MaxRaceSoftware

as the pressure difference, for cylinder head flow
measurements. 28 inches of water, is close to 1 PSI.
.
One psi does not seem nearly enough, to get
the air moving Mach 0.5 , where the interesting
stuff happens.
.
I would guess that the partial vacuum in the cylinder
would be a lot more than 1 psi.
I'm not saying the 1 psi measurement is useless,
only that real pressure differences are higher, and
that things don't scale when you get to Mach 0.5 .

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Z-Factor Inlet Mach Index
the "Best Fit" to Taylor's data regarding Induction System temperature
was approx. = 105.5 degrees F = approx. Speed of Sound= 1165 feet per sec

Taylor recommended Inlet Mach numbers from .40 to .60 Mach
from his data with various Engine types, it was discovered that
an Inlet Mach Index of .50 to .60 was "Controlling" ,
or in other words, was the RPM-HP limiting factor .

the Z-Factor Inlet Mach Index, works better with 2,3,4,5 valve Heads
with relatively low lift cams or low L/D Ratios

.60 Z-Factor times 1165 fps Speed of Sound at 105.5 F = 699 fps
DeskTop Dyno 's Book also state a limiting FPS speed of 700 fps..which is
very close to 699 fps, probably just rounded-off to 700 fps.

700 fps = 111.81 Inches of Water Flow Test Pressure
also 700 fps = 4.038 psi pressure differential = 127.5 % PerCent Potential Volumetric Efficiency

There is a tremendous amount of Research SAE Data by individual groups and Companies like GM and especially Honda SAE research papers,
and also in Books like Philip H Smiths and Taylor, all "correlating"
to the fact that .50 to .60 Mach is the "Controlling" or RPM-Power limiting
speed . (.55 Mach is average)

.50 Mach = 582.5 fps @ 105.5F = 77.4 Inches of Water
.60 Mach = 699.0 fps = 111.5 Inches of Water


700 Fps times .5 = 350 FPS , and 350 FPS = approx. 28" Inches of Water

Flow Testing at 28 Inches of Water is roughly "half" the Air Speed Velocity
in Live Engine conditions.

77.4 + 111.5 Inches = 94.5 Inches of Water average depression at Z-Index
to get a more realistic picture of Engine CFM demand thru an Intake Port or Induction System it would be better to simulate or Flow Test Cyl Heads
somewhere between atleast 60 Inches to as much as 120 Inches

it would seem the "Ultimate FlowBench" would be one that was capable
of "Wet Flow" testing at 120" Inches of Water Test Pressure

Flow Testing at only 28" Inches simulates an Engine fairly closely,
but 28" also is similiar to 238.8 mph
and a "Live Engine" at .55 Mach Choke = 436.9 mph

sure you'll discover/learn some things at 238.8 MPH
but there's more to be discovered at 436.9 MPH

sort of like a NASCAR , if you did all your Testing at 100 MPH,
the Car would look great going around Curves at 100 MPH,
but do the same Curve tests at 200+ MPH, and now you can see the Driver fighting to keep the Car turning those same Curves without spinning out
(= Flow Separation in Live Engine).

Looking at all my Flow Test data thru all the years of Flow Testing many different Brands/Styles of Cylinder Heads, to get a decent idea or correlation to a Live Engine, you need a bare minimum of 25" Test Pressure on a Stead-State Flow device like a FlowBench.

i've noticed in some Cyl Heads over the years that Flow CFM can change dramatically above 28" ....that is some Cyl Heads take a dive in Flow CFM above 28" inches. usually as you go from 28" to approx 34 inches.
So far all my data, it looks like we really need to be Flow Testing Heads
above 34" to make sure its not taking a dive....any Head that has had a Flow separation problem has shown up by 34" Inches, whereas at 28" has sometimes hidden the flow separation problem because 28" is on the verge of too slow velocity in some Cyl Head port shapes.

i mostly Flow test at 36" and spot check at 48" and use Software to convert back to 28".

OldSStroker

Thanks Larry!

MaxRaceSoftware

I am sure you guys understand that flow is not the alpha and omega of cylinder heads, velocity is just as important if not more so in some cases. Our SF-600 will support flow testing up to 48" of H2O and the SF-1020 will support testing up to 65" of flow.
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be careful with a FlowBench that has a "motor controller"
its possible for motor controller to be "disguising"
problems in port flow

"before" peak Torque RPM..you don't have enough velocity

at peak TQ RPM...you have good velocity

above Peak TQ RPM..you have all the velocity you need
now you just don't have enough TIME

at Peak HP RPM , just the right combination of TIME and VELOCITY
above Peak HP RPM, Friction/Pumping losses are cancelling out HP/TQ gains
and because there is lesser amount of TIME, HP drops off


a Port that has too much velocity will not make any HP
or even not make the correct amount of Peak TQ
contrary to what you might think.

i've experimented with the same casting number Cyl Head
with the same valves swapped in each pair of heads,
both pairs had just about identical Intake/Exhaust Flow numbers,
had identical CC port volumes, ..but with different Port Shapes...
there was over 100+ HP difference at 7200 RPM between these identical
flow number Heads. ....and "real surprise" of this series of Tests was that it was thought that the Cly Head with the highest port velocity
would atleast make the most TORQUE..but this was not the case, as
Sonic Choke was so severe in the high velocity Head, Peak Torque suffered

you need great Port velocity , but you also have to be careful of creating too much port velocity inside a Port.

contactpatch

How is a person to know if, this or that cylinder head
will exhibit Sonic Choke, and another one won't?
.
are there published numbers for 'runner cross section'
for the popular cylinder heads?, I think that would help.

BUBBA

Acknowledging that I am out of element in this forum, I will mention that my porter used 22 inches and I had to get it translated in 28 inches so that I could compare. Is it accurate to translate?