Abstract: A system for converting a spark ignition Otto cycle engine into a compression ignition or Diesel cycle engine comprises:(a) compression means connected to the engine piston end portion closest the combustion zone to effect an increase in heating and pressurization of intake air in that zone,(b) pre-combination chamber means independent of, and attached to, the engine head to communicate with said zone via the spark plug opening in the head, said chamber means having an inlet to pass pressurized fuel into the interior of said chamber means, for mixing with air from said combustion zone to pre-combust and flow to the combustion zone wherein combustion is completed, and(c) cooling means including a coolant passage adjacent said pre-combustion chamber to circulate fluid coolant in heat receiving relation with said chamber.