Flowing at higher depressions

[jakesportingservice]

Hello , I have question that i have been trying to find a answer to but i just haven't found it yet .
Ok here goes I am working on a 15 degree gm sbc head part# 615x to be exact and i was flowing at 28" depression and it flowed 370 cfm @ .700 lift and the port velocity looks great averaging around 300 fps . Now when i flow this head at 35" depression it flows 440 cfm .

Heres the question when i convert this back to 28" it says 393 cfm . and when i convert from 28" to 35" it says 414 cfm would anybody have an answer as to why i get such different numbers ? Should the cfm be closer than that when converted ?

Any help with this would be appreciated.

Thanks Jake

[jfholm]

The port is more efficient at the higher depression - maybe having problems at 28" - Larry Meaux has mentioned this quite often. This head on a real engine may be better than you think. Now if it was opposite and lost flow when you went higher then I would be more concerned.

John

[jakesportingservice]

Thanks John that was one of the answers i was looking for .I have read somewhere that Larry flows all of his stuff at 36" and converts back to 28" and he says he's been doin it that way for years. Hmmm maybe that is why his stuff runs so well.

Thanks Jake

[coulterracn]

[stef-1]

If you read aero and fluid dynamic books you will find that any orifice/nozzle/pipe etc. will have a better discharge coefficient as the pressure ratio across it goes up. Same should apply to the port. I bet if you calculated the discharge coefficients for the port at 28" and at 36" the higher test pressure would show higher discharge coefficients.
As mentioned above, if the port shows a lower discharge coefficient at a higher test pressure it's not a good port.

[jakesportingservice]

Thanks for the replys guys this is getting more interesting the more we dive in the more we learn and i guess that maybe we should look at flowing things at higher depressions than normal. I've been at this for awile but i learn new things everyday thanks to all the great people on this forum.


Thanks Jake

[emarsh]

Just out of curiosity, does anyone have any idea what the average pressure drop is across a port in a running engine at WOT in the real world?

[jfholm]

Well your Holley carbs are rated at 1.5" HG (Mercury) and I just looked at a thread over at speedtalk.com and the opinion there was that a correct sized engine should be pulling about 1.5" hg at WOT. Now you just need to convert that to inches of water. 1 psi is 27.67 inches of water or 2.0345" of HG (Mercury).

So basically 1" of HG equals 13.57 inches of H2O. So times 1.5 X 13.57 = 20.355 inches of H2O.

Trouble is it's not that simple. In a real running engine it is not steady state like that. The actual depressions in the port etc varies depending on rpm, valve opening etc.

I think that was where the original standard of 28" H2O came from. A lot of good engine builders are now flowing at much higher depressions, i.e. 36" or higher and getting better results when installing the heads on an engine.

John

[Rick360]

[jakesportingservice]

Thanks for the answer rick360. When I posted this question it had me puzzled but i have since reflowed the head at both 28 and 35 and i found the problem with it flowing higher with more depression. Error on my part that the exhaust valve was fluttering open at the higher depression. Didnt catch it on the first test but on the second one i caught it . Now when i convert the numbers are very close to what they are @ 28"

Thanks Jake

[jfholm]

Rick,
Thanks for the explanation. You have given us an answer that I had also heard else where but could not find the references. I have also heard that some F1 teams are flowing heads at way over 100" H2O. Now that may be an urban legend but backs up what you have stated.

Thanks Again,
John

[maxracesoftware]

[jakesportingservice]

Thanks for all that great info . I'm glad there are so many fine people here on this forum that are willing to share their info with the testing to back it up.

Thanks for your help , Jake

[maxracesoftware]

[msj442]

hey bruce!!! got any high depression flowbench blueprints??!!! :;):

max