Help interpret my Ignition Stress Test

Following the advice of Mike Busch from a Jan 2012 Sport Aviation column, and having recently completed my first condition inspection (at which time I cleaned and gapped all plugs), I ran an ignition stress test on cross-country last weekend. Unfortunately, I really don't have much idea how to interpret the results.

For reference, the CHT's are clustered at the bottom of the chart (with the temperature scale to the left) and the EHT's are clustered at the top of the chart (with the temperature scale to the right). Each mag was run individually for about a minute each with a minute in between to allow things to restabilize. My power setting was about 65%, as LOP as possible without getting engine roughness. In addition, FWIW, normal pre-takeoff run-up check of my Slick mags at 1800 rpm showed a similar 40-50 rpm drop on each mag.



1. What's likely going on with my #4 EGT when I do the LEFT MAG test? It spikes quite a bit higher relative to its norm than do the other EGTs.

2. What might account for no apparent EGT rise on #1 during the LEFT MAG test?

3. Most curious to me:
*On my LEFT MAG test my #1 and #3 CHT's drop while my #2 and #4 CHT'S stay the same
*On my RIGHT MAG test the opposite happens: #2 and #4 CHT drop while #1 and #3 continue their climb back to normal temps.
I would have thought my CHTs would all rise and fall together given that both mags fire plugs on all four cylinders. (???)

4. Any other observations one could make?

Thanks for any thoughts.

Stress test analysis

You didn't say if your engine is carbed or fuel injected, but I'll take a shot. The first thing to remember is normally when EGT's go up and CHT's go down, it's an ignition problem. (this is what you are checking for) When EGT's go up and CHT's go up, it's a fuel issue...plugged injector. No rise in EGT and no rise or drop in CHT...possible broken P-lead and mag is live.

It appears everything is working as it should, in general, EGT's are going up and CHT's are falling. This is what you want. As for the reason why they are not more synchronized, I don't have a good answer. If you have a carb, the mixture may be causing some weird readings. Does your engine monitor allow you to adjust the resolution on how often it samples the temps? The GRT EFIS goes into a finer setting when you set temps menu to "normalize" mode instead of "lean find". Maybe your engine monitor is in too coarse of a monitoring mode.

Just my .02 cents...

Jon D.

Oooppsss!

My statement that "When EGT's go up and CHT's go up, it's a fuel issue...plugged injector."....... is incorrect. I meant to say when EGT's go DOWN and CHT's go DOWN it is a fuel issue.

Sorry about that....

#4 EGT

I'll take a shot at question #1
You may have a stressed plug or mag wire from the LH mag. Try switching the plugs top to bottom on #4 and see what the EGT's do. If they go up on the RH mag and down on the LH, it's the plug; if the EGT's are the same as before on the LH and RH mag check, it's the wire. Also check the mag timing, as the rise and fall on the EGT's should be close when doing the mag check. Also as noted, do the check in normalize mode, cause you do not care what the absolute numbers are, just the change in values

CHT's

Question #3. Do you have the CHT's probes under the spark plugs? That could account for the CHT's numbers.

"You didn't say if your engine is carbed or fuel injected", but I'll take a shot with John D.
On a standard induction sump, the clocking of the intake tubes 1 thru 4 are in different positions to the head. On my Super Flow Bench, I also have a swirl meter that measures intake swirl by rpm at the base of the cylinder barrel. On the flow bench running intake numbers CFM at different valve lift measurements showed different swirl rpm. However, the interesting point was the clocking of the intake tubes in relation to the head in position of the sump. This did affect the swirl rpm quite a bit. Which in turn, would affect fuel atomization. Also on your mag secondary leads top/bottom, cris-crossed, or one mag fires top only and other bottom only?

Thanks for the replies. I've been really busy/gone this week.

barryrv10 said:

Question #3. Do you have the CHT's probes under the spark plugs? That could account for the CHT's numbers.

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I'm not sure what you mean here. My CHT probes go in the bottom of each cylinder just inboard of the spark plug holes.

G3i Ignition said:

"You didn't say if your engine is carbed or fuel injected", but I'll take a shot with John D.
....
Also on your mag secondary leads top/bottom, cris-crossed, or one mag fires top only and other bottom only?

Click to expand...

Sorry, I have "0-320" in my signature line but should have also put that I'm carbureted in the main body of my question.

Each mag fires two bottom plugs on one side (e.g. bottom for #1 and #3) and two top plugs on the opposite side (e.g. top for #2 and #4).

From the above replies, it sounds like more or less everything is pretty much OK, though I will try to get my mag timing checked and check on each plug again. However, I'm still very curious: Why does running on one mag causes the CHTs on one side to go down and running on the other mag causes the CHTs on the other side to go down? The drops are of approximately the same magnitude on each side, but never do both sides drop at once. CHTs for #1 and #3 drop when one mag is turned off and CHTs for #2 and #4 when the other mag is turned off. Since both mags fire both sides, I would think all four CHT's would go down simultaneously anytime I run on only one mag. ????

Thanks again for the education.

I am looking at your other thread about the imbalance of the lean limit, so I looked for all your posts to get more information on ignition system, engine provenance etc.

Just wondering about ignition characteristics here and if you got to the bottom of this?

In the middle of wiring myself and it could be easy to mismatch actual cylinder to the data logging headers. Have you checked this?

Also, have you rechecked the mags to ensure 1. that wiring is going to the correct plug? and 2. that the timing was synchronized between the mags. Timing probably not the issue, but it will be near impossible to troubleshoot the engine without good data. You may be dealing with multiple issues potentially, ignition, data cross wiring, and mixture imbalance.

Before this is over, your engine may need professional intervention.

Hi Bill.

Thanks for the input. After that post I had Mike Busch's Saavy Analysis Pro program do an analysis on the ignition system and they found nothing amiss, though they did idenify the likely poor mixture distribution and/or induction leaks. Still, will be good to go and and confirm I have the leads going the right place. If I remember correctly(?), the ignition harness caps will only go on the mags if they are oriented the right way and each of the leads was marked T1, T2, B1, B2 etc. and that's how I hooked them up.

Going on vacation for a week or so and then sending the prop in for a repitch so it may be a few weeks before I can run some more tests.

Thanks again.

Was going to start a new thread but I found this one, and figured I'd glom onto it. How does this look to the brain trust? I have a P-Mag running the A-Curve on the bottom plugs, LSE Plasma 3 on the top plugs. All plugs auto plugs. Cyl #1 Red; #2 Brown; #3 Yellow; #4 Purple


BTW, Mike Busch explains (at https://www.savvyanalysis.com/articles/in-flight-diagnostics) that "If your ignition system is healthy, when you switch to single-magneto operation you should see all EGT bars rise by 50°F to 100°F. They may not all rise the same amount; in fact, it's perfectly normal to see even-numbered cylinders rise more than odd-numbered ones (or vice-versa). What's important is that all EGT bars rise, and that all remain fairly stable at their elevated values. You will feel a small but perceptible loss of power during single-mag operation, but the engine should continue to run smoothly without uncomfortable roughness. (The engine will always run slightly less smooth on one magneto than on two, but roughness sufficient to get your non-pilot passengers to ask "what's wrong with the engine" is definitely too much")!

If I had had a passenger with me, they probably wouldn't have liked it.

Noah said:

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It looks to me like the P-mag is the more efficient ignition of the two. The reason I say this is because the EGT's with just the P-mag firing is lower than the LSE. Meaning that more of the fuel-air charge is burning in the cylinder than in the exhaust after the exhaust valve opens.

If I'm correct here, then your CHT's should show more of a drop when running on just the LSE than when running on just the P-mag.

N941WR said:

It looks to me like the P-mag is the more efficient ignition of the two.

Click to expand...

Yeah, could be that, or could be that the LSE timing lags the P-Mag timing, I would think.

CHTs fall about 20 degrees when operating on the LSE only, so yes, the mixture is ignited late and more of the charge is burning in the exhaust rather than in the cylinder, raising EGT and lowering CHT. There is no change in CHTs when operating on the P-Mag only. Maybe I need to check my LSE direct crank sensor to see if the board holding the sensor shifted.

Noah,

Different timing and advance curves could account for the difference.

If you remove the jimper on the Pmag, forcing it to the B curve, you should see even lower EGTs and higher CHTs when running on just the Pmag.