I think I outta kick your arse for making me think too hard on a Friday. j/k ya know

Increasing bore size wouldn’t increase engine rpms. The rpm “limit” is mostly dictated by the weight of the rotating assembly and its balance (provided there is enough fuel and air to carry it to destruction). By increasing the bore size, the larger piston is naturally heavier and therefore needs a heavier counter weight for proper balance. It will not turn the rpms “quicker” either. In fact (although we could be talking nanoseconds here), it would actually slow the rpm gain. Although there is more volume with more air and more fuel being burned, the rotating mass is heavier thus making it slower to gain. With keeping in mind we are talking about rpm climb losses of nanoseconds and weight increases of milligrams, this is all pretty insignificant but I feel quite confident that it’s true.

Bore and Torque and Horsepower:

I’ll tell you what I’ve done (because curiosity got the best of me) to find out what I’m now reporting. This gets kind of weird and since I could only “test” four strokes, I’m not sure how to apply it to our tiny 2 strokes.

I have DesktopDyno software. Its “virtual” I know, so one can’t really take the figures to heart but it is a great tool to see where changes of this or that changes this or that. Got me? Ok. I started off by putting together your basic single cylinder lawn mower engine. These figures are from increasing the bore only.

3.000 in. bore @3500 rpms—12 hp—18ft lbs torque. @4000 rpms—13 hp—17ft lbs torque (those were the peaks (13 hp and 18 torque) with 500 rpms difference).

3.030 in. bore @3000 rpms—12 hp—18 ft lbs torque. @3500 rpms—13 hp—18 ft lbs torque.

As I increased the bore size in increments of 0.010 each time, the number changes were bare minimal (mostly stayed the same) BUT, they moved lower and lower down the rpm scale. By the time I had the engine at 3.080 bore, and although it still didn’t ever create more than 12 hp and 18 torque, it was doing its peak much earlier @2000 rpms.

This is where it gets kinda weird.

I built your basic two cylinder engine doing the same as above. The hp and torque increases were still minimal but there was more of an increase of each with each increase of bore size. And too, it more or less fallowed a similar path of making the hp and torque at lower rpms.

I jumped up and built a 4 banger. Then jumped over a 6 and went straight to an 8 cylinder and just for the helluvit, built a V12.

During all the “testing”, the more cylinders there were, merely increasing the bore size made more and more increases in both hp and torque. Also, the more the cylinders the hp and torque increased steadily within certain rpms as oppose to the single which had little to no increase but merely moved their place in the powerband.

It makes sense but is kinda weird all the same.

I feel that the folks that are saying the .26 has more torque than the .21 are mistaken. I think that because the .26 has its peak powers at a lower rpm than the .21, the .26 pulls harder “off the line” (so to speak) and fools the eye. It seems the .21 will pull down the same numbers just later in its rpm range.

Disclaimer—The above is not based on fact but merely my findings and my speculation and theories. The debate is now open.