Ignition timing

[BOBM]

Originally Posted by hgerhardt

Sorry Bob, I'm right. If I could figure out how to post a pdf, I'd post the page out of the Lycoming Operator's Manual here. I emailed it to you at your bobvicky address.

Well, I've got an O-360 in my 172, it's 25 btdc, an O-320 in the RV6, it's 25 btdc, all of the IO-360s in this link are 25 btdc except the turbo motors.

http://www.pilotfriend.com/aero_engi...%200%20360.htm

I don't know what you're looking at, but 25 btdc is pretty standard for Lycs.

[mahlon_r]

The standard timing was 25* btdc and the change was to 20*btdc. The SI that deals with the change said it was done primarily to reduce CHT's in cruise and to increase detonation margin during cold weather operation at high power. It's SI 1325A if you are interested.
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

[BOBM]

Originally Posted by mahlon_r

The standard timing was 25* btdc and the change was to 20*btdc. The SI that deals with the change said it was done primarily to reduce CHT's in cruise and to increase detonation margin during cold weather operation at high power. It's SI 1325A if you are interested.

I checked the Lycoming website & no SI 1325A is listed. Can you provide a link?

[rv6ejguy]

On our EI systems which are user programmable, we recommend testing for best power or speed at a variety of manifold pressures and rpm. All engines are quite different and all EI manufacturers including those for non-certified engines should be doing a lot of cell/ dyno/ flight testing to determine safe timing curves for their products.

Timing would certainly vary with CR changes even on the same basic engine type so if an EI maker doesn't ask questions like what is your CR, be a little suspicious that timing may not be optimized for your engine.

As Paul stated, chamber design has a lot to do with the required advance to optimize where peak cylinder pressure occurs. For peak power, we see something like a shallow pent roof, 4 valve Toyota 4AG Formula Atlantic engine with 12.8 CR require only about 22 degrees of total timing, a Toyota 2TG with 10 to 1 CR, high domed pistons and Hemi chamber, 40 degrees, a twin plug Porsche 3.2 turbo around 28 degrees and various Continental and Lycoming engines at 26-30 degrees.

This is for max SL power. When you reduce manifold pressures either through throttling or higher altitudes, increased advance may show benefits on certain engines. Only through lots of testing on a specific engine combination can this best timing values over the whole operational range be found.

Turbocharged engines require a lot more work in this regard especially when operated on relatively low octane fuel. We sometimes have to reduce timing around torque peak rpm where the highest cylinder pressure occurs to avoid detonation, then we can re-advance a bit as cylinder pressure falls off due to lower volumetric efficiency.

This is another area where most have blind faith that the supplier did the job right or has at least been very conservative with advance.

[Kellym]

That engineer was pulling an answer out his behind.
There is a Lycoming service instruction circa 1977 that calls for changing the timing, due to problems at extreme cold temperatures, and every angle valve 200 hp engine since has been built by Lycoming at 20 degrees, and now they claim it actually makes equal or better power at that setting.
The 25 degrees was in use when Mooney selected the IO360A1A for the E model in 1963, before the A3B6 or A1B6 were on the drawing boards.
Also, the A1B6(and A1B6D) was first used on the C177RG in 1971, then adopted by Mooney in 1976, then converted to A3B6D by changing prop orientation for slightly smoother operation. In other words, the A3B6D wasn't even the original application for the engine, so the engineer was making up an answer with no basis in fact. If you haven't guessed, I have an A1A engine, which runs fine on 25 degrees, per data plate, with Bendix mags.
Kelly
A&P/IA

Originally Posted by hgerhardt

I wondered that too, as I have an IO-360-A3B6D I heard from a friend who allegedly asked a Lycoming engineer why that was, and he said it was so that Mooney (the original application of my engine) could make the performance numbers it promised. Apparently, the engine made just enough more power at 25 deg to make the numbers. T

Heinrich Gerhardt
RV-6, flying