air flow #s ?

[Alan Roehrich]

The 28" of water pressure drop is the industry standard.

However, recent testing shows that even 28" not really adequate or ideal for state of the art race heads. A few guys who port Stock and Super Stock heads say that applies even more to Stock and Super Stock, given the restricted nature of the rules.

Further, if you know anything about a flow bench, you know it is like a dyno, and comparing results from one flow bench or dyno to results from another flow bench or dyno is nearly pointless. A few years back, several flow bench owners got together and sent a single calibrated orifice around and used it to calibrate all their flow benches. The results were eye opening, to say the least.

A flow bench, or a dyno, is merely a tool, best used for back to back testing to verify work done or parts bought.

To answer the original question, in all honesty, introducing any sort of extra correction factor only adds to the error margin. If you're "correcting" results from one bench, on one set of heads, in order to compare them to results from a different bench on a different set of heads, you're wasting a lot of valuable time.

[Kevin Panzino]

Sorry, but I respectfully disagree. A flow bench which is not calibrated to measure true CFM is worthless for other than relative port work for your own usem and yes it wont correlate to other flowbenches which are properly built.

Good flowbenches will read identical. A SCFM is an SCFM, and a precision flowbench is just that, a precision measuring device that will be calibrated to indicate the exact CFM given the current atmospheric conditions. If two flowbenches do not read the same corrected flow, one of them is not of high quality, or is out of calibration. Period.

And correction between two test pressures introduces ZERO error. Its nothing more than the laws of fluid dynamics which are well known, established and accurate anywhere in this universe.

[Ed Wright]

Good column by David Reher in the latest National Dragster. It mentions something I have heard other top engine builders say about CFM. It's not the last word. As with dynos, we also don't race flow benches.

[Adger Smith]

Kevin,
If 25 is so good then why do good head guys/shops flow at 36 & 48 in developing a port?
No longwinded double talk, just a straight answer will do.

[Kevin Panzino]

Originally Posted by Adger Smith

Kevin,
If 25 is so good then why do good head guys/shops flow at 36 & 48 in developing a port?
No longwinded double talk, just a straight answer will do.

Because they want to.

[Alan Roehrich]

Originally Posted by Kevin Panzino

Sorry, but I respectfully disagree. A flow bench which is not calibrated to measure true CFM is worthless for other than relative port work for your own usem and yes it wont correlate to other flowbenches which are properly built.

Good flowbenches will read identical. A SCFM is an SCFM, and a precision flowbench is just that, a precision measuring device that will be calibrated to indicate the exact CFM given the current atmospheric conditions. If two flowbenches do not read the same corrected flow, one of them is not of high quality, or is out of calibration. Period.

And correction between two test pressures introduces ZERO error. Its nothing more than the laws of fluid dynamics which are well known, established and accurate anywhere in this universe.

You are entitled to your opinion.

However, brand new expensive flow benches, properly calibrated, will not read exactly the same, especially in different locations, even with the same calibration orifice. Compensation factors are not perfect, either.

In talking to a couple of Super Stock cylinder head guys, who do heads for very fast cars, I learned that they have found things at 36" or more that were not possible to see at 28".

You may take that for what it is worth. I know them, and I know how fast their stuff is, so I have no reason not to believe them.

[Kevin Panzino]

Originally Posted by Alan Roehrich

You are entitled to your opinion.

However, brand new expensive flow benches, properly calibrated, will not read exactly the same, especially in different locations, even with the same calibration orifice. Compensation factors are not perfect, either.

In talking to a couple of Super Stock cylinder head guys, who do heads for very fast cars, I learned that they have found things at 36" or more that were not possible to see at 28".

You may take that for what it is worth. I know them, and I know how fast their stuff is, so I have no reason not to believe them.

We can just agree to disagree. I respect you. And I respect guys who build fast cars.

But I'll stand by the fact that the square law of flow vs pressure difference is an unarguable law of physics so long as the port flow is already fully developed.

[Jeff Lee]

all sound mathematical reasoning. But don't forget the first law...
Garbage in, garbage out!

[Adger Smith]

And don't forget there are other things at play where the laws of physiscs don't apply. Kevin, On the Speed Talk Forum We did a pass around of a calibrated flow plate. Some of the guys were at The Engine Masters Challenge and spawned the idea. It is interesting the different #'s that were obtained all across the country. Most of the guys know a lot about what is going on & why.
I can tell you that a guy wanting to "sell" is more apt to use the 25 number than the 28. There are many reasons for that besides a big #. It is a shame that this industry has the "big is better mentality".
I've had ports that are very bad have better numbers at 25 than 36 or 28. Simply because of twist and turns that cause separation and swirl. You can't even tell the ports are bad until you step up the depression. (also steps up the FPM of the air flow) I don't think there is any number that can be considered the "Best" flow depression.
Hey, What is the best wine? Depends on the meal, doesn't it??
BTW: Just look at all the different numbers these carb salesmen use to sell their work. In the real world they mean nothing with the high FPM flow through a carb.
Yep, It's My .02 :~)

[Dwight Southerland]

Nobody has mentioned the quality of the air. Even with accepted procedures for correcting to STP, I have found significant differences in my own shop depending on the time of the year and even from which direction the current weather front has come from.