Josh, Lets put it this way. If said restrictor can only move X amount of air and compression and cams are a fixed number then gains will be minimal at best and lets say we are talking 3 to 4 HP difference. I can tell you for a fact that the difference between a tabletop and a pro motor is way different than that now.[/b]
True. We are seeing 15+ hp difference in the 1.6L Miata between crate and carefully prepared legal pro motors. A 3.0L 300ZX motor will show linear improvement from that.

At a certain point 27mm stalls and can not allow any further air volume to flow.[/b]
True again, but air flow alone is not the whole story. An engine will make power with a given SIR in linear relation to its specific output. That's a fancy way of saying that Josh is correct in post #33, and a pro built engine will enjoy a power advantage over the kitchen table engine, assuming they use the same size SIR. Raetech and Loyning Engine Service have shown the Club over and over again that the SIR must be matched to the engine preparation level to assure the desired output. One size does NOT fit all.

Ron you only think it's a dead deal...Look at the number of classes using them and starting to use them there is no doubt in my mind that they will be in use everywhere in the next 5 years. I personally think they would be awsome in Spec Miata....That would get rid of all the advantages of a pro motor.[/b]
It is true that we will see intake restrictors more comprehensively employed in future. Not all will be SIRs, but restrictors are the coming thing. See my comments above about the advantages of pro motors...

Thanks Chris and I competely agree that a max compression should be looked at for GT to limit how far people will go to try to cheat the effects of the SIR, Or have the sir engineered and sized based on compression number.[/b]
Both approaches are under discussion.

Stan