I bought the high/high insert to put in my 2015 Sherco SER 250 and it does everything I hoped it would. My last bike was a KTM 300 exc and I missed some of the low down punch that bike had. This insert has largely given me that type of power again. The bike now has more low end punch and more quickly makes the transition from low revs to a more solid mid range. It has really increased and smoothed the power in the low to mid range. It fit the stock head perfectly and it has not caused any detonation or other ill effects. I am at close to sea level so if that was going to happen it would happen here.