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<E30> How to install 'Trapps ??

Hi everybody,

A few weeks ago I asked about exhaust systems for
my '87 325is and everybody who responded (thanks!)
suggested Supertrapps (single or dual).  I just
have a couple more questions.

First, can you buy a complete system (cat back) or
will it have to be a custom fabrication ??  If you
can buy it, where ??

Second, what size 'trapp should I use ??  I did some
"back of the envelope" calculations on size of the
tubing needed if the pipes are run into a collector
after the cat.  If that is done, the single pipe size
should be 2-1/2 or 3". (A reasonable size for a 2.5l
engine considering that Mustangs and Camaros usually
have dual 3" systems.)  Using that pipe diameter, I
was thinking that I should get a 'trapp with a 
2-1/2" core.  is this right, or will I need to go 
even larger ??  Should I worry about 4" vs 5" disc
sizes, or just stick to the core and pipe diameters ??

Finally, how many discs are needed to sound really 
good but not obnoxiously, hideously, window rattlingly
loud ?? Will there be enough in the original set to
give me any "free flow" performance or should I buy
extras ??

Any advice is appreciated,
Jeff	jet@domain.elided