I'm glad LouW chimed in. The standing wave mentioned has the potential to improve scavenging at the tuning frequency due to a reduction of back-pressure or the formation of a partial vacuum. The tuning frequency can be anything the tuner wants, low or high. What I'm unclear on is how things like port timing and throttle position effect scavenging at the pipe tuning frequency. Does conservative timing and partial throttle erase the effect of the standing wave while aggressive timing and WOT can maximize the effect? Seems reasonable.

I considered putting one on my 1.60FX like the pattern boys but decided the benefit was slim compared to the cost.