Engine power determined by more than just cc's?

[whitenack]

What factors into engine power other than cc's? How can I compare engine power of different manufacturers with the same cc's?

The reason I ask, I borrowed my buddy's Husq 25.4cc string trimmer this weekend. When cutting normal thick grass, it would bog down. This trimmer is not even 1 month old. When using borrowing my dad's Stihl 25.4cc, there is no bog at all.

For instance..

How do you compare a 28cc 4 stroke versus a 21cc 2-stroke? I know 2-stoke is supposed to be more powerful compared to a 4-stroke of the same size, but at what point do bigger 4-strokes outperform?
Amongst 2-strokes, how much difference is there really between 21cc and 25.4cc, or 25.4cc and 28cc in the same product line?
At what point do you notice a difference in engine power? 5cc's? 10cc's?

[whitenack]

Ok, let me ask it a different way...

I see that Echo is touting a new 'T' version of some of their trimmers that boast "50% more torque" than non-'T' versions. How would you compare this to other brands?

I see in the owners manual for the 25.4cc SRM-265T a Wide Open Throttle Speed of 8,500-12,000 rpm. The corresponding SRM-265 has a WOTS of 9k-10k rpm. So looks like the torque helps out the max rpms, but that is only a 20% increase. So, torque must be measured somewhere else. Interestingly, the 28.1cc SRM-280T has the same max WOTS rpm, so a larger engine size doesn't mean a faster WOTS rpm.

My buddy's Husq that seemed to bog down has a "maximum power speed" at 9k and a "maximum recommended engine speed" of 11,700.

Looking in the OM at a Stihl FS 90, the WOTS is a much slower 7,500 rpm.

I realize that speed doesn't always equal power. You could have a very fast spinning blade that gets bogged down when it has a lot of work to do. I guess what we are wanting is speed plus power.

Just trying to make an educated purchase for a unit that I will have to live with for a long time.