All 1968 models equipped with automatic transmission, all 1969 models except the 7.0L police interceptor engine, and all 1970 and later models (regardless of other exhaust emission control equipment) are equipped with the Improved Combustion (IMCO) system. The IMCO system controls emissions arising from the incomplete combustion of the air/fuel mixture in the cylinders. The IMCO system incorporates a number of modifications to the distributor spark control system, the fuel system, and the internal design of the engine.
Internal engine modifications include the following: elimination of surface irregularities and crevices as well as a low surface area-to-volume ratio in the combustion chambers, a high velocity intake manifold combined with short exhaust ports, selective valve timing and a higher temperature capacity and cooling system.
Modifications to the fuel system include the following: re-calibrated carburetors to achieve a leaner air/fuel mixture, more precise calibration of the choke mechanism, the installation of idle mixture limiter caps and a heated air intake system.
Modifications to the distributor spark control system include the following: a modified centrifugal advance curve, the use of dual diaphragm distributors in most applications, a ported vacuum switch, a deceleration valve and a spark delay valve.