A question about mis-matched EGTs

[India Four Two]

I have been flying the same C182 glider towplane for the last three years. Recently we noticed some strange EGT behaviour. During runups, the EGTs of the even-numbered cylinders (left side) are indicating about 100º F cooler than the odd-numbered cylinders (right side). See the photos below showing the EGTs for cylinders 1 and 2 during a higher than normal RPM runup (2000 RPM).


https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/2000x1504/img_2363_406bc3220a15720e286220d16d8b4ffe4b7f7068.jpg
https://cimg8.ibsrv.net/gimg/pprune.org-vbulletin/2000x1504/img_2364_05a59032b233ef2563c2c0a3185669932e4bc81c.jpg

The engine is a P Ponk O-470-50, which is basically a carbureted O-520. The anomalous EGT behaviour goes away at full power.

The opinion amongst my club's tow pilots is that there must be uneven mixture distribution to either side of the engine. One suggestion is that there might be an induction leak on the right side, leading to a leaner mixture and hence higher EGTs.

Has anyone seen this kind of behaviour before?

[Big Pistons Forever]

Mixture distribution on carbureted engines is always going to be poor so I basically ignore EGT readings during a runup. During takeoff and climbs the big issue is keeping an eye on the CHT’s for all cylinders. A spike in CHT’s needs to be attended to immediately. However that being said if something suddenly changes the engine is probably trying to tell you something

One possibility is that the induction cross over tube at the front of the engine has a loose coupling. The other thing to look at is the baffling on the right side

A check if you suspect an induction leak is too lean the engine during a high power runup until it dies, if the EGT spread worsened as you leaned then it would be worth checking the induction piping. The lower the MP the more an induction leak affects the engine.

[India Four Two]

BPF,

Thanks for your prompt reply. I'll try your runup procedure on Sunday and report back.

We keep a very close eye on CHTs, seeing as how we spend several minutes climbing at full power and then descending at low power for a similar time. Having an EDM is essential for managing the engine, particularly the "rate of cooling" mode used during descent.

On hot days in the summer, we often have to speed up the tow to maintain the CHT within limits. Luckily all modern gliders can tolerate quite high towing speeds, but the drawback is that the rate of climb is reduced.

[Pilot DAR]

My experience is the same as Big Pistons. My EGT's settle down in flight, though there are some power settings better than others at evening them out.

As for the induction cross tube on 470/520/550 engines, they are there to tune the induction volume. As you have not recently changed it for the wrong one, it could only be a leak which would introduce a change. However, if the induction has been apart and carelessly reassembled, with the wrong part, the mixture distribution will be horrible. There are three different sizes, and when we changed a 520 from being injected to carburetted, it required a lot of experimentation with each of the three sizes to get it running properly again.

In the mean time, don't worry about uneven EGT's, remember when we had no EGT, and didn't know to care?