[Silverback]

You're partially right and partially wrong… yea, longer runners will change what rpm the intake is tuned for, and assuming that we're talking about the same harmonic a longer runner will tune for a lower rpm.

Adding a plenum spacer does not lengthen the effective runner length. It does increase plenum volume and helps top end rather then low end (it will actually hurt responsiveness slightly, but so slightly that I'd bet that no one would actually feel it). I think that most of the change is due to getting the injectors up higher helping fuel distribution and airflow…