Turbo Discussion - Trim, efficiency, Blade design, and other complicated crap

[spek1098]

This thread is specifically about the turbo its self.

That's one aspect--another has to do with the efficiency range of the turbo. Like any pump or compressor, flowrates, pressures, pumping losses, etc are all related. Turbo A may be capable of feeding 15psi to a given motor but it will not necessarily make the same power as Turbo B feeding the same pressure and flowrate.

See this is where I lose it with turbos, how does that work? If they both both put the same mass of air into the engine why don't they make the same power? Is that where the term efficiency comes into play? From my current understanding then, it must have to do with the exhaust side, not being able to fully remove all all the exhaust from the cylinder before the next cycle because of the backpressure caused by the turbo's exhaust side blades. Am I on the right track?

There is no simple answer really. There are a bunch of thermodynamic principles in play it a turbine/compressor system, and they are generally non-linear. By non-linear, I mean that changes to inputs do not produce proportional inputs to outputs.

And here we are..

So, Jay, do you know the physics of this pretty well? Do you know some of the other factors involved that cause, as you said, turbo A to give more power vs turbo B while moving the same mass of air?

 

[dmention7]

Well one factor is going to be that a turbo operating past its efficiency range (i.e. the stocker in the speed3 being pushed to 300+whp) is going to heat the charge air a lot more than one that is properly sized. This could require you to pull timing or take other power-lowering precautions to avoid detonation, aside from the fact that we all know that a given volume of hotter air at a given pressure contains will contain less oxygen than the same volume/pressure at a lower temp.


I have a pretty good handle on thermodynamics, but less so the practical applications as they show up in a turbo engine.

 

[JustROLLIN]

When you read about efficiency ranges, think about how fast the turbo is spinning to produce X amount of boost. There are other factors involved, as Jay mentioned, but this helps to understand it. With a larger turbo, the turbo is not "working" as hard to produce X lbs. of boost. Due to less "work" the turbo is doing, it will generate less heat. The lower the amount of heat, the denser the air, and in theory, the more HP you will make.

Now, with that in mind, people tend to assume that bigger is always better. But, when you increase the size of the turbo, you will start to experience that lovely turbo "lag" or the amount of time it takes to hit X lbs. of boost. Also, when using a larger turbo you can actually operate below the efficiency range. While this is not damaging to the car, it is damaging to your pocketbook. You have spent the extra money you had on a large turbo, when a smaller turbo would have suited your application just fine.

Also, something to keep in mind are you power goals. While using a smaller turbo to begin with may be just fine, always know how high you want to go with power. That way if you are debating between say a t28 and .50 trim, you will know which may be better in the long haul.

 

[mndsm]

Also, consider what you want the turbo to do for you and what kind of engine you are working with. A t28 is going to spool incredibly quickly, even moreso on a larger displacement engine. The problem is, generally the quicker the spool the quicker the turbo runs out of boost. Perfect example being the old stocker on my Talon. That sucker was spooling damn near at idle, with full boost right around 3k RPM. But, the power range was incredibly small on it, and it ran out of room @ about 5500 rpm. Now a .50 trim might not spool til 4k depending on the motor, BUT will outrun something a lot smaller on the top end, delivering much more power at 6k than a smaller quick spooling turbo. Precisely why the Supra dyno queen guys run gigantic turbos. They are generally after the taller number, without regard to power delivery. Sure they may get 1000hp off a .76 trim monster, but they aren't even at full boost til 5-6k. It's all in the final goal for the vehicle.

 

[JustROLLIN]

I wanted to share a link with anyone looking for addtional information on turbos and how they work. My Microeconomics teacher and I got into some discussion about cars one day in class. Well, he explained that his son is an engineer for Garrett Turbos and has done a great deal of studying on the subject. He also mentioned that his son wrote an article for Mazda about turbo basics..... Anyway, here is the link. There is a great explanation and incredible graphics and photos to accompany the material.

http://www.mazdausa.com/MusaWeb/dis...ommonProtege&sectionParameter=session4&bhcp=1

 

[mndsm]

Excellent find Joe.