Quote:
Originally Posted by Huntsman 
Remember, too, that V-12s are generally quite high performance affairs, and there are not that many of them. 6s and 8s are a dime-a-dozen in any number of output levels. I knew someone would bring up the flats. Ok, this isn't really all that simple. We had a lively debate in Advanced Thermo on this. There are two main kinds of 'flat' engines, the Boxer and the 180 deg V. They are not equivalent in terms of balance. 1) The Boxer, in which the cylinder motion is mirrored across the crankplane, i.e, both cylinders in each bank are at TDC and later BDC at the same time. For this to be possible, imagine the cylinders at BDC, closest to the crank. Where are the con rod big ends? If the cylinders are perfectly opposed the big end bearings would be occupying the same space, which of course cannot be. Therefore Boxers do not have perfectly opposed cylinders in each bank, they are offset slightly, requiring (as the defining feature of a Boxer) separate crank journals. Now even though the pistons move in opposite directions (inducing primary balance), there is a free secondary couple (erm, tied rotational forces) caused by that cylinder offset. That couple acts to rock the engine around the vertical axis (that is, about an axis normal to the cylinder plane). So while all Boxer configurations can have inherent perfect primary balance, that's kind of big whoop. Secondary balance is much more challenging to achieve, and Boxers can by canceling out the couples with enough cylinders. The Boxer-4 is not perfectly balanced. I think the six is. 2) The 180 degree V. The difference between a Boxer and a 180 degree or flatplane V is that the cylinder motion is not mirrored across the crank. One cylinder in a bank is at TDC while its twin is at BDC, resulting in (just like a crossplane V) two big ends per crank journal. Obviously then, perfect primary balance is not achieved for all configurations. Again, add cylinders to fix this problem. The 180 deg V-6 should be (I think), and I know that the 180 degree V-12 is balanced. However, that is for the same reasons that the V-12 is balanced, not because it is a flat engine. I'm also reasonably certain that the Testarossa uses a 180-V and not a Boxer. 3) Explanations like this are when I wish I was a better kinematician. No clue about the W's. ~ H
