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prkaye

Well Known Member
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.

23sgefq.jpg


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!

2u6zcbn.jpg


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?
 
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Not to worry

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.
 
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.
 
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.
 
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.
 
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
Click to expand...

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?
 
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.
 
More data needed

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.
 
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
 
videobobk said:
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
Click to expand...

+1 my O-320 is just like Bob's
 
There is so much learning that goes along with owning/operating a homebuilt aircraft even years after the build is finished. What a great hobby.
Thanks for your comments on this!
 
The MA4-5 used on the O-360 has an "economizer valve" that enriches the mixture just before full throttle. The MA-4 or MA-4SPA on an O-320 does not have this enrichment circuit. Trying to run LOP with a carbureted engine at high power settings rarely works due to uneven mixture distribution, which is what you are experiencing.
 
I have always wondered why they are called "Economizer" valve, when in fact they enrichen the mixture, which uses more fuel.

I think the idea is to avoid high-power operation at too lean of a mixture.

But why the name?

I'm bettin' somebody on this forum knows................:confused:
 
Norman CYYJ said:
Also at wot you are running on the enriching jets and backing off cuts them out.
Click to expand...

I just rebuilt my MA-4SPA and have reviewed the diagrams at length. I have not seen an Enriching jet or any jet/circuit that would be considered to be activated by a WOT condition on the MA-4. Could you please enlighten us on this jet and it's behavior?

Larry
 
Pete, you're just looking backwards. You get an "economy mixture" when you back off and extra rich at fullpower for cooling/detonation margin.
 
I think I've been looking backward all my life! :p

Still seems like a misnomer to me...........
 
It is not an Economizer valve, it is an economizer circuit. It is a channel and hole into the carb throat. It is a modified/engineered bowl vent. Fuel is metered through the main jet largely on the principle of pressure differential between the atmospheric pressure and the lower pressure in the venturi. This economizer circuit slightly modifes or dampens the atmospheric pressure experienced in the bowl and therefore modifies fuel delivery through main jet. It is an Engineers way of tweaking fuel delivery in a less than pure linear way as with a straight bowl vent. Its impact is throughout the flow range in the main jet (not idle or transition circuit), but allows for a different way to balance the jet size to full delivery.

I don't know the engineering or design of the MA-4, but I can assure you it is not something that is only impacted at WOT. It allows for a mixture that tapers more agressively from leaner at middle vacuum to richer at no vacuum (WOT). This allows a more economical mid-range (lower flow) without sacrificing the rich high end (high MP in our case). Hence the term Economizer circuit. Automotive carbs use other methods to achieve this, but the economizer circuit is very simple.

Larry
 
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Phil,

As with others, I chalk my similar experiences up to the somewhat normal but very poor fuel distribution of carbs. I wish I'd gone with fuel injection. That said, I've found that this poor fuel distribution can help manage CHTs which, based on your other recent threads, seems to be something you've been fighting as well.

For example, because of the poor distribution, at WOT my front two CHTs get hot on climb out but the back two stay relatively cool. If, however, I pull the throttle back at 1000AGL and continue climbing at, say 75% rather than full power, my back two CHTS heat up but my front two cool down. By periodically switching b/w WOT and throttle out a bit I can jockey the CHTs to (almost always) stay below 400F. I know this is not the ideal way to have to keep CHTs low, and it certainly reduces my climb rate, but until I figure out the source of my higher-than-desired CHTs, it seems to work.

Hope this helps.

PS, good to hear your experience with louvers. They, or cowl flaps, are likely in my future.
 
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