Does anyone know if it is more beneficial to use a non-revup lower plenum on a revup motor?

Also, on a 6MT.

 

Unmodified non-rev lower plenum, NO.

Unmodified non-rev lower plenum with 5/16 spacer, MAYBE, you gain a little in the mid but lose a little up top, personally I wouldn't run this combo.

MREV2 (or DIY version) non-rev lower plenum with 5/16 spacer, DEFINITELY. This is the combo you want, if you can't afford the MREV2 you can definitely DIY your own, the biggest thing is taking a flapper disc and taking 5mm off the top of the front two runners. Then you can port out some of the tighter spots between the runners and the edge where it meets the gasket.

 

Thank you so much for clarifying it for me.

 

Just imagine/visualize how the air is going to flow across the metal surface, you want everything to smoothly transition, if it has sharp edges the air while create a little bit of turbulence. It's sucking air from all around each of the runners so you want to smooth out the harsh edges.

The 5mm shaved off the top of the front two runners is because of a difference in clearance once the upper plenum is in place, it gets tighter in clearance around those front two runners so you need to remove the material so each runner has the same potential air volume surrounding it so they are all theoretically sucking from the same volume of air.

Z1 Motorsports sells a tapered plenum spacer that is 5mm taller in the front of the spacer, I've never actually seen a dyno on one but it technically should work as well. It's pretty simple to just junkyard a non-rev lower and use a flapper and dremel (or die grinder) and just do the porting yourself. Only costs about $100 if you already have the tools which is significantly cheaper than a new or even used MREV2.