I have a varity of glow engines both 4 strokes and 2 strokes, ranging in size from 0.15 to 1.20. I have Saitos, SuperTigre's and OS's. They all idle better and have better throttle up at lower nitro contents. Clearly peak power goes down with the nitro content, but the idle and throttle response is better, partucularly with my .40 size motors which will happily idle on 5% without loading up while the idle goes rough after a couple minutes at 20% nitro. I would have thought that higher nitro would have a better idle, not the other way around.

At the other end of the stick, my .80 and .91 Saitos are happiest at full throttle at about 10% nitro. Depending on temp and air pressure, both my Saito's have been known to backfire in flight on 15% or higher, and yes, having the valves adjusted right makes a big differnce, but does not cure the problem entirely. All my two strokes range from running better and better at full throttle on higher nitro (especially the small ones) to at least not displaying any bad habits (the big ones).

Back to this idling issue, properly tuned, the correct needle setting is always leaner with less nitro and ritcher with more, (and yes I' setting the low side as well so that's not the cause), but since the revs are higher with more nitro I would tend to think the engines are running at about the same stocastic mixture if properly tuned, regardless of the nitro content

Do I just have a couple of engines with unusual characterisitcs? Are engines just running cooler at lower nitro levels and therfore idling better? Is the brief "lean surge" as one opens the throttle less prounced with less "powerful" fuels? Is it a combo of these factors?

Dont Forget you change the timing in an engine when you change nitro..

Scott

I think skerlock means to use colder plug for more nitro and hotter plug for less.

Thanks to both of you!

Makes sense. Since you run leaner and hotter with more nitro you can/should run a cooler plug. It never occured to me before, but more nitro and /or a hotter plug should ignite the mix earlier in the stroke - advancing the timing! Too much advance and you get a backfire. So if I wanted to run higher nitro in my 4 strokes without the backfire problem, then I should go to a cooler plug - Is this correct? I wonder if a cooler plug would help the idle in my 2 strokes as well.

Dwight

dwi....there are 3 main things that determine when the fuel will ignite (the ignition point, sort of like when the plug fires in a car engine). Compression ratio, plug heat and type of fuel. You can't do anything much about compression but each blend of fuel has a point where when it's compressed enough it's ready to burn. The more nitro you use then the less compression it needs which means that you have to find a way to delay the ignition (the reverse is true as well). This is done by having a plug that is what's known as "cooler", in other words it has more difficulty getting the mixture to ignite. Some do this by putting the coil in a smaller hole so the mixture has a harder time getting to the red hot coil. Others do it by using a different thickness of wire (or a combination of the two). So playing with different heat range plugs, with the same fuel, is how you adjust the ignition timing to optimum. This optimum is when you have the highest revs at full throttle. However you MUST disconnect the power to the plug when checking max revs or you'll then be forcing the plug to run hotter than it would normally (plus you've got a good chance of burning it out).

As for the stoichiometric ratio using nitro, this goes all over the place depending on how much nitro you use. Methanol is around 6.5 but nitro is about 2.5 so you can see that adding more nitro brings you closer and closer to that 2.5 figure which simply means that the more nitro you use then the further the needles (high and low speed) have to be opened to keep a correct fuel/air mixture. Just remember that any time you change the type of fuel, even oil content, you must readjust your needles to suit.

Thanks downunder,

You've given me the missing piece of the puzzle. I was amazed at how dramatic the needle settings changed as I changed fuels, but with the difference in stoichiometric ratio between Methanol and Nitro approaching a factor of 3, I understand where the difference comes from. I've been running the same plugs as I've changed fuels and it's clear to me that I've been having problems with higher nitro across all my engines because I've been consistently running plugs too hot for the fuel.

Thanks for the info,

Dwight

Interesting thread and I always enjoy your posts 'Downunder'. I even reckon the emu looks like a friend of mine.

Not sure I completely agree that you cant do much about compression.

In the early 90's pattern scene, we used the Hanno engine. The greater the nitro content the less compression we ran. OS even provided a different assortment of rings to place between the head and liner. This allowed the pipe to be less critical, but still gain the benefits of the supposed more calorific value of the nitro.

Many a pattern flyer was running up to 35% nitro using two rings.

I'll be interested to read your thoughts.

Cheers

Phil

Sure, you can mess around with compression ratios but for the average modeller this is just another variable they can do without. With something like the Hanno you're getting into a specialised competition area where the skill level is quite high and most of the basics of engines are well understood. The main point with nitro is that the more that's used in a fuel then the less compression it can handle before it detonates. This doesn't actually mean that if an engine's got low compression that it NEEDS high nitro, just that it's capable of handling more without damaging it.

There's really only one way to find out how much nitro any engine can handle and that's by using a good tacho. Start with a low dose and check the peak revs. Then run another tank with a higher % of nitro and check peak revs again (after it's been retuned!). Keep doing this until the revs stop increasing and you've reached the maximum amount of nitro for that engine BUT only with that particular compression, plug and prop. If you change any of these then you have to redo the tests. Unless you're in a competition where you need the absolute maximum power then it's not necessary to do any of this, just use a fuel that contains anything from zero nitro up to whatever the maximum recommended by the manufacturer.

More nitro means richer mixture. Fuel dragsters use lots of nitro and fuel line that looks like garden hose!

You haven't been involved in the sport in a while have you? Now they have fire hoses, and ignition systems that look like the innards of a nuclear bomb to ignite that extremely rich mixture. They now have enough spark that the limiting factor is hydralic lock, from pumping in so much fuel that it won't fit into the combustion chamber.