[theunchosen]

Well Tas has a point that its not all about density.

It is important but its not the only thing that matters. If they air is initially at a higher temperature when the fuel ignites the heat soak in the charge will be slightly greater. In the summer or after having run the vehicle for stop and go traffic in the nether regions if you have a knock sensor you are going to defeat yourself. The higher initial temperature can cause knock which means for that cycle your engine is going to avoid that and create alot less power(but the fuel won't be used effectively). On the other really dangerous thing if the mixture ever pre-ignites because it gets a few degrees above where your coolant can handle its game over for your engine.

Also he is right the spark will advance more quickly because the molecules are vibrating faster. There is a point of lost returns here though. As long as all the fuel burns before the exhaust valves pop you're really not losing out on anything. Also if the spark advances too quickly the power comes in pulses, which you won't notice but its going to be harder on your engine than a "smoother" burn over the expansion cycle.<Edit> sorry I forgot to mention that if you look at guns and bullets the optimal load of powder will burn not all at once, but for about half the time the bullet is still in the barrell. If it burns immediately the bullet only accelerates as long as the pressure creates more force on the butt of the bullet than the drag and the friction. If the kernel goes too quickly it washes through the block(heat capacity is exceeded) and the pressure is not greater than the friction and drag of fluids. If it is just right it maintains force greater than the parisitic forces and allows the air to absorb more heat(the air can absorb more BTUs at greater specific volumes).</edit>

As someone else said. . .it depends entirely on what your car was designed to do.