NMAC BTWN 2 SMA SEL ACFT WHERE THE OTHER ACFT OVERTOOK THE RPTR DIRECTLY ABOVE BY 75 FT.

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PROCEEDING NWBOUND ON V94 AT 8425 FT MSL; I WAS OVERTAKEN BY A BONANZA (STRAIGHT TAIL TYPE). OTHER ACFT APPEARED ABOVE ME; IN VIEW EXACTLY ON MY CTRLINE; ABOUT 75 FT ABOVE. BECAUSE THE HORIZ SEPARATION WAS PRECISELY ZERO; I NEVER DID SEE HIS REGISTRATION NUMBER. HE CONTINUED LEVEL ABOVE ME; ABOUT 25 KTS FASTER THAN MY CHEROKEE-6- 300; AND PULLED AWAY. I QUESTIONED -- ZAB WITH WHOM I WAS SQUAWKING A DISCRETE CODE ON FLT FOLLOWING. PREVIOUSLY IN THE FLT; IN ANOTHER SECTOR; I HAD BEEN ALERTED ABOUT OVERTAKING TFC. THEY TOLD ME THEY WERE NOT SHOWING ANY OTHER TFC NEAR ME. AT THE TIME THEY SAID THAT; THE BONANZA WAS READILY VISIBLE A MI OR SO AHEAD. DISCOVERY EARLIER VISUALLY IMPOSSIBLE IN PA32-300 WITH NO REAR OVERHEAD WINDOW. AVOIDANCE VISUALLY IMPOSSIBLE FOR SAME REASON. DISCOVERY AND AVOIDANCE ONLY POSSIBLE USING TCASII (TCASII FOR SMALL GA ACFT). I BELIEVE HE WAS PROBABLY ON AUTOPLT WITH ALT HOLD. MY ALT HOLD HAD MISFUNCTIONED AND I WAS ON HDG AUTOPLT ONLY; FLYING ALT MANUALLY. MAYBE ITS A GOOD IDEA TO FLY NEAR BUT NOT EXACTLY ON THE CHOSEN ALT. I WOULDN'T BE HERE IF I HAD.