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In my case, it?s not so much that I know how to program an ignition curve as much as I take a very conservative approach to testing. With CPI, you can emulate the timing of a magneto if you wish, and then take baby steps from there. There?s a big difference between programing in (or buying product with) a complete curve and hoping for the best vs. adjusting the timing incrementally from a single, known flight condition. And the ability to instantly ?undo? that adjustment with the flip of a switch as in the case of CPI makes it even less of a risk.
This is not to say that I think the ?locked down? curves of the other manufacturers are generally bad or unsafe, rather, I?m just trying to illustrate that the development of a custom tailored curve with CPI is safe and pretty easy if you use a little common sense. |
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Your approach of starting with a little, monitoring the data, doing minor changes and monitoring the data, etc, etc, is the right way of programming an EI. Power vs effect is a slow process to get the best performance with the least adverse effects to heat and possible detonation. I knew a guy many years ago that installed a new Jeff Rose EI in his new Glasair. First flight, right after crosswind, his engine went into maximum detonation mode. Landed on the taxiway, lots of damage to the plane, he was lucky to have lived thru it. He later pulled the plugs to find all the electrodes were melted away. He claimed the ignition caused this problem but Jeff Rose found nothing wrong with it when tested. Later it was concluded that the installation was the probable cause. The MTH was installed 40 degrees advanced, not at TDC. Misread the instructions maybe? Point being is, misprogramming could cause catastrophic effects. Having the ability to program at will is great but only if you know how. But it's not hard. |
Agree completely on the "more must be better" trap. (I also completely relate to the huge carbs on a smallblock analogy) I think we see some of this with the "jumper in, jumper out" setting on the Pmags. Some run the more aggressive settings right off the bat because they think they are the ones that need it. I ran this curve on the -8 initially and after plenty of hours with no real issues, decided to pull it back a little. Speed stayed the same, CHT dropped a bit. That's a win in my book.
The beauty of the CPI is that you can simply advance the timing one degree at a time and observe the result. If you do it right you can observe the speed increase with advance, peak, and then fall off. Unless you are a masochist, you will not retain the "too advanced" setting because you can see with your own eyes that the engine will tell you what is required. No need to spin a fishermans yarn with how "big" your advance is - the engine needs what it needs. But yes, with the power to alter your timing to ANY setting comes the responsibility to do it inteligently. |
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I do agree that each individual is not prepared to deal with the freedom of configurability, but it seems that many in the experimental world seem willing to do the research. That said, it only takes a couple guys to show up saying that 40* at 29" is awesome and some people may follow. That's the beauty of this forum. There is usually enough information so that, if you do the research and apply judgement, it's pretty easy to stay out of trouble. Larry |
Too much is not better
I did a recent test up at 17,500' and with WOT I get 15.5" MAP at 2,500 RPM. I saw very little difference (<5%) in specific range between 30, 35, 37 and 39 degrees advance. The difference in top speed was also only a couple of knots. This was 25F LOP. More testing to see how it affects down lower and ROP.
Cheers Nige |
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Only a couple of knots
I like going fast as well, the point I was trying to make was that a knot or two is not worth blowing the engine up for. In fact the highest advance 39.2 degrees did not yield the highest speed anyway.
Cheers Nige |
Copy. More advance does not always equal more power.
However, I do not believe that the optimum advance setting (optimum = most power) is destructive. There may be some exceptions, but the vast majority of our engines will be able to remain within their structural and thermal design limits without complaint. |
Well, this is why there's no "One size fits all" in the EI mapping. There are so many variables in aviation. For the best performance and efficiency each pilot needs to build a map for his/hers needs. Altitude, MAP, humidity, RPM, fuel inconsistencies, mission usages and aircraft all affect the EI mapping. The only way to get your money's worth, per say, is exactly what Mike's doing. Figure out what your mission is for your plane and take small steps to build YOUR map.
All aircraft are designed with a compromise between stability and efficiency, the faster you can go the less stable it will tend to be. This is why a military fighter must have a computer flying it. That being said power vs speed is exponential, the faster you go the more power, exponentially, you need. The last few knots (since we were on that subject) is far harder to get to than the first 100 knots. This is why most see that same average speeds (max and VSI), then there's the few that push past the average. Barring weight, build and final trimming it comes down to horsepower. A few knots at WOT says a lot in the scheme of things. |
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Remember, the original LOP mantra was running cooler at less peak pressure; with fixed timing, LOP delayed thetaP. Being speed demons, we've twisted that into "How fast can we go on less gas?" |
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