MDT CLRED AT 1 MIN BEHIND A B747 ON TKOF. CLRNC CHANGED WHEN AIRBORNE TO TURN SW INSTEAD OF N AND E. WAKE TURB SEPARATION NOT ADEQUATE.

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MY FLT WAS TO DEPART OFF RWY 31L WITH THE IDLEWILD CLB (SID) WHICH IS A R TURN TO 090 DEGS AND CLB TO 2000 FT. THE WIND WAS OUT OF THE N AT 10 KTS; I WAS THE CAPT AND IT WAS MY LEG TO FLY. WE WERE TO DEPART BEHIND A HVY B747 WHICH WAS TO TURN L TO A SW HDG AFTER TKOF. WITH THE WIND OUT OF THE N I WAS NOT CONCERNED ABOUT WAKE TURB. SEPARATION TIME AS A R TURN TO THE N AND E WOULD KEEP US OUT OF THEIR WAKE. A MIN AFTER THE HVY'S TKOF WE WERE CLRED FOR TKOF WITH INSTRUCTIONS TO BEGIN OUR R TURN AFTER 500 FT. IMMEDIATELY AFTER TKOF; TWR TOLD US TO NOW TURN L TO A SOUTHWESTERLY HDG. I TOLD MY FO TO TELL TWR WE WERE UNABLE BECAUSE OF WAKE SEPARATION. TWR AGAIN SAID TO TURN L TO THE SW. I CAME ON THE RADIOS AND SAID 'WE CAN'T DO THAT.' TWR THEN ASKED IF WE COULD CLB STRAIGHT OUT AND MAINTAIN 500 FT. THE SID HE WANTED TO PUT US ON HAS A CLB TO 5000 FT. WE WERE ORIGINALLY ASSIGNED A CLB LIMIT OF 2000 FT. I CONTINUED TO CLB TO AVOID THE WAKE OFF THE B747 WHICH WAS 2 MI AHEAD OF US CLBING ABOVE US. THE 2000 FT CAME AND WENT AND THE ALT ALERT WENT OFF. MY FO ASKED WHAT ALT WAS I CLBING TO. I REPLIED I WAS CLBING TO AVOID THE WAKE. AT 4000 FT; TWR SAID TO CLB TO 5000 FT AND CONTACT DEP. WE DID AND THE FLT PROCEEDED WITHOUT FURTHER INCIDENT. TWR NEVER SAID WHY THE LAST AND IMMEDIATE CHANGE ON DEP NOR WHY THE DISREGARD FOR WAKE TURB SEPARATION BEHIND THE HVY. I DON'T KNOW WHAT CAUSED THE PROB. POSSIBLY ASKING FOR THE 2 MIN WAKE SEPARATION WOULD HAVE PRECLUDED THE TWR'S SCRAMBLE TO GET US OUT; WHICH I WILL ALWAYS DO FROM NOW ON; REGARDLESS OF DEP SID.