[KyleTownsend]

Thanks for the lead. I can probably find the source in the STC listings for Mooney.

We discussed this subject at some length in the recent CPA Skymaster Systems and Procedures course.

CPA recommends pressurized mags on P-Skymasters because the bendix mags are too small (physically) for altitude flying. They will cause crossfiring when near the P337 service ceiling because the reduced electrical resistance of the ambient air inside the mags versus the constant (high) pressure inside the cylinders makes this possible.

Cessna solves this problem on all of their other high flying airplanes by using larger mags. This isn't an option on the skymaster due to the limited space.

Crossfiring is extremely detrimental to the engine and even an "occasional hickup" should be avoided. Crossfiring will definitely shorten engine life.

We experience occasional crossfiring above 16,000 despite having brand new fine wire plugs gapped to 16 thousanths, a new magneto harness, and relatively young mags.

I talked to the fellow who designed the pressurized mag system for Riley (no longer with the company). He said that, when they were developing the Riley rocket, they found that they could not reliably avoid high altitude crossfire without pressurizing the mags.

I have heard the concern about "blowing junk" into the mags. There is probably some validity to this. However, we are talking about filtered upper-deck air (the same thing that is going into the cylinders), so I doubt that it is too full of "junk." This problem is further mitigated by using a good inline filter (continental and RAM both have one) on the pressurized air supply line.

CPA reported that the pressurized mags would routinely make the specified service interval, but that unpressurized mags had to be serviced much more frequently if flown at altitude.

I'm not wild about this idea. However, it seems like the only reasonable solution to the problem and, like your frind, I want to be able to use my airplane at altitude when necessary.