ORIGINAL: medicap
Totally understand, that is part of my confusion. He said that the 91 had 2.8 HP and the 120 had 3.1 HP so hardly any significant difference. He then went on to say that the four stroke was indeed a different animal. Would provide more power for verticle and slower top end speed. He ended by saying that the penalty in weight and $100 more cost would no way make up for the marginal performance increase............sorry, just repeating what the OS expert at GP told me. I believe you people however, the knowlege level on these boards never ceases to amaze me.,
Steve[img][/img]
Definitely no need for apologies! I have learned a ton from these forums, too - amazing resource.
I think the issue is with people on their end looking at purely specs. They talked about the power rating, which is a spec they get with a certain prop at a certain rpm, and is practically meaningless when comparing engines.
I did a statistical analysis of OS (2 and 4 stroke) and YS engines in school a year or two ago. The most useful thing that came from this was the fact that with the regular air breathing engines (non-supercharged, non-YS), an increase in bore directly translates to more torque and larger prop swinging capabilities, an increase in stroke gave higher power ratings at RPM (this was the opposite for YS engines, which showed torque related more closely to stroke).
Anyway, what this means and what the GP tech failed to see, is that the 120 will give you an increase in power all throughout the power band/throttle range, and definitely give you better throttle response at lower throttle settings [which is what we LOVE in pattern flying].
For example (numbers from my Sebart Wind), I will take a 18x12 at 6500rpm over a 16x10 at 9000rpm any day since we're never really running at max RPM for any lenght of time, and the 18x12 will give more 'pull', cruising speed at lower throttle, and better throttle response.