Which Mag Fires Which Cylinder?

[c170b53]

Excellent way to approach maintenance is to ask questions and be involved if so inclined. I often like voorheesh‘s comments and train of thought here but another reality is it’s hard to be on top of everything new and old even though you have the ticket for it.

[voorheesh]

I’m really glad to hear Steve’s answer and I fully support owners being involved hands on. On line messaging can be vague at times and I sometimes worry about advice being misunderstood in the blind, so to speak. Having said that, this is the best source of information for Cessna 170s and I am happy to still be a participant.

[cessnut]

Bruce still hasn't offered an opinion on why O-200s don't have staggered timing.

[Bruce Fenstermacher]

Bruce still hasn't offered an opinion on why O-200s don't have staggered timing.

I don’t have an opinion. Maybe as it was the last of the small Continentals designed the redesigned the piston crown for better burn. Maybe Continental engineeres simply forgot they had staggered timing.
What is more curious is why the A-65 also doesn't have staggered timing but is 30 degrees not 26, 27 or 28. I’ll bet the only people who could give us answer are those who hopped up and raced these engines.

[n2582d]

Here's an answer to the staggered timing question from the Cessna 120/140 Association--apparently from an A&P instructor:

First of all, it's called "staggered timing". It actually makes the engine a little more efficient.

The lower (bottom) plugs operate in an environment that is closer to the exhaust valve than the upper plugs. The air/fuel mixture there is diluted by the exhaust gasses from the previous power stroke, so the mixture there is leaner. Leaner mixtures burn slower, so the timing is set a few degrees more advanced to give it a few milliseconds longer to burn compared to the richer mixture around the upper plugs.

Continental C-series engines have all the lower plugs connected to the left magneto and all the upper plugs to the right magneto, so the left magneto is set to fire 2 degrees earlier (30* BTDC) than the right (28*BTDC). If you set the left mag to 28 you will get a bigger mag drop because the engine is already firing that plug later than optimal. The engine will also produce less power as the mixture is still burning past the optimal position of the crankshaft. If you set the right mag to 30 you sacrifice some of the detonation margin as the richer mixture will burn faster and cylinder pressures will rise earlier. Essentially, with the staggered timing the flame fronts from the two plugs will meet in the center of the cylinder, and burning on single mag operation will finish at the same time regardless of which mag is selected.

Engines that do not employ staggered timing employ one mag to fire the lower plugs of half of the cylinders and the upper plugs of the other half, and, of course, the other mag fires the other half of the top and bottom plugs. On opposed engines, one mag fires the lower plugs on one bank of cylinders and the upper plugs on the other bank. This also gives you an even mag drop between both mags, but only because half of the cylinders are operating at less than optimal efficiency during single mag operation. With both mags operating, the flame fronts of each cylinder will always meet closer to the exhaust port since the leaner mixture is not being ignited earlier as it is with staggered timing. On single mag operation, the cylinders of the bank running on the lower plugs will finish burning later than those running on the upper plugs, and so will sustain a bigger drop in power than the others.

What I don't understand is the first sentence, upon which his argument is based: "The lower (bottom) plugs operate in an environment that is closer to the exhaust valve than the upper plugs." Here's a photo of the inside of the cylinder head. It looks to me like the top and bottom plugs are equidistant from both valves.

O-300 Cylinder.jpg

Here's another question related to engine timing. Back in 1977 Continental had a problem with cylinder heads cracking on cylinder p/n 641917 -- a cylinder common to the O-200 and the O-300. AD 77-13-03 and AD 78-19-02 were superseded by AD 96-12-06. The solution in the original AD was to retard the timing to 24º BTC. So why, with a common cylinder, was the O-300 not included in this AD?

[Bruce Fenstermacher]

There are a few things that don’t fit the explanation. The A-65 uses the same cylinder as the A-50, A-75, and A-80. The A-65 still has left mag firing all the bottom plugs and the right firing all the top. But it does not use split timing. Are we to believe that the most prevalent version of the A series engines have not been run efficiently?

Maybe the max rpm of 1900, 2300, 2600 and 2700 for the A-50 through A-80 and the compression ratios of 5.4:1, 6.3:1, 6.3:1 and 7.55:1 has some effect on the burn.