flow at higher depression?

[bc]

What kind of depression does a running engine see? I was talking to a Ford engineer that said there was no point in flowing a head over 28". Has anyone ever monitored an engine to see what it would be? I know that if you hook up a vacuum gauge that its below 1". What am I missing?

[airflowdevelop]

If that was the case, you would only flow at about 10" of water...Thats about the highest measurable differencial in the intake runner on a good running motor.


The differential we are concerned about though is between cylinder and the intake runner...not the runner to atmosphere.


increased depression does a couple of things... including added resolution.

I have heard theories that the differtial at mean flow on exhaust can be > 400" of water....

Anything above 100" is a fairly new field (atleast for me and most drag racing guys I know).. It has certainly helped my valve job design. I am in the process now of adding additional capacity to flow BB heads to 120" +

Thanks
Dennis

[bill jones]

-One minor point that bc is missing tho is an inch of vacuum on a vacuum gauge is mercury and that is 13.6 times more than water which is what most flowbenches are using.
-So 13.6" on a flow bench to every inch on an automotive vacuum gauge.

[bc]

Actual I do realize the difference between the two. If you take the 1.5" that Holley flows their carbs at that only 20.4 on the flow bench. If you take the usual .5" that a race engine sees that 6.8". So the reason for flow higher is for resolution then? I can understand the exhaust side being to high.

[maxracesoftware]

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 Steady-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".

Darin Morgan previously mentioned in another Post, that IRL Cylinder Head developement was being conducted at well over 100" Inches of Water Flow Test pressure.

The differential we are concerned about though is between cylinder and the intake runner...not the runner to atmosphere.

yes its the combination of the "Flow Lag-Time" and "pressure differential" between the Cylinder -vs- the Intake Bowl/Runner ..that setsup the Mach limiting velocity relative to temperature gradient in the Induction System.

its "not" the Runner -to- Atmospheric Pressure in a Vacuum Gauge .

[putztastics]

What is the flow in cfm per port at these higher depressions and how does that cfm match actual measured cfm of the running engine?

[Boport]

I'm a little confused... I agree that testing at higher depression may be useful at some higher valve lfts. However, at lower lift ranges, the piston speed and tuning effects aren't necessarily filling the cylinder where there would be a need to test at 100+" of depression, correct? Now at higher valve lifts where the piston speed is creating a lot of depression I see a need to test that high.

I understand a need to test at that level on the exhaust side as there is going to be high pressure in the cylinder during most of the valve opening event, but why the intake?


Bo

[ozrace]

What kind of depression does a running engine see? I was talking to a Ford engineer that said there was no point in flowing a head over 28". Has anyone ever monitored an engine to see what it would be? I know that if you hook up a vacuum gauge that its below 1". What am I missing?

bc - if you think about it, the pressure you are reading in the manifold is the result of flow restriction before the valve, and has little to do with duplicating actual running depressions on a flow bench.
What you are talking about there is the pressure differential ACROSS the valve. i.e. pressure on the inlet side of the valve compared to pressure in the cylinder. How hard the cylinder is working to get air.
This will vary throughout the lift cycle, and from engine to engine, but at full lift on a Race engine, I think you would find 80 to 120 inches. (this is based on calculation using engine design software).

[Fatman]

bc

Darrin Morgan mentioned in a post that flowing at a higher depression caused them to redesign a particular cylinder head that then resulted in more power. That's what i remember but don't quote me on it.

I think this is what Larry refered to in his post.

[putztastics]

If I did the math right a 45 cubic inch four stroke cylinder @ 6000 rpm will have around 332 ft/sec average intake velocity (average over 180 degrees crankshaft rotation) through a 2.25 sq in cross section at 100% volumetric efficiency.

78cfm average flow @6000 rpm

312cfm @ 6000 rpm ((720 / 180) *78 )) (average flow during 180 degrees of intake stroke)

312 * 1728 (1728 cubic inches in a cubic foot)

=539136 (cubic inches per minute)

/ 60 (seconds per minute)

= 9885.6 (cubic inches per second)

/ 2.25 (intake port cross section area, or volume @ 1" length)

= 3993.6 (average inches per second velocity)

/12 (inches per foot)

= 332. ft./sec average velocity during 180 degrees crankshaft rotation

Up until now I always stopped at the 78 cfm (624 for eight cylinder four stroke 360 @ 6000 rpm). My mistake was not realizing the 78cfm/624 cfm is average flow and that the actual the intake flow is crammed into a much shorter time span. The port has to flow about 312cfm during the intake stroke to flow the average 78cfm.

The 332 ft.sec from above is an average velocity during 180 degrees intake stroke. The peak velocity is sure to be higher. I suppose someone somewhere has instrumented an intake port on a spintron or running engine and measured actual conditions.

Testing at high depressions never made sense to me before but it does now.

Just me keeping in the theme of doing things backwards, I have an engine dyno, but not a flow bench. I'm building a flow bench though and now I'll have to buy some additional vacuum motors for high depression testing, heh heh.

[mike_belben]

are there any books specific to these type of calculations? the name taylor was mentioned.

thanks

[panic]

The math is probably good but remember flow is not continuous, or even cyclic (i.e., repeated pattern regardless of speed).
I use: V = (B^2 / P^2) * S * R / 360
where V = port velocity, B = bore diameter, P = port diameter (assumes round port, substitute [L * W / .7854] for square or rectangle), S = stroke and R = engine speed.
Example:
B = 4.00"
P = 2.00"
S = 3.00"
R = 7000
V = 233 f/s

In addition to subtracting for time the valve is closed, you have to account for (nearly) no flow or even reverse flow during overlap when the reversion waves are not timed for that speed. IDK any way of doing this.

[thecarfarmer]

bc

Darrin Morgan mentioned in a post that flowing at a higher depression caused them to redesign a particular cylinder head that then resulted in more power. That's what i remember but don't quote me on it.

I think this is what Larry refered to in his post.

This completely makes sense to me. Watching Eric Weingarteners channel, he likes to crank that valve open to a full inch of lift, even when the engine is never going to have anywhere near that much valve lift. He does it to get air velocity up, so he can see if it goes turbulent around the short side as it turns into the valve bowl.

Seems to me, having a flow bench that would pull far more than 28 in of vacuum would be a real help. Of course, you may need to upgrade that electrical service to run the beefy fan motors required...

The math is probably good but remember flow is not continuous, or even cyclic (i.e., repeated pattern regardless of speed).
I use: V = (B^2 / P^2) * S * R / 360
where V = port velocity, B = bore diameter, P = port diameter (assumes round port, substitute [L * W / .7854] for square or rectangle), S = stroke and R = engine speed.
Example:
B = 4.00"
P = 2.00"
S = 3.00"
R = 7000
V = 233 f/s

In addition to subtracting for time the valve is closed, you have to account for (nearly) no flow or even reverse flow during overlap when the reversion waves are not timed for that speed. IDK any way of doing this.

Agreed! Also, you would need to take into account the slug of exhaust gas moving down the pipe, yanking on the intake port during the opening phase of the intake valve.

This, at least for a mathematically challenged knuckle dragger such as myself, constitutes a level of complexity that I can't even formulate the questions for, LOL

The problem as I see it is trying to use a static state test for a dynamically fluctuating system. I suspect that at best, it only gives us very, very rough data points to use as signposts - not real absolute truths about how the engine is going to run in the real world.