Started with a 2004 KTM RFS 525. Wanted more power so went with Thumper Racing 540. Worked great for 2 years but over heated in Spring easily due to the steel sleeve. Definitely more grunt than stock. Jumped up to the 570 this year and aluminium sleeve. 14:1, stage 4 cam. Stays cooler, plus I added a tunnel cooler and now I have to block a radiator to keep temps up. Stays perfect temps in spring with the cooler. At a little over 3k for each build it gets pricey but the grunt and mid range is excellent. You can rev them out when you need to but you can feel it making the most power in the mid range so you grab another gear instead. The 570 out pulls anything else I have tried. On really steep nasty hills I used to have to rev it out in 2nd and then as the bike slows and rpms drop, shift down into 1st and then was forced into a side hill out. Now no matter how nasty the hill or heavy the snow the 570 pulls 2nd gear through anything and I never have to drop to 1st. More track speed equal higher climbing ability and it shows on the hill. I am actually pulling some hills that I thought only sleds could make. (I ride sleds too) 570 makes it but at a slower pace than a sled can. My old builds would not have pulled it. I love tree riding the most but it's nice to know that when you have to pull a steep climb it can. I feel that though the carb was bored bigger it still has potential for more power. Looking at throwing in a Lectron Carb instead. You can find these bikes for around 2k and then big bore it for 3k. FMF muffler to open up the exhaust a bit too. You have to ask yourself if you are the type of person who doesn't mind tinkering and wrenching a bit for more power or do you want slightly less power and no head aches?
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