Re: suggestions for A1 TD? (busted-ass-jetta)
Good pump timing is about as far as you can go other than propane. Propane does give you a shot like NOS, but like NOS, it's hard on the engine, and only good for a short dose. You want to make sure the injection pump is timed well, and slightly advanced, but finding somebody who knows pump timing on the old diesels is hard to do.
The pump is entirely mechanical, so there are no computers to help you improve the fuel mapping curve. Added air doesn't really do much either other than improve the cloud of soot coming out the back end. It's not gonna give you much in the way of horsepower, because you already have a turbo forcing more air in, even a ram isn't gonna improve on that. Since a cam is essentially trying to do the same thing (get more air in the chamber) you're not gonna see much out of that either.
Also on the exhaust, because it is a turbo, you don't see much help from a better exhaust, other than a possible slight improvement in turbo lag.
I'm afraid you're kinda stuck with a sub 75hp engine. They get great mileage, and are essentially bullet proof if you make sure the timing belt stays in good condition. You may get even better mileage from the 5speed, but with a wide ratio, I doubt you're gonna get any better accelleration.