There is such a thing as too little back pressure.
IMHO, 2.5" exhaust tubing is far too big for a 1.6 liter engine. The Calmini complete exhaust kit, with or without the header, is made from 2" tubing. I had this arrangement (with header) on my late '94, and it worked VERY well - good performance without being loud enough to wake the dead or leave me deaf after a highway trip. In fact, it performed better, and was no more noisy, than the stock 1.75" exhaust pipe with a glasspack muffler.
Don, y'all
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