the rsx runs 11:1 and the Si runs 9.8:1. the rsx actually has the vtec we're all used to at like 5800 rpm i think (rsx owners correct me if im wrong) but the Si only has a slight change over at like 2800 rpm that is not even noticeable. they designed the Si for fuel economy and not performance. u need the rsx head if u plan on making any kind of big power out of k series all-motor wise. the rsx head was definetly designed much better. plus the vtc is gonna open up a whole new level of performance cuz u can change the timing of the intake cam anywhere in the powerband. the toda spec a kit for the rsx motor makes 225 to the wheels. thats pretty tight all motor and it still has stock driveablility. just wait til someone builds a 12.5:1 compression 2.4 liter k series with a heavily ported rsx type r head and some toda c's. id bet they can get 300 to the wheels easy