All sport engines come with VERY conservative timing and head clearance numbers.They also don't breath as well as they should. Most of the time you can drill the intake at least 1/64 to 1/32 more without having to do internal mods to get the extra R's.The extra RPM's WILL show up on a ground tach test so don't do it if you are getting close to max already. If you can't get to 17500 at all, I would do what ever mods you have to do to get it there or your SSC plane wont fly well.