Biff,
Hello and welcome!
I have an SM with the 865cc engine. I also had the previous SM model with the 790cc engine for 4 months last year (loaner from the Triumph dealers until my bike arrived).
Here is an excerpt from a post made neutral source i.e. by Graeme Lawson (RCV on here) on the "thunderbike pipes & airbox tweaks/removal/jett" thread in the "Exhausts" section.
Graeme's company make the Thunderbike exhausts for our bikes (but unfortunately they don't give a big performance boost on the 865cc motors as they do on the 790cc one - but the sound is great).
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The 865 has a flatter torque curve than the 790 and this will show in a stronger "in gear roll on" response in the higher gears, whereas the 790 likes a downshift or 2 and some revs to really rock, the 865 will grunt.
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I am able to confirm this.
