Does sonic choke affect Hemi ports?

[Donnie1126]

Or does it mostly affect ports with more of a short turn radius?

[maxracesoftware]

any Type of Cyl Head that has a portion of the Port
that causes local Mixture velocity to be too high
will go into premature Choke or flow separation before the rest of the Port.

in the case of a perfectly straight Tube,
air velocity is not infinite in speed.
it requires a certain amount of cross-sectional for a given Engine cid size
and RPM operating range.

a Hemi port is usually a nice straight shot with not a lot of Short Turn angle, but also most Hemi intake port's are pretty consistent in cross-sectional throughout its length with high velocity . If you make the port velocity too fast or put something inside the port that creates too high localized velocity, then TQ/HP suffers.

if its a NHRA SuperStock Hemi head with Port CC limitation,
and you go in a place epoxy on the Short Turn area to gain more Flow and keep CC's legal, its very possible to see Flow gains on a FlowBench but see TQ/HP losses on the Dyno and DragStrip.

if you use a FlowBench to only tell you how much CFM a Cyl Head flows
then you are not aware of all the other things a FlowBench can reveal to you .

[Donnie1126]

What would be the cause of a port showing good results on the bench but not on the engine?Would it be because the valve eventually starts to close very quickly?Or maybe the valve timing is reduced considerably in a high rpm situation?

[maxracesoftware]

What would be the cause of a port showing good results on the bench but not on the engine?Would it be because the valve eventually starts to close very quickly?Or maybe the valve timing is reduced considerably in a high rpm situation?

What "application" are you working on and what's the Engine Specs like Bore, Stroke, Rod C-C, RPM range, CR, etc. along with Cam Specs, and ET/MPH goals ??? What Dyno HP/TQ numbers so far ?
along with Flow Numbers and valve sizes ??

just for Research , i placed a small 1/8" buildup of Epoxy on the Short Turn and gained about 3 PerCent more Flow on a FlowBench...also did that to help make the Port legal in CC volume.....
Results=> Lost Mid to Top End HP/TQ in NHRA SS Race-Hemi
the FlowBench CFM numbers can lie to you in one respect and you must use a Pitot Probe to verify what you did does not create too much velocity at the wrong spot in a Port.


another example was a B/ED 346 cid Engine years back making
761 HP on my Dyno....removed the Cyl Heads and again tried placing Epoxy on Short Turn apex..again saw a Flow gain of approx 3.5 PerCent,
re-Dyno tested Engine and lost 17 HP to 744 HP...luckly we had 5 guys
helping that day, popped the Cyl Heads off , took apart the Heads, with a sharp Hammer strike on a long screwdriver, popped most of the Epoxy off the Short Turns and finished up with a Cartridge Roll ....assembled the Cyl Head backup and had the Engine Dyno testing a few hours later, ...made the 761 HP back like it was previously .

a "Saying" thats popular is =>
"Make it Flow the most CFM you can, at the smallest Port Volume"

Sounds good, but if taken too far, can be a Torque and HP disaster.

Example=> NHRA SS/IA #041x SBC Chevy heads 1.940/1.500 165.0 CC
One pair of #041x Heads were extensively Pitot Probed and all dead Flow areas and any area that could be built up and not show a Flow Loss were epoxied up...so what you wound up with was one pair of #041x Heads that on a FlowBench flowed just about the same exact Int flow numbers
as another baseline pair of #041x heads, both pairs had same exh port flow numbers and same chamber CCs, and used same valves swapped in each pair. About the same Flow specs as you could achieve, but very different Intake Port Shape and very high Pitot Probe velocities in the epoxied port head -VS- other pair of #041x heads.

From previous experience i knew the epoxied Heads should be a disaster because Pitot velocities were way too high at the pushrod area and all along the Floor and Short Turn apex...but the FlowBench showed almost the same exact Flow numbers between both pairs of #041x heads.

Dyno Test results @600 RPM/SEC were lost +105 HP at 7200 RPM,
and the biggest surprise was that all that very high velocity
made a little LESS peak Torque ! it was thought that atleast with all that velocity the Peak Torque numbers would be better,
but only 4000 RPM and lower were the TQ numbers slightly better, and 4500 RPM on up , TQ/HP numbers were progessively a disaster.

DragStrip test => RaceCar went from baseline 10.00's-to- teens to 10.60's ET with the very hi-velocity pair of 041x heads

heads were sold to another SS/IA racer "as is" and went 10.80's in his car
and eventually back to 10.60's with Car combination work.
Removed those Heads, completely removed all the previous epoxy work,
placed new epoxy into different port shape, reduced port velocities to acceptable levels while keeping CC legal, wound up with same Flow Numbers as before, but with acceptable port velocity.
Mostly the pushrod area velocity was the greatest problem in these heads.

DragStrip results=> 10.20's immediately, later ran 10 teens .


"Make it Flow the most CFM you can, at the smallest Port Volume"
...if taken too far , can be a disaster .

Pitot Probing the differences in Velocity between most 23 deg SBC Heads -VS- most Big Block Chevy rect port Heads...you'll notice right away that most BBC heads have much "slower" velocity , yet usually make the same amount of HP/TQ per 1 CFM of Flow , and make the correct amount of Torque per CID Ratio for Engine size and Compression Ratio as the much higher velocity SBC head engines.

i'm not saying you should slow down Port Velocities in all Heads,
but instead saying that if you make some spots in Int port have too high velocity , it will hurt you more than that same area with slightly slow velocity.

air velocity is not infinite in speed.
it requires a certain amount of cross-sectional for a given Engine cid size
and RPM operating range.

[Bos's5.0]

Holy crap, that was a good post Maxracesoftware.

Do you think the 041x heads would have different results had they been put on something like a 305 or something smaller where the velocity would have helped?

Also, what readings are you getting with your pitot tube that make you say, "uh oh, way too fast"?

[bc]

Ya when do you know when the velocities are getting to high? What should you look for?

[Donnie1126]

Is the velocity being too high causing reversion?In other words it's sort of causing too good a volumetric efficiency in a sense resulting in reversion.What if the two heads were tried on another engine with a different rpm capability?Is it possible the higher velocity heads could then win out under the right circumstances?

[maxracesoftware]

This SS/IA RaceCar had the "slower velocity" #041x Heads on
and set the NHRA SS/IA Record at that time at
1.330 , 4.058, 6.377 , 8.402 , 10.132 ET 3330 lbs.

http://www.maxracesoftware.com/Camaro_68_JF.jpg

but couldn't fall down a "Mine-Shaft" and run
those kind of ET Times with the extreme Hi-Velocity
other pair of #041x Heads with the same FlowBench CFM Numbers.

Do you think the 041x heads would have different results had they been put on something like a 305 or something smaller where the velocity would have helped?

Yes, to a certain extent !!
the faster the Piston speed and the greater
the Piston/CID CFM demand,
the sooner and more intense the Intake Port
will go into Choke Condition relative to
Cross-sectional area or some too high velocity
spot inside a Port -VS- Cam Lift/Flow Curve

depending on how many crank degrees of rotation
the Cyl Head stays in Choke condition will determine
how much Torque/HP you Loose to pumping losses.
The Choke condition will also affect your "Tuned Lengths"
and upset TQ/HP gains.


looking at examples of the same exact casting Cyl Head,
with exact same valve size OD, Chamber and Port volume CC's,
like in Chrysler 318,340,360 cid engine sizes

http://www.maxracesoftware.com/Volkman_ ... 00-600.jpg

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

http://www.maxracesoftware.com/Sonny_St ... aytona.jpg

the 360 cid Engine will go into Choke sooner and is more sensitive
to Port Velocity Profile , and the 318 cid, is a little less sensitive to too high velocity, but will still Looose TQ/HP and suffer ET/MPH Times losses if
Port Velocity is too high especially as Choke point is farther away from the Curtain Area point

i'm working on my 6th design Port Shape..all at 162.0 CCs 1.880 Int OD valve
all 6 Port shapes hold 162.0 CC and all 6 Port Shapes flow the same
CFM FlowBench numbers ....but all 6 Port Shapes have varying degrees of Port Velocities
and Velocity Profile Shapes along the Port Lengths.

One Shape makes more bottom to mid range Torque, but less mid to hi-rpm numbers

One shape makes same HP/TQ numbers on Dyno at 600 RPM/SEC, but uses more Fuel
with higher BSFC numbers that you can't fix without loosing HP/TQ, and
even though it makes same HP/TQ numbers on the Dyno, runs a little slower down the DragStrip.

One shape makes very good TQ/HP numbers throughout and very low BSFC numbers,
very efficient and runs fast down DragStrip.

Another Shape does the same as above but entire power curve is shifted slightly higher
in RPM range by 300 RPM. and this combo has run the fastest down the DragSrip in the
318,340,360 Engine sizes. This shape has the slowest overall Port velocity of all the shapes,
but when i say Slower Velocity,..its still very Hi-Velocity and on verge of having too mucjh velocity..i wish i had another 5 CC's to work with, as i'm pretty sure i could get another 10 to 15+ HP more by slowing down the spots that are on the verge of too hi velocity.

Also, what readings are you getting with your pitot tube that make you say, "uh oh, way too fast"?

Ya when do you know when the velocities are getting to high? What should you look for?

If your 180 deg Shaped Pitot Probe Velocity pressure readings are
the same or higher than your FlowBench Test Pressure,
then there's a real good chance your Engine's TQ/HP
will not correlate to what you expect from your FlowBench Numbers.

Back in 1979 i purchased a Pitot Probe..but the Probe was straight shape,
i couldn't make Heads/Tails of correlating Performance with Pitot Readings,
so i abandoned Pitot Probe testing for a year or so until one day
i had a pair of Cyl Heads that had better Flow Numbers but wouldn't run as fast
down the DragStrip as another pair of same castings with worser Flow numbers.

So i grabbed the abandoned straight Pitot Probe and had an idea to
hand bend the Probe back into a 180 deg loop to check Intake Velocity Pressure,
retested both pairs of Heads against each other on the FlowBench and
immediately saw the Velocity differences between the Heads or more
importantly what parts of the Port were showing hi velocity or low velocity.

What stoodout between the 2 pairs was the Cyl Heads that had more Flow CFM also had much higher velocity at the pushrod area as this was almost polished stock OEM size there.
so just to experiment, that pushrod area restriction was enlarged a lot ,
and re-Flow tested. The FlowBench CFM numbers lost 2 CFM at .600" Lift
but the Velocity was a lot slower and the RaceCar ran faster down the DragStrip.

From that point on i started including Pitot Probe velocity readings at various locations inside Ports along with and while i flow tested Heads on the FlowBench....tried to create a Database
of Velocity and Flow Numbers in an effort to realize what was really happening inside a Port and how it related to DragStrip performance....and also how you could make a FlowBench correlate more accurately to DragStrip performance.

in 1983?? started working with B/FX Camaro and in 1984 with Port mods by using 180 deg Pitot Probe set NHRA Record in B/FX
http://www.maxracesoftware.com/Meaux_Ba ... Record.jpg

so for about 20 years the Pitot Probe has been my friend

few years later on i saw Alan Lockheed's Engine Expert computer program and Manual
mentioned the Port Limiting Velocity aspects on Engine Performance potential.
and just recently, someone Emailed me articles/pictures of Pat Baer/ Sperry ??
articles on how to fashion and bend/strip a Pitot Probe to measure
Short Turn velocities and also Port velocity.
Nice to see other people discovering and verifying some of the
same things i've come across thru the years !!!

Darin Morgan also has previously mentioned the Pushrod Choke Area on other Posts here on SpeedTalk., and i've received a few Emails from others with different engines/heads
experiencing the same effects from Choke Conditions or Port Limiting Velocity.

What i thought i had discovered 20 years ago was brand new ,
was long ago discovered by other people...and a trip to
your local College Library , especially looking at SAE Papers
is a humbling experience !!

most everything you can think of , or any new bright idea you might have,
has already been thought about or discovered many years ago.
Sometimes material technology has to catch up to the Invention or Idea
for it to be practicle and useable.

Is the velocity being too high causing reversion?

No, its instead causing increases in pumping losses
as its taking more energy to create the too high Port Velocity

Another problem with too high localized velocity is
fuel/air separation with higher BSFCs and mixture quality suffers as well.

you might research Inlet Mach Index and/or Z-Factor on the InterNet
go to your local College Library and research thru SAE Papers

[bc]

So you check the velocity with a manometer and just use the numbers from? You dont look at ft/sec? If I understand you post correctly if Im flow a head at 28" and the pitot probe show 28" then the velocity is to high? If Im on the right track then where would you like the velocity to show if you are flowing a head at 28"?

[maxracesoftware]

So you check the velocity with a manometer and just use the numbers from? You dont look at ft/sec? If I understand you post correctly if Im flow a head at 28" and the pitot probe show 28" then the velocity is to high? If Im on the right track then where would you like the velocity to show if you are flowing a head at 28"?

you check Velocity with 180 deg Pitot Probe
you can look at the Velocity Pressure
or you can convert the Velocity Pressure into Feet Per Second and look at FPS if you rather .

FPS = ( Pitot_Press ^ .5 ) * 66.2
350.3 FPS = ( 28 ^ .5 ) * 66.2

its possible to run 350 fps @28" or slightly higher and not see TQ/HP losses, but the overall port shape/curves have to be just about perfect
to keep mixture attached to that Shape throughout the Port

300 FPS is usually safer and will not cause a premature Choke condition in a live engine . The Curtain Area should be the Choke point .

300 FPS = 20.5" Pitot pressure when Flowtesting at 28"

240 FPS = 13.14" Pitot Press when Flow testing at 28"
is considered the "slowest" you want velocity to be,
any Velocity pressure lower or around 240 fps can start to show
TQ/HP losses at lower RPM range of the Power Curve.
This also depends on piston speed range and rod ratio,
as to how slow this can be.
At relatively low pistons speeds and larger rod ratio,
240 fps will be way too slow, and loose Torque
At higher piston speeds and/or shorter rod ratios,
240 fps will not show any Torque losses.


350 FPS on the Short Turn apex is pushing it, but i've had Cyl Heads
handle higher than 350 fps there and not loose HP/TQ, and other times had Cyl Heads that had less than 350 fps show losses.

anything around or above 400 FPS (36.5" Pitot press tested at 28")
has always shown a HP/TQ Loss in a live running Engine, depending upon overall Port Shape, around 350 fps to 400+ fps can show losses of
10 hp to 105+ hp on a V8 at approx 600 HP level.

the #041x SBC Heads that showed +105 HP Loss at 7200 RPM
had slightly over 400 fps at the pushrod area , and was much higher
than 400 fps on the Short Turn apex Floor, when Flow tested at 28"

the higher the Pitot Press is above 28" or faster than 350 fps Pitot,
and the greater that distance is away from the Valve/Curtain Area,
the more TQ/HP losses will be.

another thing that will hurt TQ/HP is to have a very hi-velocity spot or curve on one side of the port wall that is 350+ fps, then downstream velocity to be much lower in FPS velocity or if that Port wall diverges or turns too quickly with much lower velocity downstream, will cause
atleast flow separation, fuel/air separation, and Choke.

Likewise if there is too much of a pressure or velocity fps difference between an upstream point and downstream point like the Short Turn or Curtain Area, then Choke will be more severe or premature in RPM range.

the better the Short Turn shape, the better the pressure recovery in the Chamber, the straighter the overall port shape and the more constant the port cross-sectional area, the higher velocities it can handle .

Since 28" Test Pressure is the FlowBench or Flow Test Industry Standard,
you can use simple Flow conversions to scale up or down the critical Pitot Probe velocity readings.

if you flow test at 10" Test Pressure then you don't want Pitot readings higher than 10"

at any chosen Flow Test Pressure, you don't want your Pitot Probe Velocity pressure to be higher than your Flow Test Pressure

10" Test Pressure = 209.3 fps
10" Pitot Pressure = 209.3 fps

209.3 fps at 10" = 350.3 fps at 28"

60" Pitot Pressure when Flow testing at 60" Test pressure = 512.8 fps
512.8 fps at 60" = 350.3 fps at 28"

[bc]

GREAT INFO!!

[Bos's5.0]

Yes, awesome information

Also, do you have to use an impact j-pitot tube, or will a straight one work?

[maxracesoftware]

Also, do you have to use an impact j-pitot tube, or will a straight one work?

No, the straight Probe won't work well enough as the tube itself and its length inside the port disturb the air enough to give you false readings

you need a 180 deg or "J" type Probe
on Intake sides

a long 90 deg for Exh ports

or a straight Probe can work on Exh depending upon how much room you have between Head and FlowBench

Could use a 90 or straight Probe on the Intake Valve area inside the Chamber if you were blowing out thru the Intake port and getting your pressure differential with that method ?

[Bos's5.0]

thanks for all your help. I will order up some probes when I get a chance Monday from Performance trends. If anyone has a better place, please tell me.

[VW Racer]

Hi folks, this is another one for Larry, I have been messing with my new Tubes and finding a lot of interesting things as far as the velocity in these VW heads I work on.
I have one head that has about 280-290FPS at the narrowest C/A in the port, and the velocity is fairly even accross this C/A, in other spots of the port up stream and down stream the numbers move around a lot. So I guess I have my work cut out.

At the valve seat ( Cam lift is .600" ) With the valve open say .500" what kind of velocities should I be seeing with the Pitot Tube.

Thanks