Late last year, before the Road America events, Rick's G35 started missing at high RPM under full throttle. The problem took the dealer nearly a week to fix, and was eventually traced down to a leak in the valve cover gasket allowing oil to seep into one of the spark plug tubes. During that dealer visit the dealer found cracks in the Crawford upper intake plenum, an aftermarked piece, and refused to re-install it. So, after another $300 Rick was back to stock and suffering from some horsepower loss.
In order to make up that loss, and perhaps a bit more, Rick located a used Crawford upper intake plenum (they aren't made any more), and upped the anti with a Motordyne MREV2 lower plenum as well. The Crawford has a taller front edge to deliver more airflow to the forward two cylinders, and the MREV2 is a modified 2006 lower plenum to also increase airflow at higher RPMs.
Power Output, Corrections and Other Factors:
To validate the power gain from these two modifications, we ran some power runs both before and after the install. You can see the G35 made more power in the two post-install runs, green and blue respectively, from about 4600rpm to redline. The G-Tech measured an average of 181bhp (at the wheels, minus aero drag, etc.) before-hand and 191bhp afterwards, an increase of 10bhp! After calculating the before-hand and afterward correction spreadsheets, the estimated power gain "at the crank" was 12bhp. This is pretty good power increase for the money! New, these two pieces would have cost $840, but Rick managed to buy them off G35/350Z forums for $600 total.
If you're curious, the G-Tech runs can be downloaded from the link above.