I am looking at a 89 xt600 with 14K miles. Will this motor handle a big bore kit
which takes it to 633cc and has a 10 to 1 piston. I would also like to install a
mild performance cam. Have you don this? What doe you think? Would these
modifications add 10 HP?

Thanks, Jay.

are you planning on long distance touring????

If you use the low octane foreign fuel, you will trash the high compression piston.

If you are only using the bike for blatting around GB, undoubtably it will handle the extra HP (but don't believve the claims of the people selling the bits - they are biased). Try ringing a decent dyno house. They will have years of experience.

You will find most people on this forum are interested in long term reliablity, not Horse power. - having said that I use a c90 with a lumpy cam and big carb!!

Spend the money on petrol/ and enjoy yourself.

Enjoy a reliable bike, you'll pay a lot each hp and they won't be that impressive.
XT is a torque mono, play with that and enjoy.
Power is the ability of the engine to remain on high rpm's as a supersport going upwards a hill without losing any rounds once in spin.
Torque is the raw strength of the engine, ever tried to screw a 22 nut with your fingers..take a long grip, that multiplicates the strength from your hands, torque is the same thing and is due to many things; size of the piston, weight of the crankshaft, weight of the rotor, gearbox demultiplication, clutch demultiplication etc.
Power and torque is a balance difficult to obtain by the yamaha enginers.
An XT is made very good orginally, the balance shaft compensates vibrations and has exactly the same weight as the original piston..torque is well distributed from 0- 5500rpm.
Changing the piston might give you naughty vibrations, only that makes the engine much less reliable.
You will change the engine character and in the end probably the bike will be less performant than original, high compression will create more heat to your aircooled cylinder, the vibes will torture your allready worn conrod and bearing + heat and torque on that= hello engine brake.
I won't say for sure, but I wouldn't do those changes, everyone is free of course.

Cheers,

Matt

mines got high comp piston 11to1 621cc,yoshimura cam,keihin race carbs,
gas flowed head,comp exhaust,its a hooligan runs very smooth very fast
eats ccm ktms put a wheelie wand on it to try keep front wheel on thr ground
i like it

I bet in an overlanding trip my c90 would be faster...
as in it would get there with luggage running on crap fuel!