, the preferred method was fine graph paper. Just trace the port outline on the paper. Count all the squares inside as 1, count all the squares partially intersected as .5 (the average of 1% and 99%). Add up, multiply by area of each square (measure 10" and count to get best value).

Gilgamesh taught me how to do this.

OldSStroker · Post by **OldSStroker** » Fri Feb 25, 2005 8:03 am

panic wrote:When we used a pointed stick and a wet clay tablet, the preferred method was fine graph paper. Just trace the port outline on the paper. Count all the squares inside as 1, count all the squares partially intersected as .5 (the average of 1% and 99%). Add up, multiply by area of each square (measure 10" and count to get best value).

Gilgamesh taught me how to do this.

So you also went to U. of Uruk? Were you in a frat?

Graph paper is still a quick and effective means. I use it regularly. As you said, you also need to verify it's accuracy by measuring an exact length and counting the squares in the (10") length.

panic · Post by **panic** » Fri Feb 25, 2005 11:21 am

Belonged to Sigma Kappa Pi (Sumerian & Chaldean Posse). Here's the house:

J-Rod · Post by **J-Rod** » Fri Feb 25, 2005 2:30 pm

Thnaks guys I DO appreciate all y'all input. This wa shte reason for my question. I'm trying to put as much of htis stuff in a fairly easy to use spreadsheet. As I find math and formulas I'm adding them.

I realize there is good and bad info out there, I'm just trying to sort through it.

BTW, the idea of using a scanner to calculate area is a really cool idea Rick! I'd like to see that in action sometime.

OldSStroker · Post by **OldSStroker** » Fri Feb 25, 2005 4:03 pm

panic wrote:Belonged to Sigma Kappa Pi (Sumerian & Chaldean Posse). Here's the house:

Fortunately we were there about the time the sexagesimal place value system was put into general usage. It sure made figuring large areas easier for me. I got lost when the same sign could be used in several systems. This system might help out J. Miller.

Your frat house has a style like some stuff I saw in southeast Asia. They must have copied your architecture.

FWIW, I was a Gamma Delta Iota.

Rick360 · Post by **Rick360** » Fri Feb 25, 2005 7:36 pm

J-Rod wrote:BTW, the idea of using a scanner to calculate area is a really cool idea Rick! I'd like to see that in action sometime.

It works like counting the squares on graph paper only counting the pixels and uses resolution (usually 300dpi but is a setting) to calculate area from. Does it a little faster and easier though.

I wrote it in VB so if you have the necessary files to run VB compiled programs I will send it to you or anybody that wants it. I think maxracesoftware has a download with all of the needed files for VB if you don't have them. I have a couple of .bmp files I used it on to show you how they should look.

Rick
email
rferbert (at) stjoelive (dot) com

todd8541 · Post by **todd8541** » Mon Mar 07, 2005 11:09 am

Are you trying to calculate the min. CSA or average CSA. They are different.

The average would be port volume divided by length.

Minimum CSA is the smallest diameter in the port. This one you have to actually measure and do some calculations from the shape it is made out of.

The average crosssectional area is not the same as the minimum. This will affect your velocity numbers drastically.

Think of a 1" Diameter straw as a port (for easy calculations) that is 12" long. Volume is 9.42 in^3 and the area is .785 in^2. The minimum crossectional is still .785 because the area didn't change along it's length.

If you take another straw that is 1" for 6" then changes to 2" for 6" the average crossectional area is 1.831in^2 but the minimum crosssectional area is still .785 in^2.

So it is important to determine which one you are really needing because they are not the same.

Todd

cboggs · Post by **cboggs** » Mon Mar 07, 2005 11:30 am

Todd,

really good stuff, .. thank you.

OK, is there an easy way to find the Min cross section ?
I've always done the old rubber mold, graph paper thing, ...
but i'd be real nice to take a height & width, .. and have the area.

Where I get confused of course is the corner radius and working
that into the area calc. I really, really suck at math, .. so the mold /
paper method has been a crutch for years.

Same for curtain area calcs, .. I just can't "get" the math, . I have
to use my flow software to do it, ..

I'd love to go back to school for the math instead of relying on computer
programes, etc, .. It's getting frustrating, .. I guess I shouldn't
have skipped school so much as a teen.

Curtis

todd8541 · Post by **todd8541** » Mon Mar 07, 2005 12:33 pm

It is really easy if you really stop and think about it. It is really hard to do reverse calculations as to what the actual minimum CSA is. That is why you have to actually measure it. Basically you break it down in to different types of geometry (circles, rectangles etc.)

Let me think of a good way to explain it and I'll repost it later today.

Todd

panic · Post by **panic** » Mon Mar 07, 2005 4:21 pm

IDT minimum area can be done accurately unless the ports being compared are almost identical in design. A port with a venturi-ish constriction as the min point (symmetrical low taper leading edge, generous radius at min rad, then higher trailing edge) will have higher average flow than one with otherwise same ID except min point as a sharp, abrupt biased constriction.