28" of water.

What does it mean; how is it applied to the head flow?

My understanding is that the flow bench uses atmospheric pressure
to determine how much "vacuum" is generated in the head runner.

This seems to be a standard value used to compare flow characteristics
between heads.

The valve is opened in increments of 0.100" lift and flow tested.

The measurement is xxx CFM @ 28" of water correct?

Any articles that might explain this more in depth?

Thanks!

 

That's a measure of pressure/vacuum. Do a search for manometer and you'll come up with a lot of good reading.

Flowbenches have a powerful motor to pull air through the heads. Also, if you are measuring volumetric flow rate you can raise/lower the pressure differential and greatly skew the results, so a standard had to be established for comparing different heads.


I heard that MaxRaceSoftware has a flowbench . Ask him.

Ben T.

 

28" of water.

What does it mean; how is it applied to the head flow?



you can look at "Test Depression" as your air speed or air velocity


the higher the test depression, the faster the air velocity inside ports tested on FlowBench

 

Only relative on the same port though. Your velocity(both avg and peak) for a given depression will vary from one port type to the next, right?


Some more questions though:

1) Do the majority of flowbenches in use have mechanical gauges, or full computer setups to record and calculate.

2) Can a typical computerized flowbench output the the discharge coefficient for a given port? I'd think so, but its not something you ever see anyone talk about.

-brent

 

Try these links:


http://superflow.com/support/support-flowbench-what.htm
http://superflow.com/support/support...-works-how.htm
http://superflow.com/flowbench/index.htm

 

Sweet Deal!

Thanks all. More Q's may follow...

 

GOOGLE is your friend. Visit him.

 

Only relative on the same port though. Your velocity(both avg and peak) for a given depression will vary from one port type to the next, right?


Some more questions though:

1) Do the majority of flowbenches in use have mechanical gauges, or full computer setups to record and calculate.

2) Can a typical computerized flowbench output the the discharge coefficient for a given port? I'd think so, but its not something you ever see anyone talk about.

-brent
==========================================

For a given test depression, the theoretical air velocity fps follows equation, but like you stated in your post, the actual air velocity in different locations inside port will be different, and be different for various port shapes

Air_Velocity_FPS = (Test_Pressure ^ .5) * 66.2

where FPS = feet per second
^ .5 = same as square root
Test Pressure = inches of water depression

for 28 inches H2O Test Depression ;

350.3 fps = ( 28 ^ .5 ) * 66.2

if port were ideal, then for every square inch area , air velocity
would be at 350.3 fps
but in real Cylinder Head flow tests, air velocity varies inside port
at given test pressure


2) Can a typical computerized flowbench output the the discharge coefficient for a given port? I'd think so, but its not something you ever see anyone talk about.
===========================
yes,...but you don't actually need a computerized flowbench , you can calculate the discharge coefficient from flow data and calculator manually

you don't actually see many people talk about discharge coefficient , but my software calculates this everytime i flowtest a head. ( Curtain Area Discharge Coefficient and Valve Area Discharge Coefficient )


http://www.maxracesoftware.com/Flow_Max_Intake_1.jpg

 

28 inches of water is about 2 inches mercury is about 1 PSI. This means that the flowbench works to reduce the air pressure under the head to one PSI LESS than the air outside the head when intake testing and one psi MORE than the outside air on exhaust tests that are flowing in the opposite direction.

Many veterans established this pressure drop as a standard when seeing a majority of heads run best when the intakes were ported to maximize flow per unit cross section at this pressure drop. Now exhaust and even intake can be flowed at higher or lower drops to test for other phenomenon as well by just changing the pressure drop but this number or 28 inches is usually quoted as flow comparisons between heads.

 

Buick Motor Division uses 40 inches H2O during developement of Buick V6 for NASCAR and DragRacing

Chevy Race Shop uses 41 inches

Ford uses much higher

many flowtest much higher than 28 inches , but publish or tell people they flow at 28 inches

a live engine between peak Torque and peak HP points
is between 90 to 120 inches

28 inches is barely enough test pressure to simulate real engine
primary reason why 28 inches does not correlate 100.0 % PerCent to real HP/Torque numbers

28 inches became popular by Smokey U.
and SuperFlow's 300 FlowBench where 28 inch mark was about eye level

now on SuperFlow 600 bench, the 36 inch mark is where the 28 inch mark was. and on SF-600 the 28 inch mark is awkard being many times having to struggle to look around backside of cyl head on flowbench for 28 inch readings

i flow test all heads 36 inches to 48 inches when i can ,
then Software converts back to 28 inches

every once in awhile , a cylinder head will look great at 28 inches
but take a dive in flow numbers as you increase test depression
to 36 inches or higher

then there other times while flowtesting exhaust ports , they might all flow exactly the same at 28 inches but one port will "CONTINUE" to flow a bunch more air at 48 inches
and some ports will not increase at the same rate


Your test depression sets your theoretical air velocity thru ports
the higher the test depression, the higher the air velocity.
if airflow is too slow thru ports, it can stay attached to bad turns or bad shape ...at higher air velocity, flow will unattach and choke off flow

 

There are a lot of engines that will lose hp and definitely area under the curve if you make them big enough to stay smooth at 36 inches. Can't say that the shape that still is small though even at 36 or 40 won't make peak power much later but this doesn't seem to translate a lot of times to any gains but still turbulence must stay in control but it's a balance between size and velocity I guess. Exhaust however certainly flows at more than 28 inches in reality especially in the first part of blow down! You can certainly learn something there!

 

OldSS,

THis is much better than any Google search. I only have to click
one link

Seriously, I've tried sifting through search engines. It's tough to
distinguish who/what is right and wrong.

It's nice to have all the brains in one pile to fight it out.

 

There are a lot of engines that will lose hp and definitely area under the curve if you make them big enough to stay smooth at 36 inches.
=============================================

Erik, i think you misunderstood my Post ?

i'm not making port any larger when flowtesting at 36 inches or higher ..instead flow testing at just higher air velocity .

Example=> 041x SBC NHRA cast iron heads
1.940/1.500 valves
165.0 CC intake ports (NHRA Legal CCs)

same exact castings , same exact valve sizes and intake port volumes

but "Port Shapes" are different

flow one port shape at 28 inches it looks great
flow very same port at 36 inches or higher, it takes a dive

change port shape (still holds same 165.0 cc) and now port
flows same at 28 inches but now doesn't take a dive in flow but keeps on increasing flow with every increase in test pressure
beyond 28 inches

change port shape again, now intake port flows more at 28 inches and continues to flow more at any test pressure above 28 inches (still with same 165.0 cc ports )

The last 2 examples make more HP/Torque
and run faster down DragStrip !
than 1st example when same head looked great at 28 inches but took a dive at 36 or higher test pressures

all examples at 165.0 cc ports

165.0 cc is very small intake port nowdays

i'm not using the higher than 28 inch flow test pressures to make a large port flow smoother ????

but instead, using higher test pressures to find out if port shape, roof and short turn shape are correct and able to properly handle real live engine intake port velocities

 

MaxRaceSoftware,

OK I understand you better then! You can make any port less turbulent with size but that won't usually help you but obviously a more stable port of the SAME size almost always will like you are saying. I thought you were saying that you develop the port around 36 or 40 inches. I know of only a few guys that do that and the biggest teams in NASCAR and Pro-Stock don't on intake right now because they can always make power higher by making a port stable at that airspeed but again at 36 inches average airspeed in a port is higher than it is in a real engine untill way on the backside of teh power curve so you'll be able to make power higher but you'll still actually lose overall power. I'm not talking any instantaneous drop as measured but real average airspeed. At some point the ports really do sort of mimic steady state flow which is why the flowbench has worked so well over the last few years if you know what to look for.

From what I've seen when people laugh at flowbenches and dynos they usually don't know how to operate either correctly so they think the results are less than useful even though it's these two tools that have gotten engines to where they are right now in the hands of true engine builders. I know you aren't one of those guys but just noting that. If you are even in the situation of seeing 3 inches HG or 40 inches of water on something you are running out of air bigtime!

 

If you are even in the situation of seeing 3 inches HG or 40 inches of water on something you are running out of air bigtime!
========================================

Yes on Dyno or RaceTrack ... 3+ inches Hg of intake manifold vacuum is definetly a restriction and TQ/HP killer !!

7+ inches Hg intake manifold vacuum will lose 90 to 120+ HP easily

usually, anything above 1.1 or 1.2 inches Hg. manifold vacuum
during Engine Dyno tests will start to show TQ/HP Losses as
Carb starts to act like restrictor plate

from memory, i don't think i've ever seen an engine make its "Best" peak HP at 1.5" inches HG ....usually Peak HP RPM point almost always occurs between .7 to 1.2 inches Hg manifold vacuum