Quote from Wikipedia:

"Nitromethane is usually used with rich air/fuel mixtures because it provides power even in the absence of atmospheric oxygen. When rich air/fuel mixtures are used, hydrogen and carbon monoxide are two of the combustion products. These gases often ignite, sometimes spectacularly, as the normally very rich mixtures* of the still burning fuel exits the exhaust ports and out through the exhaust pipes. [* the very rich mixtures are necessary to reduce the temperature of combustion chamber hot parts so to control Pre-ignition and subsequent Detonation]"