Tractorsport Flowbench Forum Archive

Posted: **Thu Dec 18, 2008 9:36 am**

As much as I love my flowbench, ive always thought of it as rather an unrealistic test to the operation of an actual dynamic engine: testing steady state cfm, at one lift, at one depression, into an infinite volume.
But its pretty much the best we can do...

Im in the process of ordering a pitot tube and digital velocity gauge to further my head developments, and it got me thinking more about the reality of what we are testing.
Ive been reading around, and it seems that velocities of around 300-350FPS are typically recomended. Why these figures? I guessed as this is when flow seperation occurs around ssr etc. Any other reasons?
From this, I cant really see how any specific velocity on a flowbench can be applicable to a running engine. I mean, say you fettle a head to give an even spread of 350FPS, what relevance does this velocity have to a running engine? I.e say you found peak port velocity on a running engine to be 350fps, there is obviously going to be a large drop in this as piston slows down towards/away tdc and bdc...so effectively you may have the mean velocity in the engine to low, whereas on the bench "its alright".
Or say you tweaked a head on the bench for a set FPS, then stroked the engine. This would obviously increase the dynamic port velocity. So what then?

Also what lifts and depression do you test velocities at? Is there a standard practice?

Im really trying to think how you can best draw a fixed relationship between dynamic velocities and ones we measure on the flowbench. The best solution I can come up with, and the one im going to employ myself, is to use one test engine, modify several heads for it, with several different velocity profiles, then test these on my engine dyno for solid performance data. Then from this I can better my understanding of a steady state flowbench velocity profile with regards to its relationship to ACTUAL performance data.

I guess im thinking out loud alot here!! but any comments/thoughts are welcome.

Craig

Posted: **Thu Dec 18, 2008 11:45 am**

[color=#000000]Larry Meaux has had some very good threads on this. The velocity in FPS is taken or converted to 28" h2o if you are not flowing at a test pressure of 28".

You usually take your velocity readings at near full lift.

Also the 300 - 350 fps is dependent on what type of head you are running. A 23

Posted: **Thu Dec 18, 2008 12:03 pm**

Thanks for the reply.

This kind of typifies my thoughts on it though. Why test velocities at near full lift? It doesnt really coincide with whats actually happening in a running engine??
And why better at 260FPS?

Im not questioning you personally

rather the understanding behind these figures, as I hate taking things as read.

Am I missing a big part of the picture here, or is this genuinly questionable stuff?!!!:p

Posted: **Thu Dec 18, 2008 1:47 pm**

near full lift because at lower to mid lifts the valve is in the way restricting the area and you are not using the full csa of the port until area convergence the fatsest air will be at the seat to valve relationship also max piston speed occurs depending on your combination roughly at 70-75* atdc at this point valve lift is generally 80*+ lift and this is where port shape seat angle and cross section have taken over on flow control and the air col needs to keep up to piston speed without being to slow {less ram effect at ivc} or to fast and limiting flow

Posted: **Thu Dec 18, 2008 2:26 pm**

Posted: **Thu Dec 18, 2008 4:41 pm**

Has any one here tried putting an AVG Pito in the intake runner close to the head intake parting line? I would think that if you had a decent number on your CSA for the runner in this area you could get a pretty good approximation on flow and air speed for that port at WFO.

So you test on your current setup get data, Port your heads and record all including port mapping. Derive theoretical bench racer improvements I/E 25%. Then next time out, repeat the above and compare. Did the in car improvement get close to the bench racer improvement?

Rick

Posted: **Thu Dec 18, 2008 6:49 pm**

Thanks for the replies.

Seems logical regarding testing near full lift.

Larry- ive got those Excell spreads, thanks. I also bought Pipemax a while back and have had various sessions on that. On my currently limited dyno testing with my heads, the MSCA it recomends ties in nicely with "good" dyno figures.
I guess its a case of gettign stuck further in and getting my hands dirty(er)!

1960FL--With regards to what your saying here about probing near the intakeparting line, are you reffering to on a running engine??

Posted: **Thu Dec 18, 2008 7:22 pm**

Rick, in addition to your thought, how about a top and bottom or 4 corners. Then can something like the flow computer map the speeds during a run??? Onto a computer??? Then to see how it correlates back to bench. Does it have to be avg tubes or can a basic tube work (alot smaller) slightly into the port, air speeds going across the end hole??

Craig, once you start probing you will begin to see for yourself how the ports behave. The info will begin to wrap around your brain.

I been trying to test based on a cam lobe design and go with a percent of full lift. Larry M has beat that topic to death and I ended up really looking at that very closely. I been looking at areas of around 90% of full lift depending on cam. Getting the port to work during the highest lift/area curve of a lobe. My .02$

Chris.

Posted: **Thu Dec 18, 2008 7:55 pm**

Its not so much how it behaves that bothers me, its how it behaves on the bench compared to how it behaves on a running engine. I.e. making best use of the resources by making worthwhile changes to the likes of velocity, then corrolating that and trying to apply it to various heads.

As for probing on a running engine, I would guess that you would have massive issues with sampling rates to obtain anything other than a peak value?
one thing that came to mind whilst thinking about this a while back, was hot wires across the port cross section...dont know how useable this method would be....

Posted: **Fri Dec 19, 2008 3:47 am**

Tractorsport Flowbench Forum Archive

Port Velocity probing/mapping thoughts