BE55 ACFT DEPARTING VFR IN CLASS B AIRSPACE HAD PRIMARY GYROCOMPASS LAG WHICH RESULTED IN ACFT ACTUALLY BEING 30 DEGS OFF ASSIGNED HEADING. AFTER BEING QUERIED BY 2 CTLRS REGARDING HEADING; RPTR CHKED MAGNETIC COMPASS AND DISCOVERED THE LAG ERROR.

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I HAD OBTAINED AN ATC CLRNC TO DEPART SFO CLASS B AIRSPACE UNDER VFR. JUST PRIOR TO BEING CLRED FOR TKOF ON RWY 28R; MY CLRNC WAS CHANGED TO 'L DOWNWIND DEP.' UPON RELEASE; I COMPLIED BY MAKING A CONTINUOUS WIDE 180 DEG CLBING TURN TO THE L; WELL ACROSS RWY 28L. AS I ROLLED OUT ON HDG 100 DEGS; TWR INSTRUCTED ME TO PROCEED DIRECT TO THE SFO VORTAC AND DEPART ON A HDG OF 050 DEGS. THIS REQUIRED ME TO TURN FURTHER TO THE L; WHICH I DID AT STANDARD RATE. I ESTIMATE THAT THE TOTAL TURN WAS VERY NEARLY 150 DEGS WHICH MEANT THAT I SHOULD HAVE CROSSED THE VORTAC AND BASICALLY MAINTAINED MY HEADING OUTBOUND. THE WHOLE THING WENT VERY SMOOTHLY; AND I WAS GIVEN A 'GOOD JOB' BY THE TWR CTLR AS I CROSSED THE VORTAC. HOWEVER; DURING THE 150 DEG L TURN MANEUVER; MY PRIMARY GYROCOMPASS DEVELOPED A LAGGING ERROR OF AT LEAST 30 DEGS WITHOUT ANY WARNING FLAG. THEREFORE; WHEN I CROSSED THE VORTAC (ON A HDG OF ABOUT 050 DEGS); MY COMPASS READ AT LEAST 080 DEGS. THE ARPT WAS UNDER MY WINGS AND BEHIND ME AT THAT POINT; SO I HAD NO OBVIOUS VISUAL REF TO THE RWYS. UPON XING THE VORTAC; I TURNED L TO 050 DEGS INDICATED; WHICH WAS A LOT CLOSER TO 020 DEGS; ONLY 10 DEGS OFF OF THEIR DEP RWYS 1L/R. I RECALL HAVING A BRIEF 'SA ALERT' THINKING I WAS TURNING TOWARD THE RWYS 1 DEP PATH; BUT THAT WAS IT. THEN ATC (TWR; I THINK) ASKED ME WHAT MY ASSIGNED HEADING WAS. THAT SHOULD HAVE BEEN ENOUGH TO WAKE ME UP; BUT I ANSWERED '050 DEGS' AND THEY GAVE ME 'TURN R HDG 090 DEGS.' I STILL DIDN'T WAKE UP. I WAS THEN HANDED OFF TO DEP CTL AND AGAIN THEY ASKED ME WHAT TWR HAD GIVEN ME FOR AN ASSIGNED HEADING. FINALLY I WOKE UP; CHKED THE MAGNETIC COMPASS AND DISCOVERED THE DIFFERENCE. I IMMEDIATELY CALLED DEP CTL BACK AND RPTED THE PROB. THAT SEEMED TO MAKE THEM A BIT LESS APPREHENSIVE. I ALSO NOTED THAT THE GYRO VACUUM INDICATION WAS AT THE HIGH END OF GREEN; AND BOTH VACUUM PUMPS WERE OPERATING NORMALLY. UPON ARRIVING AT MY DEST; I HAD THE VACUUM AND COMPASS SYS CHKED. BOTH APPEARED TO BE FUNCTIONING NORMALLY. IN RETROSPECT; THERE WERE 2 EVENTS THAT SHOULD HAVE ALERTED ME EARLY: 1) MY SA ALERT ABOUT TURNING TOWARD RWYS 1; AND 2) THE ASSIGNED HEADING CHK BY TWR AS I DEPARTED THE VORTAC. MY GYROCOMPASS IS NOT A SLAVED UNIT; THEREFORE; I SHOULD XCHK IT REGULARLY WITH THE MAGNETIC COMPASS; PARTICULARLY AFTER LARGE TURNS; AND PARTICULARLY EARLY IN THE FLT; WHEN COLD WX COULD MAKE THE COMPASS SLUGGISH.