At first, I only thought that by using big bore block to your bike it can only increase the amount of fuel and air that gets into the combustion chamber. Since more fuel and air get burned inside the combustion chamber this will give more power for every power stroke. I thought that is the only way the power is affected.
Now I know one more factor affecting the engine power by using big bore cylinder block that is about Compression Ratio. I have written about it here.
By using big bore block, you only increase the bore size, but the length of the stroke remain unchanged. When this happens, you actually increasing the compression ratio. Compression ratio is increased because you have bigger bore size with the same combustion chamber size between the TDC and the head. Higher compression ratio = higher power. That is why sometime we heard about conrod being broken when a rider upgrading their block to bigger bore size. This is due to the fact that high compression ratio will give higher stress to the engine parts especially the conrod. HISPEED from Thailand has a higher strength conrod for use with 135LC which have thicker rod construction for better handling of the stress of high compression ratio of the modified engine. IMO, if you upgrade your 135LC to 62mm cylinder block like the one from UMA Racing which is ceramic coated, the stress is still able to be endure by the stock conrod, but when you use bigger size bore like the 65mm X1R cylinder or the custom 71mm block, I think you need to use higher strength conrod. I also have heard about few 135LC riders use Modenas Jaguh 175cc conrod because of the strength of it.
This information is valid for all bike whether about Honda, Yamaha, Suzuki or any bikes. I only use 135LC as a reference.
Hope it helps you a bit!
