It is without a doubt one of the most complex car engines ever produced. A complexity that is as unreasonable as it is excessive and useless because V8s of similar displacement can easily rev just as fast and with 2 turbochargers only would develop equivalent power with fewer cylinders, therefore in principle with better thermal efficiency.
Crankcase and one cylinder head of the Bugatti W16
The Volkswagen Group (VAG) certainly chose to develop this crazy 16-cylinder monster since it already had the W8 and W12 in production, themselves initially developed from the VR6. Thus a W16 was only a relatively small development step from the W8 and W12 and it could use some common parts, while a large and powerful V8 or V12 would have been a totally new development. Another point that must have been influential is the prestige provided by a 16 cylinder engine: a V8 would appear too common for rich snobbish people!
Cutaway of the W12 6.0 TSI used on the Audi A8 and Bentley Continental
The W16 shares its 86 mm bore with the V6 3.2 FSI and W12 6.3 FSI. A twin-turbocharged V8 of 120 mm bore and 90 mm stroke would have a displacement of 8143 cm3 and could rev all day long at 6500 rpm. It would be externally smaller and weigh less than the Veyron’s 400 kg W16 monster while developing equivalent power and torque. A Chevrolet big block, in its most heavy all cast iron variant, weights about 310 kg (680 lbs) complete with ancillaries! The aluminum versions are at around 250 kg.
Brian Callahan's answer to What is the weight of a Chevy 454?
Edit: The main and rod bearings on these W engines of the VAG group are very much stressed. Their crankshafts are so short that the journals and crankpins have an extraordinarily small width: the rod bearings are only 11.6 mm wide and the main ones 15.8 mm. It is clear that the specific loads on these bearings are very high and that the use of an oil too fluid when hot is dangerous in this case. Those engines require a suitable lubricant which meets very special specifications established by Volkswagen: VW 503 01 whose grades are generally 0W-40 or 0W-30 but with an HTHS viscosity at 150 ° C> 3.5 mPa.s or VW 504 00 whose grade is generally 5W-30 but HTHS viscosity at 150 ° C> 3.5 mPa.s
Crankshafts of the W8 and W12 of the VAG group compared with those of conventional V8 and V12. Motortechnische Zeitschrift (MTZ) picture
The V angle between the cylinder groups of the W16 being 90°, the rod journals are not split
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