Help with some flow numbers, please.

kick2008a · Post by **kick2008a** » Mon Mar 10, 2008 5:27 pm

I've measured my port dimensions and flowed the head on a SF1020 and the resulting numbers for velocity seem high. The motor is a BBC 511, 4.375 x 4.25 with 2.25 intake valve.
I'm new at the numbers part of this, but I really want to learn. I'm an old porter from the 80's that just got back into it. I had a SF flowbench back then, but ported and tested the results at the track. I didn't really have the data numbers back then.

I've got a thousand questions but I'll try and keep them to a minumum.

Here's my numbers. This is a older Dart 308 2.25/1.88 head which had been previously ported. I did a valve job to it and blended the seats.

Figured with .50 radius in corners and at 6,800 rpm

manifold 2.475 x 1.770 = 4.327 sq in, 485.4 fps

at 1" 2.375 x 1.575 = 3.687, 569.6 fps

at 2" 2.100 x 1.640 = 3.390, 619.5 fps

at 3" 2.010 x 1.800 = 3.564, 589.3 fps

guide na x 2.100

throat 2.000 (88.89 %) 3.132, 668.6 fps
not counting valve stem area 11/32.

One more, please

The formula for the valve curtain is,
2 x 3.14159 x radius x lift = valve curtain in SqIn's

2 x 3.14159 x 1.125 x .900 = 6.36 SqIn

Is this right?

Thanks,

11secAvanti · Post by **11secAvanti** » Mon Mar 10, 2008 10:39 pm

Area of a circle is pie r square where r is half the diameter of the valve. Then times the lift should give the curtain area. To get the fps you would need the cfm and the csa of interest times 2.4. FPS = CFM divided by CSA times 2.4. Hope this is correct.

kick2008a · Post by **kick2008a** » Tue Mar 11, 2008 8:46 am

Thanks for the reply,

I see there is two ways to figure it. With engine rpm and with CFM. They give two totally different fps numbers. What is the difference or am I missing something.

I've read the posts on here until my eyes hurt and I've printed half the pages. I think I've read so much, that I'm actually confussing myself with all the formulas.

I'm on the flowbench tonight and I 'll re-measure the intake port again, since I've made a few minor changes to it. I'm going to start over with fresh numbers and use the cfm formula.

I will figure all this out. I know I can get the numbers, now I want to know the science and why.

kick2008a · Post by **kick2008a** » Tue Mar 11, 2008 9:34 am

I just noticed that I was using Port Limiting Velocity formula on Stan Weiss web page for my fps.

That should help.. <smile> I knew I was doing something wrong.

Post by **Stan Weiss** » Tue Mar 11, 2008 12:07 pm

11secAvanti wrote:Area of a circle is pie r square where r is half the diameter of the valve. Then times the lift should give the curtain area. To get the fps you would need the cfm and the csa of interest times 2.4. FPS = CFM divided by CSA times 2.4. Hope this is correct.

While this is true for area for
curtain area / Window Area = Valve diameter * Pi * lift

Which is the same as

The formula for the valve curtain is,
2 x 3.14159 x radius x lift = valve curtain in SqIn's

You are correct that FPS = ( CFM / CSA ) * 2.4