THE ACFT EXPERIENCED AN IN-FLT FIRE ON THE DOWNWIND LEG OF THE TRAFFIC PATTERN DURING A FLT TEST FOLLOWING ROUTINE MAINTENANCE. THE PLT STATED THAT WHILE FLYING AT 500 FT AGL IN THE TRAFFIC PATTERN SMOKE BEGAN ENTERING THE COCKPIT. ALLGAUGES AND SYSTEMS SEEMED TO BE OPERATING NORMALLY. THE SMOKE INCREASED AND AN EMERGENCY LANDING WAS MADE ON THE RAMP. AFTER TOUCHDOWN THE PLT REQUESTED ASSISTANCE VIA TOWER OF THE FIRE DEPT. THE PASSENGER GRABBED THE ONBOARD FIRE EXTINGUISHER AND DISCHARGED IT ON THE FIRE. THE FIRE WAS NOT EXTINGUISHED. ABOUT 2 MINUTES LATER THE FIRE DEPT PUT IT OUT WITHOUT DIFFICULTY. INVESTIGATION SHOWED THAT A CLAMP (ENSTROM P/N LW 13464) THAT SECURES THE TURBOCHARGER TO ITS EXHAUST PIPE HAD SEPARATED IN-FLT. HOT EXHAUST CASES BURNED THE COWLING AND THE FIREWALL BUCKLED. HEAT IGNITED MATERIAL IN THE COCKPIT. EXAMINATION OF THE CLAMP SHOWED IT FAILED AT A SPOT WELD. THE CHEMICAL AGENT IN THE ACFT FIRE EXTINGUISHER WAS DESCRIBED BY THE MANUFACTURER AS INEFFECTIVE AGAINST ALUMINUM OR MAGNESIUM FIRES.