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post turbo air velocity?
#721157
06/11/10 10:43 AM
06/11/10 10:43 AM
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Irving, TX
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What's a resonable upper limit of air velocity in the intake tube before the throttle body?
I read something about it years ago when I was laying out my stuff but I don't remember the details.
Obviously, subsonic velocities are going to be less turbulent but what kind of air speeds are you guys seeing?
I would imagine the type of intake and thottle body would make a difference.
A standard carbed intake would have the air splattering against the floor and distributing accordingly but what about an intake like mine? It's a front loaded plenum. What kind of air distribution issues will I run into when velocity starts getting too high?
I understand the plenum becomes a pressure vessel. Pressure everywhere will be equal. However, there is a difference between pressure and air movement.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: post turbo air velocity?
[Re: feets]
#721158
06/11/10 02:22 PM
06/11/10 02:22 PM
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north cakalaky
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super stock
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Some of the guys making 2500+ hp are using a 3.5 inch inlet and 90mm throttlebody....I feel like a 3" and 75mm TB would be acceptable till 1500+ hp....The more boost pressure and smaller the motor per hp the smaller the intake tract should be able to be....I'm going top stick with my 75mm and hope to be in the 1400hp range with my new induction system.
65 Barracuda
All aluminum Indy HEMI with some boost!
COMING TO A TRACK NEAR YOU!
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Re: post turbo air velocity?
[Re: instigator]
#721159
06/11/10 03:55 PM
06/11/10 03:55 PM
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I'm running a 3" tube and calculated my velocity. It was much higher than I expected. I know there is plenty of room to grow with my car.
No doubt the velocity across the blades is down dramatically. I went from a 1000 cfm TB to a 1700 cfm blade.
I have seen some systems that were too small for my tastes.
I wonder if there is such a thing as too little velocity before the throttle blade.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: post turbo air velocity?
[Re: feets]
#721160
06/11/10 10:06 PM
06/11/10 10:06 PM
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Milwaukee WI
TRENDZ
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The inverse is true. Velocity drops with boost. Density increases causing more friction and making it harder to "bend" air around corners. I had an intake like yours on my original engine configuration. The front two cylinders would show rich plug readings until I extended the throttle body mount plate forward about 2" and put a "lump" in the floor before the 1&2 cylinder inlets. The lump causes a low pressure area to help turn the air. It made a suprising difference at lower boost levels.
In any case, increasing the tube/ throttle plate will slow the velocity and help make the turn, but it's not boost pressure that's creating velocity, it's your engines breathing characteristics.
Last edited by TRENDZ; 06/11/10 10:31 PM.
"use it 'till it breaks, replace as needed"
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Re: post turbo air velocity?
[Re: TRENDZ]
#721162
06/12/10 10:31 AM
06/12/10 10:31 AM
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Pressure and velocity are two separate things. Just because you have pressure everywhere doesn't mean you have velocity everywhere.
Air can move on one side of a pressure vessel and not on the other.
Velocity can be calculated (roughly) by figuring out the volume of air your engine consumes in one minute and comparing that to the area of your intake tract.
Pressure has NOTHING to do with volume. A 25 gallon air tank has 25 gallons of air regardless if pressure is at 5 psi or 125 psi.
Also, your engine only consumes half it's displacement per revolution.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: post turbo air velocity?
[Re: feets]
#721164
06/13/10 03:13 PM
06/13/10 03:13 PM
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Milwaukee WI
TRENDZ
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Pressure has everything to do with volume when we're talking about gasses. There's alot more volume of air in a 25 gallon tank at 125psi than there is at 5psi.
My point is this... Density is changed by pressure. Pressure creates more volume. Volume creates friction. Friction slows things down.
Try running out of the door of a burning building with a hundred other people trying to do the same thing. Its a lot faster by yourself(less volume). Alot of people will get through that door, but not at the speed you could get through by yourself(velocity).
"use it 'till it breaks, replace as needed"
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Re: post turbo air velocity?
[Re: TRENDZ]
#721165
06/15/10 11:15 AM
06/15/10 11:15 AM
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Summit, NJ
whiplash
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Quote:
Pressure has everything to do with volume when we're talking about gasses. There's alot more volume of air in a 25 gallon tank at 125psi than there is at 5psi.
My point is this... Density is changed by pressure. Pressure creates more volume. Volume creates friction. Friction slows things down.
Try running out of the door of a burning building with a hundred other people trying to do the same thing. Its a lot faster by yourself(less volume). Alot of people will get through that door, but not at the speed you could get through by yourself(velocity).
I think your confusing mass flow and volume flow. Gallons is a unit of volume. If you said there is a higher mass of air in a tank at 125psi then at 5psi, then that would be correct. The volume is constant, mass increases.
Density is changed by pressure, pressure does not create more volume. Volume does not create friction, friction is caused by objects moving. A change in position.
I don't know if there is a practical limit to the velocity. The more mass flow, the higher your boost will be, to a point where you will start to lose efficiency in the turbo and just heating things up. Any reduction in flow resistance will be a reduction in boost and an increase in efficiency. Again, within practical limits
- 67 coronet 4dr, 383/727/GVOD, blown, EFI, daily driver
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Re: post turbo air velocity?
[Re: whiplash]
#721166
06/15/10 06:55 PM
06/15/10 06:55 PM
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Milwaukee WI
TRENDZ
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If pressure does not create more volume, please explain how a balloon works.
A given volume, lets say a gallon sized container (since a gallon is a liquid measurement. Air is not liquid, though it is fluid), will hold twice the volume of air at twice the pressure.
Mass flow indicates movement, hence the word "flow". Flowing more means more friction.
You can try to disect my logic if you want, but the fact is, the speed of the air flowing through an engine does not increase with boost pressure, it drops.
"use it 'till it breaks, replace as needed"
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Re: post turbo air velocity?
[Re: TRENDZ]
#721167
06/15/10 07:22 PM
06/15/10 07:22 PM
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north cakalaky
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super stock
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super stock
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north cakalaky
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well when charge piping starts to be made out of ballons then your therios may start to ring tru.....the ballon expands because it's wall strength is weaker than the pressure inside..an aluminum or steel pipe has the strength not to expand...a pipe with higher pressure has more mass of air than a pipe at a lower pressure, and that increase of mass is what generates power...also pressure drops will lower at higher pressure with equal mass.....
65 Barracuda
All aluminum Indy HEMI with some boost!
COMING TO A TRACK NEAR YOU!
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Re: post turbo air velocity?
[Re: 8valves]
#721169
06/17/10 10:46 PM
06/17/10 10:46 PM
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Irving, TX
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Quote:
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Pressure has NOTHING to do with volume. A 25 gallon air tank has 25 gallons of air regardless if pressure is at 5 psi or 125 psi.
Also, your engine only consumes half it's displacement per revolution.
If I went with that statement, then how would a turbo motor make any more power than an NA?
As in, you're forcing more volume of air (cfm increase with compressor wheel speed) into the same available space (either your intake manifold or your engine's displacement, whichever you prefer). That increases the pressure.
As you're well aware of, more boost in the intake waiting for the valve to open is just more CFM being pumped into the same given intake plenum.
Otherwise, everyone should stop building turbo systems and strap a 50 gallon air tank in the bed of their truck and feed their engine with a 1/4" hose at 300 psi and go enter into Pro Mod. 
You're confusing yourself.
With boost, you're not pushing a greater volume of air into the engine. You are pushing THICKER air into the engine. Big difference.
Again, a 25 gallon air tank holds 25 gallons. Period. 5 psi or 5,000 psi doesn't change it's volume. It's still 25 gallons.
The turbos/blowers move massive cfm. The engine is essentially a potato in the tail pipe. It can only move so much air volume. When the turbos push more than the engine can take it all stacks up as boost. That's the same thing as turning on your air compressor. It only holds 25 gallons. The stacked up air is denser than the outside air.
Now that you're running air with more density you need more fuel to make it burn.
CFM = cubic feet per minute.
Cubic feet is a measure of volume.
25 gallon tank is a measure of volume.
Notice it doesn't say anything about density? Cubic feet of air at what pressure? Below sea level? 30,000 feet?
A turbo inlet wheel is called a compressor. That thingie making all the noise in your garage with a 25 gallon tank attached is a compressor.
They both do the same thing. They shove more air into an opening than mother nature had in mind.
A compressor shoves more air in than your tools can use. It stacks up in the tank as pressure.
A turbo shoves more air in than your engine can use. It stacks up in the engine as boost.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: post turbo air velocity?
[Re: feets]
#721170
06/17/10 11:11 PM
06/17/10 11:11 PM
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Joined: Mar 2010
Posts: 66
SRQ, FL
8valves
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Quote:
Quote:
Quote:
Pressure has NOTHING to do with volume. A 25 gallon air tank has 25 gallons of air regardless if pressure is at 5 psi or 125 psi.
Also, your engine only consumes half it's displacement per revolution.
If I went with that statement, then how would a turbo motor make any more power than an NA?
As in, you're forcing more volume of air (cfm increase with compressor wheel speed) into the same available space (either your intake manifold or your engine's displacement, whichever you prefer). That increases the pressure.
As you're well aware of, more boost in the intake waiting for the valve to open is just more CFM being pumped into the same given intake plenum.
Otherwise, everyone should stop building turbo systems and strap a 50 gallon air tank in the bed of their truck and feed their engine with a 1/4" hose at 300 psi and go enter into Pro Mod.
You're confusing yourself.
With boost, you're not pushing a greater volume of air into the engine. You are pushing THICKER air into the engine. Big difference.
Again, a 25 gallon air tank holds 25 gallons. Period. 5 psi or 5,000 psi doesn't change a thing. It's still 25 gallons.
The turbos/blowers move massive cfm. The engine is essentially a potato in the tail pipe. It can only move so much air volume. When the turbos push more than the engine can take it all stacks up as boost. That's the same thing as turning on your air compressor. It only holds 25 gallons. The stacked up air is denser than the outside air.
Now that you're running air with more density you need more fuel to make it burn.
CFM = cubic feet per minute.
Cubic feet is a measure of volume.
25 gallon tank is a measure of volume.
Notice it doesn't say anything about density? Cubic feet of air at what pressure? Below sea level? 30,000 feet?
A turbo inlet wheel is called a compressor. That thingie making all the noise in your garage with a 25 gallon tank attached is a compressor.
They both do the same thing. They shove more air into an opening than mother nature had in mind.
A compressor shoves more air in than your tools can use. It stacks up in the tank as pressure.
A turbo shoves more air in than your engine can use. It stacks up in the engine as boost.
I'm well aware of how a forced induction system works. Please refrain from repeating your statement multiple times, it just makes for uselessly long posts.
Let's make this far simpler and see how it goes for you. We'll even use your air tank example.
Your statement, "pressure has NOTHING to do with volume".
Your 25 gallon air tank has no outlets. One inlet only. At atmospheric pressure, it is already holding 25 gallons of air at whatever altitude you want as it's open to atmosphere.
The only way to increase pressure in the tank to above atmospheric is to introduce more air, beyond it's capacity, into the tank. It doesn't have to be pressurized air, just more air period (CFM, which is a measurement of volume)and not let any out.
Voila. Pressure increased based on volume.
Aaron M
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Re: post turbo air velocity?
[Re: feets]
#721172
06/18/10 06:19 AM
06/18/10 06:19 AM
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SRQ, FL
8valves
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Quote:
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Voila. Pressure increased based on volume.
Adding volume from outside the system will increase pressure.
You're not increasing the volume inside the engine.
The volume disappears once it enters the turbo. It's now thicker air.
I can fit an entire Goodyear blimp worth of air into a smaller vessel.
What happened to that million cubic feet of volume?
*poof*
It's now stuffed into a 25 gallon tank.
Hey, what's the volume of that tank? Lookie there! It's 25 gallons. Granted, it's at a heck of a lot higher pressure than the blimp.
You can't mix volume and pressure. They are two separate things.
Do I need to repeat that or did you get it this time?
Haha! No, not needed at all, but I will certainly try to make it simpler yet for you. 
But you did get it up there, I put it in bold just in case it got missed. 
This is a matter of physics. Things don't "dissapear".
The volume is still there, it's just condensed, exactly as you were onto with the density topic.
Transfer the 25 gallon air tank at 5000 psi to a 5000 gallon air tank. It's no longer at 5000 psi, yet it fills the tank. The volume didn't "dissapear".
That's how a boosted engine makes power. That's why I've put together several 122 CID setups that make more power than your car, or any of mine for that matter. The cylinder pressure increases exponentially, yet the 1200 HP 122 CID motor is still moving X CFM, just like the 1200 HP 500 CID motor. Same CFM. Just a different pressure to get it.
Once more: CFM is a measurement of volume. It IS DIRECTLY effected by pressure.
This still has nothing to do with your original post/question, but I figure someone ought to stand up in a thread where people are discussing calculating airflow velocity theories and ideas, when this basic principal was jumped right over.
Aaron M
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Re: post turbo air velocity?
[Re: 8valves]
#721174
06/18/10 11:10 AM
06/18/10 11:10 AM
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IL
furious70
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Quote:
Haha! No, not needed at all, but I will certainly try to make it simpler yet for you.
That's how a boosted engine makes power. That's why I've put together several 122 CID setups that make more power than your car, or any of mine for that matter. The cylinder pressure increases exponentially, yet the 1200 HP 122 CID motor is still moving X CFM, just like the 1200 HP 500 CID motor. Same CFM. Just a different pressure to get it.
Once more: CFM is a measurement of volume. It IS DIRECTLY effected by pressure.
This still has nothing to do with your original post/question, but I figure someone ought to stand up in a thread where people are discussing calculating airflow velocity theories and ideas, when this basic principal was jumped right over.
I'm not sure why anyone coming across as a self appointed turbo expert would be talking in terms of volume instead of mass???
Quote:
Once more: CFM is a measurement of volume. It IS DIRECTLY effected by pressure.
While not untrue, this is a very confusing statement to make as the formula for cfm has no ability to describe this effect.
lbs/min makes a lot more sense than cfm to me when trying to accurately describe the air being consumed by an engine with a turbo on it. I would like to see your math laid out that shows the 2 engines you describe above consume the same cfm (in order to do so I believe you are going to have to make a whole host of assumptions- the very crux of the problem here- trying to use a unit of volume measure when a unit of mass measure is required)
Last edited by furious70; 06/18/10 11:19 AM.
70 Sport Fury
68 Charger
69 Coronet
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Re: post turbo air velocity?
[Re: furious70]
#721175
06/18/10 11:23 AM
06/18/10 11:23 AM
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He's talking about cfm. I guess he plans on bragging about consuming all of earth's atmosphere in his 122 CID engine. What does all the air in the world outside the engine have to do with the air that's in the engine? CFM = cubic feet per minute 25 cubic feet of air at 5,000 psi take up EXACTLY the same VOLUME as 25 cubic feet of air at 5 psi. The 5,000 psi will have a much higher DENSITY than the 5 psi but the VOLUME (cfm) doesn't change.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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