IIRC, the temps and pressure at full scream will cause nitromethane to burn without free oxygen if it's super rich and has consumed all the air. That's where the hydrogen comes from that burns out of the zoomies, burning nitro with no air makes free hydrogen gas, which would normally become water if there were more air. So the hydrogen burns AFTER it leaves the pipe. Flame temp goes down, but cyl pressure goes up. All molecules take up about the same room when they are a gas (liquids don't burn), so when H2O is made, the temp goes up, but volume goes down.
Or I might have misunderstood what I was told. I know nothing about it other than it's expensive, won't mix with diesel, and will dissolve super-glue.