Port behavior at different depressions - Do the flow patterns really change??No.

[DaveMcLain]

This topic has been the subject of much discussion as well as a lot of thought on my part. For instance, when a port is tested at 10 inches of water on a Superflow 110 and then is also tested at some other depression, say 28 inches on a Superflow 600 does it really behave any differently? I say no....

Reason? Either bench produces enough depression to force the port to be turbulent, the airflow is not slow enough to become laminar. But neither bench produces a high enough depression for the resulting velocity to approach super sonic speeds, thus the patterns do not change significantly with a higher depression.

The real value of the larger bench is in it's higher resolution esspecially on large high flowing cylinder heads. At higher depressions it's possible to see much smaller changes in the amount of flow. It's no more repeatable, it just allows you to see smaller changes.

If generating flow numbers for comparing cylinder heads is what you're doing either bench will do the job just fine as long as the differences are large enough to detect on the 110.

Some folks think that by using the square law to change the numbers from 10 inches to 28 inches they are doing all the math required but this is not exactly true. The square law is CLOSE to correct but it's not exactly right either.... Superflow never said it was but it allows useful comparisons to be made without using any mathematics that are more involved than simple arithmetic.

I thought about these ideas after talking with a friend who is an aeronautical engineer. We were discussing aerodynamics and how a shape that's good at subsonic speeds is often terrible at super sonic speeds and vice versa. He said that airflow patterns across a surface do not change much from rather slow speeds all the way up to almost supersonic speeds. I also ran this by Neil Williams the founder of Superflow when we met at the PRI show a year or so ago and he felt that the ports behave in exactly the same way.

I have experiemented with flowing a port at 7, 10 and 16 inches and when the numbers are recorded using my FlowCom and processed by my computer using the Performance Trends software they are identical when corrected to the same new depression.

Good stuff, what do YOU think???

[Nick]

That is an interesting thought. I have been told that these small block ford heads I am working with will not show there true flow unless you get the depression up around 28" I do know one thing about these crappy heads, they have some turbulence issues above .450 lift. I can lose 6cfm and not touch a thing. It is just to weird. Back off the valve and open it back up slowly and it will return to the higher number without touching anything else.

If you look at flow charts others have done on thease heads, some times the flow drops off at .450 and others it just stays the same. So I'm not the only one with this problem.

I am interested in tyring out this theory, and will post some numbers tomorrow. I will try flowing at 10".

I have read that stock small block Chevy heads have the same problem. I think It is even mentioned on the Audie site, under there flow quick.


Nick

[Rick360]

Dave,
I say YES, absolutely it makes a difference. I have seen proof in several instances. Some heads will flow fine at lower depressions but when you pull more vacuum, it can cause the air velocity to be high enough to separate from the short turn resulting in flow dropping as the valve opens more in the higher lifts.

When a port is done right, your 10" flow will convert to 28" and match what it will flow at that depression very closely. When the short turn is wrong, you can have completely different flow curves. You'll never know it if you test at 10". More than 28" is even better.

Last year I worked on a set of SBC Bowtie heads (previously ported) that had a good flow curve gaining flow to .700" lift or higher when testing at 28". I tried them at 40" and the flow dropped above .500" lift. I fixed the heads and the flow curves match from 10" to 40". The flow at 28" didn't show a change. The car picked up over .15 secs in the 1/8th (drag race) or about 30HP. This isn't an isolated thing. I've seen it other times when testing heads that were ported by someone using 10" to test.

Now I always pull as much on a head as I can and see if it converts correctly back to 28". That helps to find this problem.

I plan on my next bench pulling 50"-60" just so I can find more of these problems. The vacuum motor power does go up significantly to do this. So does the ENGINE power.

Rick

[maxracesoftware]

[DaveMcLain]

So from what you're saying, you see these differences mostly when they short turn is not correctly shaped. I would say therefore, that the problems would be most aparent on cylinder heads with an entrance that's really too low(most production cylinder heads).

I've seen those issues particularly on the Chevy Vortec head where flow at .600 lift is lower than at .500, you can hear the port change on the bench quite abruptly. Do you ever see it go the other direction, with flow getting better at high lift with higher depresions?

[Rick360]

[Thomas Vaught]

We were testing on a SF1200 bench with a 100 cfm orifice added.

The bench could move about 1300 cfm with the 16 Motors (115923)
and easily pull 48 inches of depression on most heads.

I personally believe that if you have a strong enough bench, power wise,
the more depression the better.

The Ford Motor Company (worked in engineering for them 28 years now)
has tested ports at 60 inches H20 since the 60s. They also test at the other points but their benches could pull 60 inches and they could see small differences like the .001 material change mentioned earlier.

Smokey Yunick was the guy who said that 28" was THE NUMBER and he saw no real benefit testing above that number. (His bench was not able to test at 60 inches for one thing on a big port. LOL).

Everyone has taken his comments as gospel for years and some things are just not true in all cases. He was a smart man but was not right on everything. His biggest advantage was the desire to learn new things.

JMO

Tom V.

[DaveMcLain]

What is interesting about some of this is that a few weeks ago we took an A460 Ford cylinder head that was tested on my bench over to a friends bench(SF600) to compare the numbers. The A460 head is pretty nice with a good short side and a high port entrance. His numbers and mine were identical all the way to about .600 lift where his read somewhat higher, about 3% I believe. I feel that in this case the biggest factor was that he tested with a 4.625 bore diameter vs my 4.250 bore fixture. The exhaust numbers were identical all the way to .800 lift and in that case I think that it was just because the smaller exhaust valve was not effected by the small bore nearly as much as the large intake valve.

Had we been testing a less optimum design would his number have been more conservative than mine at high lift? I don't know.....

[gofaster]

If you think that the increase came from his 4.625 bore sleeve, you can recheck on your bench by moving the head 3/8" over on your stand to unshroud the intake valve.

If you see a similar pct. of change, then it was the sleeve and not the depression that made a difference.

On a less optimum design, the cfm may have been about the same, but you would be able to hear the port go turbulent, or see it with a flag. At a lower depression, the separation may not show up.

I added a couple links that demonstrate the "Coanda effect". The fact that the writer is interested in pulse jet development is irrelevant, air is air.







Before the port goes turbulent, the coanda effect works for you, and pulls the charge around the shortside and other curved surfaces in the port. When the airspeed in the port exeeds the ability of the effect, the charge can shear away from the wall or floor. As the depression becomes greater, the airspeed through the increases and your chances of catching a problem that the running engine will see are improved.

[Mousehouse1]

I know there isn't much difference in flow between a 4.030 & 4.125 bore fixture on a small block head. But I haven't done any big block heads. Just the little one.

[DaveMcLain]

It is interesting how the port will often go turbulent above a certain lift. Thanks for the good info about the coanda effect. I'm thinking that I might be able to use a few of these ideas to help reduce reversion on my restricted induction engines.

[Tony]

I am not into competition or really high horsepower stuff, but I can well believe that higher port velocities do behave differently.

A running engine does not have constant flow, like a flow bench. Port velocities cycle up and down, and can reach peak figures far higher than you could ever hope to reach on your flow bench.

Your engine may consume 100 CFM per port on a dyno flat out, but that is not the same as 100 steady CFM per port on a flow bench. For a start the inlet valve on a running engine may only be open for something like 1/3 of the time.

As others here have already said, stable flow can become unstable above certain flow velocities. Test at as high a depression as you can, but it will still be nothing like the 0.4 Mach (peak) that a running engine may reach.

It is like designing a jet fighter aircraft, and doing all your aerodynamic testing at 30 Mph. Some of it may be valid, and some of it may not.

[maxracesoftware]

>>>When the airspeed in the port exeeds the ability of the effect, the charge can shear away from the wall or floor. As the depression becomes greater, the airspeed through the increases and your chances of catching a problem that the running engine will see are improved. <<<<

yes,
your Test Pressure (or vacuum depression)
is your "Test Speed" !!

a very crude and simple example=>

MPH = (Test_Press ^ .5) * 45.13636364

if you FlowTest at like Mullen's did at 3" H2O
thats = 78.2 MPH AirSpeed

you take your NASCAR RaceCar to the Track
and only Test it around the Track at 78.2 MPH,
sure , you'll Learn and Discover a few things

now you decide to Test your RaceCar around the Oval
at 10" Test Pressure = 142.7 MPH
again, sure you'll Learn and Discover a few things


now you Test your RaceCar again at 28" H2O= 238.8 MPH
now it gets VERY interesting...as all of a sudden,
it might spin out , CRASH

now you Test at 60"+ H2O like Real Live Engines = 349.6 MPH
....now all of a sudden, not only do you have to
radically change up your RaceCar setup....
but now you have to get them to CHANGE the "Shape" of the Oval ..to handle that Speed


DCR (Dynamic Compression Ratio)
and
Effective Trapped Volumetric Efficiency %

Question=> How "fast" must your Mixture be traveling
in order to create enough Ram-Effect to get
125.0 % + Ve like current ProStockers ????

i know you must actually include the Sound Wave Effect's
influences upon the Ram-effect as the Piston rises from
BDC to the Intake Valve Closing point ABDC....
but i'm mostly concentrating on how much speed (MPH) ?

ProStock 500 cid at 14.8:1 CR = approx. 10.546:1 DCR
add the effect of 125.0 Ve% = 13.183:1 DCR or ECR

increase static CR to 16.0:1 then DCR= 11.376:1
add 125.0 Ve effect and approx = 14.22:1 DCR or ECR

so how much more at 7500 to 9500 RPM does the
Speed (MPH) of the Mixture has to be moving
to get to ECR of 15 or 16 or even 18:1 CR

from SAE and other Data/Tests
Mixture has to be moving at the very least of
3.5 to 4.0 psi (including Wave-effect influence)

anything much more than 3.5 to 4.0 psi
and HP/TQ suffers as Pumping Losses rapidly
cancel out Ram-Effect gains .

3.5 psi = approx . 123.8 ve% = approx 96.9" H2O Test Pressure Speed in Live Engine = 444.3 MPH

4.0 psi = approx. 127.2 Ve% = approx 110.8" H2O Test
Pressure Speed in Live Engine = 475.0 MPH

so you have to design your Port Shape to keep Mixture
attached for 444.3 to 475.0 MPH
if you expect to have any chance of attaining 123.8 to 127.2 Ve%

i've been Flow Testing Heads at 36" mostly since 1984
and double-checking at 48" whenever i could
...mostly because of a Trip to Brodix in 1983
after Flow developing (2) different Port shapes on a #292 Turbo head on my SF-110 Bench at 10" inches ..then brought that #292 Turbo Head with me to Brodix to Flow Test on their
SF-300 Bench at that time...the result was the Port that Floed more on my SF-110 Bench at 10"...was the worst of the (2) on Brodix's SF-300 at 28"..just the opposite.

i had already suspected i was getting fooled by the 10" H2O
as some "small changes" i made increased Flow CFM but hurt
ET/MPH down the DragStrip.

purchased a new SF-600 Bench ..tried all those same tests
at 28" ....to see what was really happening ?...
then kept increasing Test Pressure for more trials...
when i got around 34" inches ...the Flow Tests correlated
to the DragStrip tests, and showed very different Flow
numbers than 10" on the SF-110 or Brodix SF-300 at 28"...
so from that point on i thought since 36" on a SF-600
is just about "Eye-Level" and is a Yard (3 Ft.) ..and was
a few inches above 34" ..i try it out from that point onwards...
that was 1984 year ...since then, just about every Cyl-Head
i've Flow developed at 36" Test and Higher as always
run faster down the DragStrip...where as the Heads
i've done at 10" and 28" back then..definetly did not always
run fast down the DragStrip ..some slowed down.

also since 1984 been using Pitot Probe in order to gain
more understanding of why some Heads that had better
Flow CFM Numbers did not always run faster down the
DragStrip in the real-world.

since using the Pitot Probe + Flow Testing at 36" or higher
its helped me Port Heads that just about always
go faster down the DragStrip in the real-world !

if you have ever tried to Flow a Cyl-Head
at 28" or lower on a SF-600 Bench its a pain as
28" Mark is well below Eye-Level and way under below the Head Fixture ..very akward....the old SF-300 Benches
had the 28" mark at Eye-Level

the SF-600 have the 36" Mark at Eye-Level
and have more vacuum flow/speed capacity,
so why not use the SF-600 or new ProBench
to its fullest potential to try to closer duplicate
real-world engine conditions ?

then just publish or Post your Flow Numbers
converted back to 28" Industry Standard ?
thats what i've been doing all these years

its pretty evident when you Computer Model or correlate
Engine Data from Dyno and FlowBench
that most are Choked at approx . .55 to .65 Mach....
i don't think i have any personal Data thats shown an Engine
can exceed .68 Mach, most choke around .55 Mach to .65

.55 Mach = approx 614 fps = 418.5 MPH
.627 Mach = 700 fps = 477.3 MPH
.68 Mach = approx. 758.9 fps = 517.4 MPH = 131.4" H2O
and 4.746 psi = 132.3 potential Ve%

it takes an outstanding "CURVE" to handle = 517.4 MPH
in 1 or so inches of distance and turn that Mixture without
separation

a Constant Area perfectly straight piece of Pipe
can get close to 1.0 Mach = 284 inches of water
..but you can't get a Poppet Valve in it
169.8 % theoretical Ve if you could

back to example,
a straight pipe would turn a NASCAR Race into
one big DragStrip Race...or Bonneville Salt Flats
..sure would make Porting Heads easy !

Merry CHRISTmas

[gofaster]

Larry, Thanks for taking the time to explain that all so well.

[Thomas Vaught]

Larry, I agree, very nice information.

Tom V.