Timing vs CHTs with PMAGs

[Randy]

Nigel,

I flew my 7A home to Sedona and back over the weekend, about 4.5 hours each way from Redding, so I had ample time to experiment with the technique you mentioned, noting EGT at take off power and then adjusting mixture to maintain that EGT during the climb.

I can report with confidence that when I use this technique I simply do not have any issue with CHTs, they stay well below 400F.

The increase in fuel flow takes a little getting used to but it is nice to see that I can run rich and keep it cool using the technique mentioned. Such a simple technique it seems strange that I had not used it before... Now I wonder if I can put the timing advance back where it was, 2.8 deg. more, and keep the CHTs in check. I am not sure there is any real performance gains to be had from this as I did not notice any performance degradation from shifting the curve towards less advance.

I still think I should be able to run my engine at best power mixture in cruise, ie about 100F ROP without getting CHTs above 400F, that remains a problem for me.

With your setup can you run at best power mixture and keep CHTs under 400F?

For comparison, my cruise TAS at 8500' stays between 157 to 160 knots running LOP and burning a shade under 8 GPH. ( Timing under these conditions is 30 deg.) Going ROP I need to bump FF up to around 11 GPH to keep my CHTs in check and for this extra 3 GPH of fuel burn I gain right close to 10 knots airspeed. The cost for 10 extra knots of speed at $4 gas, $12 per hour. Seems a little steep...

FYI, I do not have the restrictor orifice in my MAP line that goes to the P Mags. Perhaps it could be affecting my CHTs with out me seeing any fluctuations in timing in the EI Commander? Maybe this could be the elusive cause I have been looking for, gotta try an orifice and see what happens.

[Rick Saliers]

I'm new, building an RV-9A with O-320. Are "Pmags" a generic name for EI or a specific brand? Aircraft Spruce has an "e-mag p model ignition." Looking to have left mag electronic and right mag mechanical.

[nigelspeedy]

Hi Rick,

The 'PMag' is a specific brand of electronic ignition. E Mag Electronic Ignition make the 114 series for four cylinder Continental and Lycoming engines.

Try: http://www.emagair.com/114-series/ for the company website.

Cheers

Nigel

[lr172]

Originally Posted by nigelspeedy

Does the reduced heat from running rich really come from the latent heat of vaporization? Lets say peak EGT is at 10gal/hr and I run rich at 13 gal/hr, so 3 gal/hr is just being evaporated.

Nige

Yes.

At stoichiometric (roughly peak EGT) the complete amount of fuel in the charge (air fuel mixture) will burn. Any richer than that and you have un-burnt fuel. The richer you are the more unburnt fuel. This un-burnt fuel absrorbs the heat, as any liquid does at it's vaporization point, from the burning air fuel mixture reducing it's overall energy (heat = energy). This obviously reduces the heat transfer to your pistons and cylinder head. This reduced heat/energy also reduces your ICP, increasing detonation margin.

There is more chemistry and physics involved, as at some point rich of Stoichiometric is best power. The additional un-burnt fuel helps to produce more power up to this point, but I can't remember the details of why. Also, up to this point the extra fuel is not really cooling the charge. Past this point, the extra fuel is reducing your heat output of the burning charge and marginally reducing power as well, due to the reduced heat/energy. I should also add that the richer mixture slows the burn rate and therefore moves your peak pressure point. This also increase detonation margin as well as cooler CHTs.

When rich of stoichiometric, the exhaust will have little O2 and varying percentages of hydrocarbonsm based upon how rich. When LOP, there will be very few hydrocarbons and varying percentages of O2, based upon how lean. This is how O2 sensors in the exhaust can determine the air fuel ratio at any point in time.

Larry

[Rick Saliers]

Thank you Nigel. Does the left mag - electronic and right mag - mechanical make sense? The EI guys want you to install two EI mags. Regards.

[nigelspeedy]

Rick,

I would do two PMags if I were in a position to do it again. I have had good support from the manufacturer. They have worked without fail in my application for 250 hours. Its really easy to set the timing of both PMag at the same time. If the amount of advance concerns you it is easy to hook up a laptop computer and limit the maximum and shift the curve (retard or advance the point where more advance is added), although not with the full authority that other systems offer. If you are looking for a system to do a grand ignition timing experiment look elsewhere. If you think you want something better than traditional mags (the ability to run auto plugs & wires, easy starting, internal power generation, limited control over timing) then they are a good way to go. Unless there is a big cost saving of a traditional Mag over the PMag I would get two PMag.

Cheers

Nige

[FasGlas]

The problem with the programming on a pmag is you can't change the curve. You can shift the curve and you can put a limit on the maximum advance but you can't change the aggressive MAP curve. That's where the problem is, it can be too much advance too quickly. It's too bad they didn't allow the curve to be changed. With all my experience with pmags I would never install them again on my plane. I have installed them for customers with mixed results.

[Bevan]

I did two Pmags (144 series) right off the bat and and would do it again. 90 trouble free hours so far, super easy to time them. Sure you can't customize the MAP but you can't with a traditional mag either. I don't want to mess with timing other than a conservative advance and the other benefits of EI already mentioned.

Bevan

[Bavafa]

Now that we are shifting the subject to "one or two" I went from two Slick to one PMAG and one Slick and saw some positive improvements. Then after one year and building some confidence, I switched to two PMAG (114, V40) and have seen zero improvement. In some ways, it actually seems not as good but that is hard to measure so I am leaving it be.

[lr172]

Originally Posted by Bavafa

Now that we are shifting the subject to "one or two" I went from two Slick to one PMAG and one Slick and saw some positive improvements. Then after one year and building some confidence, I switched to two PMAG (114, V40) and have seen zero improvement. In some ways, it actually seems not as good but that is hard to measure so I am leaving it be.

The Pmag has too much advance at higher MAPs. When balanced with a mag at 25* this is not so bad. However, with two EIs, you can't run the same advance on both as you did when you had one balanced with a 25* mag. With 2 EI's, both need a lower advance setting at each step in the table. This is likely why you are losing power.

Remember, with a two spark setup, you have to consider the effect that the combination has on getting your peak pressure point to the optimum location. They really should have different tables for one Pmag vs 2 Pmags. My twin plug Porsche runs about 5+ degrees less timing than my big block Chevy with one plug.

larry