Originally Posted by
croustibat
Ok, here is how to read these maps:
vertically you have the pressure ratio, ie the difference between turbo air inlet and outlet. Watch that sheet of paper, and tell me what you see on the 2.5bar line (which is 1.5ar boost).
Horizontally you have the flow of your engine, which depends on its displacement, volumetric efficiency, the boost you are running and the rpm. As it is approximate, use 90% VE, it works ok.
The closed forms in the middle are called islands, and are the efficiency (remember it is a compression), and the "somewhat horizontal lines" are at iso turbo shaft speed. You never want to be on the left of the islands, which is surge domain, nor over it, which is overspin domain. And you dont want to be anywhere OUT of the islands in fact, because that is unefficiency domain (but that last one does not break turbos, it just kills performance)
Basically you want to stay in the center island most of the time.
On the pic i posted, someone has already drawn the flow of a CA18DET on various pressures.
The oem turbo on a CA18DET should not be run past 1bar, because it simply becomes unreliable and unefficient there. 1.5bar is just completely out of the map, so thats a no no.
R5 turbos dont run a T25, they run a T2, and they use a 1.4L engine; you cant compare them just because they have "somewhat the same size". It does not work like that. Finally, if many people do the same stupid thing, it does not make it good; it makes them stupid. If they truly run 2bars on a CA18DET turbo, that makes them completely stupid.
A GT2871r is still a T28 sized turbo, and it can nearly run 2bars ... of course you can make it produce 2.5bars. But it wont be efficient; a gt3076r will produce way more power at the same pressure (efficiency), and surely the same power at 2.1bar.
A turbo is not a magical device; it produces flow at a given pressure, and it can only do that if it is powered in some way (there comes the usual "there is no replacement for displacement"; if you want a lot of flow, you need a big engine, or you will have a very narrow boost range).
A T28 has around the same map as a gt2560r ; you can see the maps on turbobygarrett.com . It can barely flow up to 1.5bar . His is a hybrid between the 2, so limits are somewhere in between.
It is not a question of ball bearing or not, ball bearing turbos tend to be less tolerant to overboost / overspin too.