Straight pipes:

Open straight pipes perform poorly in the 2500 to 3800 rpm range. If they are 34" or longer, they do not perform really well at any rpm.

Symptoms include missing, backfiring through the carburetor, reversion (fuel dripping out of the air cleaner) and poor acceleration.

Open mufflers:

"Gutted" mufflers with stock (or stock-like) header pipes tend to perform poorly in the same rpm range as straight pipes and exhibit similar symptoms.

Long thin mufflers:

Long, small diameter mufflers with full-length baffles often exhibit the same symptoms as straight pipes, although their over-all performance may be better.


Source: http://www.mikuni.com/tg_poor_mid_range.html


High performance 2-into-1 systems:

These systems are often poor performers in the 2000 to 3000 rpm range. Most 2-into-1 exhaust systems deliver a significant torque dip at 2500 which is slightly less than 60 mph in top gear for most stock Harley Big Twins.

Header pipe diameter:

The great majority of Harley engines, of any displacement, do their best work with 1-3/4" diameter exhaust pipes. Larger pipes tend to suppress mid-rpm performance and, for that matter, seldom deliver the best power at high rpm either.