In Part-I of above, we went trough the basics and their respective definitions. We’ll now try to understand what real life implications do they have on the ‘Performance’ of an Automobile.

But before we do that, there are a few more fundamentals to grasp…..

To elaborate upon it further, the ‘Power’ that an ICE can develop is given by the following elementary relationship:

BHP = P x L x A x N - Where:
P= 'Brake mean effective pressure/Bmep' in a cylinder over its 'one power cycle' i.e. one rev for two stroke and two revs for four strokers.
L= Length of its Piston travel/stroke.
A= Bore/Piston Area.
N= 'Rpm' at which the engine can/runs.

It, therefore, follows that to get the max BHP out of an engine, given a particular 'CC' (LxA), the only two variables left with one to 'fiddle with' are 'P' and 'N'.

'P-max' in turn depends on :-
a) The ’compression ratio’ one can employ, which in turn gets limited by the Octane rating AND Calorific Value of the Fuel available (we are not talking of adulterated ones here !). For the ‘Regular’ 87 Octane Petrol in our Country, it's in the region of 8 to 9:1, going upto 10:1 for 93/100 Octane Petrol.

b) The 'Volumetric Efficiency' of the Engine. In other words, its ability to 'breathe-in’ as much fresh air and exhale as much exhaust gases as possible, without any significant 'pressure drops' between the outside atmosphere and its 'lungs/heart', that's the insides of its cylinders.

c) Its internal/frictional losses.

Similarly, 'N-max' depends on :-
d) A practical upper limit, which it can withstand without damaging itself due to the various complex forces - centrifugal and torsional for example - that go along with it. It’s also limited by the ‘valve-train’ beginning to ‘float’ after a particular/high rpm – such that they are unable to open or close ! It also depends on the ability of the Engine’s Lubrication System to maintain the desired levels at all the moving surfaces.