Wes's Onex #89

[Direct C51]

Wes, I missed the Zenith carb part. I guess you don't really ever know how far from peak EGT you are, nor do you have any in-flight control anyhow.

The likelihood of the carb causing my engine harm is probably more than offset by my not being able to harm my engine by misusing the mixture control.

But you can always learn how to correctly use a mixture control... In all fairness, the Zenith is not likely to cause a whole lot of harm either. Best of luck finishing your phase 1. It sounds like it is running great!

[Gordon]

High EGT's.........?

I agree with some of the other posts Wes.......

...keep your CHT's within limits ....and the EGT's will take care of themselves.

I expect your CHT's will drop a bit now that your Hummel is starting to break-in.

Mine is running excellent with 28 hours on it.

Gordon........Hummel 2400....Onex C-IODB

[N190YX]

I believe it was Wes who wrote something about CHT affecting EGT. It is the reverse! The higher the EGT, the higher the CHT will be. As an engine is leaned, the EGT will rise until it reaches a peak temperature and then fall as the cylinder goes lean of peak. The corresponding CHT will drop more per reduction in EGT on the lean side than on the rich side. In other words, the CHT will go down a lot with running not that lean of peak, compared to the same temperature reduction rich of peak. As long as the CHT is within limits, the EGT doesn't matter (unless too rich causing fouled plugs and buildup on valve stems, or too lean with a resulting rough running engine). In the TCM certified engine world, we consider peak EGT down to around 60 degrees (not sure about the exact figure) rich of peak a "red box" to avoid operating the engine in. With balanced injectors, we run fuel injected TCM engines lean of peak in cruise with cool CHTs. Some pilots even climb lean of peak. All cylinder instrumentation (EGT and CHT) is required to operate lean of peak. For high density altitude takeoffs, leaning to the EGT you have on a sea level take off is the procedure, assuming the fuel system is set up according to factory specs.Typically 1,250 to 1,300 degrees for takeoff. With a fixed pitch propeller, high density altitude takeoffs should be made at best RPM or a little on the rich side of that but keep the CHTs within limits, with, if necessary a richer mixture and higher airspeed climb. Bottom line - to get good life out of your cylinders, never let the CHT go above limits. Take corrective action when the CHTs get near maximum temperature by increasing airspeed and a richer mixture. Steep climbs are fun but hard on the engine with less cooling air for the engine when it is working the hardest. I recommend that, after passing any obstacles after liftoff, always climb at the maximum rate of climb airspeed or higher airspeed.

[Direct C51]

Wes, I want to show you a picture of the data log on my engine so that you can really understand that absolute EGT values mean NOTHING. This is a snapshot from leaning my engine until all cylinders peak. The red boxes indicate when the cylinder has peaked. Disregard fuel flow as my reading is sporadic sometimes.

Going by absolute EGT, you would think my #5 cylinder (Lowest EGT) is really rich, and #3 and #4 (Highest EGTs) are really lean. But that is opposite of true. #5 peaks well before #3 and #4, indicating that it is actually a much leaner cylinder. I have a 72 degree spread between the highest and lowest peak EGT. Do you know what that means? NOTHING! It means absolutely nothing. In fact, I don't even fly around with the actual EGT numbers displayed in cruise. My EFIS has a setting that will show how many degrees from peak you are. See the second picture, this is how I fly. Degrees away from peak means something. Absolute EGT value does not.

I hope this helps make the point clear.

By the way, lean of peak on a carbureted 6 cylinder Sonex is possible. The second picture is LOP!

[WesRagle]

Hi Nate,

Thanks again for the explanation. I do understand. Whenever I look at a set of numbers I always try to learn something. When I look at those numbers I see that the cylinders all peak at very near the same mixture, just not at the same temperature. I would assume that six identical cylinders should produce the same power so the temperature variation is mainly attributable to instrumentation error. Don't know.

I will continue to think about what you have said/shown, the "Red Box", ..., and try to apply it to my situation. I will obviously be running rich of peak.

I was thinking back to years ago when I was transitioning to an RV-6A with an O-360 and a CS prop.
I was taught this:

1) Take off with fine pitch and full throttle.
2) As soon as you are established in the climb reduce power to 25 squared.
3) Use the throttle to maintain 25/25 during the climb.
4) At cruise altitude reduce power to 21 in hg and 2400 RPM.

It occurs to me that if I reduced power during climb to be kind to a purpose designed O-360 why shouldn't I be at least that kind to a VW conversion?

We'll see how this all works out.

Thanks Again,

Wes

[N190YX]

"I would assume that six identical cylinders should produce the same power so the temperature variation is mainly attributable to instrumentation error. Don't know." It is unlikely all cylinders get the same mixture, induction systems are not that good. When leaning, some cylinders will get to maximum EGT and start going lean of peak prior to others. For example, even with my finely balanced fuel injectors on a six cylinder fuel injected engine, there is a .3 gallons per hour difference between when the first cylinder hits maximum EGT to the last cylinder to do so and that is considered quite good (this is while cruising at 10 to 13 gallons per hours depending on the power setting). With carburetors the fuel distribution is rarely that good, it is difficult to impossible to operate a carburetor lean of peak, by the time the last cylinder goes lean of peak while leaning the engine, other cylinders are starting to miss fire from being too lean. Another factor in CHT is uneven cooling air flow past the cylinder, different cylinders get different volumes of cooling air. Bottom line, it is normal to see different EGTs and CHTs among the cylinders, so you need to control the CHT of the hottest cylinder. Which may well be a different cylinder in climb than in cruise. (True in my TCM engine)

[WesRagle]

Hi N190YX,

BTW, what's your real name?

Humor me a little here. Lets assume that the difference we are seeing in Nate's numbers are primarily due to instrumentation errors. To remove the majority of that error take a look at the first cylinders to peak (#1 and #2). Now slide across to the last cylinder to peak (#4). Notice that there is only an 11 Fahrenheit Deg. difference between when the first cylinder(s) peak and when the last cylinder peaks. That's pretty good. That's the difference I would attribute to imperfections in the induction system. Nate has already said his FF isn't that accurate so we can't get the "Gami Spread" but I'll bet it's not much.

With such a small spread between the first and last cylinder peaking it would appear that, as Nate said, he could run lean of peak without rough running on the leanest cylinder.

What do you think?

Edit: I just realized that the screen shot of the EFIS shows a 19 Deg spread while running lean of peak.

Wes

[builderflyer]

Wes, I want to show you a picture of the data log on my engine so that you can really understand that absolute EGT values mean NOTHING.

Going by absolute EGT, you would think my #5 cylinder (Lowest EGT) is really rich, and #3 and #4 (Highest EGTs) are really lean. But that is opposite of true. #5 peaks well before #3 and #4, indicating that it is actually a much leaner cylinder. I have a 72 degree spread between the highest and lowest peak EGT. Do you know what that means? NOTHING! It means absolutely nothing. In fact, I don't even fly around with the actual EGT numbers displayed in cruise. My EFIS has a setting that will show how many degrees from peak you are. See the second picture, this is how I fly. Degrees away from peak means something. Absolute EGT value does not.

By the statements contained above as well as those contained within your earlier messages with regards to absolute EGT values, it appears that you have accepted the Mike Busch concept of "mixture" theory that he applies to Continental and Lycoming engines as being applicable to your Jabiru 3300 engine as well. I'm not convinced that this is a good idea. The Jabiru factory is quite clear in providing absolute EGT values that are not to be exceeded for various power settings. Why? Because running a Jabiru exhaust valve at too high of an absolute EGT value for too long of a period of time has shown to contribute to the developmen of transverse hairline cracks in the valve stems and, ultimately, their failure. Roger Lewis of Power Engineering Consultancy, a prominent Jabiru engine expert in Europe, has in the past verified this phenomenon in the many engines he has torn down. He strongly recommends checking for valve stem cracks whenever an engine is opened up and especially if the engine has been operated with excessively high absolute EGT values in the past.

More than just my opinion,

Art,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,Jabiru 3300 #261

[N190YX]

1. That very small 19 degree EGT spread while lean of peak is really good, and the small difference from the first to the last cylinder to go lean of peak is also really good! I'm looking forward to operating a Jabiru 3300, hope mine will be that good. Aero Carb or ? I want what Nate has!
2. Thanks for writing about the difference between a Jabiru, and Lycomings and Continentals, to avoid high EGTs with the Jabiru due to possible valve stem cracking when subjected to high EGTs. Good to know!

Stan (N190YX name here), my Waiex serial number is 0190, but that N number is reserved by someone else now so I will not be using it.

[Direct C51]

Hey guys, sorry to burst your bubble, but I don’t fly a Jabiru.

I fly a CORVAIR!!!!!!!

I am absolutely astounded at how good the mixture distribution is. I did nothing to the engine itself besides the normal conversion. I’m running a Marvel Schebler MA-3 SPA. I did do a little modification on the intake manifold that comes with the conversion, but that’s it. It operates wonderfully lean of peak, as my data shows.