Originally Posted by downwind4final View Post
The OE system has a 8.5" opening on the rear deck for an OE sub. I have a 12" sub suitable for IB with a 9" cone area. My plan is to build a "step-down" spacer (12"-8.5") and mount the sub there.
Can I get away with not enlarging the hole without causing any issues? Basically, a 12" sub firing through an 8.5" hole.
A two to one compression ratio is no problem, air is easily compressible.

I've run compression ratios of ten to one and ran into issues, but even that is doable if you add a coupling chamber in front of the woofer to reduce turbulence.