FLT DEPARTING LGA ON 90 DEG HDG CLRED BY DEP CTLR TURN TO 270 DEGS. CAPT DID NOT START TURN UNTIL HE OBTAINED CONFIRMATION OF COURSE REVERSAL.

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NY DEP AMENDED OUR SID WHEN WE CHKED IN -- FLY A 90 DEG HDG IN LIEU OF A 40 DEG HDG. WE MAINTAINED OUR INITIALLY ASSIGNED ALT AND HDG 090 DEG. SEVERAL MINS LATER NY CALLED; 'FLT XXY TURN L 270.' WE WERE FLT XXX SO I CALLED TO CONFIRM INSTRUCTIONS WERE FOR XXX. NY ANSWERED; 'XXX TURN L 270 DEG.' I WAS UNEASY WITH INSTRUCTIONS FOR A 180 DEG COURSE REVERSAL; BACK INTO THE DEP FLOW OF TFC FROM NY SO I AGAIN QUERIED THE 180 DEG COURSE REVERSAL. NY WOULD NOT ANSWER AND CONTINUED TALKING TO OTHER ACFT ON THE FREQ. I WAS VERY FRUSTRATED AS I CONSIDER INSTRUCTIONS FOR A COMPLETE COURSE REVERSAL EXTREMELY RARE BUT I AM NOT IN THE HABIT OF IGNORING ATC INSTRUCTIONS. FINALLY AFTER 3-4 CALLS; NY ANSWERED; 'XXX; L 270 DEG NOW!' BTWN 30 SECONDS AND A MIN WERE WASTED. OWING TO THE VERY UNUSUAL INSTRUCTIONS I THINK NY SHOULD HAVE FOLLOWED UP ON MY REQUEST FOR A CLARIFICATION IMMEDIATELY. IT IS VERY UNFAIR OF A CTLR TO BLURT OUT AN ODDBALL INSTRUCTION AND THEN IGNORE REPEATED PLT REQUESTS FOR CLARIFICATION. THESE SORT OF SITUATIONS LEAD TO ACCIDENTS. I ADMIT THAT ATC INSTRUCTIONS ARE TO BE FOLLOWED IMMEDIATELY BUT MY CONSIDERABLE EXPERIENCE OPERATING IN THIS AIRSPACE GAVE ME REASON FOR DOUBT.