"Restrictive" is relative. What works at mid revrange on a 4 valve head engine that is set up in all ways to work with that turbo is one thing. On a less efficient engine that needs longer duration cams to flow it's not a perfect match.
The Saab engines are high CR (9.3:1) and can take huge amount of back-pressure due to the fact that the cams are really short in duration. when you put on a larger turbo it all needs to be changed to adapt, you would probably get less lag if the engine was adapted to suit it. I have a Saab too but woth the GT17 turbo which is about the size of a fist. It's
useless over 4000rpm and I STILL think it's laggy. My 155 V6 is WAY quicker at all revs.
You sure can put a TD04 on an alfa, I had a 16T with a #7 turbine on my 75T and while the top end was improved due to the larger compressor the turbine was very restrictive and was glowing after just short drives. I felt that the turbo was better then the standard unit but it was still not perfect for the top end performance and it was difficult to get the boost level stable and the same at all gears and loads.
If you want to build a caravan tower, do it.
Anyway, the turbine numbers are the entry area in centimeters square.
and those Saab engines would show a lot higher power figures with some real turbochargers.
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