For series 1 the quad rear is the pick. Side pipes on the S1 are an expensive option - at the factory it's reputedly a 5k job..

On s2 and later going to side pipes is cheaper (I had my s2 converted - around 1500 about 7 years ago). On the later cars I would go side pipes (especially on 4.8 engine)...

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Maxxed out!