mikeburgin said:
the reason people run higher nitro is to increase octane levels, octane not only gives you more bang it also lets your motor run cooler, therefore letting you run your motor leaner to give more power, why would you run more nitro if you have to run richer and loose performance ????????????
once you start getting over 30% then nitro starts working the other way
Not sure I agree. You can't directly compare alcohol based fuels to petroleum based fuels. Glow fuel does not contain any octane or heptane so you can't equate RON ratings.
Nitromethane is an oxidising agent, not an octane booster (anti-knock agent). It performs an equivalent function to laughing gas (NOS) in a petrol engine. i.e. it introduces a higher volume of oxygen in the combustion chamber without resorting to forced induction and this allows you to burn more fuel (a richer mixture) and therefore effectively increases the volumetric efficiency and thus the compression ratio. More fuel = more energy if burnt at roughly the correct air/fuel ratio and more compression = more torque if you can avoid detonation.
With a methanol fueled engine the air/fuel ratio is less important. Basically the engine will burn as much methanol as the oxygen in the combustion chamber will support. The rest disappears out of the exhaust unburnt.
The reason that R/C 2-strokes run "better" on a leaner mixture is the reduction in the windage (drag) caused by the lubricating oil in the fuel mixture in the crankcase. Knife-edging the conrod and using a racing blend with less oil content helps to reduce windage. Ultimately, performance is dictated by how little oil you can get away with before the engine overheats. These engines don't produce much more torque on a leaner mixture, they just rev higher. Power increases with engine speed as long as the torque does not drop off. A higher revving engine means a faster top speed.
Cooling is primarily provided by the evaporation of the atomised methanol during the induction phase and by cool air passing over the crankcase and cylinder head fins. Some heat is also absorbed by the unburnt oil and carried out of the engine.
Increasing the nitromethane content of glow fuel also helps smooth running at idle speeds, where the LSN metering is very approximate due to low air speeds through the carb venturi.
In summary, in two identical engines with identical carburettor needle settings, one running on 30% nitro content and the other running on 20% nitro content, the engine running on the higher nitro fuel will have better idle and torque characteristics (and therefore a higher power output) due to the superior volumetric efficiency and compression ratio. Maximum engine speed may also be slightly higher as the higher torque will help to overcome windage losses.
That's my 2c worth. Healthy discussion and alternative theories are most welcome.
