Dean when I built the duster I had a Performer laying around so put it on. A while later found a LD4B and replaced the Performer with it and the performance increase was almost unbelievable. I couple of years later I was able to acquire an LD340 and the performance difference was not near a much as when I went from the Performer to the LD4B. Yes the performance point increased but not as markedly. The basic difference in the LD4B and LD40 is that the runners are reversed and the runners taper down at the head ports.