To start lets talk about engine theory. The 4 cycles of the internal combustion engine are, intake, compression, power, and exhaust. Theses events are made possible by the opening and closing of the intake and exhaust valves at the proper time by the camshaft(s) and valve springs. The events start just before the piston reaches top dead center by a few degrees of crankshaft rotation, when the intake valve starts to open. Just after TDC the exhaust valve closes. This period of crankshaft rotation is called the overlap period. When the pressure in the exhaust is greater than the intake manifold pressure, exhaust gases will travel past the intake valve and back into the cylinder. Now the intake valve gets a coat of unburned fuel (carbon) from the exhaust gases. The DI engines as we know have no fuel in the intake manifold to wash off the intake valves.

Now. The N-14 engine has variable valve timing on the intake cam only. Thus the timing of the opening and closing(duration) of the exhaust valve is fixed. Not allowing for a big spread or reduction in the over-lap period. Thus more chance of carbon build up on the intake valves. The N-18 engine has variable valve timing on both intake and exhaust cams, as well as variable valve lift on the intakes. I've read where at idle RPM the intakes only open .008 of an inch. This too would shorten the intake valve duration. It might even be possible that at lower RPM's there would be no overlap period and thus less chance of carbon on the intake valves of the N-18 engine. The BMW engineers might be able to tell us that.

So, what is the fix for this? Maybe hard chrome on the backs of the intake valves, and polished combustion chambers? Don't really know for sure. One thing I do know for sure is there has always been carbon on the back of intake valves on all engines I ever worked on since the 50's. A long time before DFI. One 1960 Pontiac comes to my mind in 1963. Very poor performance and hard starting. I performed a leak down test on all cylinders and on each one air came out the carb. The intakes were so choked up with carbon they couldn't close all the way. We threw away the intakes and had to install new. The early performance V-8's had a lot of valve duration and thus overlap. Even the turbo piston aircraft engines I worked on in the AF in the 50's had this same problem. Don't think the PCV has much to do with it!

That's my story and I'm sticking to it. Looks like the walnut shells will have to be the way to go¡¡