PTS Flowbench Forum

Will these things affect port velocity to the point that velocity will change?
1. Altitude (does that really affect velocity?)

2. Temperature

3. RPM (this is a given)

4. Barometric pressure (does this have any effect?)

any other things that can affect velocity?

My self i would love to here an answer as i live at high altitude and really wonder just how much it affects my bench as well as the porting affect.

Chris your right, it is small.....if you do the math for the speed of sound at 60* and sea level. Then figure it out at 7000 feet and 60*..........the speed of sound really don't change much............But it still make me wonder..................

I guess the big question for me is the altitude......aka air density(of course this applies to temp. and bar. as well)
On one side of the coin, the lower you go in altitude the more air...or the more dense or heavy-r the air becomes........so, in my mind, that makes that border line fps air speed, over the short side more prone to skip off the turn.

On the other hand if that same port was run at a higher altitude,.....yes it would make less power because the fuel would have to be leaned out do to lack of air. BUT the air is thinner aka lighter, as well as the fuel charge would be lighter............If it stayed attached to the ssr(and there was less fuel separation) it could create a better air fuel mixture in the combustion chamber, with the potential of a faster car even though the air speed were the same,(flow bench tested air speed)
The dynamic compression is lower and there is less fuel to burn at a higher altitude..........................but what if that made a cleaner burn(more efficient/complete burn) and there for, made more power...........

am i chasing my tail, trying to get the ssr air speed below 430-450 fps
Is the ram affect suffering when i slow the ssr down to 350-380?

I think about this a lot, can you tell

feel free to pic my theory apart
I don't mind
Really, would like every/any ones opinion on this.

Some good questions. On the Bp I would think yes. Velocity should be higher at sea level pressure vs port velocity at high altitudes like Denver. On a bench if you check velocity at 28" and check at 20" the speed is higher for 28" So I would think the same principle applies to the port at different altitudes in a running engine. However I was informed that velocity does not change in boosted engines over on Speed Talk. Never got an explaination on why but that was the answer feedback. But it certainly changes on my bench. So it should also change at significant altitude changes since the Bp changes.

Flash and I think about the temp density because we flow our stuff in the sub zero temps of the winter. There may be something about how well the air navigates at low temps even though the speeds are the same. Density higher through temps not pressure. Im still pondering this one too.

200cfm wrote
On a bench if you check velocity at 28" and check at 20" the speed is higher for 28" So I would think the same principle applies to the port at different altitudes in a running engine.

Yes, it would be interesting to set up a set of head or just a port, to have "X" fps at "X" lift and @ 28", at the ssr, at sea level, on a correctly calibrated bench.
Then check the fps at 28" at 7000 feet..........
would they be the same?

An even better test would be to have a head that that backed up at X lift at sea lever
then see where it backed up at a much higher altitude....................

This would be and almost impossible task as you would then have to get the air temp/humidity exactly the same as well.

I thought maybe this would help to understand more of what is happening at higher altitude
hope it does not make it more confusing here is the link http://www.grc.nasa.gov/WWW/k-12/airplane/atmosi.html Use the( AtmosModeler Simulator)

I really hope that can help answer some of your question Flash But on the other hand it may raise more Question or maybe of no use here to help.

Malvin,
Thanks so much for putting this link here. Very helpful! I also noticed that you could download the apps there for use on your computer.

The one thing I noticed is the air density has the biggest effect on the speed of sound. At my altitude here in Salt Lake City at 4300 ft the speed of sound is roughly 750 mph while at sea level it would be about 761 mph. So my question is how does this affect my port size? Or does it? If you are aiming at .55 mach in the port for my head, a 23º SBC that would be 418.55 mph at sea level and at my altitude it would be 412.5 mph. Now convert that to feet per second. 418.55 mph = 613.87 feet per second and 412.5 mph = 605 feet per second.

Now this would be in a running engine where the running pressure would be higher than 28" H2O that we use in our flow benches. That is why we are usually aiming at 300 fps or so. As you can see the difference is only a few fps between high altitude and see level.

Fun to think about this and mull it over in our heads.

John

blaktopr wrote: Work the port for correct cfm flow as close to piston demand as you can get.

How would this be calculated?

If you have a program called "Pipemax", put your eng spec's and the max rpm of the eng and it will tell you. if not, here is the math,

Ah, can't find it right now, and late for work.
I'm sure Chris or one of the other sharp pencils will post it up soon

varnish wrote:

blaktopr wrote: Work the port for correct cfm flow as close to piston demand as you can get.

How would this be calculated?

Flow = ( CID * RPM * .000978474 * Ve% ) / ( Cylinders * 127.5 )