The shape of the Wankel combustion chamber is more resistant to preignition operating on lower-octane rating gasoline than a comparable piston engine. The combustion chamber shape may also lead to incomplete combustion of the air-fuel charge using gasoline fuel. This would result in a larger amount of unburned hydrocarbons released into the exhaust. The exhaust is, however, relatively low in NOx emissions, because combustion temperatures are lower than in other engines, and also because of exhaust gas recirculation (EGR) in early engines. Sir Harry Ricardo showed in the 1920s that for every 1% increase in the proportion of exhaust gas in the admission mix, there is a 7°C reduction in flame temperature. This allowed Mazda to meet the United States Clean Air Act of 1970 in 1973, with a simple and inexpensive "thermal reactor", which was an enlarged chamber in the exhaust manifold. By decreasing the air-fuel ratio, unburned hydrocarbons (HC) in the exhaust would support combustion in the thermal reactor. Piston-engine cars required expensive catalytic converters to deal with both unburned hydrocarbons and NOx emissions.