I know this thread is older, but was wondering if you also have torque curves on top of hp? I think the useable torque makes a big difference. For instance my 99 LS1 camaro was reprogrammed and put 3.73 rear in, hp was increased 10% but torque was more like 20% and the low end at 3000rpm was crazy, couldn't get a big enough tire to keep from spinning.
Also, i suspect you might have a large range in 066 or 660 you test, even if all stock and they probably publish on the high side. The original 066 was rated at 7.4 hp and with dual port (read as magnum) was listed as 7.8hp, at least in my vintage Stihl catalog from 1994 i believe.
What i do know is my newish 066 round top, pretty much same as 660, with the older style thumb size exit muffler, no screen, but no dp front, with HD2 air filter is only just a tad slower than my 394xp, both with new chains on a 36" bar, and let me tell you 394 is noticeably heavier, though av is better. Biggest difference i found was in frustrating oiler on the stihl, have to switch that one out, then would be close to even.
Workin my tail off lately. Put over 300 miles on my service truck today. Thankfully the repair in the field only took 2.5 hrs.
