Mag Timing

[uwochris]

Hey guys,

Does the term "magneto timing" mean the same thing as "ignition timing?" ie) if you change the mag timing, does this mean that the spark that is delivered to the spark plugs in the cylinder will ignite the mixture either earlier or later? I know the role of the magnetos is to generate current and to deliver that current to the spark plugs so the engine can run. I'm just a little confused as to what the term "magneto timing" means.

I had our 152 go into maintenance a few weeks ago. This a/c has always been very difficult to start. The mechanic said the "mag timing" was way off, so he changed it. It doesn't seem to have helped much, so I was just curious as to what exactly "mag timing" referred to.

Any comments?

Chris.

 

[Mmmmmm Burritos]

Yeah, it's pretty much the same as ignition timing. Mags are like a distributor in a car (except a distributor needs an external source of power). By setting them you set where the spark plug fires in relation to the cylinder moving.

 

[A Squared]

Yeah, it's the same as ignition timing, it's the timing of when in the cycle the spark occurs. About the only difference I can think of is that in automobiles the ignition timing changes depending on rpm and throttle setting. This is done by centrifugal or pneumatic (vacuumn) devices on older engines and electronically on newer engines. On most aircraft, however the ignition timing is fixed, it is set by the mechanic and never varies.

 

[avbug]

Magnetos have two "timings." One is timing to the engine, and it works the same as the distributor on your car. The top of the magneto is the distributor, and it's exactly the same as what's on your car. It's doing nothing more than distributing an electrical impulse to each spark plug. It's timed in the same manner as the distributor in your car, too...merely by loosening a locking or tension device, and rotating the magneto...just like rotating the distributor housing on your car.

The magneto requires internal timing, too, just like timing the points on your distrubutor in the car. The top half of your aircraft magneto is the distrubutor. The electrical difference between your aircraft magneto and that of your car is the source of the electricity...the magneto produces it through an internal magnet and in a car it's merely delivered from the battery and enhanced by the action of the distributor assembly and the ignition coil.

Internally your aircraft magneto requires timing of the breaker points, which are set to a condition known as "E gap." This timing is critical to the ability of the magneto to produce a hot spark, as well as the timing of the spark, or when the electrical impulse is produced in relation to where it's attempting to deliver the spark. The magneto timing that you're probably most familiar with, the rotating of the magneto in relation to the engine, is timing the spark to the position of the piston in the #1 cylinder.

Internal timing is opening of the internal points, which collapses an electrical field and causes the sudden rise in voltage that is what arcs across the spark plug electrodes, only looks at the relationship of the points to the distributor position and magnet/rotor inside the magneto. The external timing only looks at the relationship between what the magneto is sending out, and what the engine is doing. Both are critical to successful functioning of the magneto. Different methods are used to time different magnetos, both in and out, but the end result is the same. Hard starting, engine roughness, etc, could be the fault of either internal or external timing.

 

[A Squared]

Good point on the internal timing, I'd always thought of it as "setting the e-gap" but it is timing of a kind.

 

[TurboS7]

If you are having a hard time starting it more than likely is a bad impulse coupling. If you have a 'shower of sparks" it could be that the timing is off for the shower. Check it out. I have had impulse couplings go bad and the only way we found it out was to change it. With the new coupling it worked like a champ.