for the question of an engine size, the whole deciding factor is
the aims of the designer. A V12 has more cylinders, hence the gap
timing between each of the cylinders' firing order is smaller,
hence it is smoother. But there are so many factors to
consider.
In terms of more cylinders, 12 vs 10 vs 8, in general more
cylinders means more valve area, so u can in theory get more
airflow in an engine per revolution. More cylinders also tend to
have lighter pieces for a given capacity, so u r often able to run
them to a higher RPM. And both of these things mean more airflow,
to which you put more fuel, and that means you add more base
power.
But what u do is u take off some things too. More cylinders give u
greater losses thru friction. Though u are theoratically producing
more power, because there are more moving parts so it takes more
power for them to move, so in theory u get a situation where a 12
vs 10 vs 8, will each respectively have more power but a lot more
losses, so that the actual bhp is different.
But there is obviously a lot more to it, because of the losses.
More cylinders are less efficient, so you have to use much more
fuel. In the end u've got a lot of things going against much more
cylinders and only a few going for them.
Also considering the bore and stroke, in a V8, it has a large bore,
which is god for generating power, and a short stroke, which is
also good for boosting revs because the piston has less dist to
travel, It has good torque. For a V12 though, the capacity must be
much larger, because in any case, if u compare a V8 and a V12 both
with the same capacity, the V12 is going to get better top end
power because of the lighter pistons moving up n down, but it lacks
low end torque.
Another problem is packaging. In designing a car, u have to decide
how to package the engine. A V12, though more powerful, is longer
and heavier, used more fuel, and its crankcase was so much longer
that it is fundamentally less torsionally stiff. There are also
more exhaust pipes to package. Also, operating with more friction,
it is less heat efficient, which would imply that larger radiators
are needed, which presented more packaging problems. This would
also lead to increased drag, which could be undesirable for the
want of a slightly superior top end power.
