I have 2 exhausts here. One stock.
One DG.
What a difference in diameter the stock one is.
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the id is what matters. a lot of stock pipes of that era have an inner 'layer' for sound deadening. the tzr for sure does. dont read too much into it.
The stock pipe is double walled. Did you notice they weigh twice as much as the DG pipe?
IIRC the stock pipe will be better for top end.