Plug ignition firing sequence for P-mags?

Hi.


We're just about ready for first start for my IO-360 with dual P-mags and I have a question: (which is possibly very stupid...)

In the P-mag manual, it say's to confirm plug wiring by taking out the sparkplugs, grounding them to the engine case, turning the prop by hand and observe when the plugs fire.

We did just that, and the plugs fired like it says in the manual:
Plugs from the two front cyl (cyl 1 + 2) fired at TDC # 1 and plugs from the two rear cyl (3+4) fired exactly 180 degrees later.

My question is:
Is this really how it schould be??
On the engine case, the igntion order are listed as 1-3-4-2 (might be I'm not remembering the sequence correctly) but the plugs are actually firing in PAIRS AT THE SAME TIME...?

I've never heard of an engine with TWO cylinders fire at the SAME time...?

I'm sure there's a logical explanation to this, and maybee I'm stupid too, but I really would like an answer to this....?
And like they say: "If you don't ask, how are you gonna learn..?"

Thanks from someone who has a VERY steep learning curve these days...

YUp

Thats exactly what they do...Actually its very common..Its called a wasted spark system.

Have fun learning!

Frank

Alf, me again. Yes, every other spark is wasted in each cylinder. I have not thought about why this is so. I guess it is much cheaper to make the electronics that way. Sparks are cheap. I think it saves gearing the mags to revolve at half speed, but thats not thought through.

One other puzzle which is still ahead of you. When I first did mag drops, because it is a tail dragger, and everyone has said dont run up to 1800, I did the first few around 1500 and got very large drops > 100rpm. But very smooth and identiacal.

Yesterday when I did mag drops I did them at 1750, the tail is showing no sign of lifting with the stick back, and I got a drop of perhaps 90rpm. For a reason I havnt fully worked out, it seems a bigger drop at lower rpm.

I am on car plugs. And you?

With Cylinders 1 and 2 at TDC, Cylinder 1 is on its compression stroke, Cylinder 2 is on its exhasut stroke. So, when cylinder two fires, with one at TDC, it is not doing anything for it.

In the automotive world, they use the "Wasted Spark" to burn off any unburned gas that may still be in the cylinders in an effort to reduce emissions. Just another trick from our friends in Detroit.

One other thing, remember to set your engine to TDC when timing the P-mags. This is different than a standard mag. The P-mags will figure out when to fire the plugs.

Oh, and I still have a new 114 P-mag for sale, if anyone is intrested.

Aha...

So THAT's why.... I'll be....

Well, that sure was a relief to know. Sounds like a smart system those guys in Detroit have made up.

Steve: yes, I've installed the autoplugs and harness which came with the P-mags. I'll begin with them and see what happens...

The timing is set at TDC # 1 as closely as I possibly could and it looks very good (to me at least...)
The P-mags have recently been in the US for the latest upgrade, so I set the timing with the "blow-in-the-tube-metod" according to Brad's advice.

Engine run-up RPM:
I haven't decided how much RPM I'll use for the run up yet, but I think I've read somewhere on this forum that the P-mags needs above 1700 RPM to avoid the big drop?

What does other folks with P-mags use for run-up RPM?
What would be the max RPM to safely use for run-up with a TD RV?

P-mag

Bill,
How much do you want for your new P-mag? Aden.

Aden,

I sent you an email.

Sorry, but I have to object to the statement that the wasted spark is used in cars to ignite unburned fuel and clean up emissions. The gas in the cylinders between the exhaust and compression cycles has already been burned and is at low pressure. Depending on the spark advance and the amount of intake overlap, engine rpm, etc, there might be some new air-fuel mixture present from the intake port. However, it's in the presence of very high temperature gas. Adding a spark isn't likely to help anything.

The reason for the wasted spark is to halve the number of coils and coil-drive circuits in a distributor-less ignition system. It also allows one to take the timing reference from the crankshaft (or a same-speed shaft as in the p-mag). In this sort of ignition, each end of the coil's secondary winding is connected to two spark plugs; as opposed to one end being tied to ground as it is for coil-over-plug systems and engines with distributors.