I recently had catastrophic coolant loss with overheating and engine knocking that needed a tow home. I did a headgasket job with a new timing belt and water pump but the engine doesn't develop full boost and knocks with preignition when the accelerator if halfway down. Is my engine lower cooked and damaged? It idles fine and powers okay if the throttle isn't pushed more than half way. Compression is good 120 to 130 each hole. The turbo spins easily. Could my cat be clogged or O2 sensor damaged by the oil gas coolant released by the blown headgasket? Last question. Would the timing belt being off by one notch cause preignition upon acceleration? I heard that if you were off by a notch the car would not even idle. Is this true? Can anyone describe the symptoms of a timing belt off a notch. I've changed ECT, knock sensor and checked the turbo hoses and still this preignition rattle. I have retarded (or advanced) the distributor setting to the max to settle the idle. Any thoughts out there. Thanks alot.