AN MD90 PILOT DEPARTING SNA REPORTS AN NMAC WITH A LIGHT TWIN AS IT RESPONDED TO A TCAS AT 8000 FT.

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DEPARTED SNA ON MUSEL DEP; DAGGETT TRANSITION. I WAS THE PF. WE WERE CLRED TO 8000 FT; ON RADAR VECTORS. WE WERE IN A CLBING R TURN; PASSING 7000 FT; WHEN THE TCAS ADVISED; 'TFC.' I GLANCED DOWN AT THE SCREEN AND SAW A YELLOW TARGET SHOWING '+9.' THE TCAS THEN ADVISED 'REDUCE VERT SPD;' FOLLOWED IMMEDIATELY BY 'DSND; DSND;' FOLLOWED IMMEDIATELY BY 'INCREASE DSCNT.' DURING THE ADVISORIES; I CLICKED OFF THE AUTOPLT AND AUTOTHROTTLES AND AGGRESSIVELY PUSHED THE NOSE OVER. WE PEAKED AT ABOUT 7500 FT. THE LIGHT TWIN WENT DIRECTLY OVERHEAD ABOUT 400-500 AWAY. WE ADVISED ATC THAT WE WERE COMPLYING WITH A TCAS RA. THE CTLR SAID 'I SHOW TFC BEHIND YOU AT 8000 FT.' WE SAID; 'YES; THAT'S THE GUY -- YOU CLRED US TO 8000 FT.' THE CTLR SAID; 'ROGER.' I CALLED THE SOCAL TRACON AFTER WE LANDED AND THE OPS MGR HAD REVIEWED THE INCIDENT. HE SAID THE ACFT WAS NOT IN CLASS B AIRSPACE; WAS NOT TALKING TO SOCAL (NOT REQUIRED); AND HAD AN INTERMITTENT MODE C READOUT. WE HAD A CAPT ON THE JUMPSEAT; AND ALL 3 OF US AGREED THAT HAD IT NOT BEEN FOR TCAS AND THE CORRECTIVE ACTION; WE WOULD HAVE COLLIDED WITH THE LIGHT TWIN. I CREDIT 2 THINGS FOR SAVING US FROM COLLIDING. 1) TCAS; 2) THE TRAINING I HAVE RECEIVED AND THE CONSTANT REMINDERS FROM MY AIRLINE'S SAFETY DEPT ABOUT INSTANTLY COMPLYING WITH RA'S. 'DON'T THINK -- FLY THE JET' IS THE MESSAGE WHEN IT COMES TO RA'S; AND IN THIS CASE IT SAVED US.