Thought this was an excellent explanation as this question comes up from time to time;

From a post off Meaux Racing Heads forum with permission from Larry to post here

MaxRace Software

ET_Analyst ( DragStrip Simulation Software )

PipeMax ( Engine Simulation Software )

http://www.maxracesoftware.com Quote:

Why is 28" any more valid then 10" if it still isnt anywhere near real world depressions for accurate data

if every Port shape were a perfectly straight round tube that magically did not need a Valve and Stem inside, 10" WC would be about all you needed to get good accurate Data.

10"WC will still give you decent Data in most Port shapes

28"WC will give you more accurate Data in a greater majority of Port shapes

36"WC is still even more accurate, very seldom will the FlowBench fool you at 36"WC +

and from what i've seen, 45"WC to 48"WC should be more the Flow WC Standard than 28"

10" Water Column Height (WC) = 209.343 fps or 142.73 mph potential molecular speed in the centerline of a perfectly straight round tube

28" WC = 350.297 fps or 238.839 MPH

48" WC = 458.647 fps or 312.714 MPH

105" WC = 678.348 fps or 462.51 MPH

Analogy->

Suppose you are designing and testing a Formula One RaceCar , and you do all your Suspension/Tire setups and testing at 10" WC or 142.7 MPH well you will get good Data but probably not good enough to Win the Race .

at 142.7 MPH is probably going to be slow enough speed that just about any good Formula One Car can handle a normal shaped Curve without spinning out and blocking out most other Cars in that Turn.

Now suppose you Test and design for more realistic MPH Speeds of Formula One and you Test at 28" WC = 350.297 fps or 238.839 MPH and now those same Curve "Shapes" that you easily went around at 142.7 MPH, now at 238.8 MPH you spin out or barely make it thru.

i know the above is not the Worlds best Analogy to give you , but i think its the easiest to understand ?

Now suppose again you have the ability to change the Road Course into a straight line, which is analogy to straight round tube, then it becomes a DragRace and all the Cars ( molecules) don't have much a problem going straight at 142.7 MPH to 238+ MPH , so its who gets to finish line first , Wins

getting back to Curved Shapes,

lets make the Formula One Cars tow a Trailer with heavier Fuel on it ( Wet Flow ) and see how fast and how many Formula One Cars that can make it around a Curved Shape ??

FlowBench = Dry AirFlow testing , no heavier Fuel suspended in AirFlow stream trying to make a turn in your Tests.

there are some Port Shapes that definetly need to be tested on a FlowBench at 45" WC upwards to get accurate enough Data,

and

more Port Shapes that when tested at 28" to 36" WC give very accurate correlation to Real World

Another major problem with a given FlowBench Test Pressure is that Test Pressure is the theoretical average pressure thru the Port CenterLine or the theoretical average Speed thru the Port Shape,

but

because a Port has a Valve/Stem and Curves in its Shape, and changes in CSA area, the actual local FPS speeds will be drastically different than the theoretical speed.

Lets say your Flow Test Pressure = 28" and use Pitot Probe your Port Shape in many areas inside, and you can see Pitot speeds/pressures 10+" inches higher than your Test Pressure.