Ok I've got a 159 2.2 JTS which we're thinking of increasing its capacity.
Either through a bigger bore and/or change in piston length + Port & Polish.
Any idea if anyone here has done this or where do we start?
Remember that engine volume is defined as swept stroke x area of bore. By adjusting conrod length or piston height does not influence swept stroke - it changes the compression ratio only. Stroking by definition means to lengthen the crankshaft. Increasing the piston bore does relate in increasing capacity.
By increasing compression ratio - you increase the thermal efficiency of the fuel burn thus better power for each cc of fuel used
The problem comes in with pre-detonation (a.k.a) pinging (a.k.a) knocking.
By Flowing the input ports you increase the air velocity that the air/fuel can enter the combustion chamber, thus increasing potential power. By Flowing the exhaust ports, you increase the air velocity of exhaust gasses leaving the combustion chamber, thus leaving a cleaner environment for the new air/fuel to burn.
Now increased air velocity only helps a small bit to get more air/fuel in. By getting more aggressive cams you lengthen the time to port stays open to let more air/fuel in. With Inlet / outlet cam overlap you allow the new air/fuel to push out the old exhaust - but overdoing that allows unburnt air/fuel to go into the exhaust and increases fuel consumption more than the power it adds. By increasing the valve size you increase the volume of air that can enter for a certain valve duration - it speaks for itself, but difficult to adjust
So if you can increase all 3 air characteristics - air velocity (port flowing) - valve open time (cams) and valve open area (bigger valves) you will optimize quite a bit.
Now Back to CR and capacity... If you have poor flow as defined above, increased capacity will not help much as you are not getting optimum air/fuel in... Higher CR will always increase thermal efficiency, but by not adjusting any of the other items - it in itself does not add that much to warrant opening the engine...