MOONEY PLT ENCOUNTERS ICE NEAR PRC; AZ. UNABLE TO MAINTAIN ALT DUE TO PWR LOSS; DECLARES EMER; DSNDS; COORDINATES WITH ARTCC THROUGH PASSING ACFT AND DIVERTS TO ANOTHER ARPT.

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W OF PRESCOTT PRC COULD SEE CUMULUS BUILD-UP AND WENT OFF FREQ TO CHK ON WX. FLT WATCH SAID THE TOPS APPEARED TO BE ABOVE 30000 FT AND SUGGESTED DEV TO THE N. CTR APPROVED. FLYING AT 11000 FT BEGAN ENCOUNTERING MODERATE RIME AND SOME CLR ICE. REQUESTED LOWER FROM CTR BUT THEY COULDN'T APPROVE AT THAT TIME. DUE TO IFR CONDITIONS THE CTR CTLRS WERE VERY BUSY. NW OF PRC THE ENG BEGAN TO CUT OUT AND I BEGAN TO LOSE ALT I IMMEDIATELY KILLED THE AUTOPLT; TURNED ON THE BOOST PUMP AND SWITCHED TANKS. THIS HELPED A LITTLE BIT BUT I STILL COULD NOT RETAIN ALT. I DECLARED AN EMER AND NEVER HEARD DIRECTLY FROM CTR AGAIN. FORTUNATELY; A CITATION ABOVE RELAYED CTR'S VECTORS TO THE NEAREST ARPT (VALLE 40G). I BROKE OUT OF THE CLOUDS APPROX 700 FT AGL AND BEGAN TO REGAIN PWR. FLEW TO ARPT AND LANDED. IN RETROSPECT; IT WAS NOT THE ICE THAT BROUGHT ME DOWN; BUT PROBABLY THE FACT THAT I HAD LEFT MY RAM AIR OPEN WITH ICE AND FREEZING RAIN BLOWING INTO THE ENG COMPARTMENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE FUEL INJECTED MOONEY HAS A RAM AIR SYS TO INCREASE MANIFOLD PRESSURE AT HIGHER ALTS. IT IS RECOMMENDED THAT IT NOT BE OPEN IN RAIN. ON PREFLT THE NEXT DAY IT WAS FOUND TO BE OPEN EVEN THOUGH THE RPTR SAYS HE NEVER USES IT BELOW 5000 FT. HE ASSUMES IT WAS OPEN AT ALT AND WAS LEFT THAT WAY DURING DSCNT AND LNDG. PWR WAS AVAILABLE BELOW THE ICING CONDITIONS BUT WAS NOT NORMAL THROUGHOUT THE APCH AND LNDG.