Didn't we already do this one? It goes along with the 'black first six feet of the exhaust plume' argument. The F1 engines were cooled by running the relatively cool 800 degrees exhaust from the fuel pumps between the inner and outer skins of the engine bells. Gaps in the inner surface were left 'leaky' to allow this gas to mix with the exhaust gasses along the inside surfaces of the bells. This kept the inner surfaces 'cool'. The fuel pumps ran fuel rich, and the exhaust has been described as the 'dirtiest greasiest gunk imaginable' by an engineer who worked on the pumps. Once mixed with the air, this 'gunk' burns to produce the smokey exhaust of the F1 engines. It also produces the 'black first six feet'.