Engine Smoothness

All horizontally opposed engines (boxer engines)

All boxer engines, regardless of no. of cylinders, provide perfect balance because the movement of a piston is exactly counter by the corresponding piston in another bank.

However, they are too wide for good packaging, and is more expensive due to more parts used, thus the usage is limited to Porsche and Subaru today.

Volkswagen VR6, W12 and W16 engines

For conventional V6, a narrow 15° Vee angle would have required extraordinarily large spray angle between split crank pins, hence special strengthening. However, in contrast to many believes, VR6’s crankshaft is more like an inline-6. It has 7 main bearings and independent crank throws for each cylinder, (this is possible because VR6 is longer than a conventional V6), thus avoid the crank pin problem.

Don’t think a 15° V6 must generate a lot of vibration ! on the contrary, the VR6 is inherently a well-balanced configuration because it is nearly identical to an inline-6, just differs from the latter by a very narrow angle separating each pair of 3 cylinders. As a result, it generates no end-to-end vibration like conventional V6s and is actually nearly as smooth as an inline-6.

Cut-away VR6. Note the 7-main bearing crankshaft W12's cylinder block W12's crankshaft. Note the slightly split crank pins

If VR6 is a version of inline-6, then W12 must be a version of V12. It is made by mating two VR6 at 72°. The corresponding cylinders in different banks share the same split crank pins. Thanks to the 72° V-angle, the splay angle between split crank pins can be so small that no additional strengthening is required. (see photo) You know, the short engine with 7 main bearings can hardly find space to add strengthened flying webs between the split crank pins.

W16 is identical to W12 except that each bank consists of 4 cylinders instead of 3. In other words, it is made from 2 VR8 engines.

The only mistery left to me is the V5 (formerly called VR5). It's also 15°, but how can it manage to balance between the banks ? one bank has 2 cylinders and another has 3 cylinders. Unfortunately after a lot of time spending, I still fail to find sufficient information about its detailed configuration. If you have its detailed technical specifications or even service manual, please kindly inform me :)

Volkswagen W8 engine

For installing to smaller and lower price cars, W8 is a logical development from the W12. However, W8 does not balance inherently, unlike W12. Like all W engines, the W8 has 5 main bearings and 4 crank throws like a conventional V8. It should have balanced as well as any conventional V8s, but space reason doesn’t allow. If you remember, conventional V8s can have 2 configurations:

1) flat-plane V8: some high performance V8s use this configuration. It works like a combination of 2 inline-4 engines so that there is quite a lot of second order vibration generated. However, these engines usually employ short stroke and lightweight pistons / con-rods to enhance power, thus vibration is also largely reduced.

2) Cross-plane V8: most V8s - including all sedans V8s - employ this configuration. It uses large counter balances incorporated in the crank throws to cancel the force generated by pistons and con-rods, thus all vibration is eliminated.

Because W8 is far shorter than conventional V8s, its crank throws are machined extremely thin and have no space for adding large counter balances. As a result, it employs the flat-plane configuration. Therefore W8 does not generate the "rumble bumble" noise like conventional V8s.

On the other hand, being a mass production sedan engine, W8 would have actually generated more vibration than the high performance V8s from Ferrari, Lotus and TVR, because:

1) Long stroke: the narrow-angle configuration does not allow bores too large, otherwise combustion chambers in adjacent banks would have been overlapped. Moreover, a sedan engine requires to generate a health amount of torque. No wonder Passat’s 4.0 W8 has a stroke measuring 90.2mm, longer than the bore of 84mm.

2) Heavier pistons: while Ferrari uses lightweight forged pistons and titanium connecting rods, the W8 can only seek more cost-effective method to cut weight from pistons.

To sports car V8s, a bit vibration means nothing. To the sedan-use W8, the only choice is to add twin-balancer shaft like many inline-4 engines.