© Chevelle SS Guide

Chevelle SS Guide

Controlled Combustion System

Exhaust emission control was here to stay.

All 1968 and 1969 L34 350-hp and L78 375-hp engines were equipped with GM’s Air Injector Reaction (A.I.R.) system; essentially an air pump that forced fresh air into the exhaust to allow for a fuller combustion of exhaust gasses. The 325 horsepower engine with manual transmission was also fitted with A.I.R.; only the 325 horsepower engine with one of the two available automatic transmissions was exempt from A.I.R. Both 325 horsepower engines with automatic transmissions were outfitted with Controlled Combustion System, or C.C.S.

The Controlled Combustion System (C.C.S.) introduced for both 325 horsepower 396 engines changed in the areas of carburetion, distributor, and cooling. The C.C.S. system increases combustion efficiency through different carburetor and distributor calibrations and higher engine operating temperatures. Although he C.C.S. system imposes no additional components other than a special air cleaner assembly that is used in conjunction with it, C.C.S. system carburetors are calibrated leaner and timing is retarded during low speed or deceleration operations. The system was individually developed on each engine-transmission-rear axle ratio combination. Adjustments are not the same for each engine-transmission-rear axle ratio combination.

Carburetors using C.C.S. have an “idle fuel limiting” feature. This is nothing more than a fixed orifice at the base of the idle mixture screw insuring that even if the idle mixture screw is turned out too far, the fuel enrichment will not greatly affect exhaust emission reduction. Both 325 horsepower 396 engines used ported spark advance; the vacuum for spark advance is taken off a point above the carburetor’s throttle plate. At idle this port has no vacuum available, hence retarding the ignition timing.

Air cleaner assembly on a C.C.S. equipped engine has a thermostatically controlled air cleaner system that is designed to keep air entering the air cleaner housing (and hence the carburetor) at approximately 100° F. when underhood temperatures are less than 100° F. The system is composed of a special air cleaner and heat stove that simply traps heat from the exhaust manifold and routes the warm air to the air cleaner snorkel tube to a valve that routes the warm air to the air cleaner housing. Once the ambient underhood air temperature is sufficient, the valve to the heat stove closes allowing normal air flow from outside the snorkel to flow into the air cleaner housing.