Mine's a 2.1 petrol, running most of the time on LPG. I just went through a huge amount of research into engine options (see other threads elsewhere on this forum). I considered Subaru, Golf GTI, Audi 5, Audi V6, and various Diesels including TD and TDI *(both 1.9 and 2.5 5-cyl). In the end, after much deliberation I've decided to stick with the 2.1 DJ and am just waituing for it to be returned with a slightly uprated wasserboxer engine and the LPG system uprated to closed loop.
The 2.1 wasserboxer is a lovely engine. Sounds good, and had characteristics that suit the van. Not the most powerful or torquey but if that's important, better buy a more modern van!
Subaru conversion; Nice engines and a logical upgrade but expensive - doubts about valve seats and LPG too!
Golf GTi conversion (and also the Audi 5): not enough advantage over the wasserboxer and the engine doesn't fit the engine bay as 'nicely' as the wasserboxer.
Audi V6: Nice but doesn't fit easily and is really more power than you need.
Diesels: (I should say I have a SEAT Ibiza TDI, which is excellent, so I'm not anti-Diesel BUT..) Diesels are not at all like the wasserboxer. They have great torque but are an 'industrial' engine - nothing like as smooth. Lesser ones like the AAZ are inferior to an LPG wasserboxer in every way and the TDI -engined vans I've experienced are pretty unimpressive really, unless you want to go serious off-roading. Because of transmision limitations they are nowhere near as good as the Wasserboxer on the motorway (don't expect a TDI-engined Type 25 to perform like a TDI engined T5!).
Enough. I've made my decision and am looking forward to many more wasserboxer powered years..
