Experimental measurements of the instantaneous exhaust gas temperature, mass flowrate, and hydrocarbon concentration have been made in the exhaust of a single cylinder research engine. The temperature measurements were accomplished using an infrared optical technique and observing the radiation of the exhaust gas at the 4.4 μm band of CO2. Instantaneous exhaust gas mass flowrates were monitored by placing a restriction in the exhaust manifold and measuring the instantaneous pressures across the restriction. Time-resolved hydrocarbon concentrations were measured using a fast-acting sampling valve with an open time of 2 ms. From these measurements, the hydrocarbon mass flowrate is calculated as a function of crank angle. The hydrocarbon mass flowrate calculations show that approximately 40% of the hydrocarbons leave the cylinder early in the exhaust process, due to the exit of the head quench layer, and 50% leave the cylinder near the end of the exhaust process, due to the exit of a hydrocarbon-rich vortex produced by the upward motion of the piston. The remainder of the hydrocarbons leave in the middle of the exhaust stroke. The measurements explain why a reduction of the quench volume, created by the clearance between the piston and the first compression ring, reduces average hydrocarbon concentrations.