andrew lott

An interesting report and one which was largely discredited. It was a classic mistake of forming a theory and then going about proving it instead of the correct method of finding out all the facts then drawing a conclusion. Read John Deakins article in The Pelicans Perch. "Whyalla - Junk Science?" Seems the same culture still exists in the ATSB

This depends on how rich the mixture is. 50degF ROP will cause the highest CHTs. 80 - 100 deg F ROP will produce best power. The position of Peak Pressue relative to crankshaft rotation TDC is the big factor in CHTs. PP and its position is determined by the speed the cylinder charge burns and this depends on the fuel/air ratio. Lower and later PP can occur both LOP and ROP. The charge has to be very rich on the ROP side compared to the LOP side to get the same effect. Check the APS website for more detail.

Agree. Sadly you do not have that option in the Jab or most small aero engines of that group.

The peak pressure can be changed by fuel air mixture ratios eg Ratios that produce EGTs of 50degF LOP compared to 50degF ROP, result in a lowering of PP and a delaying of the peak several degrees After Top Dead Centre. The peak occurs ATDC anyway. I think it changes from 12 deg ATDC to 18deg ATDC with the mixture change (my figures may not be completely accurate but close) Have a look at the graph put up by Ultralight on pg. 1 which shows engine parameters relative to mixture control and the relationship of Internal Combustion Pressure to Cylinder Head Temperature

"Yes they may be but they have also had the benefit of 80 years of development. Yes they are heavy both in weight and in dollars!!!"

You are quite correct that all parameters have to be looked at, and in the Jabiru, cooling could be improved. For example the baffling around the cylinders of all air cooled mainstream aero engines and industrial engines is quite extensive and considered critical to cylinder cooling. The Jab doesn't have that level of cooling sophistication. A Jab engine is really pretty advanced in its power output. If you scaled its volume up to the equivalent of the Continental O-470 it would produce 50 more horsepower . This applies to both the 2200 and the 3300. Quite remarkable really. I don't agree with the Continental being a big heat sink. While it is larger, it has to get rid of much more heat than the smaller engine and once it is up to temp, it runs at the same temps as any other air cooled engine, be it a Jab, a Lycoming 540 or an 1820 Wright. CHTs result from a number of factors including also, the cooling arrangement, the mixture setting, the position of Peak Pressure in the cylinder (refer to APS info re that one.)

Yes. Have a JPI 700 with CHT & EGT on all cylinders, fuel flow, carb inlet temp, EGT difference,, oil temp, volts, and other fuel management (linked to GPS) functions. The unit is needed to run LOP which I do all the time in cruise. At max. power in the climb EGTs range from high 1200f/700c to 1470f/800c with very rich mixture. CHT is what I watch for. In cruise and leaned out to LOP, the EGTs could range between the 1400f and 1500f range. Because of the awful inlet system I have, it is not uncommon to have a difference in EGTs of 100 -110*f, however provided the engine is running smoothly, it does not matter. Carb inlet temps of 50*f/10*c is the optimum temp for mixture distribution between the cylinders. I use carb heat to get that temp if needed. If the engine is rough after leaning, applying the correct amount will smooth it up again. Remember that you have four or six single cylinder engines bolted to a common crankshaft. Unless it has tuned Gami style injectors, you are bound to get mixture distribution differences and hence differences in EGT readings. Any roughness or vibration is because one cylinder is not producing the same power as the others. When it is running smoothly, all the cylinders are producing about the same power. You can still have EGT differences with a perfectly smooth running engine. There always differences between the cylinders performance and indeed each cylinder. Personally I consider that as long as my engine is running smoothly, the CHTs are below 380f and the EGTs are close enough, then Ops are normal. The best info on engine operation is at the Advanced Pilot Seminars and there is one graph from them in an earlier post. Look up their website of the same name and there is a section that is open to the public. Also look up John Deakin, Pelicans Perch, as well as The savvy Aviator- Mike Busch. Those three sites have some remarkable reading on engine matters.

Seems like a lot of effort being made to even up some EGTs which in my humble opinion may not have any beneficial effect on the engine with the fuel management systems that you are stuck with. What is critical is the CHT which directly affects engine health and is not directly related to EGT. EGT has many variables so is useful mainly for establishing peak EGT for the purpose of leaning or richening your mixture. After over 3000hrs of running a big bore Continental with the worst induction system (carby) devised, I have found that EGTs are all over the place but have no measurable effect on the running etc. Mine regularly exhibits temps (and higher) to those you are concerned about but no bad effect so far. I religiously keep CHTs below 380f (193c) and the cylinders last. Cooling is critical.