Im a new pilot and have two trainers that I have been flying and landing successfully. I have always used 5% nitro fuel in my O.S. 40 engines. Will the engine run differently on 25% or 35% nitro? Will it fly faster, or better? Any insight would be helpful. Thanx in advance.

Kat ... the higher the nitro content the more powerful the engine will be ... the more powerful the engine will be the shorter it will last. Higher nitro also assists in the transition between low trottle to high ... its a lot smoother. Idling is also better with higher nitro content. 5% or 15% is just fine for everyday use (I prefer 15%) ... don't bother with 25% I tried it and love it but there is no real benefit for average flying, just higher operating cost.

As a beginner try this link ... its great ... http://www.bolly.com.au/book

As far as flying "better" on higher nitro the answer is no. It may fly faster but seeing how trainers are not the most efficient planes u may not notice it. You will probably notice a better climb rate though. While you are learning stick with the 5%. I use 5% and have found no need to change. Its better to spend your money on more fuel rather than more nitro.

Higher nitor content will also make your engine run hotter, so will require richening the needles. You will also burn more fuel.

It's much harder on an engine, so unless you're racing that trainer, I'd go with no more than 10%.
Dennis-

I found that Cool Power 15% nitro worked well in my OS 46AX. I now have a couple of Saito 4-strokes as well as the OS's and found that they love Byrons 15% Nitro 20% Oil so I use that in my OS's as well and it is even better.

This in an everlasting discussion subject in R/C circles. "Nitro, what it does and does not do."

Some of the theories are tales of legend which keep alive some amazing myths, many of which I have read on this website.

Basically, Nitro is a power enhancer. By its chemical composition, it increases the amount of oxygen available in the fuel for combustion. Use of small amounts will help the engine run smoother, especially at idle.

Nitro WILL NOT increase RPM. But in the combustion process, it helps produce more energy, so the engine will produce more TORQUE, allowing the use of a bigger propellor, or better operation under high load.

It's use is common in competiton R/C aerobatics (Pattern),where competitors use high nitro content fuel, so they get better performance increase in vertical climb.
5% more Nitro content, can be the difference between running a 12 x 11 prop or a 12 x 12 prop at 10,000 rpm.

The downside of all this is that the more nitro in the fuel, the more heat is produced by the engine in the combustion process, so cooling also becomes and issue.

It also contributes to rusting of the engine internals, caused by the acidic by-products of the combustion process. This is especially so in 4 strokes, where a lot of "gunk" residue is left in the crankcase (for the engine bearings to bathe in) when the engine is not running.

Just how much more effective higher nitro will be for your engine will also be determined by its compression ratio. High compression 2 stroke engines from Europe, where nitro is expensive (MVVS etc ) don't need much at all. They are designed to run on FAI fuel, but will tolerate up to 5% o.k. Anything above that and your wasting $$$$.

2 stroke Engines produced in Asia and North America generally have lower compression ratio's and are usually designed to run between 5 and 15 % nitro content. Some engines, such as Dubb Jetts, may even need more.

There are some exceptions to this however. For example, Super Tigre, which although now made in Asia, is a European design and needs little if any nitro (0-5 %) and MDS which although originating in Russia, follows the Asian design practice (needs 10 % and higher.)

Larger, lower revving 4 strokes (with the exception of Laser's) are designed for between 10 - 20 %.

The best recommendation is: read your engine manufacturer's recommendations and go with what they say. After all, no one will know more about an OS engine, than OS!! Even on this website.

The engine manufactuers don't know squat... we, the modelers know best, don't we?
I use YS 20/20 in most of my engines 2 and 4 stroke however, the big 2 stroke engines like the Moki 180 and 210, MDS 148, OS 160 and others can use only 5% nitro fuels, 10% in cold weather only.
I was surprised to find most guy's here in Gardnerville Nevada, 4730 feet elevation using 30% heli fuel in most engines, even in LA 40's. I notice a huge difference between 5 and 20 % nitro in my OS 46 engines and others.

In simple terms. Higher nitro fuel is MUCH more expensive than the 5-15% fuel. You will burn more of it. Your engine will not last as long, and will actually be more difficult to adjust. The typical sport engine works great with low nitro fuel. So, until you are flying something where the extra power matters, it's not worth it.

I agree there is no need for more than 10% for most people. If you are above say 5000 feet then I can see 15%. For trainers you don't need that much nitro. You are just wasting your money.

Mark Shuman

For trainers & sport fliers, high nitro content is essentially an exercise in wasting money (in every way that you can imagine). Unlike racing engines, sport engines gain little, if anything, from nitro content above 15% & show remarkably little difference in performance between 5% & 15%.

I agree for the most part with the others. I started with 10% but change to 15% nitro; I think my idle is more reliable (I use 4-strokes exclusively) . If 5% has worked, why change? But >15% is throwing your money away, especially on a trainer.

Great reply Woody, I'd only add that nitro percentage also serves to advance the ignition timing in an engine design that has all the mechanical parts fixed in relation to each other. It does this by decreasing the flashpoint of the fuel (higher nitro content) causing it to ignite slightly earlier in the combustion cycle. This is partly the reason for greater torque and power. There is a great article in one of the recent magazines, either Model Airplane News or RC Modeler, I forget which, that talks mainly about glow plugs, but it mentions nitro content as well

Thanks khodges.
I know that guys who fly at altitiude (like in Colorado) will benefit from higher nitro as the thinner air does make model engines as well as full size ones, lose some power.
As for those who claim that engines like OS LA. 46's and also Super Tigre's need 20 % plus nitro fuel at or near sea level?
Well yes, as I said, there are many myths and legends generated on this website about nitro, which keep me entertained for hours.