PTS Flowbench Forum

SWAirflowServices wrote: It's funny that you say that. I just had this discussion with the guy who taught me to port. People are always so focused on CFM and Velocity and I don't really see people talking much on the fact that not only does the valve close each cycle and completely shut off the flow but also that the piston accelerates the flow and decelerates it as well.

<<Groans>> Please don't make it any more complicated than it already is!

SWAirflowServices wrote:

jfholm wrote:Also watch the fuel coming in and how it sputters at the end of the intake cycle and the beginning of the intake. If you hit the pause button as it goes through the cycle it is interesting what you can see.
John

It's funny that you say that. I just had this discussion with the guy who taught me to port. People are always so focused on CFM and Velocity and I don't really see people talking much on the fact that not only does the valve close each cycle and completely shut off the flow but also that the piston accelerates the flow and decelerates it as well.

Think of how a turbo pressurizes air, then think of the port and it's velocity profile.

blaktopr wrote:Think of how a turbo pressurizes air, then think of the port and it's velocity profile.

The charge pressure from a turbo is relatively constant from one intake cycle to the next so I don't see how that relates to the acceleration and deceleration of the air column from the piston. Are you possibly referring to the build up of pressure on the back side of the valve from the velocity of the air column when the valve closes?

Yes. I say things like that to kinda simplify them. But I see that you took it both ways. Thats cool. But yes to a certain amount. As the mixture expands through the bowl, it increases in pressure. Shut the valve and with inersia, and Bernoulli, (compressible flow), pressure is built up behind the valve. Then depending on RPM, there is the wave front too.

Here is a long shot using smoke and using it with dye like Flash said and possibly seeing this phenomenon. This is what I thought. Do a 1/4 scale port in acrylic with a valve and a chamber. Set up a way to pop the valve like with a cam. Spin the cam to 3500 rpm. Put it on the bench and crank up all the motors with the valve open. Get a high speed camera. Turn off the lights and start filming with the cam poping the valve and the dye smoke. You may be able to see all this happening when you watch the replay. Anyone know someone with a high speed camera?

Out of interest why a 1/4 scale port, I would have thought actual size easier and better?
I ask because I started work on my clear model of a head and am just waiting for some more materials to arrive, I'll start a different thread later on it so you can make comment.

SSR. I figure a smaller port for depression. Look at it this way. Say a bench can pull 450cfm @28. I know you can pull 110 cfm @50+ inches. Not only that, less material to work with. One would just have to shrink the numbers. It is not like you have to recreate an existing port, but something to see what happens with port to deck relationships, short sides, etc. especially if going to try using the valve and high speed camera.

Please do post with the clear port.

Smaller ports = higher depression, higher cfm and port speed which means we are closer to a 'real life' situation right? If that is so I understand and agree - thanks.

"It is not like you have to recreate an existing port"

Yes I do! I work this head more than any other, so it will be useful to see it in action like I have never seen it before.

I'm going to do this at actual size because of the work involved in making a scaled down version and because I'm most of the way there already. Also, I'm kind of 'tooled-up' for doing this size of port at the moment, although I have done a few successful motorbike heads in the past I find them very fiddly and time consuming in comparison. I'm not 100% sure how the mould will work out too so we'll call this Mk1 and maybe I'll do some variations later.
I've got this port in various stages too and I've been pondering which to use, I think I will use the most advanced one.

SSR wrote:Smaller ports = higher depression, higher cfm and port speed which means we are closer to a 'real life' situation right? If that is so I understand and agree - thanks.

Right