strange behaviour in cruise - explanation?

[prkaye]

I was playing around with power settings in cruise at 8000ft, and encountered a really strange behaviour in my CHTs. Look at the first screenshot below. This is in cruise, leaned-out, with the throttle just a bit backed-off, say about 1/2" throttle movement back from WOT. CHTs are all very well balanced. This is after enlarging my jet and adding cowl louvers.



Now look what happens after I push the throttle forward to WOT, changing nothing else. CHT on Cyl number 2 drops significantly, and CHT on Cyl 4 rises significantly! All of a sudden there is a 63 degree spread!



This is reproducible... I repeated the experiment several times, on two different flights. With WOT leaned out in cruise I get that massive spread between Cyl 2 and Cyl 4 CHT. I back the throttle off just a bit and they come right into line.
Any ideas what's going on here?

[N427EF]

What you see is called mixture maldistribution.
A slight deflection of your butterfly valve can cause enough "twirl"
up the intake pipes to unevenly distribute the mixture to each cylinder.
This is more common in carbureted engines but is still noticeable with fuel injected engines. Of course it is only air distribution in a FI engine.
I can achieve a near perfectly even EGT and CHT spread on full take off power
but at 8000 feet and full throttle with the prop backed off, I see a spread of almost 100 on EGT and maybe 20 to 30 on CHTs.
If I back the throttle off just a bit the EGTs and CHTs even out again, much like yours do.
I am not obsessed with chasing even numbers on CHTs and much less on EGTs
but looking more for consistent readings from flight to flight.

[Mike H]

You are too lean for that power setting. You cannot set mixture for a particular power setting then go full throttle without resetting the mixture and expect good results. Looks to me that cylinders #1 #2 and #3 are just past peak on the LOP side, #4 is very close to peak EGT.

[Dbro172]

Mine is pretty much the opposite of yours... Pretty even ant 65% power and at WOT but very offset at 75% power! rear cylinders much hotter that fronts.

Just goes to show you.

[crabandy]

Yup, same for my O-360. Full throttle gives me a pretty unbalanced mixture distribution, just like your example (I assume you're ROP) the hottest CHT is also the leanest cylinder (highest EGT) and the coolest CHT is the richest cylinder (lowest EGT).
If I back out the throttle a tad my mixture distribution evens out with EGT's +/- 30* and the CHT's even out to within 10-15 degrees. Generally at cruise altitude this puts me less than 65% power and I run LOP.

[prkaye]

Well I'm relieved to hear I'm not the only one experiencing this.

A slight deflection of your butterfly valve can cause enough "twirl" up the intake pipes to unevenly distribute the mixture to each cylinder

So you're saying that at WOT the butterfly valve gets in just the right position to cause this phenomenon, explaining the sudden and drastic change in mixture distribution with a very small change in throttle position?

[Norman CYYJ]

Also at wot you are running on the enriching jets and backing off cuts them out.

[N427EF]

Ordinarily one would expect the most even distribution with a fully open
butterfly valve and almost undisturbed mixture flow. There is still quite a ways to go for the mixture to reach the intake valves and a lot can happen to the airflow along the way.
A slightly deflected butterfly valve can change the flow just enough to make the distribution nearly perfect or worse. Every time you change throttle settings
you change the pressure or vacuum "manifold vacuum", we call it manifold pressure, you also change the pattern of mixture flow.
Trying to achieve perfect mixture distribution under all power settings in a carbureted engine is nearly impossible and difficult at best in a fuel injected engine.
We can perfectly match fuel injectors and flow match the cylinders but there is
still the butterly valve to disturb the intake airflow.
Racers go to great length to "tune" intake pipes as well as exhaust pipes to achieve the most even fuel/air distribution to each cylinder.
Rich or lean mixture is the least of the variables impacting maldistribution
of fuel/air mixture.

[BobTurner]

To make sense of this you need to repeat the experiment, but this time slowly lean from well rich and note the order in which each cylinder reaches peak EGT. Do this for wide open throttle, and throttle slightly closed. As someone has already noted you may be engaging tne 'auto rich' feature at wot so lean for each case.
TCM is different from Lycoming but when I had a 182 with TCM O-470 this effect was very apparent. At wot the fuel-air did not mix well, and the fuel droplets tended not to make the 90 degree turn into cylinders 1 and 2; they ran lean. All the fuel droplets piled up at the end of the intake manifold, which was at #5 and 6; they ran rich. Slightly closing the throttle plate induced enough turbulence to better mix the fuel and air, and evened out the EGTs (and subsequently the CHTs). By evening out I mean the EGTs peaked closer together.
On cold days a little carb heat had the same effect, by encouraging better fuel vaporization.

[videobobk]

On my O-320, I see much of the same thing, but a touch (maybe half way) of carb heat evens everything out nicely and lets me lean more without roughness. You might try a little carb heat and see what happens. There seems to be a couple of "sweet spots" where everything is lined up. They are high and low cruise for me. YMMV.

Bob