MOONEY M20R HAS A MALFUNCTIONING ENG DURING APCH.

Narrative

I WAS ON AN IFR FLT PLAN. ON THE DSCNT INTO ZZZ; I WAS ADVISED THAT I WAS #6 FOR THE APCH AND COULD EXPECT A LONG DOWNWIND VECTOR. I WAS IN CLASS E AIRSPACE IN VFR/VMC CONDITIONS AND ADVISED ATC THAT I WANTED TO CANCEL IFR AND PROCEED VISUALLY TO THE ARPT. ATC REPLIED; 'NEGATIVE; REMAIN IFR.' I DID NOT BELIEVE THAT ATC HAD THE AUTH (OR THE JUSTIFICATION) TO ISSUE SUCH A CLRNC. I COMPLIED WITH THE CLRNC AND CONTINUED ON MY NBOUND VECTOR. WITHIN 1 MIN; MY ENG STARTED RUNNING SO ROUGH THAT I DECLARED THAT I NEEDED TO LAND IMMEDIATELY. BECAUSE THE SHAKING WAS SO VIOLENT; I WAS UNABLE TO ESTABLISH COM WITH APCH; IN SPITE OF ATTEMPTING ON 2 RADIOS WITH 2 DIFFERENT HEADSETS. I COULD HEAR APCH STATE THAT I NEEDED TO CONTINUE NBOUND. I CIRCLED ADJACENT TO THE ARPT WHILE LOOKING FOR A LIGHT GUN SIGNAL. I WAS ABLE TO ESTABLISH COM ON A HAND MIKE WITH APCH AND RESTATED THAT I NEEDED TO LAND IMMEDIATELY AND WAS SWITCHED TO LCL CTL. I LANDED IMMEDIATELY AND WAS TOLD TO CALL APCH. MY ACFT WAS TOWED TO A MAINT FACILITY WHERE IT WAS DETERMINED THAT MY MAGNETO FAILED IN SUCH A WAY AS TO CAUSE MISFIRING. APCH WAS UPSET WITH ME AS THEY INCORRECTLY BELIEVED THAT I 'FAKED' AN EMER TO EXPEDITE MY LNDG CLRNC. I EXPERIENCED AN ACTUAL INFLT EMER THAT COULD HAVE BEEN MADE MUCH WORSE BECAUSE OF THE INCORRECT AND PETTY ACTIONS OF ATC. BECAUSE OF THE COMPRESSED TIMEFRAME THESE EVENTS UNFOLDED; I WAS UNABLE TO USE THE WORD EMER OR MAYDAY UNTIL AFTER TOUCHDOWN.