Fuel Change

| Would it be ok to drop 5% in nitro fuel? Say from 20% to 15% or 15 to 10%?

You haven't given many details, but generally this is not a problem.

Sure, you'll need to re-tune your engine, but you generally have to do that any time you change fuel, or even if the weather is hotter today than it was the last time you flew.

Mostly, nitro just makes our engines easier to tune (more forgiving), and gives them a little extra power. It's not essential, and in Europe they generally use little or no nitro just because it's expensive -- but they do just fine.

As a rule of thumb, 4-stroke engines prefer higher nitro content while

2-stroke engines are a little more ambivilent to nitro content.

Dropping 5% in nitro content won't affect most 2-stroke engines, they'll run similarly with 10% or 15% nitro, for example. With 4-stroke engines, you'll want to provide at least 15% nitro for optimal performance and probably only run 10% nitro if no other fuel is handy.

Higher compression 2-strokes run fine on 5% or even no nitro fuel.

Hopefully these general guidelines will prove useful. The rest of the guys here posting to this newsgroup will now point out the myriad of exceptions to these general statements as well as explain why they disagree with everything I've said.

We generally drop to lower nitro content as the weather warms up. We have difficulty with the lower 10% fuel when the temperatures are 30-40 degrees.

Ed, I differ. Before I began mixing my own fuel, my general rule was 5% in the summer and 10% in the winter. Now, since it is more work, I just use 10% year around. I tried 40% in a ThunderTiger 25 once, and it really did not perform any better. That is misleading...it did not perfom well and the 40% was an attempt to find a way to make it not be a very small boat anchor. The test failed.

The higher nito some use in 4 bangers is to squeeze more power out of them. I have heard that the Saito 180 on helicopter fuel (30/30 nitro/oil or better) is a real hoss...

Ed, I've heard that for a while now (mostly since I started running a couple

4 strokes last couple seasons) but I'm not so sure... Empirical evidence does not seem to indicate any noticeable difference in running 10 or 15% nitro in either my 72 or 82 Saitos or lately in my 91 OS. I haven't done any tach level testing and with all the variables of humidity, air temp, cylinder temps, etc... I'm not sure how accurate I could be anyway. I've been running a 10% Nitro, 20% oil blend in all of them and can't tell any major difference between that and the same fuel with 15% nitro as far as performance, ease of tuning, ease of starting or anything else significant. I wonder if the fuel folks just make a few more dollars off 15% than 10%. ;-)) Or is this one of those things that just isn't as big a factor with the more modern design engines as it used to be? I haven't tried anything lower in Nitro as my preferred 2 Stroke fuel is 10% as well so I can use 10/20 in all my engines so I don't have to keep two fuels on hand if I don't want to! Also both manufacturers say that 10% Nitro is within the recommended range of fuels. I am considering going to a pure synthetic oil content just to see how that works and to avoid the gunk buildup inside my expensive 4 strokes that seems to be the fate of any such engine that is running a fuel with Castor content... or is that just another myth! We'll see I guess.

Jack

Jack, I use only synthetic oils for exactly the reason you are considering. For 2 strokes, castor is fine. However with 4 strokes, it builds up on the exhaust valve and causes problems. Part of the reason is that 4 strokes run hotter than 2 strokes.

YMMV

The higher temperatures are exactly why castor is recommended for 4 strokes. Castor retains it's viscosity long after synthetics have given up the ghost.

Someone posted a link to a very good article here a couple of months back. I don't remember the subject line from the thread but I believe it had castor in it. A search might turn it up or someone may repost it.

Dave

"Six_O'Clock_High" > couple 4 strokes last couple seasons) but I'm not so sure... Empirical

While Castor works in high temp environments, it causes excessive carbon build up... BTDT and now straight synthetic with no problems, thank you.

(as I said before) YMMV