I have a Duncan big bore with a short rod and love it. It's a 340cc PC2000 with a power valve. I just recently figured out how to get the most out of it. It has a ton of torque and revs to the moon. Theoretically the short rod should rev out a bit quicker than the long rod, but the long rod should have a bit more torque. I like a rev-happy engine so I went with the short rod.