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Old 02-09-2009, 12:41 AM   #1
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Default Turbo Size - I'm not convinced

Ok. Can somebody explain this to me:

Why does the 2560 make more power at Xpsi than the 2554 at Xpsi?

I've heard the firehose vs gardenhose analogy, but I'm not convinced. If the manifold pressure is Xpsi (Assuming it holds Xpsi to redline), then it shouldn't matter where that pressure is coming from, the same number of molecules are going to push their way through the intake valves, and each molecule is capable of making a fixed amount of power.

I'm not denying that the bigger turbo will make more power, I just want to know why. Is it because of the turbine size instead of the compressor size?
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Old 02-09-2009, 12:47 AM   #2
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When the 2554 compressor is running close to its max capability; say it's flowing 220 whp worth of air, the compressor is demanding more power from the turbine than the GT2560 would, and thus the turbine is presenting more backpressure to the exhaust flow out of the exhaust ports. This reduces the engine's volulmetric efficiency. Its airflow is reduced for the same intake manifold pressure, thus it makes less power.
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Old 02-09-2009, 12:49 AM   #3
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Originally Posted by JasonC SBB View Post
When the 2554 compressor is running close to its max capability; say it's flowing 220 whp worth of air, the compressor is demanding more power from the turbine than the GT2560 would, and thus the turbine is presenting more backpressure to the exhaust flow out of the exhaust ports. This reduces the engine's volulmetric efficiency. Its airflow is reduced for the same intake manifold pressure, thus it makes less power.
This makes sense, so why do I hear things about firehoses vs garden hoses?
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Old 02-09-2009, 12:52 AM   #4
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Ok dude sit back and enjoy this it is simple.

The same number of molecules are not making there way through and the intake velocity is not the same becouse of heat. Through the compression of the ambient air to be forced into your car alot of heat is captured. This expands the repsective mollecules and slows the verall effeciency of your engine. Then even if you cool all the heat out and have optimal intake velocity u run into the problem of the actual compressor inlet saze and maximum wheel speed. In physics u learned only X amount of matter can pass through Y amount of space at any given rate this holds true on turbos to essentialy alil past the effeciency range of the wheels to suck in air and move it the inlet itself becomes a major restriction and with this your ability to make power. just like say u have a vac cleaner and u measure the flow with just the open hose and with a atachment to clean corners. The attachment will create more vaccuum but ultimatly flow less do to its smaller opening. Turbine size is alll about spacing out the speed of your compressor. If you do it just right the turbo runs in the optimal RPM for maximum flow and minimal strain and you make more power. I hope this helps man
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Old 02-09-2009, 12:53 AM   #5
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the fire hose and garden hose is a simple way to show that the same pressure under a bigger orifice will always flow more.
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Old 02-09-2009, 01:00 AM   #6
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All else equal... if there is 10psi in the intake manifold, the orifice size at the compressor doesn't matter... the manifold can't see that. I understand how backpressure at the turbine would make a substantial difference.

If we take our engine to be the control volume, from just before the intake valves, to just after the exhaust valves, the only difference between one turbo and another (at the same PSI and intake temp) would be on the exhaust size... yes?
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Old 02-09-2009, 01:18 AM   #7
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lol that holds true if you arentconsidering the motor is using the air. So essentialy the air the motor uses becomes less than the turbo can provide man we run out of hp and boost preformance etc will go down. its balance man with a stuffed cat or restricitive exhaust u could make 60 psi and still no real hp but with a open downpip and a small *** compressor u wont make alot of hp either the trick is finding a balance u can live with.
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Old 02-09-2009, 01:20 AM   #8
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Quote:
Originally Posted by skidude108 View Post
Ok. Can somebody explain this to me:

Why does the 2560 make more power at Xpsi than the 2554 at Xpsi?

I've heard the firehose vs gardenhose analogy, but I'm not convinced. If the manifold pressure is Xpsi (Assuming it holds Xpsi to redline), then it shouldn't matter where that pressure is coming from, the same number of molecules are going to push their way through the intake valves, and each molecule is capable of making a fixed amount of power.

I'm not denying that the bigger turbo will make more power, I just want to know why. Is it because of the turbine size instead of the compressor size?
Yeah, what they said.

Basically the compressor is sized to move a certain mass flow of air. Say 20-25 lb/ min of air. Well, to do it, it needs a certain amount of power. The turbine is sized to provide this power. Say the compressor needs 20hp to move 20 lb/min of air and needs 25 hp to move 25 lb/min of air.

The turbine provides work (power) to the compressor. The amount of work out of a turbine is equal to the difference (delta) in enthalpy across the turbine. Enthalpy is a property that consist of other properties. That is enthalpy, h, is equal to internal energy, u, plus pressure, p, times volume, v. h= u+PV.

To get more work out of a turbine you need to increase the difference in either u, P, or V. Adding a bigger down pipe drops the pressure after the turbine. This creates a bigger difference in pressure across the turbine, hence a bigger difference in enthalpy. More work, more spoolage. As you change the throttle and RPM, the volume going through the turbine gets bigger, increasing the difference in enthalpy.

So a small turbine, say a .48, is more restrictive as to cause a greater pressure difference across the turbine, or a bigger enthalpy difference. This means more work out of the turbine sooner to drive the compressor. But this comes at the expense of greater back pressure to the engine. No free lunch. But now you're moving 20 lb/min of air.

A bigger turbine, say a .78, is less restrictive. So the delta P across the turbine is reduced. That means the delta h is also reduced, so you get lag. But once it's spooled, there's less backpressure against the motor. So you're actually moving more MASS FLOW through the engine as there's less back pressure pre-turbine. Even though you might be running the same boost pressure, there is more mass flow. IE-25 now instead of 20, even though the boost pressure is still the same.
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Old 02-09-2009, 09:16 AM   #9
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Quote:
Originally Posted by patsmx5 View Post
Yeah, what they said.

Basically the compressor is sized to move a certain mass flow of air. Say 20-25 lb/ min of air. Well, to do it, it needs a certain amount of power. The turbine is sized to provide this power. Say the compressor needs 20hp to move 20 lb/min of air and needs 25 hp to move 25 lb/min of air.

The turbine provides work (power) to the compressor. The amount of work out of a turbine is equal to the difference (delta) in enthalpy across the turbine. Enthalpy is a property that consist of other properties. That is enthalpy, h, is equal to internal energy, u, plus pressure, p, times volume, v. h= u+PV.

To get more work out of a turbine you need to increase the difference in either u, P, or V. Adding a bigger down pipe drops the pressure after the turbine. This creates a bigger difference in pressure across the turbine, hence a bigger difference in enthalpy. More work, more spoolage. As you change the throttle and RPM, the volume going through the turbine gets bigger, increasing the difference in enthalpy.

So a small turbine, say a .48, is more restrictive as to cause a greater pressure difference across the turbine, or a bigger enthalpy difference. This means more work out of the turbine sooner to drive the compressor. But this comes at the expense of greater back pressure to the engine. No free lunch. But now you're moving 20 lb/min of air.

A bigger turbine, say a .78, is less restrictive. So the delta P across the turbine is reduced. That means the delta h is also reduced, so you get lag. But once it's spooled, there's less backpressure against the motor. So you're actually moving more MASS FLOW through the engine as there's less back pressure pre-turbine. Even though you might be running the same boost pressure, there is more mass flow. IE-25 now instead of 20, even though the boost pressure is still the same.
So it has more to do with the turbine than the compressor... which is what I thought.
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Old 02-09-2009, 09:19 AM   #10
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Quote:
Originally Posted by skidude108 View Post
So it has more to do with the turbine than the compressor... which is what I thought.
Egg Zachary!
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Old 02-09-2009, 09:30 AM   #11
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It's a government conspiracy funded by big turbo lobbyists.
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Old 02-09-2009, 10:06 AM   #12
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Originally Posted by SloS13 View Post
It's a government conspiracy funded by big turbo lobbyists.
I love that a GT2560 is the "big turbo", only miata guys could pull that off

As far as psi vs psi, maybe I'm mistaken but I think this is being way overcomplicated. The answer as I understand it is "heat". As a turbo gets out of its efficency it creates more heat. As the temperature rises the air becomes less dense and makes less power. It's the same reason a car will be much faster when ambient temperatures are 50 degrees vs 100 degrees.
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Old 02-09-2009, 10:10 AM   #13
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Quote:
Originally Posted by skidude108 View Post
All else equal... if there is 10psi in the intake manifold, the orifice size at the compressor doesn't matter... the manifold can't see that. I understand how backpressure at the turbine would make a substantial difference.
pressure is a just a measure of restriction.

10psi does not tell you if one compressor is moving 20 lb/min and the other is moving 30 lb/min. Only that you have built 10psi of pressure built up in front of the intake valves.

The larger compressor will be moving more air at the same pressure level.
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Old 02-09-2009, 11:05 AM   #14
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Quote:
Originally Posted by Braineack View Post
pressure is a just a measure of restriction.

10psi does not tell you if one compressor is moving 20 lb/min and the other is moving 30 lb/min. Only that you have built 10psi of pressure built up in front of the intake valves.

The larger compressor will be moving more air at the same pressure level.
This is the part I am not convinced of... There should be the same restriction regardless of the source of pressure.
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Old 02-09-2009, 11:16 AM   #15
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Change out your 14b for a "big" 16g, run the same amount of boost and you'll feel the difference
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Old 02-09-2009, 11:20 AM   #16
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Quote:
Originally Posted by skidude108 View Post
This is the part I am not convinced of... There should be the same restriction regardless of the source of pressure.
The pressure is the same, but the amount of air getting compressed and forced in the motor within the same volume varies due to the larger compressor size.

this is why you'll see setups like y8s's where he's making 260rwhp with only 9.5psi, he has a huge 76mm compressor, but he shares the same turbine as a 2860 or 2871.
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Old 02-09-2009, 11:25 AM   #17
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The larger compressor will be moving more air at the same pressure level.
Not if they're feeding the same engine. They will be operating at the same point on the compressor map. (assuming that the intercooler will be negating the effects of different outlet temps, and assuming the engine somehow sees the same exhaust backpressure.)
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Old 02-09-2009, 11:31 AM   #18
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Quote:
Originally Posted by Braineack View Post
10psi does not tell you if one compressor is moving 20 lb/min and the other is moving 30 lb/min. Only that you have built 10psi of pressure built up in front of the intake valves.

The larger compressor will be moving more air at the same pressure level.
This has always bothered me, and I think it's an oversimplification. For the same turbo installed on two different engines with varying amount of headwork, it's true that 10 PSI ≠ 10PSI. But for two different turbos installed on the same engine, it is less true.

Assuming identical intake tracks, valves, porting, etc., the primary difference between 10PSI out of turbo X and 10PSI out of turbo Y is heat. The volume flow ratio (cubic feet / min) will be comparable for the two turbos blowing into the same head at the same PR. However turbo X, being smaller, is making more heat at 10PSI, so the air charge is less dense. Thus, for identical volumes of air, the bigger turbo will be putting slightly more mass into the manifold. Assuming that both turbos were exhaling through a hypothetical 100% efficient intercooler, then they'd both be flowing roughly the same mass and volume at the same PR.

patsmx5 nailed an important variable, which is backpressure due to varying turbine configurations. During the exhaust cycle, a smaller turbine will create a larger restriction, thus causing a larger mass of burnt gas to remain behind in the chamber, reducing effective VE.

IOW, there's no one single reason for large turbos to make more power than small turbos at the same PR. Many variables interact.
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Old 02-09-2009, 11:43 AM   #19
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so let me get this straight:
Turbine affects power mainly due to the restriction it presents to the exhaust.
Compressor affects power mainly due to air temperature.

Makes sense to me. So my t3 with .63 hotside should put out a bit more power than if i had the .48 hotside. But if I were to increase the compressor size within reason, I may or may not gain power through coooler intake temps depending upon the intercooler efficency

edit: the compressor would also need less power from the turbine to make the same boost, so more power from that too.

Last edited by akaryrye; 02-09-2009 at 11:57 AM.
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Old 02-09-2009, 11:49 AM   #20
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Quote:
Originally Posted by Braineack View Post
pressure is a just a measure of restriction.

10psi does not tell you if one compressor is moving 20 lb/min and the other is moving 30 lb/min. Only that you have built 10psi of pressure built up in front of the intake valves.

The larger compressor will be moving more air at the same pressure level.
This is sometimes true, but it's not just because the compressor is bigger. Again, it falls back to turbine efficiency. If you have a small compressor maxed out moving 30 lb/min at 20 PSI, it's way out of its efficiency range and it takes a lot of power to drive it. That power comes from the turbine. The wastegate has to stay further closed as to increase backpressure, increasing the pressure difference across the turbine to get the needed work to drive the compressor.

So it's possible that adding a bigger compressor that's more efficient (key word) for this particular setup will make move the same 30 lb/min and do so using less power from the turbine. It needs less power, so the wastegate opens more and there's less back pressure. Less restriction, less boost needed to move the same 30 lb/min. Or if you run the same 20 PSI, you'll net a higher mass flow, say 35 lb/min.

I'll also expand this and say it's possible that you put a bigger compressor that's "too big". It may be so big (think T3/T4 turbos) that the turbine can't efficiently produce enough power to run it. In this case, once again the wastegate has to stay closer to shut to increase the backpressure and delta P across the turbine to get enough power to drive the comp. A bigger compressor is not always better.

Figuring efficiencies gets messy. There's a lot of variables that affect everything. Turbine specs, compressor specs, shaft speeds, engine's constantly changing VE, exhaust backpressure and it's constantly changing as well, IC efficiency, and so on. No one thing can be changed without affecting other systems.

Last edited by patsmx5; 02-09-2009 at 12:22 PM.
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