C-85-12 Timing

[8348]

Just curious why the C-85 has top plugs timed different than bottom plugs. Per TCDS the bottom plugs are timed for 30* BTDC, and top 28* BTDC. I am not aware of any other engine I have been around where this was so.


Thank you,
Paul

[6643]

I'm so glad you asked!

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.

Quiz tomorrow...

[2066]

And, I'm so glad you asked, too, Paul!! I've been waiting 7..(well, a lot of years! ) for a definitive answer on this question & have heard a lot of opinions. I'm confident (and, happy!) with the Professor's explanation (Thanks, John!). Ready for the quiz! Mac

[8348]

Awesome! Thank you for the reply and thorough explanation, I feel I'm ready for the quiz . It's great to have such knowledgeable people to be able to learn from. Thanks again!

Paul

[6298]

The next question is: Why do the later small Continentals use equal timing on both magnetos?
Preparing for the Quiz..

[6643]

The next question is: Why do the later small continentals use equal timing on both magnetos?

I'm guessing because too many people asked too many questions...

[4004]

If one compares the specs on the progression from the C-75 to the 0-200 series, the C-85 is essentially the C-75 but increasing the RPM another 300RPM. Then they got smarter, more experience, better instrumentation, new dynamometer, more/less integratory, more/less FAA oversight, marketing push for product upgrades, airframe people needs, -- who knows - possibly some of all - but without a lot of capital investment for a new engine design, they started changing the "breathing" of the engine by camshaft profiling which change the valve timing -opening and closing the exhaust/intake valves which increased the efficiency and affecting the combustion process as Prof. John has so adequately covered. Plus increased RPM from the C-90 to the 0-200.

On an off-thread note but edification for some, I learned in one of my earlier careers that one doesn't really learn a subject until one has to teach others! That's why Mac addressed John C. as "Prof" - he used to teach this "stuff"!! Well there goes my grade on the Quiz!

[6643]

Don't get me started on grading! What we used to call an "F" is now called "Emerging Understanding." (OBTW, in our FAA approved Mechanic's school, anything below a 70 was failing. Better than a 60, but, who wants 30% of their mechanical work to be substandard?)