As I understand it from the few flattrack folks that run these motors, somewhere around 80-85hp they tend to break cranks pretty regularly and they last less than 200 miles of racing.
"I've been roadracing EX500s since 1999. Won a few natl twins titles, with a fast rider.. Johnny Staska. You guys can google up that stuff.
My 540cc EX engine makes about 70 dynojet HP. My 600cc EX motor makes 80. The 600 will break a crank in two, usually some time after 250-300 race miles. I've got a (stiffer) crank stroker 650cc EX500 in the works, which should make closer to 90HP... With a 32mm (in) valve head. Who ever said that these bikes don't respond to porting, should be discounted immediately. I've spent hundreds of hours porting and flow testing EXs. Stock heads flow just over 100CFM @ 10in H2O. A good 32mm head will flow 140. My stroker 500 mis on the back burner, since we built a Ninja 650 Racebike.
The Ninja 650 made 98HP with 12.6 to 1 compression. I just revised to a single ring piston and 13.8:1 compression. Haven't been back to the dyno yet.
BTW, I've got 3 complete racebikes for sale, including the championshp winner... with a fresh 600cc engine. Also have a couple of nice race engines for sale."
"We do not use high compression pistons. JE pistons weigh more than stock. Heavier parts move slower. We run stock pistons, with a milled head and cylinder. Depending on the track, we use different combos of Versys and 650R cams. (A cheap route, is to install a Versys intake cam in your 650R, and put the 650R intake cam in the exhaust.) We have billet cams ready to grind, but until we start getting some requests, they will stay in the unground form. Proper porting & compression nets the biggest gains.
Stock rods stretch a lot faster. Not a big problem, until you start getting over 75 hp. My engine builder is currently building some flat track engines. One is a 700cc, the other 750 using stock bore/rod length, with stroked crank and custom built pistons, with pin location moved. Those should be in the 90+ hp range."