Another P-mag question, timing

[Paul K]

I am running an IO-360, 8.5:1 cylinders, dual P-mags, fuel injection, cold air induction, and a WW200RV prop. I have about 80 hours on it and I feel it is well broken in.

The question is, if I remove the timing jumpers on the mags, what should I expect? Performance change, fuel burn, speed, idle, etc.

For those who have done this, what can you tell me?

[RV10inOz]

Break in was achieved about 1/10th the time ago

What should you expect? Higher CHT's. Leave it where it is.

If you ever get to see an aero engine on a really well instrumented dyne like the one at GAMI, you will see what happens to HP with more advance. Depending on where you started, the HP usually drops. Contrary to popular belief.

Apart from the already advancing curves in the EI's, the "Effective Spark Timing" from an EI is a few degrees advanced over a magneto already due to the lag in a magneto.

You really do not need more, and you do not need more CHT.

[Toobuilder]

A dyno is at SL however. Lean mixtures in thin air mean a slow flame front, meaning a need for timing advance (compared to "fat" air). While its true that an engine needs what it needs, testing on the dyno isn't likely to tell the whole story.

And I run the Pmags in the "advanced" setting, but for the record have not tried it any other way.

...Guess I should and see what happens.

[Paul K]

So standard mag allows advance up to 26 degrees
P-mag A setting, jumper in, allows up to 34 degrees
P-mag B setting, jumper out, allows up to 39 degrees

Controlled by MAP. If I unplug the MAP tubes, it reverts to standard mag 26 degrees or so advance.

Just as a base line, I am a flatlander, field alt 760. Rarely ever go over 12000 and everything is running really good.

Unless I hear a good reason to change things, the jumpers wil stay in.

[N941WR]

Typically you will see an increase in CHT's and a decrease in EGT's.

How much depends on your engine and leaning technique.

Give it a try and if you see your CHT's getting too high, push the red knob in. Then, if you don't like the results, put the jumper back in. Better yet, buy and install an EICommander.

[RV10inOz]

Originally Posted by N941WR

Typically you will see an increase in CHT's and a decrease in EGT's.

How much depends on your engine and leaning technique.

Give it a try and if you see your CHT's getting too high, push the red knob in. Then, if you don't like the results, put the jumper back in. Better yet, buy and install an EICommander.

You will get a drop in EGT and a rise in CHT. The science says so regardless of your leaning technique.

There is nothing to be gained in higher CHT.

The area under the curve of work in Vs work out is what actually matters and moving the peak pressure to a different angle (Theta PP) and the resulting higher peak pressure is what you get. So higher cylinder pressure, highet temperatures and no gain in HP and maybe a loss??

Tell me why that will be good. These are basically fixed RPM machines thus they do not need the advance like cars do. There is more than enough advance in the standard curve as it is. And remember they fire off earlier due to less mechanical lag than a mag in most cases.

Go try it for yourselves, in your own flying dyno. And by the way you can replicate less or more air in a Dyno, especially one built for the task

[miyu1975]

I have dual pmags and still running the A curve. I have been planning to pull the jumper and try out the B curve... I had considered running the jumpers to a switch on my panel. That way I could easily make change any given time..(not during flight).. Is there any benefit in putting in a switch, or do you find once pick a curve you pretty stick to it all the time?

[Andy Hill]

After advice from others, I never even bothered with the A curve.

NB at full power, the P-Mag does nothing to the advance - it leaves it where a Mag would be, so as above, "Full Power" is unaffected, as will CHTs at Full power.

At cruise (Higher Alt, reduced MAP / RPM) is where the advance kicks in. You exchange higher CHTs / lower EGTs for improved efficiency / fuel consumption. The B curve to a slightly greater extent than A. If your cruise CHTs are an "issue" then the A curve would seem better, but not an issue for me.

There is nothing to be gained in higher CHT.

The higher CHTs (and lower EGTs) indicate the rather strange concept of burning the fuel in the cylinders to produce power, rather than wasted in the exhaust So I would disagree with you to that extent - the whole idea of EI is to achieve this. Clearly, if the CHTs get anywhere near a limit, you will need to take action to resolve - but my experience is that the P-Mag kicks in at those flight phases where high CHTs are not an issue.

Just my 2cs worth I reckon getting 12%, maybe 15% better fuel consumption over Mags. I am sure my limited understanding will be pointed out by others however

PS

I had considered running the jumpers to a switch on my panel. That way I could easily make change any given time..(not during flight)

The advice to not bother with the A came from someone who did this, albeit he only ran the switch to near enough the oil door to change it prior flight. I suspect after 1 flight on the B curve you will stick to it...

[RV10inOz]

I reckon getting 12%, maybe 15% better fuel consumption over Mags.

The comparison was over the A&B curves....not vs Mags.

[N941WR]

Originally Posted by RV10inOz

...Tell me why that will be good. These are basically fixed RPM machines thus they do not need the advance like cars do. There is more than enough advance in the standard curve as it is. And remember they fire off earlier due to less mechanical lag than a mag in most cases....

Keep in mind, that under full power/full load the P-mags act like a standard mag in that they dial the advance back to either 25 or 30 degrees, depending on the curve selected (A vs. B). When cruising at altitude, even at 75%, the timing will advance to the limit set by the curve (or custom programming). For those few people who fly in the teens you can really dial in some additional advance to maximize your power. This works because at high altitudes there is very little air in the mixture and the flame front takes longer to propagate across the cylinder. That's why Clause will run 40* BTC or more on his long cross country flights.

One major advantage of

Originally Posted by miyu1975

... I had considered running the jumpers to a switch on my panel. That way I could easily make change any given time..(not during flight).. Is there any benefit in putting in a switch, or do you find once pick a curve you pretty stick to it all the time?

Correction

Originally Posted by Emag Ignitions Manual, page 15

Note 2: The ignition looks at the jumper state at power-up only. You cannot route
these jumper terminals to a switch and go back and forth between curves while the
engine is running. EICAD does allow you to change timing (Advance Shift) while the
engine is running.

What I said about adding a switch may work with the 114 ignitions but I'm not sure.
I still suggest you bring the wires into the cockpit so you can run their EICAD program or install an EICommander at some later date.