Engine Supercharger Upgrade

This blog covers preparing the engine for the supercharger and some of Marwen's work and discoveries about Rotax 912ULS cylinder heads

Overview
The selected engine had a total of 140 hours when it was removed from a wrecked Foxbat which as a result of the accident suffered a prop strike. The engine was inspected and signed back into service by an approved Rotax agent at the time of purchase but it was decided to verify.

To achieve this, the engine was stripped, cleaned, painted black and fitted with a full set of O rings etc to bring the engine back to nearly zero time. One issue noted was that this engine had seen a lot of service running LL Avgas and this required about two [2] hours of cleaning by Joe from Marwen.

Checking engine mechanicals

Engine assembly

All the gears and crank checked out to specification so Joe [ex-race bike mechanic] decided to check the final compression ratio as he always had done when racing. While a relatively simple operation it revealed a static compression ratio of 9.6: 1 which would create an effective compression ratio close to 13.6: 1 [worst case].  

Joe was concerned and contacted me, we looked at options like knock sensors but the real issue was the compression ratio. 

The starting ratio should be 8.5: 1 - where was the error?

Note: Worst case was defined as the smallest combustion chamber - they varied by about 3 cc and due to the small swept volume this had a large effect.

Joe checked his figures again & again and they came out the same, there was an error somewhere. Joe next calculated back to the factory compression ratio, it was 11.1: 1 with the accepted ratio for a 912ULS generally understood to be 10.5:1.  This was answered when Rotax released it's current heavy overhaul manual which clearly states the compression ratio is now 11.1: 1 for these engines.

Extract Rotax Heavy Overhaul manual - page 12

Modifications 

Marwen Spacer 1.5

We have decided to decompress the motor further with the original 1.2 mm spacer rings replaced with 1.5 mm rings and the heads CC to match the largest measured volume. 

This realized a compression ratio of 9:1 and gave a effective compression ratio of 12.7:1 with a 6 psi of boost.

The next issue was that the original 1.2 mm spacers left the hydraulic lifter with only had 0.8 mm of travel and now with a 1.5 mm spacer it would be 0.5 mm [0.020''], we were edging close to mechanical lock up. 

This issue is to be addressed by manufacturing custom lifters placing the hydraulic lifters back onto their mid position [+/- 2 mm]. 

CC combustion chambers as part
of setting the final compression ratio

Comment: 
The comment below was taken from the Pro Power site but generally, any search will yield the same comments at many sites - edited here for clarity with this case.

"For 9.5:1 EFI/TPI applications running without an intercooler, boost levels above 5 psi will require the use of ignition/timing retard on pump gas, and will produce horsepower gains of 35-45%.  Of course, lower compression motors will be able to run more boost, and higher compression motors should run less boost, everything else being equal. 

For carburetor motors, the rules are slightly different. 

Carburetors deliver the vast majority of fuel in a liquid state, and as this raw fuel atomizes from liquid to gas, a chemical state change actually occurs. Due to this endothermic reaction, which draws heat and cools the incoming air, a carburetor motor can safely handle more boost than a comparable EFI/TPI motor. For carburetor engines with compression ratios of 9:1 or less and boost levels in the 8-14 psi range, pump gasoline works very well. "

Joe cool and sidekick

Does the current arrangement work, well yes but with the measured increased compression of 0.6 points an intercooler should be considered. Also as we intend to improve fuel delivery so any protection created by over fueling may not be there.

This one is open to comment .!

For this aircraft, we decided to err on the side of less power to help stave off any chance of detonation on those very hot Australian summer days.

Methanol injection would be an option, Joe Cool loved racing on methanol.