ORIGINAL: DarZeelon
Dave,
The torque and power curves of an engine are by far more dependent on the timing numbers, than on the nitro content of the fuel. In timing numbers I include intake bypass(es), exhaust port(s), shaft valve port and compression ratio.
In an engine with given timing numbers... Well, let us look at it this way:
With more nitro, the mass, the density and the viscosity of the intake charge are larger, since more fuel is needed for a given amount of air.
Also, these qualities are also true of the exhaust gasses.
These would both tend to make the engine breath more efficiently at slightly lower RPM levels, thus lowering the RPM, at which peak HP and peak torque appear.
In the case of four-stroke engines, the lower compression ratio needed for higher nitro, decreases the induction efficiency, since vacuum builds up more slowly, as the piston descends. This would also lower the peak HP and Peak torque RPM levels.
This has little effect, if any in a two-stroke engine, since there is no actual induction (sub-piston pressure pushes the fresh mixture into the cylinder).
All these effects are minor at most...
Increasing the nitro content has the effect of advancing the engine's timing; one of the means by which it increases power. Nitromethane has a lower flashpoint and faster burn rate than methanol. Increasing its proportion in the fuel mix allows the air/fuel charge to ignite at a lower temp and burn faster. Consider the compression stroke and ignition sequence:
piston is on its way up, fuel charge is being compressed, compression causes heating, there is residual heat in the glow plug from previous ignition, plus catalytic reaction of platinum element and methanol, until heat becomes great enough to ignite air/fuel charge. the sooner this happens before top dead center, the more ignition advance there is, and the faster the charge burns, the higher the combustion pressure, the more power is developed with each cycle. In addition to lower flashpoint, a hotter plug will contribute to ignition advance due to more retained heat.. This is true in 2- and 4- stroke engines, and the trend of higher nitro and/or hotter plug can continue---(along with increasing rpm as well) to the point where there is enough residual heat and a low enough flashpoint that the fuel ignites as soon as it enters the combustion chamber, resulting in preignition, or a high enough nitro content can burn fast enough in a fully compressed state that it virtually explodes, causing detonation. Both conditions are detrimental to power output and engine longevity, detonation being worse because it occurs when valves and ports are closed, and there is nowhere for the explosion to vent, so it usually holes a piston, bends a rod, or can even crack the cylinder wall. This is the reason high compression engines can't tolerate high nitro as well as lower compression ones. Preignition can also damage engines because the charge is burning too early and causing undue stress on the still rising piston/rod. If it is a 2-stroke and any of the port area is still uncovered, or a 4-stroke and intake valve is still even partly open, cylinder pressure won't climb enough to cause damage, but engine will blow-back, or backfire. There is, somewhere in this maelstrom, a happy medium for every engine, where nitro content and ignition heat and rpm combine to create the optimum power output for the engine. All we have to do is experiment our entire life to find it, just before we crash and tear it all to hell anyway.