Naah. The eight is rougher, and the twelve is impractical.
I actually thought the ... "Straight 8"s and "Straight 12"s were smoother, even the one "Straight 16" that was in production for a short time.Nope.
Here, from [link|http://www.autozine.org/technical_school/engine/smooth3.htm|AutoZine][*]:
As shown in the picture, straight-6 engine is simply two 3-cylinder engines mated symmetrically together, thus piston 1 is always in the same position as piston 6, piston 2 the same as piston 5 .... in other words, the engine is balanced end-to-end and requires no balancer shaft, unlike 3-cylinder engines.So basically, the straight-twelve is just as smooth as a V twelve -- no less, but no more either. The only difference being, it's impractically long. The straight eight, with its number of cylinders being a multiple of four but not three, will (AFAICS) have the same problem as a straight-four -- second-order mass movements not cancelling each other out; thus, vibrations. (Bugatti, Alfa Romeo, Mercedes and the others who built straight eights did so for other reasons than any particular smoothness.)
What about vertical / transverse forces? like 3-cylinder engines, the vertical and transverse forces generated by individual cylinders, no matter first order or second order, are completely balanced by one another. The resultant vibration is nearly zero, thus inline-6 is virtually a perfect configuration.
Inline-6 is not the only configuration can deliver near perfect refinement, but it is the most compact one among them. All boxer engines are perfectly balanced, but they are two wide and require duplicate of blocks, heads and valve gears. V12 engines also achieve perfect balance, but obviously out of the reach of most mass production cars. Automotive engineers knew that long ago, that's why you can see most of the best classic engines were inline-6, such as Rolls-Royce Silver Ghost, Bentley Speed Six, Mercedes SSK, many Bugattis, Jaguar XK-series and BMW's various models.
V12 engines
Theoretically the best balanced configuration for practical use. It is simply a duplication of inline-6 (therefore achieve the same perfect balance), with corresponding cylinders in both banks joined at the same crank pins. V12 is better than inline-6 just because it has more cylinders, thus doubling the firing frequency and smoothen power delivery.
And for all I can see, the same would go for a straight sixteen... Not that I can quite put my finger on what you're talking about there. Did Packard[+] build one of those, or are you mixing it up with the old thirties Cadillac V 16[\ufffd]? (Or even the Cizeta Moroder of the late eighties / early nineties? Naah, didn't think so. :-)
[*]: Thanks, Other Scott, for bringing that site to my attention!
[+]: This must obviously have been before he teamed up with Hewlett.
[\ufffd]: The one the show prototype a year or two back was a homage to.