Supersonic port flow??
Supersonic port flow?? I've been thinking about the offhand mention here of supersonic port flow and have been having trouble with the concept. I started doing some math tonight, and here is the result.
Given a 346 inch motor, we need 346 * 7000 / 3456 = 701 CFM for 100% VE at 7K. That would be about 88 CFM per cylinder. That is average, of course, and the valve is only open for, say 280 degrees of 720, so the average during that time would have to be about 226 CFM. Since the valve does not open and close instantly, the peak must be higher than that. The valve motion is sort of sinusoidal, but a triangular approximation can't be off by much and would almost surely be high. That would be a peak of no more than 452 CFM, and surely less, but more than 226. Since the intake port is about 5" long, a 225 cc port would have an average area of about 2.75 sq in or .019 sq ft, so 450 CFM is about 395 fps. That is nowhere near supersonic. I double checked and triple checked the math, but maybe I made a mistake. The throat under the valve could be less, but by the old 80% rule, it should be about 2.56 sq in which isn't much smaller. The thing I can think of is that flow might go supersonic when the valve opening is small, like as the exhaust cracks open or as the intake is closing.
Can anyone set me straight?
Given a 346 inch motor, we need 346 * 7000 / 3456 = 701 CFM for 100% VE at 7K. That would be about 88 CFM per cylinder. That is average, of course, and the valve is only open for, say 280 degrees of 720, so the average during that time would have to be about 226 CFM. Since the valve does not open and close instantly, the peak must be higher than that. The valve motion is sort of sinusoidal, but a triangular approximation can't be off by much and would almost surely be high. That would be a peak of no more than 452 CFM, and surely less, but more than 226. Since the intake port is about 5" long, a 225 cc port would have an average area of about 2.75 sq in or .019 sq ft, so 450 CFM is about 395 fps. That is nowhere near supersonic. I double checked and triple checked the math, but maybe I made a mistake. The throat under the valve could be less, but by the old 80% rule, it should be about 2.56 sq in which isn't much smaller. The thing I can think of is that flow might go supersonic when the valve opening is small, like as the exhaust cracks open or as the intake is closing.
Can anyone set me straight?
I see your math and looks correct and interesting. I use this to figure min. and max. port flow on any given v8 engine: engine size x peak rpm / 3456 x .333 = min. intake port flow and then I double that number and use the average of that and that would the max. intake port flow that that engine size can take advantage of at that rpm. Example...
346 x 7000 = 2,422,000 / 3456 = 700.8 x .333 = 233.3 x 2 = 466.6 and the average of 233cfm and 466cfm is 350cfm so low side is 233 and high side is 350cfm. Funny that the stock head flows around 233 and most high end ported LS6 heads flow around 350cfm.
346 x 7000 = 2,422,000 / 3456 = 700.8 x .333 = 233.3 x 2 = 466.6 and the average of 233cfm and 466cfm is 350cfm so low side is 233 and high side is 350cfm. Funny that the stock head flows around 233 and most high end ported LS6 heads flow around 350cfm.
6600 rpm clutch dump of death Administrator
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According to Larry Meaux sonic choke takes place @ .54 to .63 of speed of sound.
http://www.speedtalk.com/forum/viewtopic.php?t=329
http://www.speedtalk.com/forum/viewtopic.php?t=329
>>>An observation:-
The 3.178 psi, or 88 inches H20, is the result of the kinetic energy in the air stream being stopped by the closed Intake Valve.
This is different to the differential pressure across the valve seat when the valve is open - which is what you would want to simulate in a Flow Test. <<
the Pressure Differential and the Tuned Sound Wave moving thru the mixture is probably causing atleast the last 221 CC's to be moving at 620 fps to get 121.6 Ve% or a chance at turning an 14.8:1 CR into a dynamic 18:1 CR engine ....This last 221 CCs has to make it inside before the Intake valve closes, when it does, it should give 3.178 psi above atmospheric pressure inside the cylinder at the IVC point, and 121.6 Ve% approx
where is the last 221 CC's located ??
just before the Short Turn Apex towards/including the Bowl Area and Valve Job area ...the most important parts of the Port, besides the Total Induction Lengths/Cross-Sectional Areas,..where the Valve Job/Curtain Area, Chamber walls have the last chance to influence Flow
it might not be possible to make a Steady-State FlowBench make a Port go into Actual Sonic Choke at .55 Speed of Sound (613.8 FPS/ 86 Inches)
With a FlowBench, you don't have a Sound Wave moving back and forth,
and the effects of Induction Length
the FlowBench is a "Steady-State Device"
and the Engine is a "Batch-Processor Device"
being a Batch-Processor, Inertia /lag times will cause greater pressure differentials in a Live engine causing Port to go into Sonic Choke as early as .55 Speed of Sound , especially with a Fuel like gasoline or methanol
inside the Port with its Inertia influences and its Temperature/Cooling effects, affecting the Speed of Sound inside the Port
1116 FPS = 284.2 Inches of Water, Speed of Sound in Air at 59F/Sea Level
the Speed of Sound = ZERO in a vacuum
The 3.178 psi, or 88 inches H20, is the result of the kinetic energy in the air stream being stopped by the closed Intake Valve.
This is different to the differential pressure across the valve seat when the valve is open - which is what you would want to simulate in a Flow Test. <<
the Pressure Differential and the Tuned Sound Wave moving thru the mixture is probably causing atleast the last 221 CC's to be moving at 620 fps to get 121.6 Ve% or a chance at turning an 14.8:1 CR into a dynamic 18:1 CR engine ....This last 221 CCs has to make it inside before the Intake valve closes, when it does, it should give 3.178 psi above atmospheric pressure inside the cylinder at the IVC point, and 121.6 Ve% approx
where is the last 221 CC's located ??
just before the Short Turn Apex towards/including the Bowl Area and Valve Job area ...the most important parts of the Port, besides the Total Induction Lengths/Cross-Sectional Areas,..where the Valve Job/Curtain Area, Chamber walls have the last chance to influence Flow
it might not be possible to make a Steady-State FlowBench make a Port go into Actual Sonic Choke at .55 Speed of Sound (613.8 FPS/ 86 Inches)
With a FlowBench, you don't have a Sound Wave moving back and forth,
and the effects of Induction Length
the FlowBench is a "Steady-State Device"
and the Engine is a "Batch-Processor Device"
being a Batch-Processor, Inertia /lag times will cause greater pressure differentials in a Live engine causing Port to go into Sonic Choke as early as .55 Speed of Sound , especially with a Fuel like gasoline or methanol
inside the Port with its Inertia influences and its Temperature/Cooling effects, affecting the Speed of Sound inside the Port
1116 FPS = 284.2 Inches of Water, Speed of Sound in Air at 59F/Sea Level
the Speed of Sound = ZERO in a vacuum
Very good information, I always have thought it would be cool to be able to have a "pulsing" flow bench to simulate and engine drawing in air instead of a "steady state" flow of air normally seen in a flow test. Maybe a shudder wheel of some sort could be used in a flow bench.
J-Rod
Good info and good reference. There are some pertty clueful guys on that board.
Where do you find all this stuff? Are you one of those guys that runs on 3 hr sleep?
One thing though, all the research I have seen found speed of sound in the intake tract to be close to normal, not half of normal. Mabye something else is at work here?
Good info and good reference. There are some pertty clueful guys on that board.
Where do you find all this stuff? Are you one of those guys that runs on 3 hr sleep?
One thing though, all the research I have seen found speed of sound in the intake tract to be close to normal, not half of normal. Mabye something else is at work here?
Originally Posted by Bo White
I see your math and looks correct and interesting. I use this to figure min. and max. port flow on any given v8 engine: engine size x peak rpm / 3456 x .333 = min. intake port flow and then I double that number and use the average of that and that would the max. intake port flow that that engine size can take advantage of at that rpm. Example...
346 x 7000 = 2,422,000 / 3456 = 700.8 x .333 = 233.3 x 2 = 466.6 and the average of 233cfm and 466cfm is 350cfm so low side is 233 and high side is 350cfm. Funny that the stock head flows around 233 and most high end ported LS6 heads flow around 350cfm.
346 x 7000 = 2,422,000 / 3456 = 700.8 x .333 = 233.3 x 2 = 466.6 and the average of 233cfm and 466cfm is 350cfm so low side is 233 and high side is 350cfm. Funny that the stock head flows around 233 and most high end ported LS6 heads flow around 350cfm.
Thanks
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Originally Posted by J-Rod
the Pressure Differential and the Tuned Sound Wave moving thru the mixture is probably causing atleast the last 221 CC's to be moving at 620 fps to get 121.6 Ve% or a chance at turning an 14.8:1 CR into a dynamic 18:1 CR engine
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Verdad Gallardo 6600 rpm clutch dump of death Administrator
Joined: Dec 2001
Posts: 4,983
Likes: 13
From: Texas
Like you said, there are some pretty smart guys there.
I spend a lot of time digging throught hte back pages of the internet, and looking around on sites. Its amazing all the info thats out there, if you just dig for it...
I spend a lot of time digging throught hte back pages of the internet, and looking around on sites. Its amazing all the info thats out there, if you just dig for it...
Teching In
Joined: Sep 2004
Posts: 31
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Originally Posted by gollum
I am dumbfounded. Thanks for the Speedtalk site.
and what average depression the head will "be running at". id think it has more to do with the engine speed, or really rate of accleration, since time is involved and the cross sectional area in the port. has not a damn thing to to do with how much air it flows.if the head flows more, the depression created by the piston descending in the intake stroke will be less, thats a known fact.
dave
Originally Posted by J-Rod
Like you said, there are some pretty smart guys there.
I spend a lot of time digging throught hte back pages of the internet, and looking around on sites. Its amazing all the info thats out there, if you just dig for it...
I spend a lot of time digging throught hte back pages of the internet, and looking around on sites. Its amazing all the info thats out there, if you just dig for it...
I also picked up over there that nobody pays much attention to stroke/rod length.
