pressure vs power when setting boost psi - Page 3 - Miata Turbo Forum - Boost cars, acquire cats.

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Old 03-12-2009, 04:47 PM   #41
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Originally Posted by johnhanson View Post
I'm glad you understand this now.



Uhhhhh....



Ok, I guess you don't quite get it. You acknowledge that:



Might as ask where said work to drive aforementioned compressor comes from?

Food for thought: blah blah blah look how smart I am
I always understood it. You're just nitpicking.

I'm counting the wastegate outlet and the turbine outlet together as a single orifice of variable size. This is true for most manifold/turbo setups, at least as far as the engine is concerned. When you have a less efficient compressor, you close the wastegate more to increase the amount of pressure differential across the turbine wheel. This draws more work from the engine- it has to push the exhaust gasses through the smaller orifice.
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Old 03-12-2009, 05:00 PM   #42
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Originally Posted by JimAtFSU View Post
I always understood it. You're just nitpicking.

I'm counting the wastegate outlet and the turbine outlet together as a single orifice of variable size. This is true for most manifold/turbo setups, at least as far as the engine is concerned. When you have a less efficient compressor, you close the wastegate more to increase the amount of pressure differential across the turbine wheel. This draws more work from the engine- it has to push the exhaust gasses through the smaller orifice.


You've said several things that were 100% wrong in this thread so far. So no, you didn't always understand it as you made a few false assertions that as I mentioned, would fall under 'common knowledge'.

You're responses are slowly starting to make more sense though. However, you must realize that "it drawing more work from the engine" is not exactly the big picture. IE-it doesn't just eat up 20 HP. Rather, the higher back pressure reduces VE, and you simply don't make those 20hp.
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Old 03-12-2009, 05:23 PM   #43
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Look, it's difficult to express the operation of the running system from beginning to end using ordinary english words. To make it more easily understandable, I've tried to simplify it down by discarding as much as possible that doesn't have a big effect on miata setups.

You obviously feel I've oversimplified, but all of your complaints essentially amount to "you left out this or that detail." Nevermind that every model I used to describe the operation of the system in this thread CORRECTLY describes why it works. At the end of the day compressor design has a much bigger effect on our cars than turbine design. I know why all this happens the way it does, but I didn't feel like taking a long post and making it longer. It was boring enough already.

Each configuration of engine + turbine + compressor + plumbing produces a different set of behaviors across the rpm range of the engine. Going into excruciating detail about exactly how each setup effects the volumetric efficiency of the engine at each rpm point is unnecessary. Leave volumetric efficiency and all these other considerations out of it entirely. Just discuss compressor efficiency and go do something else with the extra time left over.
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Old 03-12-2009, 05:37 PM   #44
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Originally Posted by JimAtFSU View Post
Look, it's difficult to express the operation of the running system from beginning to end using ordinary english words. To make it more easily understandable, I've tried to simplify it down by discarding as much as possible that doesn't have a big effect on miata setups.

You obviously feel I've oversimplified, but all of your complaints essentially amount to "you left out this or that detail." Nevermind that every model I used to describe the operation of the system in this thread CORRECTLY describes why it works. At the end of the day compressor design has a much bigger effect on our cars than turbine design. I know why all this happens the way it does, but I didn't feel like taking a long post and making it longer. It was boring enough already.

Each configuration of engine + turbine + compressor + plumbing produces a different set of behaviors across the rpm range of the engine. Going into excruciating detail about exactly how each setup effects the volumetric efficiency of the engine at each rpm point is unnecessary. Leave volumetric efficiency and all these other considerations out of it entirely. Just discuss compressor efficiency and go do something else with the extra time left over.
lol.
Quote:
Lets pick a magic rpm, say 4000 rpm.
X psi of boost from any turbo is going to produce the same amount of flow into the engine. The pressure differential between the intake plenum and the cylinder is the same so the flow should be the same. And it is.
Detail: 100% wrong.

Quote:
-if you are running a very small turbine this can choke off flow at higher rpms but this is IMO a relatively minor influence compared to compressor size.
And
Quote:
First off, I'm measuring pressure at the intake manifold plenum, not the turbo. So I'm not wrong. The laws of physics are pretty straightforward- cavity > passage > cavity with a pressure differential between the two cavities = a certain amount of flow between them. Unless you change the characteristics of the passage between the plenum and the combustion chamber, this isn't going to change.
Detail: 100% wrong.
Quote:
No, I was talking about flow into the combustion chamber, not through the engine.
Detail: First law of thermodynamics violated.
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Old 03-12-2009, 05:48 PM   #45
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I've tried to be polite, but **** you.

No one here measures boost at the compressor outlet unless they are mentally retarded. Everyone except ITB people (who don't generally have turbos) measure boost after the throttle body but before the intake runners.

Stating that a given psi measured at the plenum produces a given amount of flow into the combustion chamber at a given rpm doesn't violate any thermodynamic laws. It's a simple statement that a pressure differential causes the air to flow from high pressure area into low pressure area. There's no physical law that cares whether that pressure was produced by a bottle of air, a centrifugal compressor or some sort of screw based compressor. It's just compressed air. There aren't any "big turbo particles" mixed in with it. I was making this point because I felt some people were saying that 12 psi of boost produced from a big turbo would somehow produce more stress on the internals than an identical amount of boost from a smaller turbo at the same RPM. If you didn't say this then congratulations, I wasn't talking to you.
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Old 03-12-2009, 07:39 PM   #46
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Quote:
Originally Posted by JimAtFSU View Post
I've tried to be polite, but **** you.

No one here measures boost at the compressor outlet unless they are mentally retarded. Everyone except ITB people (who don't generally have turbos) measure boost after the throttle body but before the intake runners.

Stating that a given psi measured at the plenum produces a given amount of flow into the combustion chamber at a given rpm doesn't violate any thermodynamic laws. It's a simple statement that a pressure differential causes the air to flow from high pressure area into low pressure area. There's no physical law that cares whether that pressure was produced by a bottle of air, a centrifugal compressor or some sort of screw based compressor. It's just compressed air. There aren't any "big turbo particles" mixed in with it. I was making this point because I felt some people were saying that 12 psi of boost produced from a big turbo would somehow produce more stress on the internals than an identical amount of boost from a smaller turbo at the same RPM. If you didn't say this then congratulations, I wasn't talking to you.
nice av, power-queer.
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Old 03-12-2009, 09:12 PM   #47
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nice av, power-queer.
wat
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Old 03-12-2009, 09:35 PM   #48
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Quote:
Originally Posted by JimAtFSU View Post
Lets pick a magic rpm, say 4000 rpm.
X psi of boost from any turbo is going to produce the same amount of flow into the engine. The pressure differential between the intake plenum and the cylinder is the same so the flow should be the same. And it is.

Pressure (symbol: p or sometimes P) is the force per unit area applied to an object in a direction perpendicular to the surface. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.

Mass flow rate is the mass of substance which passes through a given surface per unit time. Its unit is mass divided by time, so kilogram per second in SI units, and slug per second or pound per second in US customary units. It is usually represented by the symbol .



12psi @ 20 lb/min 12psi @ 40 lb/min
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Old 03-14-2009, 12:19 AM   #49
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Quote:
Originally Posted by JimAtFSU View Post
I've tried to be polite, but **** you.
I'm not trying to be an ***, so sorry if I came across that way. I'm just debating with you, nothing more.


Quote:
Originally Posted by JimAtFSU View Post
No one here measures boost at the compressor outlet unless they are mentally retarded. Everyone except ITB people (who don't generally have turbos) measure boost after the throttle body but before the intake runners.
Ok.


Quote:
Originally Posted by JimAtFSU View Post
Stating that a given psi measured at the plenum produces a given amount of flow into the combustion chamber at a given rpm doesn't violate any thermodynamic laws. It's a simple statement that a pressure differential causes the air to flow from high pressure area into low pressure area.
You said pressure differential causes the air to flow. Hold onto that thought. This is key.


Quote:
Originally Posted by JimAtFSU View Post
There's no physical law that cares whether that pressure was produced by a bottle of air, a centrifugal compressor or some sort of screw based compressor. It's just compressed air. There aren't any "big turbo particles" mixed in with it.
lol at the big turbo particles.


Quote:
Originally Posted by JimAtFSU View Post
I was making this point because I felt some people were saying that 12 psi of boost produced from a big turbo would somehow produce more stress on the internals than an identical amount of boost from a smaller turbo at the same RPM. If you didn't say this then congratulations, I wasn't talking to you.
Uhhhhh... Bigger turbo's do cause more stress than smaller turbos for the same pressure. Duh.

And now I will somewhat explain why you are wrong.

The piston rising upward approaching TDC at the end of the exhaust stroke, the working fluid has some temperature and pressure that is no less than the pressure and temperature of the exhaust gases pre-turbine. (neglecting any scavenging effects)

Let's assume we're running a small turbo like a GT2545. We'll assume we're running 15 PSI of boost and seeing 30 PSI of back pressure pre-turbine. And let's also assume that we have an idealized valve train such that the exhaust valve is held wide open until the top of the exhaust stroke, at which point it instantly shuts and simultaneously the intake valve fully opens. And 9:1 compression.

At exactly TDC the exhaust valve slams shut while the intake valve simultaneously fully opens. Freeze time.

Now let's move forward in time 0.0000000001 seconds. What's happening? Air's going in the cylinder right? Wrong. The pressure in the combustion chamber is higher than that in the intake manifold. Like you said, flow occurs from high to low. In our 0.00000000001 second time frame the piston has not moved down much, so the gases have no place to expand other than through the intake valves and into the intake manifold. So you've got hot residual gases flowing into the intake manifold, heating up and diluting your fresh cool air/fuel mixture. How lame.

As time goes on, the piston drops, increasing the volume and pressure drops, and eventually it creates a vacuum such that flow reverses and the mixture of air/fuel/residual exhaust gases begins to fill the cylinder again.

Fact is, you can not do anything about the volume of the residual gases that will be mixed with the fresh charge. At best it's a combustion chambers' worth, but in the real world, it's more than that because we don't have a perfect valvetrain. With 9:1 compression, at best you will be mixing 1 part of say 1800*F residual exhaust gases at 30 PSI with 8 parts air/fuel at 15 PSI and say 100*F. Net result will be some air/fuel/residual gas mixture that's not as cool or dense as the charge exiting the IC. Actually, it will be MUCH less dense than the charge exiting the IC because that 1 part of 1800*F at 30 PSI residual gases are gonna expand and take up a lot of space. How much? I'll leave it at "a lot". I can do the math if you insist though.

This is why larger turbines are key.
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