How Important is OMC Ignition Timing?

[legendre]

quote VinTin:

Would it be a correct assumption that max motor output would be achieved by setting the timing just before the onset of spark knock? I’d like to get the max out of my 57 35hp OMC but of course don’t want to cause engine damage. I understand that spark knock is difficult to hear in a wide open outboard motor. Thoughts?

Per my understanding, general theory states that, for a given RPM, more advance is better, so long as it’s not causing spark-knock / detonation. But there are a lot of variables and there’s no way to simply say that more advance will or will not benefit your engine.

If you want to play, I’d trial & error it.. while watching cylinder head temperatures (if possible) and frequently checking the plugs. A higher octane fuel will also tend to tolerate more advanced timing. +Shrug+

[legendre]

quote legendre:

quote VinTin:

Would it be a correct assumption that max motor output would be achieved by setting the timing just before the onset of spark knock? I’d like to get the max out of my 57 35hp OMC but of course don’t want to cause engine damage. I understand that spark knock is difficult to hear in a wide open outboard motor. Thoughts?

Per my understanding, general theory states that, for a given RPM, more advance = more torque, so long as it’s not causing spark-knock / detonation. But there are a lot of variables and there’s no way to simply say that more advance will or will not benefit your engine.

If you want to play, I’d trial & error it.. while watching cylinder head temperatures (if possible) and frequently checking the plugs. A higher octane fuel will also tend to tolerate more advanced timing. +Shrug+

ETA: Not sure how I managed this, but I’ve tried to delete the post several times. System says I can’t.

[phil-b]

Timing of the spark is only a range relative to TDC.

The timing of points opening ("event #1") is a fixed relationship between the coil "shoe" and the position of the magnet (which has a fixed relationship to the high point on the cam, since both the flywheel and the cam are keyed onto the crankshaft. This is why you can adjust the points at any throttle setting.

Timing of the spark relative to TDC at a given throttle position (& carb butterfly position) controlled by the "link and sync" adjustment cam.

[legendre]

quote Phil B:

Timing of the spark is only a range relative to TDC. (…) Timing of the spark relative to TDC at a given throttle position (& carb butterfly position) controlled by the “link and sync” adjustment cam.

Sure.. but what defines the upper & lower bounds?

Let’s just ignore the fact that the ignition timing changes with mag lever / throttle position.. and view the motor at full speed, max RPM and hence, max ignition advance.. just as with any motor, running at full speed and full advance.

What defines that final, high-RPM timing angle figure? Is it a certain, factory-specified number of degrees BTDC (as with every other engine), or just wherever it happens to end up, if ‘timing’ is set for the moment of max spark energy – on an OMC?

Are you saying that OMC timing theory is literally different from virtually every other engine out there? Every other engine I’ve encountered has ignition timing specified in degrees BTDC – but per you, and those who agree with you, that’s out the window. You’re claiming that the spark should fire at the moment of greatest energy, and the actual ignition event just falls where it may.

I don’t buy it. And again, ask yourselves: Why does the OMC timing tool exist?

[phil-b]

quote legendre:

What defines that final, high-RPM timing angle figure? Is it a certain, factory-specified number of degrees BTDC (as with every other engine), or just wherever it happens to end up, if ‘timing’ is set for the moment of max spark energy – on an OMC?

I don’t buy it. And again, ask yourselves: Why does the OMC timing tool exist?

The timing tool, or "gapping the points" exist to adjust timing of the spark (the points opening) relative to the position of the coil & magnet.

Timing of the spark relative to TDC (advance) is controlled by the position of the magneto plate (which also controls the throttle via the magneto plate cam)

[legendre]

quote Phil B:

The timing tool, or “gapping the points” exist to adjust timing of the spark (the points opening) relative to the position of the coil & magnet.

And in all other instances I’ve ever encountered, the timing is set relative to crankshaft position – in terms of degrees BTDC.

quote :

Timing of the spark relative to TDC (advance) is controlled by the position of the magneto plate (which also controls the throttle via the magneto plate cam)

Again, this is a red herring. Ignore the fact that the timing follows the throttle lever position.. and answer this:

What determines the final timing advance figure, when the engine is running at full speed? Is it simply a matter of where the spark is strongest, or where the spark is most appropriate to the crankshaft position in terms of degrees BTDC (as with +every+ other design)?

And no one has answered the big question – Why does the OMC timing tool exist, when the "timing" is simply a matter of aligning the magnetic TDC with the coil’s pole shoe? Why would you need such a tool, when the relationship you desire can be set with a ruler, or with a set of factory-applied marks? Does the OMC timing tool index on the magnet position, or on the crankshaft position?

(Hint: It’s because the magnetic TDC isn’t what we’re after… it’s a particular crank position, as with every other timing spec.)

[phil-b]

quote legendre:

What determines the final timing advance figure, when the engine is running at full speed?

The position of the magneto plate (the "stator")

quote :

And no one has answered the big question – Why does the OMC timing tool exist, when the “timing” is simply a matter of aligning the magnetic TDC with the coil’s pole shoe?

Think of the tool as a "gap setting jig" it doesn’t control what you think of when you hear the word "timing"

quote :

Why would you need such a tool, when the relationship you desire can be set with a ruler, or with a set of factory-applied marks?

Uh, the tool depends on factory applied marks. It’s just a pair of "pointer arms" to connect the crank position with the marks on the magneto plate.

If your ruler has a round hole in the center, with a slot for the crankshaft key, and ends which narrow down to a point and droop, you have your own timing tool.

quote :

Does the OMC timing tool index on the magnet position, or on the crankshaft position?

YES (both). The magnet is in the flywheel, which is keyed onto the crankshaft.

To use the tool, you remove the flywheel, and put on the tool, while has a slot for the crankshaft key, just like the flywheel does.

[legendre]

quote Phil B:

YES (both). The magnet is in the flywheel, which is keyed onto the crankshaft.

Are you sure that the magnet is always in the flywheel? On the OMC magneto I know, it’s in the rotor, which is a separate part from the flywheel. While the flywheel must be removed to access the points, the rotor may remain in-place. In fact, it must, as one of the tuning parameters is rotor / coil shoe gap..

Right?

And once again, why not just put a mark on the rotor, to align with a coil shoe, if our only concern is that the magnet(s) properly time the ignition event to the point where the magnet(s) – on the rotor – are properly aligned with the coil pole(s)?

Do you not see how this would obviate the OMC timing tool? Did OMC omit those marks, to prevent the average owner or handyman from accurately setting the ignition timing, unless they were willing to purchase a tool that was never widely distributed? Huh?

None of this makes any sense.. since when is an engine timed, based solely on peak spark power? Have you ever encountered any other engine, or family of engines, that didn’t time the ignition relative to crankshaft position (in degrees, BTDC)?

[chris-p]

Timing is fixed. That is all there is to say about that really.

The timing fixture was not meant to be a common household item I believe. It was for the techs in the shop. Frank just happens to make them so we can all reap the benefits of using them, compared to simply using feeler gauge.

[The Boat House]

• This reply was modified 6 years ago by The Boat House.
• This reply was modified 3 years, 9 months ago by The Boat House.

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