I would assume that with the rx series the new compressors pushing more air into the improved combustion chambers that is raising the compression ratio and also the amount of fuel being burned. The clearance on the NGV with the kero start see a slower more uniform temp rise on start up and all the parts get a chance to expand at roughly the same rate. With the older propane starts the driven wheel saw high temps before the EGV and that would cause a rub. So with tighter clearance comes more power and more temps.