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PaulR

Well Known Member
Yesterday, while just out enjoying my new 9A and flying off some hours, I tried something I hadn't ever done. I was at about 5000 DA and just cruising along at about 2400 RPM. I've been kind of battling what I consider high CHT's during a climb and was watching them in cruise. All were between 360 and 380. # 3 is always the hottest.

I have a Slick on the left and a Pmag on the right. I reached down and shut the Slick off and ALL the CHT's cooled off by at least 15 degrees and #3 by at least 20. I turned the Slick back on and they climbed back to where they were previously. So I shut the Pmag off and they didn't change at all. Stayed at the hotter temps. The Slick is timed at 25BTDC and I don't know what the Pmag's timing is at that rpm and Manifold Pressure.

What would make the CHT cooler on the Pmag and hotter on the Slick? It just seemed odd to me.
 
Paul,

When you turned the Slick mag off in flight, you're losing power, just like it does during the mag check on the ground. Since you're getting an incomplete burn of the fuel/air charge due to only one plug firing, you will of course be running cooler. However, with the P-mag the effect is less because of the advance and hotter spark. It almost sounds like you have them wired backwards. Is this possible?
 
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Sounds to me like the Slick is firing well before the P mag.
Slick off, timing is retarded, less power and lower CHT.
P mag off, not much happens because the Slick has fired so much earlier that the burn front has already passed by the P mag plugs.
 
Interesting theories. Paul, I think I see from previous posts you're running a Catto prop and carburetor - is this correct? What were the respective RPM losses when you switched to just the Pmag, and then just the Slick? If the timing theory is correct and the Slick is way out ahead of the Pmag, I would think the RPM drop would have been greater with just the Pmag running.
 
If The Slick timing were advanced to far, this would show up on an ignition drop ck during run-up. Since the P-mag is usually advanced beyond 25BTDC @ this setting. With the EI switched off there would be a smaller RPM drop running on the mag. Running on one ignition in flight will usally drop the CHT and increase the EGT due to the incomplete burn if ignition system is set up properly.
 
Well they (switches) aren't wired incorrectly. Timing on the Slick was 25BTDC just 36.1 hours ago.:) I don't know what the Pmag is at this setting but like Thomas said, I would have assumed that CHT's would drop running on either one by itself.

Mag drops @ 1750 are about 60 with Pmag grounded and about 40 with the Slick grounded. Catto and Carb confirmed.
 
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Did the pmag actually go off? If you turn the pmag off it runs on its own steam, so did you actually ground it out?

You would see an egt rise and cht drop on all cylinders.

Interesting note, all the CHT problems (higher than normal) I see are those with one mag and one EI, and I have found it is always from too much advance that is not actually achieving much.

Even on the ground, take off, in cruise. There is a tell tale sign, lower than expected egt.

Just food for thought.

Paul, Have you a data file you can end me?
 
Data File

David,
I haven't down loaded it yet, but will tonite and email it to you. I know the Pmag was grounded and not just turned off. Same way I do a mag check during run-up. It baffles me a little that the cht would go down while running on the pmag alone and UP while running on the Slick alone.
 
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