Velocity or Suction?

[nm9stheham]

OK, so I have been around for a few decades in this and always accepted the idea that flow velocity is important in cylinder head ports. But I got to wondering if flow velocity is the really important thing or is it suction through the ports that is important. I don't pretend to know the answers here... just looking for knowledge.

Port flow is not constant unless you are high enough RPM to get near to a resonant situation. Flow starts-stops-starts-stops..... So if you are at lower RPM's and the port flow has to be started when the intake opens, does a smaller port allow the cylinder suction to work better to start drawing the A/F mixture? Is it like drawing a drink through a straw....too large a straw makes it hard? (Forget milkshakes.... that is viscous flow!)

Or, is it really flow velocity that counts? This would seem to only be effective in the closing half of the cylinder filling cycle, to help jam more mixture in after it gets flowing. A low velocity, large air column would have less kinetic energy in it (1/2 mv^2)than a smaller, high velocity air column..... (but you have to put more energy into it to get to that state).

And, do we talk about port velocity because that can be measured on a bench in a controlled, repeatable fashion, and that somehow correlates to better performance aspects, but is not really what is happening?

Like I said, just looking for knowledge....tnx.

 

[RustyRatRod]

What difference does it really make as long as the durn thing runs? Now you've opened up another ten page thread so the "experts" can argue.

 

[moper]

Look a little deaper. I think you've oversimplified "suction", and I think you're confusing testing vs reality. We can test using certain parameters and tools ie. flow benches and pitot tubes. We cannot even measure some components that go into "reality" because there's so much going on that what we would measure with can't function properly. Even the best engine dynos are still sampling x-number of times a second and using preset values and software that will "connect the dots" of those millions of samples in order to produce a result we can interpret.
Flow test results give an idea of how a port might work in a form we can easilly compare to other ports. Velocity is one of those indicators, although overall port velocity doesn't mean much aside from more cfms indicated. It's localized velocity within a port that really makes things work (or doesn't).
Sometimes you just have to accept it...lol

 

[nm9stheham]

Yeah, I know, RRR... not just trying to start a new drama thread....LOL. Just trying to gain knowledge to understand and predict things better. If you can believe it, I understand things like how the electromagnetic field impedance changes from an antenna to free space (crazy, hunh?).... so I want to improve my engine breathing concepts.

Yeah, Moper I know I have not hit it right on the suction idea.... I just threw out my early thoughts for discussion. But again, to accept the test results and never look deeper into the real what's and why's does not advance one's knowledge. I will give some thought to the localized velocity idea; I do understand that flow is great in the middle than at the walls from other flow theory.

What you say about tested velocity being an indicator of performance is where my thinking is at this time. But you see folks talking about how better flow velocity from smaller ports makes for better low end performance. Higher flow velocity from smaller ports and runners implies that something close to the same air charge is being pulled through small versus large ports/runners, but smaller ports/runners have higher flow resistance. So why would the same or greater charge be pulled through a smaller port/runner? And don't say 'because the velocity is higher'! I want to know why the velocity is higher....IF that is indeed the case. Or the low end improvement due to something else, like moving closer to a resonant situation, or the cylinder suction 'coupling' through the system to the intake plenum better with the small ports/runners at low RPM's with longer cylinder "dwell" times?

Thanks for the replies. No problemo if there is no interest in this here. Mark B.