Well I have one and I measured a few things.
Inlets are 2.000-2.005" in diameter. I was surprised that several measurements with my calipers had that kind of repeatability on both sides of the plenum. The diameter going to the throttle body is approximately 3". My calipers didn't reach far enough inside to get accurate measurements so we'll say that's within 1/8" or so.

Other notes:
The casting on the inside is rougher than the outside. There are grinding marks on the inside where the casting lines from the mold were ground down. And there were a couple of very slight edges (flaws) that I ground down flush with a quick touch of a grinding stone on an air tool. I'm considering a 200 or so sanding wheel on said air tool to smooth it out some on the inside. Things I won't do are grind anywhere to port it, grind the divider at all, or polish the inner radius of it as the "y" part of it is expanding in diameter near the divider as I doubt it would do any good. I also don't think polishing it on the outside would be a good idea either as it's not the most symmetric piece out there and polishing would show the various bumps and inconsistencies with the casting.

OK despite a critical sounding review, I'm happy to have the piece. I'm just particular about things that flow gases and anything thermal since it comes with my normal job during the day. I'm very much a function before form person when the hood comes open.

So after a bit of touching with a sanding wheel (and again, I don't think it's necessary for any "normal" person to want to do it) I'll put it on and then measure some things with the stock inlet. That said it's not on the pepper yet so if anyone has any questions or wants me to measure something I'll try to answer them.

 

So a silly question...why spend all that money on a product only to take a grinder to it?

 

Again, I'm not grinding on it to make it bigger. There were two sharp edges poking up from the casting, probably from the manufacturer where their tool contacted the surface that I felt needed to be removed. I just want to smooth out some of the interior surface because it's pretty rough for something that just has to move air.
And mine was used so I didn't pay the full retail price.

 

I can appreciate your efforts of "fine tuning" the Cayenne Turbo IPD Plenum but that should not have been necessary for any added performance or enhanced fit. We have even extrude honed some IPD plenums and found VERY minimal gains. I myself tend to tinker with (aka improve) manufacturers products giving them that little extra TLC that a manufacturer can't afford to incorporate into the price of the product; it's all part of the fun. But that being said all the IPD plenum should be good looking, good fitting and good performing parts right out of the box. I know you mentioned that you purchased it used so I'm a little weary of what may have happened. As always ALL IPD plenums come with a 100% satisfaction guarantee. If you feel your plenum is sub-standard please send it back and we will exchange it for one that has our seal of approval. I'm glad to hear that you like the performance though. If you have any questions or concerns of any kind feel free to give us a call anytime.

Best Regards,
Greg @ RSS

 

Pretty much sums it up on the "why" for me. Thanks for the offer to swap it out, but this one is OK.

 

Had a chance to measure the stocker and the inner inlet diameter is roughly 1.85". So the area for the IPD is about 15% bigger at each inlet. Diameter just in front of the throttle body is about 3", same as the IPD. The biggest difference is the divider area. You can look through the stock inlet and see a good part of the opening on the other side. On the IPD, you cannot see the other opening when looking through. This is more or less confirmed when you consider the outside dimensions of both are about the same, but the distance from the TB opening to the divider is about 3.75" vs. the 3" on the IPD, so the divider of the IPD is definitely longer. Or really, the bends are longer than stock to direct the flow into a straighter path into the throttle body.
As for the pics, the one interior shot of the IPD shows where the casting is ground down from the factory. The other thing to see is the tab/boss that holds a clip for a hose is about 1/2" closer to the vent nipple on the bottom side. If you put the clip on there, it put it right over the nipple so you can't use the clip to hold that one particular hose.

 

A quick update on how it's doing. I have to say I'm not sure if it's the plenum or cleaning the MAF sensor, but it drives much more smoothly starting from a stop and part-throttle acceleration. This part of the throttle response is greatly improved and the engine no longer feels like it's lugging/bogging ever so slightly. And due to the smoothness of the engine the shifts are more friendly as well. I haven't really had the chance to punch it for any length of time (I'll hit the track for that), but just from a daily driving standpoint the pepper runs much smoother. After reading some of the threads on the throttle plug-in mod, the IPD has done basically the same thing to fix some of the hesitation in the throttle. So far so good.