We left part 1 of this tail with a repaired Cub after my friend in his Speedy RV-8 zoomed off to fetch parts to repair the Cub. I was off to the Copperstate to enjoy sunshine and airplanes. About 50 miles out of St Johns, AZ, I hear my buddy in the RV-8 call that he was returning to St Johns due to high engine temps. Hmm. Now that was odd as his engine has never run warm. I called and asked what was hot. He replied that his oil temp was indicating 280?. Wow! Now that's hot. I offered to return back to St Johns to help, but ultimately decided to proceed on to Casa Grande. Once on the ground in Casa Grande, I called and asked what had happened to his engine. He was unsure, but said that his oil temp topped out at 280? and his hottest CHT topped out at 500?. This engine typically runs with an oil temp of 180? and a CHT of 300 - 325?. I was concerned as this is an engine I built for him a number of years ago and I have never had an aircraft engine that I built fail. My initial diagnosis was a possible failed vernatherm, but that doesn't explain why the CHTs went so high. He also described lots of loud banging coming from the engine while it was overheating so bad.
I spent a lot of my weekend at the fly-in on the phone walking my friend through diagnostics and inspections on his engine. He drained the oil, pulled the inlet screen to check for metal, cut apart the oil filter and checked for metal, pulled the spark plugs to inspect the piston tops for damage, replaced the vernatherm, did a successful 30 minute test flight over the airport, cut apart the new oil filter to check for metal again, installed a third oil filter, then flew the plane home the next morning. The only anomaly noted was that two of his cylinders seemed a bit soft. After he got home, he ran a compression differential test and confirmed that the cylinder that had been the hottest (#3) was now showing 57/80 and the other back cylinder (#4) was now 65/80. The front two cylinders were still normal. This engine is a 200 HP angle Valve IO-360 Lycoming with 10:1 pistons that I built for him some 500+ hours ago. This engine has had chronic ignition issues with the single drive dual Bendix magneto ever since he finished the plane. When the mag failed again a few months ago, he cried uncle and gave up on it. So the magneto was replaced with dual EFI electronic ignition systems that would allow the ignition to dial in an additional 4? of timing advance at cruise throttle settings. He had dialed up the ignition timing to 26? with a max advance of 30? at lower altitudes. That is 1? more advance than called out in the manual with an additional 4? advance at low manifold pressures. Pretty conservative compared to my electronic ignition that tops out at 39? advance or others that run up to 41? advance. With the new ignition systems, he was also able to change over to NGK Automotive spark plugs. So, why did this engine suddenly overheat so badly, and how much damage did it do? I didn't get a chance to go look at the engine until the following weekend. My expectation was to see either hammering signatures or melted aluminum at the tops of the pistons and I was fully prepared to break the news to him that we would need to split the case for a full inspection. When I got there and started going through the engine, I found very little I could complain about. The rod bearings were still as tight as they were when I put the engine together several years ago. The wrist pins and rod bushings were still a snug fit. The one cylinder that had been the hottest showed some pre-ignition scarring in the lead deposits on the piston adjacent to the bottom spark plug, but no damage to the piston itself, and there was some very mild scoring in the cylinder that will disappear with a fresh honing. So what happened? Can't prove anything, but here's what I think happened. High cylinder pressures from the 10:1 pistons are a contributing factor. Some additional timing advance further increasing cylinder pressures is a second contributing factor. This problem only happened taking off from St Johns when the engine was heat soaked, so heat soaking the engine and lower altitude are also contributing factors (this was his first time below 6500'). Each of these factors contributed to additional heat at the cylinder head/spark plugs. When I looked at the timing curve for his electronic ignition system, it was bringing in the timing advance way too early and had the full additional advance rolled in while the engine was still above 75% power, although the timing numbers themselves were still pretty conservative even for a 10:1 compression engine, it still adds to the cylinder pressures and heat. Add to that a high energy ignition system that holds the spark through 30? of crank rotation during the firing cycle and there is a tremendous amount of heat loading at the spark plug tips. This engine should have been running a spark plug 2 heat ranges cooler than what was in there rather than the stock plugs intended for 7.0:1 to 8.5:1 compression engines. I don't think the engine ever went into detonation. I believe what happened is that it overheated the plug tips and went directly into pre-ignition on compression in #3 cylinder, and possibly in #4 cylinder. Running in pre-ignition (dieseling) for very long will often times melt holes through the pistons and usually causes burning of the exhaust valves. His pistons showed no damage other than some pre-ignition scarring in the lead deposits on top of the piston. That scraped off with the lead deposits. The scarring was right at the lower spark plug, which would be the hottest one. This was consistent with the banging noises the pilot described hearing coming from the engine. the two hottest cylinders do have some burning to the exhaust valves and seats. Dieseling/pre-ignition explains why the CHT went so high, but not the high oil temps and why didn't it melt the piston? I was really looking for an explanation for this. What I found is that this engine doesn't use the bottom of the rod to put a little squirt of oil into the opposing cylinder to cool the bottom of the pistons like my Continental. Instead, this engine has case mounted spray nozzles to spray a lot more oil than other engines at the bottom of the pistons. That accomplishes two things. It does a much better job of cooling the piston, and in this case kept the piston head cool enough to prevent melting. In the process of doing such a good job of cooling the piston, it also transferred all that excess heat into the oil. That's why his oil temp went to 280?F despite running two oil coolers. Most engines would not have survived that kind of abuse, let alone come out of it virtually unscathed other than some minor burning to the exhaust valves. Once again, I came away thoroughly impressed with just how rugged these Continental and Lycoming engines are built. The spark plugs in his engine are NGK BR8ES (heat range 8), which is the same plug shipped for a 7:1 compression Continental O-200. I recommended that he go to a heat range 10 plug (140 - 200?C colder heat range). If he starts building deposits on the spark plugs, then consider moving to a heat range 9 plug. Each heat range number makes a 70 - 100?C difference in the temperature of the spark plug tip during operation. The additional advance and the higher compression each adds roughly 70 - 100?C heat load to the spark plugs. A heat range of 10 should have been used in this engine due to it's higher compression and dialing in the additional advance. For some additional reading about spark plug heat ranges, heat transfer and spark plug pre-ignition issues, there is some really good data on the following web sites. <http://www.ngk.com/learning-center/article/207/i-have-slightly-modified-my-motor---do-i-need> <http://www.ngk.com.au/spark-plugs/technical-information/heat-range-explanation> <http://ngksparkplugs.com/tech_support/spark_plugs/index.asp> Jeff Scott Los Alamos, NM
