It is my understanding they are to help reduce damage over the metal surfaces of the engine from running. Vibration against a high tension lead can lead to voltage bleed very quickly. They are not heat shrink, they are simply a protective barrier for the wire.
Just a note, the 9 series engines did not always have this sleeve. It was introduced a very long time ago however.
I just checked to confirm. If you look into the respective parts manuals you will note that the sleeve is called a "protective hose". Also looking at the type 912iS engine parts you will see no sleeves. This is because the coils are mounted on the intake manifolds and the wires do not run where they can rub on the engine like the standard 912/914 does.
The 912iS wires do have sleeves, it’s listed as a “glass-fiber silicon sheeting”, #18 in the diagram 74-20-00-2 of the IPC. Last year I bought a complete set of wires for my 912iS and it came with the sleeves installed and number/letter coded for each position.
Due to addl info I found in the forum regarding the sleeves and shielding, I ordered the Ignition Shield Braid, along with a length of 7mm Plug Wire from our favorite supplier. I also found a great posting on a different forum. Hope this helps.
Scott..perhaps read the thread. The 912 they were talking about was the earliest version of the CDI, vintage 1992. They used an aluminum suppression box and the modules, coils, were all inside. The original wires were carbon core and had tons of problems. Due to this Rotax developed the newer systems. I would not recommend any suppression type wires. attached is the information when they moved away from the old wire and plugs...from 1991.
Cheers
36827_2_SI-01-1992_Engine type 912 UL ignition unit with resistor spark plug 1.pdf (You do not have access to download this file.)
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