The only difference between a 180 deg V6 and a boxer six is the number of crank planes and therefore the overlap of the power strokes.

As alluded, a 180deg six is rough and rarely used; unless you count two of them as in the Ferrari 312 racing 12!

Straight sixes are more easily developed from four-bangers (ask Alex von Falkenhausen, Karl-Ludwig Brandt et al.). If correctly designed, they needn't be 50% heavier, since the crank doesn't need to deal with as much vibration.

Interstingly, BMW publicised a direct V6 version of their 3.0 straight six, they'd made as a direct comparison, since they could share it with Rover.

Packaging wise, it was only about one pot shorter (good in a shunt) but the extra V6 width made it almost a square, so it didn't really save any length whichever way you put it in the car. And the width caused problems in the 3er, which had a narrow bay. It wasn't that much less refined, or less powerful, really. So they concluded they'd best keep the S6 & ditch Rover, I suppose.

You can make a V6 by sawing two pots off a V8 and using offset crankpins to smooth it out, as with the Buick and PRV engines, too.

The NSX's DOHC heads meant the car had to grow about 150mm in overall length, over the Rlegendo-engined SOHC prototype. So those doubled-up heads are expensive in many ways.

 

That's because they're two-stroke

Wait 'til you have to have both timing chains replaced. There's only one on an S2000 and you don't need to drop the lump, so there's a saving!

 

That's no problem, that's what a warranty is for so not bothered if it goes pop . I take it you have a warranty too

 

I seem to recall reading a piece by the late, great LJK Setright that the main reason for the development of the V6 configuration was for packaging reasons - the shorter block can have a profound effect upon the styling constraints imposed in a front engine configuration and it is a much more efficient use of space in the engine compartment than an inline unit.

An inline 6 has better primary balance than a V6 however.

 

BMW gets around that by canting their engines 30 degrees.

But packaging a V6 behind the front axle to minimise polar moment of inertia is easier.

 

Right, a bit more time to read and reply now (answer was a bit brief cause I suddenly realised I was a bit short on time at lunch)

There are 4 primary engine layouts used for reciprocating internal combustion engines (I'm going to ignore rotarys/radial engines since they're not used much in Automotive)

The 4 types are:
Straight/In-line - All cylinders have a common axis (Vertical or canted at a slight angle when mounted
V/Bent - 2 banks of cylinders canted over so the axis forms a "V" at the crank centre line
Flat/Boxer/Horizontally opposed - Essentially a V engine at 180 degree angle so the two axis are flat. Nicknamed a boxer due to the continual "punching" motion of the pistons
W - 4 banks of cylinders around a common crank. Essentially 2 V engines mounted side by side around a common crank

Typically engines start at 3 cylinder straight engines and go all the way up to monster W16 engines.

Hence I'll start with:

In-Line 3 Cylinder
Lacking in dynamic balance compared to a 4 cylinder due to the lack of a fourth cylinder to counteract the vibration (paired cylinders tend to help eliminate vibration). Lighter and cheaper than a 4 cylinder and more economical due to reducing the drag of one cylinder. They're also lighter and shorter.

Makes them ideal for small light city cars where economy rather than high speed refinement are most important.

In-line 4 cylinder
4 cylinders giving a power stroke each 180 degrees of crank turn matches well with the 4 stroke engine cycle meaning one cylinder is always firing. The crank is flat which means it helps to give good balance in the engine and the power strokes make for a smooth torque curve. Since everything is pretty much in the same axis costs are low, the single cylinder head/valve system and flat plane crank all help this. This is the traditional configuration for the majority of cars since it's the best compromise for most applications.

Horizontally opposed 4 cylinder
Very slightly better balanced than the straight 4. With similar torque characteristics. Low which makes it useful for rear engine mountings or for use where you're trying to keep a low centre of gravity. Packaging can be a problem though since you have to make the mounting wide enough for the cylinder heads for servicing. Also the additional cylinder heads add to the cost

V4 engine
Generally pointless. Extremely compact but normally outweighed by it's extremely poor balance. Almost always requires a balance shaft to help bring vibration to an acceptable level making it expensive and not as suitable as a straight 4 which isn't much bigger.

In line 6 cylinder engine
Longer than the 4 cylinder but suitable if space is available once you get above 2.5L. Has excellent balance compared to the 4 and has the advantages of the single cylinder head

Horizontally opposed 6 cylinder engine
Shares the advatages of it's straight sibling and also it's disadvantages to a more significant level (even more room required for clearances). One additional advantage though is the shorter crankshaft allowing for a shoter engine and more space lengthways. Again, once you're adding in additional cylinder heads cost and complexity begin to rise.

V6 engine
The biggest issue in both of the above engines has been in packaging to fit a vehicle without compromising other features such as cabin space. The V6 does this by keeping itself almost as narrow as the Straight engine but as short as the flat engine.

Straight 8 engine
Extremely long and rotational speeds have to be fairly low to keep twisting of the crankshaft within acceptable levels. Good dynamic balance though. It's most commonly used as a truck engine layout or similar where the length isn't so much of an issue

V8 engine
Once you're into 8 cylinder and above engines you're really looking at bent forms to keep the length manageable. Cruciform crankshafts are a preference since they give a significant balance advantage over single plane in this engine type.

W16 engine
V-16 engines begin to run into the same issues that straight 8s run into and hence you have to shorten the crank again. To do this another pair of banks of cylinders is added to make a double V V mounted around a common crank and hence the W designation (essentially the same length as a straight 4)

 

I've heard people say that V10 is imbalanced, and V12 super smooth. Is this due to harmonics/vibrations?

And why the hell do VW make a V5, albeit with a narrow angle?

 

Straight and narrow angle 5's work quite nicely. The 5th cylinder helps to smooth off the cycle somewhat giving a smoother acceleration. Plus the added bonus of an extra cylinders worth of power.

12's work a treat since they essentially give you the benefits of 2 x 6's. i.e. more powerful but still silky smooth. 5's are harsher than 12's but they're not dreadful by any means.

Essentially vibration is down to balance of forces. For example on the straight 4 cylinder you have two pistons on the down stroke at the same time as two on the up stroke which helps to naturally damp vibration. The V 4 has those pistons moving at all sorts of odd angles with little to balance them so hence it tends to be all over the place

 

The S5 and V10 require massively heavy sranks, in order to contain their unbalanced forces.

You cannot enhance torque by using the backpressure pulse principle, which gives good torque on fours and excellent torque on V8s. Indeed, S5s reallt need puffers, which is why most are rattlers.

Honda did an interesting racing bike V5, with the 5th pot giving an out of phase 'pulse' to the crank. The idea was the engine was so powerful, smooth power pulses would light up the back tyre too easily. The out-of phase pulse gave a form of cadence acceleration, which made the thing less difficult to ride.

 

Not half as interesting as the Honda NR - a siameased V8 with four oval pistons and 8 valves per 'cylinder'. A super cool way to circumvent the prevailing racing regulations requiring a 4 cylinder configuration. That was the plan at least...

The twin cylinder Duke's 'big bangs' were supposedly the reason for the success of their bikes - the longer interval before each power stroke gave the rear tyre longer to recover than with the more frequent (albeit smaller) bangs of the competing four cylinder engines.

Fascinating stuff. But I reckon it would've been more effective to simply to fit wheels and tyres from the 'Tron' light-cycles.