The theory would be the same for turbocharging a GT versus a Manta, only the actual fitment differs. A GT will have far more heat issues, and far tighter confines to work within. Regarding turbocharger selection, if you are trying to match a junkyard turbo to your application, then the best bet is to establish honest criteria regarding HP levels. Trying to make a 75 hp engine into a 200 hp engine is not easy unless you have the right hardware. You
could just boost the hell out of a stock engine, but you'd better have spares!
Choose a turbo from a vehicle with a similar displacement, and a realistic power goal. A common turbo (Garrett T3) to use for a 1.9-2.0 litre displacement engine is a Saab turbo. Commonly producing around 160 hp in a Saab, this would effectively double your hp, if you played your cards right.
As I mentioned in another post, I decided to run a turbo on my father's GT, but some preliminary measuring means I have to do a complete rethink of my manifolding to fit a GT, the ideas I used for my friend's Manta won't work. My initial reaction is the turbo will end up where the stock air filter housing is located (with a custom manifold of course), the intercooler will make up the lower grill opening, and the radiator will be a VW type, mounted at an angle further up in the nose of the GT. I will end up cutting the stock radiator 'firewall' away most likely for more plumbing room.
One aspect that will give me more room is the use of a 2.2 head, with a short-runner custom EFI intake manifold. This raises the intake ports away from the exhaust pipes substantially (2.2's have raised intake ports, by about 5/8"). More freedom of fabrication thanks to this feature....
My turbo of choice will be either a GT28RS or a GT2871 ball-bearing turbo. Neither came in any production cars, they have to be bought new, and they're around $1200. But, they have a flow capability of 350 hp and 400 hp respectively, yet with the modern aerodynamics and ball-bearing center, plus the large displacement engine (2625 cc's), they should provide nearly lag-free boosting plus a smooth power curve.
If you want to get a 'big' turbo that is cheaper, yet can still flow some air for big HP numbers, I suggest considering an IHI VF22 turbo, it's the largest 'bolt-on' turbo made for the Subaru WRX. Costs around $600 new if you look around (try my friend's shop,
www.xxtuning.com), and can flow enough air for about 380 hp. It is also a ball-bearing center section, so it spools nicely. I just built a manifold to adapt one of these to my friend's 1.8 litre VW rally car, it comes on boost by 2200, and pulls HARD to 7000 rpms. Made 231 hp to the wheels at only 12 psi boost. Check out the manifold at
www.metalmarks.com
HTH,
Bob
edit for spelling...
2nd edit, VF22 turbo is $589 on special this week!