Turbo questions Options

[scorp28]

I am trying to get a little better understanding of turbos. I understand the concept of how they operate and all but I was curious about this.

Alot of people go with the saying bigger is better, but is that always true. For this discussion I will just use generic terms to avoid the differences in manufacturers.

We have turbo A (small turbo) and turbo B (big turbo). For the sake of keeping all things equal lets say that both turbos are efficient to at least 15psi, and in this scenerio we are only going to be running at 15.

Not here is where I am trying to fully understand. Is 15 psi the same 15psi for both of them in which case the smaller turbo would be better because it would spool faster therefore reaping is benefits sooner, or since generally speaking a larger turbo is capable of running higher boost pressures more efficiently the air charge would be cooler than the smaller turbo (since it would be nearing it's peak efficiency and therefore be heating the air more) and therefore more dense in effect pushing more air at that same level of 15psi.

I hope you know what I am attempting to say.

I never claimed to be a physics genius or anything but just close to genius on an IQ level

[akamiami]

I wasn't able to type up a really good response as I was in a hurry for work...

As I can see it, here's the part where it gets tricky. Turbo efficiency.

If a big turbo pushes more air at slower wheel speeds then it would seem to make sense to just set it at a lower pressure. So if a t25 flows 20 lb/min of air at 18psi and a 50 trim flows 20 lb/min at 7 psi, then it would seem to be a lot better to set a 50 trim at 7 and not overly stress a small turbo and risk getting compressor surge or other such nastiness. To go back to my fan analogy, it would be better to put the big fan on the low setting than crank up the juice on the small fan and risk frying the circuit. But this is not the case at all.

You've got to consider the mass of the wheel and the energy it takes. If you look at a turbo map, you'll see what look like islands on a topological map that represent turbo efficiency. Of course more efficiency is better, so you want a turbo with a broad map that relates to the air flow you'd like to see and the operating range you'll be within. So to relate this to the previous analogy, lets say that you are given only a certain amount of juice to feed the fan (which would be the case with a small displacement, specific output engine). Now a large fan isn't going to get the most out of that energy, but a small fan could excel in this situation due to it's low inertia (less mass for fan blades, or compressor wheel).

The thing I still don't quite understand to this day is how a rotary engine gets by running what I would consider a ridiculously low amount of boost and still having a very solid power output and similar dyno charts to a conventional piston engine running more boost. I just read an article about a rotary with a GT42 pushing 30psi, and on evom, there's a huge thread about AMS's 800+ hp customer car with a GT42 at 45psi. So what gives? I doubt the FD in the first article was putting down that kind of power, but why bother with a large turbo if you're not going into it's efficiency range? Do rotaries distort turbo maps or do they just use an entirely different portion?

Here's some turbo efficiency maps to look at.
http://not2fast.wryday.com/turbo/maps/

Here's a much better, technical explanation worth reading. It covers very fundamental topics like volumetric efficiency and gas laws that are crucial to properly understanding these concepts.
http://www.gnttype.org/techarea/turbo/turboflow.html