Hal was down visiting Wayne of Southland Rotax and we started talking about more power in the 900 series engine. The rumor was the German’s have a big bore cylinder. Why can’t we make one? Wayne gave Hal some cylinders and then started looking for pistons.
The limiting factor on the bore was the 91mm bore in the case. After some consulting with sleeve and piston manufactures we decided an 88mm bore would give us about a 1.5 mm wall thickness to protrude into case. Hal built jigs for the lathe and bored stock cylinders out and then installed iron sleeves.
We installed the first kit in a 1000HR 80 HP 912. The static RPM went from 5300 to 5800 RPM. Take off roll was shortened several hundred feet and climb increased 500 FPM. Top speed went from 105 MPH to 120MPH.
After building 3 kits with substantial power gains, it was time to look at manufacturing. The forged pistons on iron sleeves had to have .005 clearance to run properly. This causes some oil burning–about 1 quart every 6 hrs or so. We looked at hyperutectic (high performance cast pistons) and cast pistons but there were no molds for our application. Then we found the Porsche racing engines with air cooled cylinders and water cooled heads using billet nikasil plated cylinder. The Rotax is cast pistons and cast nikasil cylinders. The Porsche racing boys found the billet cylinders with low expansion JE pistons could be clearanced at .0015 and run much cooler with no oil burning. The Rotax was able to use the same blanks as the Porsche.
The billet cylinders have 35% more cooling using 14 fins instead of 9 on each cylinder. The billet is much stronger and lighter than the cast cylinder. The piston, rings and wrist pins are 50 grams lighter than a 912S, 410G versus 460G. With the lighter assembly the engine will lose 3lbs of weight.
All of these parts are designed and made in the USA. Replacement parts are available and cylinders can be replated if the need ever arises. There are two 1484 zipper kits available. A 9.7 to 1 compression that will run on 87 octane auto fuel. This will produce 105 HP. We call this the 1484 Zipper High Torque. The second kit is an 11 to 1 compression piston that will need 91 octane or avgas. This produces a whopping 114 HP. The 1484 zipper High Torque or High Power are both plug and play on the 912ul or 912uls. If you want the 114HP Kit a Heavy Duty Case, slipper clutch and Heavy Duty Starter are recommended. Rotax started the HD Case about mid 2006. If yours is not we can machine it to HD for you. If you want other compression ratios or bore sizes, give us a call.