Exhaust hydrocarbon concentrations are known to be very high during deceleration relative to other vehicle operating conditions. Because of this, considerable effort has been spent in developing devices which would reduce exhaust hydrocarbon emission during deceleration. The most promising of these was a manifold-vacuum limiting device; however, its adaptability was hampered by a serious loss of engine braking when the device was operating.An improved device has been developed to reduce the hydrocarbon content of automobile exhaust during deceleration. The new device utilizes the previously-known principle of manifold-vacuum limiting to reduce exhaust hydrocarbon concentration, but in addition, uses retarded engine spark timing to overcome the serious loss of engine braking encountered with previous devices. Retarding spark timing to about 10°ATC during deceleration restores engine braking to the same level as that of a standard car and thereby overcomes the major drawback to the use of deceleration devices of this type.For best over-all performance, the device should be “tailored” to match the operating characteristics of the particular vehicle. Data are shown to illustrate how this is done.Several cars were equipped with the deceleration device and were tested under the conditions of the California Standards for Motor Vehicle Exhaust Emissions. California composite hydrocarbon concentration results indicate significant over-all reductions when the device is used, although these reductions are greatly lowered when the increased exhaust flow caused by use of the device is taken into account. The device is considerably more effective during decelerations beginning at 50 mph than during decelerations beginning at 30 mph.