Yup I'm stumped, my oil feed and drain is essentially identical to yours in inside diameter. The PCV is blocked off, the other valve cover vent doesn't run into the intake anymore, it has a filter on it now.

Update: I've got good news and bad news, the good news is the turbo is fine, I removed it from the car and i am still seeing lots of oil smoke from the exhaust. The blades look fine and has very little shaft play still. Bad news is I can't diagnose what is going on inside the engine yet, I did a compression test and the numbers are the same as from when the car wasn't smoking before the turbo install. I'll do a leakdown test next to see if the problem is the valve seals or the rings. I am most likely looking at picking up a decent used 94-97 engine and swapping it in at this point. It's odd because if it wasn't for the massive amount of smoke coming from the exhaust the car drives fine with plenty of power.

Compression numbers from after oil smoke/turbo install/deinstall on left, before turbo install on right.
https://www.miataturbo.net/attachmen...ine=1342393067

Update 8/12/12:

I have narrowed down the likely source of my oil burning to a stuck oil control ring on one of the pistons. I will try a combustion chamber soak to free the stuck control ring before I resort to pulling the engine.

Some products I might try for the combustion chamber soak are the new WD-40 Specialist , Marvel Mystery Oil in the crankcase, and possibly Piston Kleen as well.

so what was the final result?

I tried a combustion chamber soak for a stuck oil control ring to no avail. I then replaced the valve guide seals, without pulling the head as seen here: (this job was way easier with compressed air in the cylinder instead of rope, but riskier if the air fails and the valves drop. The valves cant have any give when you try to pound the 2 piece seal keepers onto the valvestem, and the rope was giving me fits until I threaded compressed air into the sparkplug hole.)

But that didn't fix it either. While the camshafts were off the head I finally found a threaded/quick-release hose to connect to compressed air, I pressurized the cylinders with 120psi and #1 and #2 held pressure with minimal pressure drop. #3 slightly leaky but was acceptable, #4 was shooting air past the seated exhaust valves and past the rings, I could feel the air coming out the exhaust and heavy sounds from dipstick hole. The air coming past the rings on #4 was expected, but the air shooting past seated exhaust valves may indicate a chipped valve or seat, in any case it wouldn't be the cause of an oil leak, especially since it has fresh valve guide seals now.

At this point I have narrowed it down to bad rings on #4 cylinder, and a bad head on #4 too. Rather than a rebuild this motor or get a used engine, I am leaning towards purchasing a low milage '94-95 miata with good leakdown that already has a torsen, and swapping my parts over to a new chassis.

update? Having similar problem.

Bad piston rings, motor needs a rebuild.

Damn bro... good luck on the rebuild.

I put a another engine in the car, and tore the old one apart to see what was wrong...

https://www.miataturbo.net/attachmen...ine=1365044233

It turns out Piston #4 had a significant amount of metal missing from 2 places on the edge of the piston face, and when those chunks broke free and got pulverized they messed up the exhaust valve seats on the way out of the engine. I should thank my lucky stars the pieces of piston didn't seem to damage the fins in the turbo on the way out.

It goes to show how robust these engine are that the motor still made nearly 200WHP with such damage internally.