SMA PLT TOOK AVOIDANCE ACTION IN N90 CLASS B AIRSPACE AFTER IT APPEARED A HVY ACR ACFT PRESENTED A COLLISION HAZARD.

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I WAS ENRTE FROM ACK TO BOM; WITH VFR ADVISORIES BEING PROVIDED BY NEW YORK APCH ON 135.9. I WAS CLRED INTO CLASS B AIRSPACE; AND HAD PASSED OVERHEAD JFK; ON A VECTOR; WITH INSTRUCTIONS TO MAINTAIN 4500 FT WHILE IN CLASS B AIRSPACE. THE CTLR ALERTED ME TO HVY JET TFC CLBING AT MY 2-3 O'CLOCK POS. I RESPONDED THAT I HAD THE TFC IN SIGHT. IMPORTANT NOTE: I WAS UTILIZING MY AUTOPLT TO MAINTAIN HDG AND ALT; IN PART SO THAT I COULD MAXIMIZE MY 'HEADS UP AND OUTSIDE' TIME. BOTH I AND MY PAX (ALSO A PLT) WATCHED THE HVY JET CLB. AT A CERTAIN POINT; IT LOOKED AS THOUGH IT WAS HEADED DIRECTLY FOR US. IN RETROSPECT; THIS MAY HAVE BEEN AN OPTICAL ILLUSION; BUT IF IT WAS; IT WAS EXTREMELY CONVINCING. BOTH MY PAX AND I EXCLAIMED 'HE'S HDG STRAIGHT FOR US' AT THE SAME TIME. IT SEEMED THAT AN EMER EVASIVE MANEUVER WAS REQUIRED. I OVERRODE THE AUTOPLT AND DIVED IN A STEEP L BANK (JET APCHING FROM R). AS WE DSNDED THROUGH 4000 FT (JET NOW CLRLY LEVEL WITH US); THE CTLR TOLD ME TO CLB IMMEDIATELY BACK TO 4500 FT; AND SAID THAT THE JET WAS LEVELING AT 4000 FT. (MAJOR FACTOR: THE APPEARANCE THAT THE JET WOULD COLLIDE WITH US WITHIN SECONDS.) I RETURNED TO 4500 FT; AND COMPLETED THE FLT WITHOUT INCIDENT.