Replies

Minor correction to that last - a V8 has five main bearings.

What VW did is to build a relatively short stroke V6 that had a 15 degree angle. This made the engine much narrower than a traditional 60 degree V6, yet it allowed them to keep the length shorter than an inline six while reducing internal stresses, vibration to near I6 levels. It also, incidentally, let them put medium displacement six cylinder engines in cars that normally would have had to make do with a large displacement (and buzzy) four cylinder. A cutaway picture of the beast can be seen at http://heckteck.de/vr6.jpg - it's a very clever job of engineering. What does this have to do with the W12?

Most V12's are essentially two inline sixes running off a common crankshaft ("siamesed"). Since the VR6 is sort of a hybrid between an inline six and a V6 and leans more towards the inline side, VW decided to see what happened if they siamesed two VR6's together. What they got was the W12, which turns out to be a very good engine indeed - it's a lot shorter than it is wide, which makes for interesting packaging options. VW has further adapted this technique to create a W8 and a W16; the W8 has already been used in the Passat, which wouldn't be able to accomodate anything larger than a conventional V6 under normal circumstances.

The W12 is 72 degrees between the two VR6 modules that comprise each bank. It's 512mm long, 710mm tall, 715mm wide, and weighs just 239kg. Compare that to a "traditional" BMW V12 which is about the same height and width but twice the length and you can see the advantages. It also uses quite a bit of higher technologies - variable valve timing, exotic metallurgy, lots more.

There are disadvantages to this idea, though. The casting and machining is more difficult to successfully achieve than, say, the BMW or Mercedes V12s (with a corresponding relative increase in production price somewhat mitigated by the fact that it's just two VR6's) and the valvetrain can be a problem to design (getting four cams into that space can get ugly). In addtion, special attention has to be paid to the lubrication system design to ensure that weird things do not happen - and you have to be very particular about what kind of oil goes into the beast.

Any questions? :)