We had an enquiry from a diesel engine manufacturer in Adelaide to run a diesel at 12000 ft. Apparently they had a sale in New Guinea on top of a mountain for a generator if they could prove the engine would run. Set it up in a Cherokee 6 behind the pilot with a hole out the roof for the exhaust. Flew it to 12 grand and started it up and ran it for the desired period. Ran ok but the aircraft vibrated a bit.