CFI AND SPI IN AN EMER OFF ARPT LNDG DURING A NIGHT OP IN A C172. FLT ASSIST FROM POLICE ROTARY WINGED TFC TO LIGHT UP LNDG AREA.

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AFTER AN UNEVENTFUL TRAINING FLT FROM VNY TO ST PAULA WITH MULTIPLE TOUCH-AND-GO LNDGS; WE CLBED UP TO 3500 FT MSL AND STARTED BACK. JUST AS WE WERE RECOVERING FROM A PRACTICE APCH TO LNDG STALL (PWR OFF STALL); AT THE POINT OF PWR APPLICATION; THE ENG MADE A LOUD SOUND AND BEGAN VIBRATING. AS WE HAD PREVIOUSLY PRACTICED SIMULATED ENG FAILURES; I HAD MY STUDENT CHK: SEAT BELTS TIGHT; FUEL ON BOTH; MIXTURE RICH; CARB HEAT ON; MAGNETOS L/R/BOTH (NORMAL DROP); PRIMER LOCKED; AND I CALLED VNY TWR; INFORMING THEM I HAD A PARTIAL PWR FAILURE; WAS DECLARING AN EMER AND HDG DIRECT FOR THE ARPT. WE WERE ABOUT 15 NM WNW AND THE TWR GAVE US A SQUAWK CODE AND ASKED IF I THOUGHT I COULD MAKE IT TO THE ARPT. I SAID I WOULD DECIDE AT THE E SIDE OF THE LAST SUITABLE OFF ARPT LNDG SITE (CHATSWORTH RESERVOIR -- A DRY WASH WHICH I KNEW HAD 2 DIRT ROADS). HAVING LOST 1000 FT IN ABOUT 5 MI; I TOLD TWR I WAS LNDG IN THE RESERVOIR AND REQUESTED ASSISTANCE IN CASE THINGS DIDN'T WORK OUT. BY THE TIME I TURNED BASE FOR A DIRT ROAD I COULD BARELY MAKE OUT; A POLICE HELI ARRIVED AND LIT UP A LNDG AREA AND TREE I WAS ABLE TO AVOID BEFORE TOUCHDOWN. THERE WERE A FEW SCRATCHES FROM BUSHES I SCRAPED; BUT OTHERWISE NO INJURY OR DAMAGE. ONE SPARK PLUG WAS FOUND MISSING; REPLACED AND THE ACFT RUN UP AND INSPECTED. AFTER MEASURING AVAILABLE SPACE AND DETERMINING IT SAFE TO TAKE OFF; I FLEW THE PLANE BACK TO VNY. I MAY HAVE BEEN ABLE TO MAKE IT TO VNY ON THE NIGHT OF THE OCCURRENCE; BUT ELECTED TO LAND OFF ARPT.