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.

Re: air flow #s ?
 

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

Re: air flow #s ?
 

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 :~)

Re: air flow #s ?
 

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.

Re: air flow #s ?
 

While I did not mention it specifically, it was what I was alluding to when I mentioned locations and conditions.

Re: air flow #s ?
 

Wow! Someone finally brought it up.. Thanks Dwight.... Fall fronts are different from Spring fronts. You may have the same DA's, but you can get the same DA's with a totally different set of air/weather readings. If the program is not set up(written) to allow for this, you will see variations in the final #'s. That is just one of those things as well as the human element that keeps numbers from repeating exactly or being exactly the same. Then I might add the computer differences. Not all the same Algorithms are used to do the different programs used for correction. Small differences are there yet the Laws of Physics are not cheated and are still in place.

Re: air flow #s ?
 

Yep; there is a lot involved to get the right numbers and it can get a bit complex many times when you start to apply CFD concepts.

Just attended a class on Computational Fluid Dynamics (CFD) sponsored by NASA and it confirms the many things that can affect flow numbers.
Had an interesting discussion with the instructor on how temperatures can effect the final numbers and behavior. Also discussed the importance about velocity and its effects on obtaining real CFM numbers..

Re: air flow #s ?
 

spSS6 I was going to say that NASA was probably the only group that really get it and do it right. The real problem is we (the avg guy) can't afford to buy the resources they have. We have to use the systems/programs made available to us at the affordable price. (And some not so affordable)
In a perfect world what Kevin is saying could and should happen, but we don't live in that kind of world. I'm not saying everthing is wrong. I am saying there are those among us that try to make things (corrected numbers) closer to right or as close to right as they can be. Then there are others (salesmen that usually don't understand) that use the numbers just for a sales pitch. I'm sure it's tough to be a customer and wade through all of it.

BTY: There are two books I keep on my headboard for those non sleeping late night reads.
Books from my college Fluid Dynamics & Applied Physics

Re: air flow #s ?
 

Adger,
NASA has these CFD seminars for free on the web several times a year.
They are called "Webinars" and they are taught by a company named COMSOL and sponsored by NASA.
Next time I get an invite to attend another CFD Webinar, I will post it here so anyone interested can sign up and attend...and it is free.

Re: air flow #s ?
 

Great! Thanks. Nice to know that . I'll never be too old to learn!

Re: air flow #s ?
 

Gosh, Those guys are really into the training, even worldwide..
Did a Google on them. Interesting!! I might have to dig back into my 40 year old College... LOL!

Re: air flow #s ?
 

Absolutely the weather correction is key. But correcting simply for density will get you 97% there. Yes, there are small differences. But there should not ever be large differences if both flowbenches are quality pieces, calibrated and properly operated. Of course you will see small variations... But a while ago, there was a dicussion that so and so's flowbench reads 275 CFM, while some other guy's reads 250 Cfm, both at 28"... THat is just plain garbage data there... One of them (or both, with one higg and one low) is clearly WAY off.......

And Yes, NASA, Boeing, Airbus, and any steam turbine/gas turbine manfuacturer gets it, and does it right, thats for sure.

And regardless of the density, the square law is... well a LAW :D (sorry, had to get that in one last time ;) )

Good conversation, gents.

PS: Adger, I have the same sort of books on my nightsand too... My wife thinks Im nuts... She says "nothing like a little "light" reading before bed, huh hon ?? " ....


Kevin

Re: air flow #s ?
 

Funny how so many of us are cut from the same Cloth. & yes my wife will be reading her favorite paperbacks when I'm into the "HD" reading!! LOL!

Re: air flow #s ?
 

Going from memory here, but the 25, 28, 35, etc debate is related mean inlet Mach number. Higher test pressures I think will be more realistic to real world conditions, especially at high rpm/flow numbers or low lift flow.

Re: air flow #s ?
 

Chris,
You are correct in your statement.

Re: air flow #s ?
 

In my case, I am sent to sleep in the guest room. :)

Re: air flow #s ?
 

great discussion. let me throw this ? out. if you had a port flowed at 25" and let's say it flowed X.CFM, then changed to 28" [all else remaining equal] and checked the port, would it flow more or less?
thanks Rod in AZ

Re: air flow #s ?
 

Sure, it would flow more, and to be precise, it would flow the square root of (28/25) times the original X cfm..

And that right there is what started this thread off in the first place !:D The square law of compressible airflow !!! ;)

Kevin

Re: air flow #s ?
 

Kevin,
Using math to extrapolate from 25 to 28 is correct. The laws of Physics allow us to do that, but you assumed there was nothing wrong (called a Problem) with the port that the increase in air speed didn't magnify or make show up.
There, that is the Real World problem with extrapolation and with assumptions.
As I've said or aluded to.... That is one of the reasons why there is a lot of salesmanship in below 28 numbers. There is no way in He** I would buy a head that was flowed @ 10 or 25 or anywhere in between and the numbers extropolated up to another flow number. That is IF I was buying based solely on flow numbers (which I wouldn't do, not a good idea!)

Re: air flow #s ?
 

It's kind of like using barometric pressure, humidity and temperature to predict ET without taking into account oxygen content in the air.

Re: air flow #s ?
 

x10!!!