Rotax manufactures the 914 F2/S1 variant, which is (at least) used in Stemme's S10 and S12 self-launching gliders.

What are the differences between the F2 and the F2/S1?

Bonus question: Why have any differences at all?


1 Answer 1


The Stemme S12 maintenance manual discusses this:

The engine of the STEMME S12 is based on the ROTAX 914 F2-01, which has been certificated by BRP-ROTAX® GmbH & Co.KG, Austria. STEMME modified the arrangement of some accessories (induction and exhaust system including turbo-charger, engine mounts etc.) to adapt the systems to specific installation requirements of the S12. These modifications are certified in the STEMME S12 as engine model ROTAX 914 F2-01/S1.

and later in the manual:

The ROTAX 914 F2/S1 is modified by STEMME and based on the type ROTAX 914 F2. The modification was completed as a co-operation with ROTAX and with approval by ROTAX.


Due to special requirements for installation in the center fuselage, the following modifications were made:

  • Relocation of the turbocharger unit to the aft of the engine to stay within the outlines of the S12 fuselage. The turbocharger unit is supported by five struts aft of the engine. A supercharger inter-cooler is installed. Relocation of the turbocharger unit required modifications of the exhaust system. The exhaust headers are attached to the turbocharger by springs, positioning them in spite of the high thermal stress.

  • The layout of the oil pipes for the turbocharger are modified.

  • The exhaust headers and the muffler are shrouded by temperature-resistant material, thus thermally isolating the system from the engine bay. The turbocharger unit and the air box are isolated by radiation protective shields.

  • The original ROTAX engine mounting is not used. STEMME specially developed an engine mounting for a center installation, consisting of two upper and one lower supporting element.

  • In the liquid cooling system, the combined function of the expansion reservoir and refill container was split up in two separate containers, with the expansion reservoir located above the engine and the refill container on the left side of the fire-wall.

  • The ignition unit is installed above of the engine slightly behind the original ROTAX-position.

  • The throttle levers on the carburetors have been slightly modified (modification does not affect the throttle rigging of the original engine) and additional springs have been installed (pulling towards full-power position) to compensate for friction due to the long control cables between cockpit and engine bay.

So, the placement and mounts appear to be different, and the major reason is (presumably) for certification of a narrower fuselage.

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    $\begingroup$ This is great! I took the liberty of copy-pasting the exhaustive list of modifications, in case sometime in the future the link is no longer live. If you don't like the changes, feel free to revert to your original form. $\endgroup$ Commented Nov 4, 2022 at 12:08
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    $\begingroup$ @kennsebesta totally fine. I was trying to decide how much to paste. I thought I'd only be able to find the first paragraph but the list is very detailed. $\endgroup$ Commented Nov 4, 2022 at 15:05

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