How much boost can a stock '90 740 turbo handle? I know a guy that has one, with an exhaust and a boost controller. He claimed that he was able to run ~25 psi of boost. I thin he is full of it...is he?
He may be able to run 25psi of boost for a short time, but not for very long, and I'm almost certain that the stock turbo can't handle that much boost effectively. He'd need some kind of programmable fuel system to deliver enough fuel, which is quite expensive. When raising the boost, it's a good idea to have a calibrated boost gauge, as well as an air/fuel ratio gauge. A quick look into his car will tell you if he's got extra gauges, and if not it's a good sign that he's full of it.
By 1990, the B230FT had gone back to larger connecting rods, so the engine can handle quite a bit of power. I think that most people who raise the boost go up to about 12-15psi, and that's with minor modifications like a larger exhaust system.