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Old 01-04-2009, 02:18 PM   #41
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Is this in fact true?

This is one area of turbo math where I start to get a little hazy. We've basically got a closed system with three variables- temperature, pressure, and air mass.- all of which are related. If you increase temperature for a given pressure, mass decreases. If you increase pressure without an increase in temperature, then mass goes up. Etc.

It seems to me that for any two turbos on a given engine, if temperature after the intercooler is the same, and manifold pressure is the same, then air mass per unit volume is the same.
This should hold true even for a GT25 vs. a GT40, assuming the hypothetical intercooler is 100% efficient.

So, if this is true, then any difference in power output, efficiency, timing requirements, etc., between the two turbos should be ascribable to some other factor. Maybe the back-pressure applied against the exhaust side of the cylinder by the turbine. It'd certainly affect scavenging and cylinder fill. Smaller turbine, more backpressure, less efficient exhaust cycle, more unburnable waste gas left after the exhaust cycle, less power. Kinda like EGR for boost.

Or maybe I'm completely wrong. But then, what's the real explanation? I've read many times that "X is true" but I really can't understand why in this case.
The bold part is correct, but I think you're confusing mass per unit volume (density) with mass flow rate. Think about it. Assume Savington and I are both on a dyno with a 100% efficient intercooler. He's running a GT2554, I'm running a GT3271. Both running 12 PSI. Identical motors and all. Who do you think is gonna make more power?

The turbine is not being considered in your hypothetical analysis. Consider how an A/C system works. In such a case, the compressor is the compressor on the turbo and the turbine is the orifice tube (throttling device) Given the same temp/pressure/density of the working fluid, actual flow rates will vary depending on the size of the restriction.

And once you understand this, it's easy to see why smaller turbos can run more timing than bigger ones. For the hell of it, I'm gonna put 87 in the tank when I fill up next time.
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Old 01-04-2009, 05:46 PM   #42
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The bold part is correct, but I think you're confusing mass per unit volume (density) with mass flow rate.
I don't think I am, and here's my reasoning.

Why do turbos produce boost? Because they are attempting to push air against a restriction- namely, the intake side of the engine.

So with MAP and MAT being a constant between the two turbos, the mass of air per unit volume in the intake manifold will also be the same. So far so good.

Now, given that two cars, one with turbo A and the other with turbo B both have identical conditions inside the intake manifold (temperature, pressure, and density) why would the mass flow rate of air entering the cylinder during the intake cycle be any different for one vs. the other?

Like I said earlier, I can see that the size the of the turbine would have some effect here. A smaller turbine would create more backpressure, the reducing the efficiency of the exhaust cycle and causing the cylinder not to be as completely scavanged as one exhaling into a lesser restriction. But what does this have to do with cylinder filling? Is it so bad that there is signifigant reversion out the intake at the beginning of the intake cycle owing to residual cylinder pressure and/or valve overlap?

I don't question that there is a difference, I've just always had difficulty fully conceptualizing why.

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Think about it. Assume Savington and I are both on a dyno with a 100% efficient intercooler. He's running a GT2554, I'm running a GT3271. Both running 12 PSI. Identical motors and all. Who do you think is gonna make more power?
Depends. Do you both have the same number of PCPros?
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Old 01-05-2009, 12:25 AM   #43
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I don't think I am, and here's my reasoning.

Why do turbos produce boost? Because they are attempting to push air against a restriction- namely, the intake side of the engine.

So with MAP and MAT being a constant between the two turbos, the mass of air per unit volume in the intake manifold will also be the same. So far so good.

Now, given that two cars, one with turbo A and the other with turbo B both have identical conditions inside the intake manifold (temperature, pressure, and density) why would the mass flow rate of air entering the cylinder during the intake cycle be any different for one vs. the other?

Like I said earlier, I can see that the size the of the turbine would have some effect here. A smaller turbine would create more backpressure, the reducing the efficiency of the exhaust cycle and causing the cylinder not to be as completely scavanged as one exhaling into a lesser restriction. But what does this have to do with cylinder filling? Is it so bad that there is signifigant reversion out the intake at the beginning of the intake cycle owing to residual cylinder pressure and/or valve overlap?

I don't question that there is a difference, I've just always had difficulty fully conceptualizing why.

Depends. Do you both have the same number of PCPros?
Yes, the turbine is the reason why they're different. On a typical, every day turbo setup, the exhaust pressure pre-turbine is ~ twice intake manifold pressure.

Let's say you're running 12 pounds of boost and have 20 PSI of exhaust pressure pre-turbine.

Motor fires, and the exhaust valves open. At first, the charge is around 80 PSI of pressure give or take, so as the piston is descending, the charge is actually flowing out. Eventually the piston hits BDC and begins its assent to expel the engine of residual gases from combustion.

There are several problems. For one, no matter what you do, you'll never get pressure inside the piston/cylinder arangement to drop below what the exhaust pressure is (not considering scavenging effects obviously). Now remember there's some overlap, ~17* or so IIRC. So what happens when the intake valves open and the exhaust valves are still open? You guessed it, reversion. This dilutes the air/fuel mixture that's about to go into the engine. Even if there was NO overlap whatsoever and the exhaust valve magically stayed completely open until exactly TDC, and then at that exact moment it shut while the intake valve opened, the pressure in the cylinder is still higher than in the intake manifold.

So the actual volume of charge that is drawn into the cylinder is a function of how much exhaust is left in the combustion chamber and how much is reintroduced into the cylinder during overlap. And how much exhaust is reintroduced during overlap is a function of the pressure differentials between the exhaust and intake charge, in this example, 8 PSI.

If you think about it, that's HUGE! I'd be willing to bet that during the intake stroke, the pressure differential isn't nearly that big as the piston doesn't actually pull a vacuum. It just creates an area of low pressure that causes the charge to flow in.

And of course the pressure differential between the compressor and turbine is dependent on the turbine selection. Given the same compressors, the turbine has to make the same amount of work for a given boost pressure. The smaller turbine uses less mass flow and more delta P, the bigger turbine uses more mass flow and less delta P to get the same delta h, enthalpy.

EDIT: And I'm pretty sure we're both running the same number of PCpros.
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Old 01-05-2009, 03:37 AM   #44
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Makes sense. Whenever I've heard the matter discussed, it's always been in the simple form of "X PSI for turbo A is not the same as X PSI for turbo B" without really going into detail as to why. I'd always looked solely at the compressor-to-chamber side of the equation, without properly considering the effect of the turbine on the cylinder fill process.

I always hated fluid dynamics.
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