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

[wortdog]

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?

Easy, a 13B rotary is the equivalent of a 2.6 liter piston engine, plus it probably rev's higher than AMS's EVO. And, the 'peak' effeciency island of a 'little' 94mm compressor wheel GT42 is from 12.5psi to just over 30psi, while the peak effeciency island of a 'big' 104 mm compressor wheel GT42 is from 7psi to 23.5psi. Just because a turbo is bigger doesn't mean it is more efficient at higher boost levels, but it can flow more air that a larger/higher revving/more efficient engine might need. Interestingly, the efficiency island on the larger GT42 stretches much farther and allows the big bad 104mm to top 50psi and still be moderately effecient, while the little guy is stuck around 45psi.

Boost numbers mean NOTHING between different setups on the same engine, let alone between different engine layouts/sizes. Just because a turbo CAN flow X air @ X psi doesn't mean it will on your motor. Also, the compressor efficiency maps have nothing to do with how hard the wheel has to be spun to compress the air, they only have to do with how much heat is placed into the air by the act of compressing it.

If you had a hypothetically perfect intercooler(such that the charge air temp was cooled to ambient after being compressed), and a T-25 and a 50-trim with the same exact exhaust housing and wheel, you'd see almost exactly the same horsepower PSI for PSI until the T-25 chokes out on the compressor side, somewhere around 14psi in the upper RPM ranges.

Transitioning into real life, some of the gains you see at the same boost level when upgrading turbos come from gaining effeciency in the compressor side, while most of the gains typically come a more efficient exhaust housing that sacrifices spool for power. That's why the small 16G and big 16G make very similar power at the same boost levels on nearly stock cars, as they have the exact same turbine wheel and housing.