Efco cyl pics.

[Stihl Crazy]

156 vs 165.

The 156 is rated at 4.1 hp, 46mm bore, 34mm stroke, 56.5cc
The 165 is rated at 4.3 hp, 48mm bore, 35mm stroke, 63.2cc
The 156 is the better performer. All ports are smaller on the 165.

 

[Timberwolf]

Something does not add up, 11% more displacement for 5% more power. Would be interesting to know some more details, if the 165 had a .035 squish and 160 psi compression and the 156 0.025 squish and 175 psi I could see that accounting for the difference.

When porting them it's not always the fastest stock saw that makes the fastest ported saw.

 

[Stihl Crazy]

I have no doubt that 165 can be made to out run the 156. It has alot of room for improvement. Stock the my 165 is a dog. My 625 Jonsered blows it away.
Don't have a 156 here at the moment, but have a JD cs62 to compare with.
61.5cc, 48mm bore, 34 stroke, 4.7 hp.