Sonic Choke 2

[ashmig]

maxracesoftware, You have listed on this site the problems created by sonic choke, specifically, Dyno HP below levels predicted by the raw CFM numbers from the flow bench. Does this mean that veloctity, as measured by J type pitot tube, should not be above the numbers you have listed before. I am currently modifying a port. The velocity on the floor of the port along the short turn radius is 220 fps at 10 inches. The roof of the port in the same area is 155 fps. The velocity at the entrance averages 150-170 fps with the velocity higher on average along the left side or cylinder wall side. The port is rectangle 2.1" X 1.25" and 4.9" long, 2.02 valve, 45 degree seat. After moderate modifications, i.e. Combustion chamber, bowl, raising roof, and pushrod wall, produced little results, I then lowered the floor at entrance and widened short turn, which produced huge difference, but really increased velocity in the area of the Short turn radius. From your previous post you stated that velocity >209 fps at 10 inches can lead to Sonic Choke, is this value an average or does it mean any measurement, anywhere in the port? On average the port velocity is around 260 fps @ 28 inches @ .396 L/D. A second question, before .25 L/D, when the valve velocity and port velocity become the same, the calculated valve velocity is 350 fps at 28 inches .050 L/D, to around 220 FPS @ .198 L/D does this also relate to sonic choke? Thanks for any info.

 

[MaxRaceSoftware]

>>>The velocity on the floor of the port along the short turn radius is 220 fps at 10 inches.<<<

if most of the entire Port floor's Short Turn apex area is at 220 fps at 10 inches of Test Pressure ....you are very close or already into Sonic Choke on a live engine and may already be seeing HP losses

if you make a FlowBench CFM gain in this ST-area ,
but see a LOSS in HP when on a Live engine on the Dyno
...yes it went into Air/Fuel separation and possible sonic choke. The worse the Port-Axis -to- Valve-Axis centerlines the sooner in FPS it will go into choke or separation as you make the speed faster there.

The Port's velocity is more critical than the Short Turn velocity

you can loose 5 to 25+ HP on the Short Turn even at the same time you see FlowBench CFM gains

but you can loose over 75+ HP with too much Port velocity
on a 8 Cylinder near 600 HP output , almost 13 % Power
even though you see FlowBench CFM gains


<<<< A second question, before .25 L/D, when the valve velocity and port velocity become the same, the calculated valve velocity is 350 fps at 28 inches .050 L/D, to around 220 FPS @ .198 L/D does this also relate to sonic choke? Thanks for any info.<<<

yes, but its the ST velocity and more important the port velocity FPS that can hurt HP/Torque
don't worry about low-lift valve velocity fps

you need to solve any ST or Port velocity problems
as these spots that will affect the engine intake stroke when the piston is beginning to move the fastest speed down the bore all the way to 180 deg+...if the Intake Port goes into Sonic Choke earlier than this or at that period...pumping losses increase and there will be less air in the Cylinder reducing HP

Theoretically the Curtain Area = Valve Area at .25 L/D Ratio

before or at .25 L/D Ratio the Port velocity "does not" equal the valve velocity...the Port velocity is slower

typically on a ported head, the Port velocity is max near and from .37 L/D Ratio on upwards to .42 L/D Ratio
..and in this same range of L/D Ratios, the curtain area velocity is slowing down as the port velocity is increasing
usually, as you increase Valve Lift and if the Port keeps flowing more ..then Port Velocity is also increasing


and where to measure the Port velocity ?? and at what valve lift or L/D Ratio are the most important ??
---i've already posted way too much Public Info on this
you'll have to experiment to find out where and when




Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

 

[MaxRaceSoftware]

a Port has "zero velocity" with the Valve closed/seated

to make decent use of Port Velocity, you should have a Cam with at least .25 L/D Ratio and upwards to "get on the Port"

a small block Chevy head with 2.200 OD Intake valve and .310" stem diameter
Flow Test

Valve-----CFM----------Curtain Area-------------Valve Area
Lift------@28"-----Sq.Inch---FPS---CFM/SqIn------CFM Sq/In
.200-----150.8-----1.049"---344.9--143.7------------40.5
.300-----214.9-----1.574----327.6--136.5------------57.7
.400-----268.1-----2.099----306.6--127.7------------71.9
.500-----314.2-----2.623----287.5--119.8------------84.3
.600-----353.6-----3.148----269.6--112.3------------94.9
.700-----380.8-----3.673----248.9--103.7------------102.2

lets say the Port entrance = 3.360" Sq.Inches Area
Valve------Port Entrance Velocity
Lift-------in Feet per Second (Theoretical)
.200-------107.7
.300-------153.5
.400-------191.5
.500-------224.4
.600-------252.6
.700-------272.0

2.200" OD times .25 L/D Ratio = .550" Valve Lift

to get decent Port Velocity you need at least .550" Lift
with a 2.200" OD intake valve (238.5 fps)





Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

 

[LJW]

Thanks AGAIN Larry !
The posts above are the most concise explanation of this topic I have seen.

 

[Viper488]

This guy has some thoughts on what I think is the same subject as above. So put the link and a partial quote for those of you more versed in this subject to peruse:

http://mototuneusa.com/super_sonic_nozzle.htm

Part of what he says is: In 1994 I had a conversation with 3 guys from an engineering college that were in the Formula SAE competition.?This is a contest in which the students build a race car using a motorcycle engine, and then compete against teams from?other schools all over the USA.?Anyhow, as we were driving down the road in my van ...? I told them about how my customers were winning everything in sight at the motorcycle races?using my High Velocity Ports. The first thing they all said was:?"Yeah .but what about 'Supersonic Nozzle'??" Super - Sonic - Nozzle ??? " That was the funniest phrase I'd heard in a long time, and the more I thought about it, the funnier it got. Pretty soon I was laughing so hard,?I almost?lost control of the vehicle....!!!"No, seriously", they said, "supersonic nozzle is why small ports don't work ... we learned it in engineering school !! When a gas reaches the speed of sound (supersonic) the flow becomes really turbulent, creating a huge nozzle effect (restriction)." ...? ****************************************MotoMan (the author) Says: I'm not saying that the supersonic nozzle effect doesn't exist. For example, if you wanted to get the maximum amount of air to pass through a pipe as fast as possible, you wouldn't want to restrict it with supersonic turbulence. Of course the solution would be to make the pipe bigger !!But, the problem with applying that idea to engines is that it assumes that engine airflow works the way it does on a flow bench ... that port flow is a constant steady stream. It also assumes that the cylinder is only filled by the?action of the?piston, and ignores the charging phenomenon.?This is an example of "half science"....?Which is when a phenomenon is observed and found to be true,?and then incorrectly?applied to another situation.? What's Actually Happening: It's true that at the peak of intake port velocity, high velocity porting does cause the airflow to reach and even exceed supersonic speed, and the "nozzle" causes the port to not gain any more flow at that point in the cycle. The flow is momentarily restricted, but it's important to note that that doesn't mean it stops flowing air !!?When the port velocity slows to below supersonic, and the piston is returning up again, the charging effect overcomes the loss created by the dreaded SuperSonicNozzle !!?The overall effect is a net gain in cylinder filling over the intake period. Back To The Engineering StudentsThe big problem is that the students had automatically closed their minds to exploring & testing that area of engine technology ...... because a textbook "expert"?said it wouldn't work !!??The saddest thing of all is that now 8 years later those engineers could be on the cutting edge of the automotive industry ! They should be leading the way and in control of their own careers, instead of just?"following the flock".