NMAC. CAPT OF AN EMBRAER EMB120 MADE A FULL PWR CLBING TURN TO AVOID AN ONCOMING NORTH AMERICAN T6 CLBING INTO CLASS B AIRSPACE ENGAGED IN AEROBATICS MANEUVER.

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ON MAR/XA/00 FLT FROM LAX TO ONT; EXPERIENCED AN NMAC WITH A PVT ACFT OVER SEAL BEACH VOR AT XA00. THE FLT WAS NORMAL EXCEPT FOR MODERATE TURB DUE TO SANTA ANA WIND CONDITIONS FROM THE NE. THE PLTS SWITCHED FREQS AS THE ACFT LEVELED OFF AT 6000 FT APPROX 3 MI W OF THE SLI VOR TRAVELING NWBOUND. JUST BEFORE CHKING IN WITH THE NEW ATC FREQ; THE PLTS NOTED A SMALL ACFT AT JUST OFF THE R ABOUT 20 DEGS AND ALSO NOTED THE TCASII DISPLAYED THE ACFT 500 FT BELOW. THE SMALL ACFT WAS OBSERVED PASSING TO THE L HDG NWBOUND APPROX 1/2 MI AND THE COPLT CHKED IN WITH THE NEW SOCAL APCH CTL FREQ AND STATED THAT THE TFC PASSING R TO L WAS IN SIGHT AND LEVEL AT 6000 FT. THE CTLR ACKNOWLEDGED THE XMISSION WITH 'ROGER.' THE COPLT THEN LEFT THE FREQ TO OBTAIN THE ATIS AT ONT. THE CAPT STATED THAT HE HAD THE #1 RADIO WHILE THE COPLT WAS LISTENING TO ATIS. THE CAPT THEN GLANCED TO THE L OUT THE DV WINDOW AND SAW THIS ACFT (NOW IDENTED AS A WWII T6) PERFORMING AN AEROBATIC MANEUVER IN A NEAR 90 DEG L CLBING TURN ONLY 200-300 FT AWAY JUST IN FRONT OF ACR X'S L WING AND STARTING TO ROLL WINGS LEVEL ON AN EXACT COLLISION COURSE WITH THE ACFT. THE CAPT NOTED JUST BEFORE THE CLBING R TURN WAS STARTED THAT THE VFR TFC WAS ALSO AT 6000 FT; WHICH WOULD PUT THE T6 IN CLASS B AIRSPACE WITHOUT A CLRNC. THE CAPT COMMANDED 'SET 100% TORQUE' AND ROLLED INTO A STEEP CLBING R TURN TO AVOID IMPACT. AS THIS MANEUVER WAS BEING PERFORMED; THE CAPT NOTIFIED ATC THAT AN EMER CLBING R TURN HAD BEEN INITIATED TO AVOID THE VFR TFC. AS SOON AS THE R TURN WAS INITIATED THE CAPT LOST SIGHT OF THE T6; BUT MULTIPLE TCASII 'TFC; TFC' ALERTS WERE HEARD. THE PLTS NOTED THE ACFT PASSING BEHIND AND STILL CLBING ON THE TCASII AS THE FLT PROGRESSED NEBOUND TO THE PDZ VOR. AFTER THE CAPT'S INITIAL RADIO CALL TO ATC; THE CTLR STATED THE VFR TARGET WAS NOT IN REGULATED AIRSPACE AND WAS NOT IN CONTACT WITH SOCAL APCH CTL AND THERE WAS NOT MUCH THAT COULD BE DONE ABOUT THE CLOSE CALL. THE CTLR; NO MORE THAN 1 MIN LATER; CALLED BACK TO THE ACR X CREW AND STATED THE VFR TARGET HAD CLBED INTO THE CLASS B AIRSPACE AND THAT THE TARGET WOULD BE TRACKED TO LNDG FOR FURTHER INVESTIGATION. THE ACR Y FLT PROCEEDED TO ONT AS NORMAL AFTER THIS OCCURRENCE. AFTER LNDG IN ONT; THE CREW THOUGHT MAYBE THE T6 CLBED SO RAPIDLY THAT THE ALT ON THE RADAR SCREEN WAS LAGGING BEHIND THE ACTUAL ALT OF THE T6 AND IT TOOK A FEW SECONDS FOR ATC TO GET AN ACCURATE READOUT.