[Elarson]

The power potential of nitromethane as a fuel comes from the fact that it is introduced into the engine as a liquid instead of a vapor. Since the liquid is more dense, more molecules of fuel can go thru a given port size. As a rough comparison, a 500 inch Pro Stock engine might be making 1500 hp and the Top Fuel 500 incher makes 10,000 horsepower; so using liquid fuel (instead of vapor) gives you over 6 times the power potential.

But....liquid fuel won't burn. So nitro has to phase change to vapor before it can burn. If all is well, the spark plug finds enough vapor to start a flame and then the flame front heats the liquid, creates a bit more vapor, burns some more, creates more vapor, etc. This phase change makes the burn process last a long time. So the spark has to fire anywhere from 54 - 70 degrees BTDC to get started. On the other end, combustion is still going on when the exhaust valve is opening, which explains the noise and flames out the pipes. This also means that the exhaust valvetrain has to be much stouter than the intake since it is opening against combustion pressure. The other effect that happens is that combustion is pushing on the piston for a lot longer after TDC so the engine pulls much harder.

Common misconception....the blower (on a nitro engine) isn't there to make power. The fuel quantity injected sets the power level. The blower is just there to push in the corresponding correct amount of air.

Recall that normal combustion in any engine is a smooth burn of fuel across the combustion chamber. Detonation is the sudden combustion of the entire slug of fuel because the temperature/pressure in the chamber gets too high. From our previous calculation, detonation in a liquid-fueled nitro engine has over 6 times the energy potential of an NA gas engine. So the gas engine detonation beats up the pistons and bearings, and nitro detonation grenades the entire engine and tosses the cylinder heads at the spectators.

Nitromethane is shock-sensitive, so a sudden spike in cylinder pressure can cause the entire load to detonate.

Tuning nitro is walking a tightrope. If the mixture is too rich, the extra fuel cools the combustion chamber too much, then there isn't enough heat to vaporize fuel and the spark plug can't fire anything. Then you have a "dropped cylinder". Watch video closely of nitro cars and when you see mist suddenly appearing at the pipes, you know a cylinder has dropped. On the other hand, if you lean the mixture too much, it accelerates combustion too quickly, a violent pressure spike occurs and the next thing to happen is detonation; recognized from the stands as a wallet-draining engine fireball.

Decades of experience have taught nitro tuners that compression has to be low (6:1 in our case....mid-7's for the big show cars that can afford to blow stuff up), in order to spread out the combustion event and prevent the engine-killing pressure spike.

One more thing to help you wrap your head around what's happening. The typical shower head in your home runs about 3 gallons per minute. Our fuel pump is pumping about 25 gpm of 90% nitro at 8000 rpm....so your home shower head shows you about the flow of nitro in each individual cylinder.

Hope this was entertaining,
Eric