Ive wrote many articles on this discussion.
Backpressure at the valve, on the head, at the exhaust port.... is bad.. always, from a power perspective. Its true you can burn a valve with too little BP, but then you have a wimpy engine.
But... I've never burnt a valve, even with 2" straight pipe on my 3 Cyl 1.0L sprint at 7500 RPM... which I drove for 60,000 kms.
However, too little backpressure in your exhaust can create more backpressure on the head at the exhaust valve, specifically at lower RPM.
Without delving into a 3 page discussion.... when you have too little backpressure in your exhaust system, the exhaust pulse (which is followed by a vacuum pulse) rushes out of the exhaust leaving a vacuum behind it, and fresh air rushes INTO your exhaust to fill the vacuum, and collides with the next exhaust pulse, creating backpressure at the head.
This goes away at higher RPM for a HP gain, the best exhaust is horn shaped like on a motorcycle. Narrow at the begining and slowly widening, this gives you the most area under the TQ curve, or in other words, the most overall HP in your powerband.
For me, this was the best setup, and still is I cant believe how well it works:
http://x90.zukiweb.com/bbs/index.php?topic=644.0The sound is awesome at 5000 RPM but so quiet below that, and yet great overall TQ. (I think the reason is the dual setup, one pipe is about a half wave length longer then the other pipe, so at lower RPM's the noise cancels out)
-Steve