[steve25]

Just keep in mind that a highly developed induction system on a pro stock motor or a Indy cars motor can see up to a 120” H2O depression.
But this has to be tempered with average numbers also.

I don’t know if they still do, but Ford has tested at 65” which is in my opinion why they thought that the oversized intake ports on a Boss Cleveland head where the way to go on street 302 cid motor.

Pontiac at least though that those same size ports where good for a 303 motor that could buzz to 8200, but even that was wrong!

Just keep in mind that a steady state flow bench test does not show the effects of the cycles that a running motor places on an induction or a exh system.

A good marker for making HP in regards to induction system that’s tested at 28” is that for every 100 hp your looking to make you then need .100” of lift at the valve.

[GTO-relic]

Smokey's home made bench could pull 1500 cfm at 28" back in the 1960's. He had the most powerful flow bench in the entire USA. There was no shortage of flow capacity power whatsoever. The machine was a monster. Another description made 1998:

"I went to Detroit on Hudson business and get to examine equipment used to check various parts for air flow. I want to buy a book with pictures, dimensions...that tells me shapes, sizes. I'm shocked to find out some manufacturers don't own or use air flow equipment. Some like Buick, Olds use equipment that ain't worth schit. Chevy air flow bench 1958 was a 6-71 supercharger driven by 10hp electric motor- so help me God. I sat down called every place that had anything to do with air flow equipment, even the aviation engine people. No one made anything that was acknowledged accurate in a 7,000 rpm range. I bought 5 SAE papers on air flow- but I became confused, all were different from each other. By 1960 I decided nothing published could be proven correct...decided to figure it out for myself. I decided to be my own counsel in the experiment- discussions with peers distracted, confused me. Back to the peyote.
I decided to get a pump big enough to flow 1500 CFM of air at 28 inches water, negative pressure, vacuum mode. I got this CFM number guesstimating the biggest engine/highest rpm I would ever test. In next 2 years more shock treatment- a company called Stuterbuilt makes such a pump, but it takes a 100 horsepower electric motor to turn it, and $25,000 to own one. Well I've sold an invention and got $50,000.
One year fast forward- flow bench is complete- dry flow/wet flow. I can handle stuff up to 75 cubic inch per cylinder, 8,000 rpm. $85,000 is now gone.
BUT I'VE GOT THE BIGGEST, MOST POWERFUL, MOST ACCURATE FLOW BENCH IN THE UNITED STATES.
THIS FLOW BENCH RUNS DAMN NEAR YEAR ROUND FROM 8AM TO 2AM FOR 20 YEARS, 7 DAYS A WEEK.
This SOB makes so much noise, it takes a pair of mufflers that weigh a ton each to half way quiet it down to about 100 decibels at 20 feet.
I quickly learned best control valve was like a camera iris, the best port was straightest, that air flow around a valve was controlled by subtle things like radius, sharp edges/angles were better than a radius alone if over 9 degrees of direction change was required. I learned there is no way to change one parameter, without adjusting every other variable.
Most importantly I learned the Otto cycle engine, no matter how perfectly done, would never give maximum performance longer than a narrow rpm range- unless valve timing, port shape/size/length could be altered on the fly.
I LEARNED THE OTTO CYCLE ENGINE COULD NEVER BE PERFECTED.
It is a basket of flawed systems. But it's also as of today August 1998, the best most viable system known to man, considering every aspect/parameter to be met, to be state of the art of viable surface transportation.
I learned the Otto cycle engine would be replaced with a machine at least 2-2.5 times better in thermal efficiency. I learned the entropy of the Otto cycle is flawed because of the tremendous increase in the size of the combustion cavity.
But man...for a quicky deal about 100 years ago, we owe Otto a helluva lot more than we gave him.
I've learned it's possible to have roughly no emissions, and consume nearly all the hydrocarbons (with external combustion), but in a confined environment.
I learned if we don't consume all hydrocarbons completely, this world won't be fit to live in, in 100 years. You know what 100 years is in real time ? About 100 seconds.
Same goes for the carbon in wood and coal, or any other combustible solid.
I WANT YOU TO CONSIDER THIS ONE SENTENCE CAREFULLY.
Water is wonderful as it is, and as we understand it. Maybe one day in the near future, water may contribute a solution to our current trajectory of world genocide by atmospheric pollution.
Well, my flow bench got to meet many various engine manufacturers engine components regarding air/fuel conduits, cylinder heads, intake/exhaust manifolds, carburetors, air cleaners. Small, affordable flow benches are now offered to the racers. Nowadays all racers of any stature have a flow bench. So air flow is as big a topic in race publications as chassis technology. It's too bad most of it is bullschit.
The technology of air flow is no different than a liquid. Air is of course not possible to totally observe visually, but the physics is the same- the difference being the specific gravity of each. I put the "what if" to it. What if you colored the air ? Yup, it worked. You can see what's happening.
I could teach you in 2 days how to build correct size intake/exhaust manifolds, and correct size carburetor, if the parts were not cluttered up with pistons, cylinder walls, head bolts, water passages, push rods, and rpm range.
It turns out the best pushrod cylinder head will be the best known series of compromises. Assuming all parts are not adjustable, the maximum operating range will at best be 300 RPM for maximum efficiency."- Smokey Yunick 1998

[GTO-relic]

[QUOTE=Mr_GTO;2200399]Could someone just explain that in detail for me. Thanks. [img]/infopop/emoticons/icon_biggrin.gif[/img]

Being no one else went into detail with the OP question, I will- it's a good question, with an even better reason why, 28" became the standard. It was a long, arduous, expensive, time consuming, cut-and-try method, discovering a hidden secret of physics: how a flow bench has a sweet spot test pressure, that directly correlates with how a naturally aspirated engine inhales air/fuel, using atmospheric pressure, and creating vacuum. THEY FINE TUNED THE FLOW BENCH RESOLUTION TO THE DYNO RESOLUTION, AND MADE BOTH LOCK TOGETHER, CORRELATE. They went on a treasure hunt out of curiosity, and struck paydirt- because nobody else up to that point including the Big 3 automakers really knew how to do it:

"Next thing we had to determine how much pressure depression- 'vacuum'- to use pulling air through test fixture. When we started our research, we found most flow testing at other facilities was being done at around 10"-12" of water. So we followed along with everyone else. Before long we changed our mind about this.
Flow balance between all head ports is important. You want each intake port to have the same operational efficiency, so each cylinder receives the same quantity/quality of mixture. To confirm this, you must flow all intake ports and compare them.
WHEN WE WERE FLOWING PORTS WITH A 10" WATER DEPRESSION, WE FOUND WE COULD MAKE VERY DRAMATIC CHANGES BETWEEN 2 PORTS, AND YET THE MEASURED AIRFLOW WOULD NOT CHANGE SIGNIFICANTLY.
We then decided to undergo a series of tests to detect valid differences between ports. We started with tests at 10" water and checked airflow variations between ports that were OBVIOUSLY DIFFERENT. Then pressure drop was increased by 2" and same test performed again. This step/repeat procedure continued all the way up to maximum drop of 34".
When the test series were completed, we found at any pressure drop less than 26" water, there was little discernible variation in port balance. BUT AT LEVELS ABOVE 28" , DIFFERENCES REALLY STARTED TO SHOW UP. In addition we noticed as drop was increased beyond 28" water, the percentage of change- per increase in drop- was quite small. SO WE SELECTED 28" WATER AS A BASELINE LEVEL- and started checking results with back-to-back dyno testing.
We would measure flow on bench, make a change to improve flow, take the head to dyno to see if engine performance actually improved.
After considerable testing we became convinced the correlation was almost totally linear. Virtually every improvement on the bench (at 28" water), would show up on the dyno as a measurable increase in engine performance. For the past 30 years, all our flow testing had been done at 28" of water depression."-
Smokey Yunick 1983