air flow #s ?
 

if you had a flow sheet for a set of heads and the test was at 22 inches of water...is there a correction to get flow #s at the now standard 28 inches of water?
Thanks Rod in AZ

Re: air flow #s ?
 

Yes, Take the square root of the pressure difference (28"/22") and multiply that with the flow number.

Example:

You have a set of heads that flows 215 CFM at 22". They would flow 242.6 CFM at 28"


And by the way, I contend the 'standard' is still 25"...
28" only came to be because of unscrupulous head companies that were looking to be able to advertise more CFM than their competitors, with the idea that most folks wouldnt understand the difference between 28" and 25".....

And testing at 28" offers no advantage at all over 25" , or even as low as 10" for that matter (the average cyl, head port easily has fully developed flow even at 10" H2O.....)



Kp

Re: air flow #s ?
 

I remember reading years ago that Smokey Yunick started the 28" deal after doing some testing at different pressures. Some experts say that 36" is the best test pressure but my flow bench is complaining at 28".
thanks and regards, Roland

Re: air flow #s ?
 

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.

Re: air flow #s ?
 

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.

Re: air flow #s ?
 

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.

Re: air flow #s ?
 

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.

Re: air flow #s ?
 

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.

Re: air flow #s ?
 

Because they want to.

Re: air flow #s ?
 

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.