Hi

Any engine will be a compromise of friction' date=' reciprocating masses, thermal efficiency and weight.

Optimum for thermal efficiency in petrol engines is said to be around 325cc (according to GM) per cylinder. If the cylinder gets smaller, the ratio of thermal loss to combustion chamber size goes up. If the cylinders get bigger, the thermal losses get smaller but the combustion efficiency (due to flame front / burn rates) gets lower.

Internal friction (the total amount of friction losses) goes up as the number of cylinders goes up. However, it also goes up if the bore and stroke go up. For example a 2 litre engine may have similar internal friction in 4 and 6 cylinder configurations. The 4 cylinder engine will have higher reciprocating mass loadings, longer stroke hence higher piston speeds and bigger vave gear dispalcements. The 6 cylinder engine will of course have more cylinders and more vavles. The losses will often equal out pretty close.

Where a six cylinder engine will prove superior to an in line four is in refinement. The Vibration will be less, (a V6 has good primary balance) the exhaust noise easier to silence (smaller pistons = smaller bangs), and the sound is somehow much less tiring than in a four cylinder.

When it comes to tuning, the reciprocating masses come into it again, along with piston speeds. Bigger cyinders with bigger valve gear just cannot be made to move as fast as smaller cylinders. Optimum again appears to be around 300-350cc/ cylinder.

Chris[/quote']

I'm sure i read this in evo :confused:

I'm sure i read this in evo :confused:

These principles are common sense and common knowledge for anyone looking to increase the power output of engines. The EVO tech articles are pretty useful laymans introductions IMO and give a bit of an insight into the physics and principles that make our cars go (and stop). Mind you, they are not always strictly accurate of comprehensive.

Chris