well hold on there isn't a big difference. if you pay for it, you shouldn't. it is a very small and subtle change. if you were looking for some measurable increase in performance you won't find it. a lot of people feel a big difference and then, they also feel a major difference by installing a k&n air filter. butt dynos are VERY inaccurate. a timing advance is not worth paying for but if you have the chance to get it done free it isn't something i would pass up....