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P-mags and CHT at full power.

S

Steve Sampson

Well Known Member
My -4 with an O-320 and P-mags is still new to me, a little over 25 hours.

The P-mags are superb for me, but clearly behave differently to normal mags. I cant sustain a climb at 2700rpm and full power for very long (2 to 3 mins) without the CHT climbing fast also. (Admittedly I am babying the CHT because its a new engine and changing somethng to control the temps when they hit 400F.)

What i am finding though is a small reduction in the throttle (manifold pressure), together with a reduction in the rpm to say 2600, makes a very significant reduction in the issue. I am presuming this is because the presumably fully advanced mags start to move back from the max (of I think 39 degs.) Does this ring bells for other P-mag users?

I would be interested in other P-mag user comments.

PS I have the jumper in place, so the standard factory advance table.
 
Steve Sampson said:
My -4 with an O-320 and P-mags is still new to me, a little over 25 hours.

The P-mags are superb for me, but clearly behave differently to normal mags. I cant sustain a climb at 2700rpm and full power for very long (2 to 3 mins) without the CHT climbing fast

(snip)

I am presuming this is because the presumably fully advanced mags start to move back from the max (of I think 39 degs.) Does this ring bells for other P-mag users?

I would be interested in other P-mag user comments.

PS I have the jumper in place, so the standard factory advance table.
Click to expand...

My guess is that you are running too lean at WOT and/or have insufficient cooling airflow at climb speeds.

My understanding of the E/Pmag advance map is that there is little or no advance from 20 degrees under the conditions you describe. The EIs typically aren't aggressively timed under very high output conditions, to increase detonation margins.

Magnetos, because they have fixed timing, are set for "worst case" detonation conditions and are not optimized for partial throttle settings, such as high altitude economy cruise.

The various EIs begin to advance the timing as you retard the power, so they remain more or less optimized. The real value is in cruise, not so much on takeoff (except maybe for a hotter, longer duration spark).

Others understand this better than I do <cough> Ross </cough>, so I welcome corrections.
 
Sounds normal

Hi Steve,

I don't believe the Pmags are creating the temperature issues. My 7 is ECI IO360 equipped, dual Pmags. I operate in high temps in AZ. I will climb at generally around 1200fpm set on the auto pilot. I'll watch the CHT's. Every 1000 ft I'll lean to get the EGT's to around 1350 and hold that. As the CHT's climb through 400 I'll adjust the rate of climb to keep them at or below 400. If the temp is hot 36-42C I'll use 425 as the don't exceed temp. Even then, I can still get 800 fpm easily.

Try to adjust your rate of climb and monitor your EGT's as well as the EGT's. With some practice you can find that sweet spot.
 
Steve Sampson said:
My -4 with an O-320 and P-mags is still new to me, a little over 25 hours.

The P-mags are superb for me, but clearly behave differently to normal mags. I cant sustain a climb at 2700rpm and full power for very long (2 to 3 mins) without the CHT climbing fast also. (Admittedly I am babying the CHT because its a new engine and changing somethng to control the temps when they hit 400F.)

What i am finding though is a small reduction in the throttle (manifold pressure), together with a reduction in the rpm to say 2600, makes a very significant reduction in the issue. I am presuming this is because the presumably fully advanced mags start to move back from the max (of I think 39 degs.) Does this ring bells for other P-mag users?

I would be interested in other P-mag user comments.

PS I have the jumper in place, so the standard factory advance table.
Click to expand...

I'm running an IO-360, standard pistons, dual P-mags. After the initial five or so hours on the engine, I have never seen a CHT over 400. My prop (Hartzell -7496 blade) has a restriction that operation about 2600 RPM is limited to takeoff, so I usually do my climbs at full throttle and 2500 RPM at about 110 kts. With those settings on a 95 degree day, my CHTs peak at about 375 and start dropping after the first few thousand feet if I don't lean in the climb.

I've never had a problem with CHTs. Oil temp, however, is a different story -- thanks to that standard (small) cooler that I haven't replaced yet because I don't want to rework the baffle. This has been covered a thousand times on this forum, but in case anyone new is reading: Install the biggest oil cooler that fits. You can always cover it up later if it's too cold.
 
Thanks for the various replies. Perhaps I am expecting too much, but I would like in principal to be able to climb with everything forward for as long as I want and not go above 400F.

If I pull the rpm back to 2500 I probably can, so it would seem I am not so different to you Geoff. (Where I am different is I do have the standard small cooler and wish there was a smaller one because my only problem there is low oil temps.)

RV7guy - your climb rate of 1200fpm appears quite conservative. That must be giving you quite a high airspeed even at gross? If I pulled the rpm back to 2600 I think I could match that although I would be quite fast.

Flyeyes - In E-mags advance/rpm dia. they show max advance at max rpm, which makes sense to me. What they dont show and I dont understand is how they vary the advance as the MP alters. My assumption was that the max advance occurs with everything wide open. Is this wrong? It is clear to me that by pulling the power from 2700 back to 2600 lots less heat is produced. The fuel flow is about 1% more than the Lyco spec for the engine at full power.

I am puzzled because as I said above I really dont have an oil temp problem. The cooler is remote from the rear baffle fed by a 3" SCAT. I need to reduce that air flow because I doubt in the first 25hrs the oil temps have been over 180F. To me this implies there is plenty of air with a good differential pressure above the engine to flow the cyl fins into the lower cowl. I dont have any obvious leaks either, though a traditional cowl seal is not perfect.

Thanks again,
 
Incorrect

Steve Sampson said:
What they dont show and I dont understand is how they vary the advance as the MP alters. My assumption was that the max advance occurs with everything wide open. Is this wrong?
Click to expand...

Yeah, it is. Above about 75-80% power, the advance is 25 degrees (same as a standard magneto). When you reduce the manifold pressure, the advance increases up to 34 degrees (or 39 degrees if you have the jumper wire removed). They won't share the actual tables with you, but if you call Brad on the phone he'll give you a rough estimate of what it's doing at various points in the MP/RPM spectrum. (IMHO I think they should share the tables, but that's another issue...)

You can find more information here. Read the "Ignition Dynamics II" article. http://cafefoundation.org/v2/research_reports.php
 
Forgot-

Steve Sampson said:
Thanks for the various replies. Perhaps I am expecting too much, but I would like in principal to be able to climb with everything forward for as long as I want and not go above 400F.

If I pull the rpm back to 2500 I probably can, so it would seem I am not so different to you Geoff. (Where I am different is I do have the standard small cooler and wish there was a smaller one because my only problem there is low oil temps.)

RV7guy - your climb rate of 1200fpm appears quite conservative. That must be giving you quite a high airspeed even at gross? If I pulled the rpm back to 2600 I think I could match that although I would be quite fast.

Thanks again,
Click to expand...

I forgot to mention, I climb at 120kias and adjust everything for that.
 
Steve,

I wouldn't be expecting to be able to climb full forward except for a timed air test.

I would normally climb at 25/25, leaning for altitude - that is what I've always used on medium and big non turbo Lycomings for years.

If you want to go full power, you need to keep the speed up and get the air flowing or you will start to cook the back cylinders at least.

As Scotty said - "ye canna change the laws of physics Captain" :D
 
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