Originally Posted by willwren
91PA, going to a larger downpipe on a mostly-stock L67 results in an immediate loss of low-end torque.
Downpipes are something that happens MUCH later in the modding game. Typically with either a cam or rockers, and PEM'* or Headers.
DP is bad advice on a nearly stock or bone-stock L67.
a 2.5 inch DP looses power ok
well ill quote ZZ Performance
"Stock downpipes are double-walled tubing with an inside diameter of just over two inches. Following the restrictive downpipe section, the stock unit subjects the exhaust gases to the choking effects of the catalytic converter. After the cat, it flattens out into a section dubbed the infamous "U-Bend," which is a section of pipe crushed into a "U" shape by GM in order to fit a rear O2 sensor. This section has less volume than the same length of 2" pipe, and approaches the volume of a 1.75" pipe with the rear O2 sensor in place! Our downpipe eliminates all of the aforementioned problems! Gains of over 15HP have been seen, along with a substantial drop in knock. Most users can drop a pulley size and/or eliminate 5-7 degrees of knock. This part can typically be installed in under an hour".