FLC IN A B26B ON IFR DEP FROM VNY ARPT ENCOUNTERS STRONG UP- AND DOWNDRAFTS.

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ON THE MORNING OF FEB/XA/00 AT XA30; I DEPARTED VNY FOR A FLT TO TRM. THE WX WAS IFR AS A SERIES OF WX FRONTS WERE MOVING THROUGH SOUTHERN CALIFORNIA. I HAD FILED AN IFR FLT PLAN AND WAS IN CONTINUOUS CONTACT WITH SOCAL DEP. I PENETRATED THE CLOUD BOTTOMS AT APPROX 2500-3500 FT MSL IN THE CLBOUT USING THE GLENDALE 8 DEP SID. I THEN EXPERIENCED A SERIES OF SIGNIFICANT UPDRAFTS AND TURB. I REDUCED PWR AND IMPLEMENTED A SERIES OF DOWNWARD PITCH CHANGES TO THE MAX EXTENT POSSIBLE WITHOUT EXCEEDING THE PRESCRIBED SAFETY LIMITS FOR THE ACFT. DESPITE MY BEST EFFORTS THE ACFT CONTINUED TO CLB TO 4600-4700 FT. THE UPDRAFTS AND TURB LASTED 20-25 SECONDS AFTER WHICH TIME I DSNDED TO MY ASSIGNED ALT OF 4000 FT. I ACKNOWLEDGED TO THE SOCAL CTLR THAT I KNEW MY ALT WAS TOO HIGH AND THAT I WAS CORRECTING IT. I DO NOT BELIEVE THIS PROB WOULD HAVE OCCURRED HAD I ADVISED THE CTLR OF THE UPDRAFTS INSTEAD OF STATING I WAS 'CORRECTING MY ALT.' UPON LNDG; I CONTACTED SOCAL TRACON AND ADVISED THE SUPVR OF THE INCIDENT INCLUDING THE TURB AND UPDRAFTS.