The title of this thread is about compression and how that relates or not to exhaust colour.

We have been hearing the old saw for a great many years about how 1/2As run too cool by some very knowledgeable and competent people in the engine business.

Bob Davis is the first I've ever heard of that set that one straight, that in fact, 1/2As tend to run too hot and he did a good job of explaining why. It has to do with mass. This illustrates a high level of knowledge and understanding about physics, engines and fuels. I'm willing to bet that Bob's knowledge is a result of a lot of hard work, research and not the least, an intellect capable of getting to the heart of a problem.

This thread started out well and then fell apart. I was hoping that the feedback from the gang might provide some insight into what NOT to do when dealing with such a diverse bunch as we all are. You have to be unique in mindset and approach to appreciate what diesel combustion can do for you in your love of the hobby. Dare I say that Bob epitomizes this to a degree that even we, in the SAME camp have yet to measure up to and are finding hard to understand. Yet it must be said that if your love of a thing is also your business then business concerns inevitably will taint your reaction to opinions and approaches to issues.

I honestly don't think that any one of us were doing any bashing. I do believe that we were just trying to point out where the thing went wrong. No one, absolutely no one involved in this has any mean spirited motives. We're all seeing the issues from our own unique perspectives. We all have our own personal biases, experiences and points of view along with varying levels of knowledge and understanding.

I've noted differing exhaust oil colours and never thought much of it because I was always dabbling with the chemistry. I assumed that the fuel mix had a lot to do with it and as long as the engine ran, I was happy. Except for rod wear, I've not seen any engine failures related to the colour of the exhaust oil residue on the plane.

But here's something to ponder about colour. After I settled into using the 40/40/20 mix for the Norvel .074 I had occasion to run out and just didn't have any ether left to make some more. Sunday and all and industrial outlets are closed on that day. So I grabbed a can of old fuel just to get in some air time and not let a good evening for flying go to waste.

I wasn't expecting much because this was a quart can in which I had soldered a brass tube in the cap that ran all the way to the bottom of the can. The other brass tube was a short one and this was the fill outlet. The long tube to the bottom was the vent line so that an inverted can could still vent and I could fill the tank by squeezing the can. This was a diesel fuel management system I had made up many years ago based on what I read at the time.

That can of fuel was partly filled with some fuel that was at least 5 years old. That can was also my place to dump excess fuel from the bit left in the bottle left over from a days flying. That can also got a shot, now and again of the remnants of some of my more bizarre mixes. It ended up about 3/4 full and sat like that for at least a year. By then, I was using a transparent poly bottle for fill up and when I did, I noticed that the fuel was a deep, rich, transparent green. Darker even than Cool Power. None of the ingredients that ever went into this can were green.

As it turned out, that fuel was some of the best that the .074 had ever ran on. It started easily and throttled very well but based on verticals that I could now do, it was estimated to be 25% more powerful . Stock fuel will just hover the MiniSport. THIS fuel would allow a gently climb from a hover.

Green fuel. What could have caused that? The only time we see green in a fuel is when those brass tubes and clunks corrode when we leave them soaking in fuel. Copper oxide? Could this, if that's what it is, be a power enhancing additive?