Detecting sonic choke on the flow bench

[Donnie1126]

If we keep upping the test pressure #'s on the flow bench...there should be a point in which sonic choke can be detected right?Flow separation should be a factor at some point causing the flow pattern to distort.Ok given two heads that flow exact numbers but different velocities can we assume that sonic choke will occur at a lower test pressure for the higher velocity head if it has the same or less port cc's?Given that.... can we determine the percentage difference in engine speed that the two will make peak power?The higher velocity head should peak at a lower engine speed on the same engine right?

[cboggs]

Donnie,

Read through this thread, .. Larry gives the best description on this
work I have ever read, .. . ..

You can't actually "make" sonic choke on the flow benches we have
today cause we can't "pull" enough, .. but you can look for indicators
that will tell you if the head will go into choke in a dynamic condition.

read this thread, .. it's very good.

http://speedtalk.com/forum/viewtopic.php?t=840

Curtis

[Rick360]

From my understanding of what is being referred to as "sonic choke" is that it is strictly a dynamic phenomenon that wont happen on a flow bench at any pressure. This happens in a running engine when the velocity gets too high, about mach .55-.60 . Calculating velocity and measuring with a pitot tube on a flow bench will show you when it is happening or likely to happen. How to do this has been explained very well by Larry Meaux in other posts.

This should really be called something else, perhaps "velocity choke", since actual sonic choke occurs at mach 1.0, the speed of sound which wont happen on the intake side of an engine.

Air separation causing turbulence over the short turn is a different problem, although both are velocity related. This can be seen as flow loss at higher lifts when testing at higher pressures on a flow bench.

Rick

[cboggs]

Hey Rick,

speaking of short turn problems, .. have you seen this on heads flowed
at low test presures? Say 10" h20??

Having a discussion on another forum about this, .. and I also have
a small bench I use for 4 cyl heads, .. seen some small block
heads have problems on it but not on big bench at 28". ???

Curtis

[PFM]

Rick360,

As for "sonic choke" and not hitting Mach 1.0 I am not so sure, I worked with some guys on a Formula SAE deal some years ago the motor had to run one 20 mm restrictor (I think, memory in question here) 600 cc motor with turbo, on alki with lots of RPM and I think the math said darn near 1200 fps and the temp was up in the 300 degree F range, not quite mach on average but well over .6 mach.

PFM

[Bos's5.0]

Hey Rick,

speaking of short turn problems, .. have you seen this on heads flowed
at low test presures? Say 10" h20??

Having a discussion on another forum about this, .. and I also have
a small bench I use for 4 cyl heads, .. seen some small block
heads have problems on it but not on big bench at 28". ???

Curtis

very interesting.. are you saying you are finding issues as lower test pressures that aren't showing up on 28" benches? I'm very curious about this since my little bench can't pull anywhere near 28", but my E7 ford heads are showing all kinds of issues at the much lower test pressures.

[cboggs]

very interesting.. are you saying you are finding issues as lower test pressures that aren't showing up on 28" benches? I'm very curious about this since my little bench can't pull anywhere near 28", but my E7 ford heads are showing all kinds of issues at the much lower test pressures.

Well yes in one case, .. I have a head that goes all kinds of turbulent
at 10" h2o but doesn't seem 1/2 as bad at 28" ????????????

I know a bunch of things don't shop up at low presures that do at high, ..
but I'm wondering if it works both ways???

I think there's issues keeping the flow attached at low presures, ..

This came up cause my custom bench for high flow is being worked
on, .. so I've put a few things on the little bench I wouldn't otherwize
have tried.

Curtis

[Rick360]

Hey Rick,
speaking of short turn problems, .. have you seen this on heads flowed
at low test presures? Say 10" h20??
Curtis

When I first started flowing heads I used 28" except on some large heads when I couldn't get that much depression, then I would drop to 25" or however low I had to drop it for my bench. Recently started flowing higher when I can. I have never flowed a head at or around 10"H2O.

I would think it odd to have a turbulence or short turn issue only at lower depressions, when the velocity is slower when the air should turn much easier. I can't think of anything that could cause this either.

Is it turbulent or just down on flow compared to higher depressions?

Rick

[maxracesoftware]

If we keep upping the test pressure #'s on the flow bench...there should be a point in which sonic choke can be detected right?

i'm posting from my Home Computer, all my equations are on my Shop Computer, but from memory , you would need a FlowBench to
pull atleast 192 Inches of Water minimum at room temperature just to be right on verge of real Sonic Choke on a Steady-State FlowBench device.

i'm probably not doing a great job explaining...
but you can checkout an Intake Port with a Pitot Probe
at 10", 28", 36" or whatever Test Pressure...(although i'd prefer to Probe a Port at 28" or higher Test Pressure),..and get a very good idea when that Port will go into premature Choke condition in a Live Running Engine.

in a Live Engine, you have various and different temperature gradients along length of Induction System, different Speed of Sound, and you have Sound waves moving back and forth inside Induction System, plus you have a Air/Fuel Mixture also inside Induction System....and dynamics of Crank/Rod/Piston CFM demand of each degree of crank rotation, Mixture inertias, Mixture Lag times,ETC.
there's too much going on in a Live Engine that can't be simulated on a Steady-State FlowBench device.

all you can do with a FlowBench is to Pitot Probe a Port while testing on a FlowBench and then try to correlate at what Pitot reading will correlate into a premature Choke or TQ/HP Loss condition.

and it happens that if you Flow a Cyl Head at for example 28" of Test Pressure, and while you are 28" Test Pressure, if your Pitot Probe reading is near equaling or exceeding your Test pressure, then theres very good chance that the Engine will go into premature pumping losses (Choke Condition), and there's a very good chance that you are already starting to loose HP/TQ and are not aware of the beginnings of these losses unless they are severe or you have a baseline for comparison.

and you won't be able to see or tell from FlowBench CFM Numbers, but only from a Pitot Probe reading !!

you can keep increasing Test Pressure, and Flow CFM Numbers will still increase on the FlowBench even if Pitot Probe readings are sky high.

The premature Choke won't show up in a FlowBench CFM Number !!
the Sonic Choke won't show up under 192 plus Inches of Water Test pressure as a CFM Number.

The Choke will only show up on the Pitot Probe,
and that in turn will only show up on a Live Engine accelerating down the DragStrip or Dyno Test.

as it turns out anything over 350 FPS or close to 350 FPS
will probably cause a premature Choke in a Live engine.

any kind of disturbence or protrusion or wall divergence or too quick a turn when air is moving that fast , will lead to premature Choke or TQ/HP Losses.

the real trick is to keep the Mixture moving as fast as you can and not go into premature Choke..that's going to give the greatest chance of making really great Volumetric Efficiency..but it takes someone really skilled like Darin Morgan or Carl Foltz or others plus a really great Cyl Head Port design to start off with.

when you get Mixture velocity that fast in a port, and protrusion or disturbence will set off the Port into premature Choke !!

i had a well known Engine Builder send me a GM-Chevy 15 deg SBC Head
for B/SR ...on the Dyno the Engine would fall off quickly after 9000 RPM,
the Intake port's Entry were "shifted" towards center/Carb, so in a few ports the valve cover stud protruded a lot into the roof of the Intake Ports,
and most of the gasket was slightly protruding into the intake ports,
all that was causing the Engine to go into Choke as well as the speed was already very fast there at the Entry and Pushrod area.

i enlarged the Sheetmetal Intake at the flange area and the Cyl Head port to slow down the speed to reasonable FPS, ground the valve cover stud protrusion flush, told the Engine Builder to make dead sure that the gasket's edge didn't protrude into the port as the Velocity was still high there...and the Dyno results were that the Engine made 5 or 6 more Peak Torque, 15 more Peak HP, but over 50+ more HP after 9000 RPM...
down the DragStrip, the Car was 1.5 tenths and almost 3 mph faster and set NHRA B/SR record that year.

then there are other times when you can install a Big Block Chevy rectangular port Intake manifold on an Oval Port BBC head and make more HP and TQ even with a tremendous port mismatch...but what really counts is what's the Pitot Probe Velocity FPS in the Oval Port at or near that area or what's the fastest Port's Veocity measured on the Pitot Probe?? is it exceeding 350 fps or close to it ??
the answer is NO on an Oval Port ..so it works most of the time

you want to keep Port Velocity as high as you can and stay attached
but not over 350 FPS if Flow Tested at a test pressure of 28"
its going to take a very great shape to handle 350 fps or higher velocity and not see a premature Choke on the Live Engine.

i'll post some Dyno numbers later on of "premature Choke"

[ozrace]

From my understanding of what is being referred to as "sonic choke" is that it is strictly a dynamic phenomenon that wont happen on a flow bench at any pressure. This happens in a running engine when the velocity gets too high, about mach .55-.60 . Calculating velocity and measuring with a pitot tube on a flow bench will show you when it is happening or likely to happen. How to do this has been explained very well by Larry Meaux in other posts.

This should really be called something else, perhaps "velocity choke", since actual sonic choke occurs at mach 1.0, the speed of sound which wont happen on the intake side of an engine.

Air separation causing turbulence over the short turn is a different problem, although both are velocity related. This can be seen as flow loss at higher lifts when testing at higher pressures on a flow bench.

Rick

Very well put - I have thought for some time that "Choke" is actually a misleading term here.
David Vizard has called it "Power Limiting Port Area", a more accurate description, but unlikely to catch on.

[maxracesoftware]

Note=>all 3 of these Heads were tried on
the same Short Block with all the same pieces
and Dyno Tuned for best possible HP/TQ Curve
with those pieces.


#041x SBC Heads = 165.0 CCs
1.940/1.500 valves
these are the very Hi-Velocity Heads
with too much velocity everywhere inside
the Intake Port, same FlowBench numbers

and the "BEST" Dyno test with them
600 RPM/SEC , i don't have the Sheets
that we started at 3000 RPMs and all the
rest of the Sheets , but only kept the
Copies that stood out, and these are every
200 Hundred RPM increments as its too much Info to type
every 100 RPMs, but it should give you enough Info ?

note=> at 600 RPM/SEC you get a little Needle/Seat
action showing up especially with small gas bowl
chamber in Q-Jet, so look at Fuel Lbs/Hour trend
as well as rate (Same Q-Jet Carb all Dyno Tests)

RPM---TQ----HP----Fuel Lbs
4500-419.3-359.3--178.4
4700-438.1-392.1--171.5
4900-449.1-419.0--177.2
5100-451.0-437.9--169.8
5300-445.8-449.9--174.1
5500-443.2-464.1--188.3
5700-441.0-478.6--209.8
5900-429.6-482.6--222.1
6100-424.3-492.8--227.4
6300-413.9-496.5--214.9
6500-412.7-510.8--200.8
6700-407.6-520.0--210.3
6900-388.8-510.8--221.9
7100-363.7-491.7--236.1
7300-345.3-479.9--239.8
7500-325.1-464.3--233.4
7600-312.6-452.4--226.1

Avg=>406.5-464.9--206.0

[maxracesoftware]

with #041x Heads back-to-back on same Short Block
same basic Flow CFM Numbers, same valves, same CC's
but with Port Velocity slower and more acceptable
throughout the entire Intake Port

RPM---TQ----HP----Fuel Lbs
4500-449.5-385.1--166.1
4700-444.2-397.5--164.5
4900-455.2-424.7--177.0
5100-456.2-443.0--158.3
5300-464.1-468.3--169.6
5500-471.1-493.3--192.9
5700-470.2-510.3--199.2
5900-468.2-526.0--199.3
6100-465.0-540.1--204.5
6300-459.8-551.5--209.7
6500-456.6-565.1--216.3
6700-442.6-564.6--223.1
6900-432.8-568.6--217.2
7100-426.6-576.7--215.3
7300-418.3-581.4--224.5
7500-401.8-573.8--238.2
7600-394.1-570.3--231.0

Avg=>445.7-514.1--200.4

===================================================

with #462 castings 1.940/1.500 162.0 CC Ports
differences just 3 CCs can make when ground out in the
correct places, again FlowBench CFM between the
#462 and the other 2 #041x Heads were very close
and CFM numbers don't indicate the HP/TQ differences observed
and Ports had different Velocity Profiles.
Same ShortBlock and all pieces the same.

RPM---TQ----HP----Fuel Lbs
4500-443.0-379.6--168.3
4700-441.8-395.4--159.8
4900-450.6-420.4--164.4
5100-456.9-443.7--169.8
5300-459.2-463.4--183.9
5500-465.9-487.9--190.6
5700-464.1-503.7--192.0
5900-463.0-520.1--195.2
6100-460.6-535.0--196.6
6300-454.9-545.7--206.5
6500-446.8-553.0--216.1
6700-438.0-558.8--225.1
6900-428.4-562.8--220.8
7100-422.0-570.5--223.9
7300-410.1-570.0--219.5
7500-395.3-564.5--226.4
7600-388.8-562.6--231.1

Avg=>440.6-508.1--199.4

[maxracesoftware]

Very well put - I have thought for some time that "Choke" is actually a misleading term here.
David Vizard has called it "Power Limiting Port Area", a more accurate description, but unlikely to catch on.

i just use the word "Choke"
because sometimes the Engine will be Choked by an Area
and sometimes by the same cross-sectional, but now has one of the walls
with too much local velocity FPS and/or diverging too quickly on 1 wall

in the above Dyno Test examples the
one extreme hi-velocity #041x SBC Heads
is using more Fuel, but if you try to lean it out, you loose even more Torque and HP...notice it makes Peak TQ and Peak HP lower and runs out quickly with rapid rising BSFC numbers as rising RPMs show Choke problem even more.

the 2nd pair of #041x heads
make more Peak TQ & HP and at higher points,
and don't lay over top end.

the #462 castings with 3 less CC's
make Peak TQ at same point, but past Peak HP point start to layover
more than the #041x

Fuel consumption is about identical

Same FlowBench CFM Numbers
but different Intake Port Pitot Probe profiles/velocities

[SBC]

Thanks for posting this data, Larry

It took you TIME to obtain those numbers.

[Donnie1126]

Yes thanks alot Larry.You have very informative posts.It's you afterall that's got me to thinking about all this.