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A couple of weekends ago I was in the shed listening-in to a few guys discussing the percentage of nitromethane they use in their fuels. The conversation went like this:

“I use 5%”.

“Is that all - I use 15%”.

“That’s too much - I reckon 10% is about right”

It seems to me there’s a lot of misinformation out there about nitro. Let me see if I can clear some of it up.

Yes, NITRO = POWER - but it doesn’t add power because it is a “hot” chemical. In fact the methanol in fuel (methyl alcohol) is by far the most flammable ingredient - nearly twice as flammable as nitromethane. If you put a flaming match into nitro the match would go out.

Well, how does nitro add power? Every internal combustion engine burns a mixture of air and fuel of some sort - in our case a liquid glow fuel. The purpose of the carburettor is to meter these two ingredients in just the right proportions, and every engine requires a specific proportion of liquid fuel and air to perform at its optimum. If we try to push more liquid in without enough air, it won’t run.

However there is a way of running more fuel through our engines without increasing the air supply - by adding nitromethane to the methanol/oil mix. An engine can burn almost 3 times as much nitro to a given volume of air than it can methanol. Voila! More power! That’s all there is to it.

However there are a few practical aspects to consider. Not the least of these is fuel cost .

Nitro costs around $20.00 a litre and ready-mixed 10% nitro fuel costs about 30% more than “straight” fuel.

I know some flyers who don’t use any nitro at all. Pylon racing engines designed for international FAI competition run on no nitro at all, due to their rules. Yet they go much harder then engines running on high nitro fuels. This is because these engines have compression ratios, intake and exhaust timings etc designed especially for FAI fuel (4:1 methanol and oil). Even then they won’t idle at all and can be a serious bitch to tune and run - just ask Ranjit!

A popular misconception is that nitro gives you an immediate power jump. In the 5% - 25% nitro range you will probably see an rpm increase of about 100 rpm static (sitting on the ground or in a test stand) for each 5% nitro increase. However in the air the engine will unload and achieve a greater increase, and it will idle better too.

At the other end of the scale it’s possible to use too much. When I was running powerful racing engines in Old Timer competition, where optimum power is important to get maximum climb from a short motor run, I found virtually no incremental improvement in performance with nitro contents above 30%.

Most of our popular 2-stroke sport engines are designed to run on 5% to 10% nitro, 4-strokes 10% to 15%. Most European engines will run successfully on less, because they are built to do so. In the UK, nitro costs between $200 and $300 a gallon! Reason enough?

Conversely engines made in Asia, as most of those we run are, are designed to run on nitro-containing fuels. The vast majority of model engines manufactured in Asia end up in USA, and nitro is very cheap there. This is because the only manufacturer of nitromethane in the Western Hemisphere happens to be in the USA. ‘Nuff said.

Going back to the beginning, how much nitro do you really need? From a practical standpoint, virtually all our everyday sport flying can be done on fuels containing from 5% to 20%. If you’re flying something like a trainer or a Cub with a 2-stroke engine, there’s no reason why 5% won’t work perfectly well.

Need a little extra power? Move up to 10% or 15%. Four strokes need a little more - 10% to 20%. OS 4-strokes are adjusted at the factory for fuel containing 10% nitro.

I wouldn't recommend going higher than those percentages. It won’t do you much good and it’s a waste of money.

I have found 5% in 2-stroke fuels and 10% in 4-stroke fuels is about right for me, giving easy starting, good top-end performance, reliable idle and instant pick-up. If I could afford it I’d up these percentages by 5%. However I use about 50 litres of fuel a year, so adding more nitro would add substantially to my fuel costs.