The V12 is smooth, but does not quite achieve the "natural balance" of a straight six because the V angle creates the same kind of vibration that a V6 offers, just at higher speed. The sheer mass of a V12 helps raise that harmonic well above the useful part of the power band in most engines.

It is interesting to note that one of the smoothest engines is the I-3 . . . in 2-cycle form. It is still used in mid-sized outboard motors, and I believe it was used briefly in a motorcycle or two - Suzuki, I think. Because it fired on every stroke, it imitated an in-line 6, although the rotating masses had to be carefully balanced.

 

I'm going to disagree with you on that one. V6s vibrate because there's not an equal balance of force between the two banks at all times.

Additionally, Straight 6, flat 6, and V12 engines are free from all 4 types of forces and moments of vibration:

free forces of the first order
free forces of the second order
free moments of the first order
free moments of the second order

(Bosch Automotive Handbook, Sixth Edition, pp 459-463)

 

you need a larger engine bay for a I6 it has to be longer than a V6 bay

 

Yes, especially when crash/safety criteria (including crush space) is included.

 

I knew somebody would pull up some real engineering data on this subject eventually instead of the usual general auto knowledge that everybody already knows. But from a practical standpoint a 60 degree V6 can be silky smooth even though it's a V configuration, examples being the modern Lexus V6 engines.

 

To be honest, I was hoping someone would. The reason I started this thread was to gather more info on engine configuration. After more research, I plan to move on to rotaries.

I have a very ambitious idea for my senior project, which involves building my own version of an engine.

 

The main reason for the switch to V-6s is b/c it is compact, thus it can fit in more vehicles and spread the development costs. BMW is strongly rumored to be dumping its I-6 for a V-6 in the next M3 (with turbos). So bottom line, COST or rather, less cost.

Probably the first example of this was the Nissan VQ engine which went in everything from the Z to vans to Infinit's. Nissan created a strong/big V-6 that could be tuned for different vehicles and fit in nearly all its vehicles.

Using one engine in multiple vehicles instead of multiple engines in multiple vehicles saves an auto company hundreds of millions.