My assumption, Wossner pistons result in the same compression ratio as stock Kawasaki KLR650 pistons, is in error!! (NOT the first time my assumptions have been found so.)
Here, direct from the US distributor, is this information:
So, sports fans, if you're serious about hopping up your KLR650, here's one proven path to additional power and increased fuel mileage: Raise the compression ratio.
No, I haven't done it . . . YET! (I'm tempted.)
US stock of Wossner pistons in various sizes (stock to oversize) is varied but small; additional sizes are available from Germany with an attendant delay.
Now, would the higher compression ratio suggest higher octane fuel? Reckon so!
Would a higher compression ration increase the heat load of the engine? You bet, but . . . this conservative increase ought to be within the stock cooling system's capability, I'd imagine. Verwah reports, on this forum, operation within safe limits with a Wossner 11:1 piston on his KLR650, with stock cooling system.
Will durability and reliability suffer with a higher compression ratio? Certainly, components will be stressed more fully, but . . . so would they with any other hop-up techniques and procedures ('cept maybe the PCV valve mod! (I'm kidding.)).
No, I haven't done it (as stated); no, I have no dynamometer "before" and "after" runs, but . . . conventional wisdom states, increasing the compression ratio on an engine increases its power output. Know of no reason why an exception should occur here.
Stock compression ratio: 9.5:1. Wossner piston compression ratio: 11:1. "Delta," or difference: > 15 %. How much power increase probable from raising the compression ratio over 15 %? Don't know, but . . . the popular "685" big-bore kit raises displacement by about 5 %; GREAT increases in power are reported by some who have performed this mod.