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 VELOCITY formula 
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Joined: Sat Oct 03, 2015 1:38 pm
Posts: 75
Morning guys,

To increase the CFM I don't want to lower velocity. Is there a formula that can be worked out by digital manometer pressure readings from both side of the orifice.

"To translate velocity pressure into actual velocity requires either mathematical calculation, reference to charts or curves, or prior calibration of the manometer to directly show velocity. In practice this type of measurement is usually made with a Pitot tube which incorporates both static and total pressure sensors in a single unit."

source: https://www.dwyer-inst.co.uk/Products/A ... uction.cfm

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Thu Jun 29, 2017 2:13 am
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Joined: Sat Oct 03, 2015 1:38 pm
Posts: 75
Have I got this wrong?

CFM stands for Cubic Feet per Minute; way of describing the volume flow rate of compressed air.

So the higher the CFM , it auto means it is going faster?

So if you port you engines and increase the CFM, how do you work out if it the air is going slower.


Sun Jul 02, 2017 3:36 pm
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Joined: Mon Feb 23, 2015 6:10 am
Posts: 264
Location: Northern NSW, on the wrong side of the Great Divide.
CFM is, as you say, Cubic Feet per Minute. It has nothing to do with compressed air because a naturally aspirated engine doesn't compress the air until the valves are shut.

Speed, or velocity, of airflow is a by product of multiple variables. Let me give you an example using 3 different sets of heads on 1 engine.
Mark 4 Big Block Chevs (BBC) come out with 3 different types of heads, Oval Port (great for street use), Rectangular Port (not so great for street use but great for high RPM racing use), and Peanut Port (best for low RPM haulers). If, by some sort of magic, a head porter was able to get a peanut head to flow the sheer numbers a rectangular port head flows the peanut head would have a higher air speed (or velocity). Why? because the same amount of air is being forced (either by vacuum or by pressure) through a smaller orifice. It is the exact same as a 1/2" water hose flowing the same amount of water a 3/4" hose flows, the water will travel faster (have a higher velocity) and go further once it has been released from the hose nozzle.

Making air flow faster isn't just about making a port bigger. Sure a bigger port, in a perfect world, will flow more air at any given pressure but we don't live in a perfect world and ports have turns (Short Turn Radius) and restrictions (pushrod bulge, valve guide, valve stem) that make the air flow in directions that are contrary to where we want it to flow.

Also you have to match the port size (flow) to the engine size and its intended use. A bog stock peanut port BBC head will never flow the numbers a bog stock rectangular port BBC head will flow so you wouldn't put a peanut port head on a historic NASCAR racer that is still competing against other historic NASCAR race cars. If you did your engine will run out of puff at approximately 3000 RPM and your competitors will keep going past 7000 RPM.

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Sun Jul 02, 2017 11:12 pm
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Joined: Tue May 14, 2013 4:24 pm
Posts: 11
A couple of formulas I have run across: Velocity fps = (square root of pito pressure) x 66.2
Velocity fps = (cfm x 2.4) / cross sectional area


Fri Aug 18, 2017 5:04 pm
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Joined: Wed Mar 03, 2010 3:40 pm
Posts: 1271
Location: Melbourne, Australia
As Storm says, you need everything to be matched to the intended application and purpose.
Its not just the all important port, but also the manifold, valve timing and exhaust system that must all work together in perfect harmony.

The very best thing to get you started in all this would be to get yourself the ($75) "Pipemax" computer program, it will be the best money you ever spent.

That will give you all the data you need, critical valve and port sizes, tuned runner lengths, induction CFM, velocities, optimum valve timing points and even the expected power and torque levels you can expect over a range of RPM.

The trick seems to be look at the whole package, see what you have, and decide what part of it when changed can give you the biggest gains. Hogging out the ports may not get you the gains you expect if some other part of the system is restricting performance. Pipemax will help you to nail down a really good combination for what you want the engine to do. Its vastly more than just the simple pipe tuning program the name suggests.

http://www.maxracesoftware.com/PIPE395.htm

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Fri Aug 18, 2017 6:16 pm
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Joined: Sat Oct 03, 2015 1:38 pm
Posts: 75
I will buy this program but first I want to make my bench work properly.

I am stuck. I am going to make a new thread about this.


Sat Aug 19, 2017 12:48 pm
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Joined: Mon Feb 23, 2015 6:10 am
Posts: 264
Location: Northern NSW, on the wrong side of the Great Divide.
I 2nd Pipemax, it is good, however without a dyno and datalogging you'll never get the best out of it. If you know this and are willing to work with it anyway you'll get good results.

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Sun Aug 20, 2017 1:42 am
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