Ignition Timing Fun On a Dyno - Lycoming

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Toobuilder

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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 the data plate value (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.
 

Victor Bravo

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Any guesstimate or opinion on whether this general principle (6 degree retardation only costs a little power but adds a lot of margin against nuking the motor) would hold true on a stock certified engine using far more prehistoric fuel and ignition systems than what you have... or does your gut tell you that it's your newer systems that are delivering this phenomenon?????
 

Toobuilder

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I believe it's a charactaristic of the tractor like Lycoming combustion chamber and easy to light off fat mixture. Not much magic required with rich mixtures and thick air!

That said, retarded timing would KILL power at altitude and/or lean.
 

Toobuilder

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Not the point of this. It's to find out where power falls off relative to timing. And the data supports that one can radically retard timing with almost no penalty. And since the 8.5 pistons in this engine can already deal with auto fuel on fixed magnetos (as demonstrated by countless other Lycoming drivers using auto fuel), my ability to retard timing simply buys me MORE edge distance from the already adequate detonation margin.
 

Toobuilder

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Yes, I run LOP a whole bunch. Of course for that condition, ignition timing is VERY peaky. A hot spark and a bunch of advance is needed to recover power from a slow burning lean mixture
 

simflyer

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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.
........
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 the data plate value (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. .....
Hi Toobuilder :)
some kind of similar info, even it wasn't done for racing, could be found here:
It is used on VW based engine, which is normally carbureted and interresting is ise of very wide ign. timing and purposes for it.
 

Chris Matheny

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Did you have an EGT probe in on this Dyno? I'd be interested in what the temperature rise was as you retarded the timing.
 

Toobuilder

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I'll see the data sheet later when I pick up the engine, but I doubt this parameter was collected since I didn't ask for it. The behavior is well understood anyway through my many flight tests: Advancing timing generally decreases EGT and increases CHT, and the reverse is true. It's not a bunch, and there is no limit on Lycoming EGT anyway, so the behavior is just a curiosity to me.
 

TFF

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As you know, CHTs rise with more advance because more burn is contained in the chamber instead of escaping during exhaust stroke. Rich is standard airplane takeoff for cooling and detonation. Also written in pilot manual so you don’t have to think about it. Piston helicopters almost always have to be leaned for best power on takeoff with added fuel for a margin. I have gotten use to this so I do it with airplanes if I know I have adequate cooling. Timing is the same way. If timing is retarded on a piston helicopter 4 degrees, You might not make the power to take off. Manifold pressure is monitored differently With a helicopter. Retarded timing easily using max manifold pressure to take off when it should have needed 4-5 inches less. An airplane would only see something like this on a real short runway where you needed max acceleration. 6000 ft of runway, most would have never thought of it. 0 ft of runway, you have too.

Detonation retard is a nice feature for auto gas. Excess power that can be converted into cheap gas is ideal.
 

Chris Matheny

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fall? is EGT only a relative measurement based on probe location,....is the absolute temperature difference accurate?
You can retard timing only to a point then the exhaust valve starts opening while the combustion is still in growth stage and egt will start to rise and the exhaust will start glowing.
 

Bill-Higdon

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Hi Toobuilder :)
some kind of similar info, even it wasn't done for racing, could be found here:
It is used on VW based engine, which is normally carbureted and interresting is ise of very wide ign. timing and purposes for it.
Sadly LeBurg ceased to be an option sometime ago
 

Toobuilder

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Sadly LeBurg ceased to be an option sometime ago
...however, the SDS CPI looks to be the architectually similar system. With the added benefit of being completely user programmable, the addition of the MP curve, provides a direct readout of some engine parameters, and is available today.
 

speedracer

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The Lightspeed Engineering manual says to set the timing at 20 degrees BTC for engines normally set at 25 BTC. I have dual LSE EI's in my 360 powered Long EZ. I have an spark timing readout window also. At 17,500'/200 mph TAS the window shows 39 degrees spark advance. IMHO magnetos are just a really bad idea.
 
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