ORIGINAL: hobbsy
I have been following this with great interest, I have broken in AB? engines completely wrong, before I knew better, completely right, and some not at all. I run several ABC type engines as Diesels which according to some operate too cool for long ABC engine life. Dub Jett says the engines have a certain amount of ability to adjust to operating at differing temperatures, if this were not true you could not fly at part throttle to just cruise or land without damaging the engine. I believe Jett also recommends starting his engines 1/2 turn rich from your normal run setting to avoid shock heating the engine, then let the engine run a few seconds to stabilize then reset to your normal setting and fly. Bob Brassel can correct me if I'm wrong here. I think I read that when Dar was challenging Bob about their breakin and start up procedure.
Yeah. thats about right.
Although on the R/C sport engines, I rarely open the needle to start the engine - just start it at high-idle, let it warm up 20-seconds or so (about as long as it takes to put the starter and battery back in the box, and move to a safe spot behind the engine), then slowly advance to full power. Not really different than Id start any other engine.
(the racing engines are a slightly differnt story - not a good comparison toward the sport engines)
The aac/abc engines can and do adjust for varying rpm/temperature/fuel/ etc. Otherwise they would not throttle well. A well designed engine of this type will keep the relative fit and temperature of both the pistion and sleeve in proper balance.
The break in has more to do with achieving an optimum fit at max power setting. My proceedure for AAC/ABC involves running an engine with an undersized prop, up to the RPM I wish it to work at when I am done. Break-in involves running it at that RPM. The resulting mixture to keep at that rpm is always a bit on the rich side. After a few tanks (15 min or so) I switch to the flying prop, and finish setting the low end and then find the peak of the high end needle. Its ready to fly. Using the throttle during break-in is ok. Usually that very first run is the tough one - but Dub does those on the bench before anyone ever sees and engine from him.
Yes, you can run the engine REAL fat (4 cycleish), but it all that does is burn fuel... does nothing to improve the fit or operation of the engine at that point. With a brand new AAC/ABC engine, it simply will not run 'that' rich. Kinda self-determining factor there. The way the Jett engines (and most modern abc type engines) are constructed, the fit is pre-determined at the factory. The break-in period provides that final fitting.
The Ro-Jett CL engines are different. Different taper, materials, timing - plus they are designed to run at constant rpm. They tend to be fit tight when delivered, and require quite a bit of time on the test stand before flying - but then they run flawless forever.
Bottom line, is if you follow the mfg's break-in proceedure, it is unlikely you will have any trouble.