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Timing Sweep On A Dyno - Interesting Numbers
Some may know about my governor failure and major prop overspeed on my Rocket last year. Well, the mess is nearing resolution and a significant milestone was reached yesterday when the rebuilt engine ran across Ly-Con’s dyno. Since this engine has a fully programmable SDS EFI system, this allowed me the rare opportunity to validate “rich mixture” ignition timing insensitivity on an instrumented dyno. This experiment is a continuation of the flight testing I did in Death Valley a few years ago looking for any “peak” ignition timing point at 100% power. In that experiment, I did a 10 degree sweep from 20 to 30 degrees looking for any gains in speed. I was unable to discern any advantage based upon the IAS – it remained essentially flat. I opined that the airplane was hard against the drag wall and simply did not have the resolution to measure slight changes in power. It was admittedly a crude measurement.
Fast forward to yesterday when I once again did a 10 degree sweep, but this time I wanted to see how far of a retard from peak timing I could go before I took a big hit on power. Since I plan to run auto fuel in this thing, I want to retard my timing as a measure of detonation protection, but need to know where the “sweet spot” is. The results validate much of my in flight testing to date, but now I have hard numbers. Disclaimer: I’m not going to defend the peak numbers shown here. This is Ly-Con’s data and I'm taking it at face value. What I’m more interested in is the percentage of power change with retarded timing. This data should translate well across the PV 320/360/540 fleet. For general info, this is an otherwise stock 8.5 cr IO-540-D4A5 with the following modifications: SDS EFI with 80mm throttle body Ly-Con CNC ported heads Piston oil cooling jets The data as provided by Ly-Con: Timing TQ HP 25° 620 320 20° 618 317 19° 615 316 17° 610 313 16° 606 310 15° 600 305 In this case, the peak power timing matches Lycoming's data plate value of 25 degrees. What’s interesting is the fact that I can pull a whopping 6 degrees out of the timing with almost no loss of power. So this tells me that I can program a huge retard into my takeoff curve and gain substantial detonation margin for no performance penalty. Anyway, I thought you people might like another data point. |
Informative post Mike.
Amazing insensitivity to timing on these engines. Appreciate you sharing this data. |
That IS interesting. I'm surprised.
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Michael---excellent results. It will be great to have the engine back in the plane, so you can compare these dyno numbers to real world flight data. Will be interesting!
Tom |
Tested on 100LL? I'd bet you'd see vastly different numbers on mogas.
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Break. Everyone note, Mike's motor is a parallel valve. |
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Yes, this was 100LL. I did not have the opportunity to try mogas.
But Bob, what numbers do you think will change, peak power, peak timing, or timing insensitivity? |
Michael, great effort, and this is exactly as I would have expected.
I have mentioned this many times before, so while HP will be fairly static the peak pressure varies quite a bit, and this is why when fitting EI's it is a good idea to knock off a degree or two. The traditional mag has a bit of lag in it. The spark plug fires on average a degree or three later than the static timing. The dual mags on a chieftain even more so. EI's hit the mark, and the SDS system is direct sensor, so even more so than the gear driven EI's. If you decide to run on MOGAS, knock a couple more out, because HP is not the real issue here rather peak cylinder pressures. If I don't forget to do it, I will dig in to some dyno runs and give you a guide to the differences you might expect with MOGAS, mainly with ICP, as this is the important factor. |
Since Mike has an IO-540 parallel valve running 8.5:1, those who have a parallel valve 360, should see the same results as the cylinders are the same.
I would also suspect that a standard compression (8.5:1) (I)O-320 parallel valve (There is at least one angle valve 320 out there.) should also see the similar results. Great info Mike, thanks for sharing! (Bummer you didn't go to 26 degrees to see what the drop off would be up there, but it is understandable that you were conservative with your engine.) |
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