ORIGINAL: downunder

I had to go back and read everything again and I didn't notice anything about running rich is bad...maybe there was some misinterpretation about getting the engine up to 200-240F for the running in process. However there's a very wide spread belief that running a new ABC type engine rich is bad for them and because of this belief no one takes the risk of running them rich so the belief perpetuates.

I've never really believed rich running would harm a new engine but I had no way of knowing if I was right or wrong until I managed to pick up a brand new ABC very cheap so I could consider it to be somewhat expendable. It had a very good pinch so I decided to run it extremely rich right form the start and monitor the pinch between runs to see what happened. Also between runs I'd take off the muffler to check as much of the piston as I could see.

I ran it for 45 minutes in a slobbering rich 4 stroke with a head temp of 145F and the pinch stayed exactly the same as from new. I stripped it down completely and the piston had virtually no sign it had even been run. The conrod however was running in beautifully . Curiously, although the piston itself looked brand new, the chrome liner was also running in beautifully. I'll keep this short because if anyone's interested in the full test with photos I have it in the engines forum at http://www.rcuniverse.com/forum/m_28..._1/key_/tm.htm

OK, so why didn't it ruin the piston because everyone knows an ABC has to be up to temp to expand the liner and relieve the pinch (which is an interference fit)? This took me some time to figure out and I can only come up with one answer. Our engines are cooled in 2 ways, one by airflow over the fins and the second by internal cooling from the evaporation of fuel in the crankcase. Now when you run rich (or very rich in this case) there's a lot more fuel to evaporate in the crankcase. The piston spends most of its time down in this fresh mixture so the inside of the piston is continually wet with raw fuel which then evaporates off it's inner surface including under the crown and cools it down. The upper part of the liner however only has contact with heat from combustion (and some heat from compression) so it runs hotter and expands away from the piston. Leaning out the mixture reduces the internal cooling of the piston allowing it to expand more.

Since doing this experiment I've changed my method of running in an ABC type engine. I start with 4 stroking rich to start the rod bedding in then gradually lean out to slowly build up heat in the piston and let it gradually adapt it's size to the liner. My son used this system in his car and it worked extremely well.

I'm not saying this is the only way to run in an ABC because most seem to survive the usual hot method quite well, basically the experiment was just to find out if rich was bad. It isn't.