Here's some really good stuff from KRnet lurker Jeff Scott: Sent: Wednesday, March 17, 2004 10:21 AM Subject: Octane, Mogas, etc
> Mark, > > Excellent write up and article reference regarding octane, Avgas, and > Mogas. As one might expect, I've done a fair amount of testing with fuels > over the years. For the most part, the I've never been able to tell the > difference in performance using 80/87, mogas or 100LL fuels when running > them in A-65, C-85, O-290, O-300, O320 (150 HP) or Franklin 150. The > biggest caveat to using Mogas in aircraft is that they tend to sit. When > mogas sits, it leaves a coat of varnish. > > When I worked as a mechanic in a snowmobile and motorcycle shop it was > inevitable in the fall that I would be vatting snowmobile carbs that were > gummed up from sitting since the previous spring, and in the spring would > be vatting motorcycle carbs that had been sitting since fall. The only way > to keep them clean was to start them regularly regardless of the > season. It would be noticeable in that the fine orifices for the idle > circuits would be clogged, so they wouldn't idle. However, on tear down > the main jet orifices were almost always at least partially blocked due to > the varnish from the fuel. If one was willing to run their bike or sled a > little lean for a while, the washing action of the fuel moving through the > orifices would eventually wash away the varnish provided that there was any > flow through the orifice. If it was completely blocked, the carb almost > always had to be torn down and vatted. I have helped to resurrect many > planes that have sat for years with fuel in the carb, but have never seen > any sign of varnishing in the carbs from Avgas. > > While I have used Mogas extensively in some of my planes, I have converted > both of my current planes to 100LL only engines. I don't care for the > smell or the handling problems of dealing with Mogas, but that's my > choice. If I was on a tighter budget, the smell might suddenly become > tolerable. :o) > > Ok, here's another side to the coin. We found that some VW engines > converted for aircraft use and run on a diet of straight diet of 100LL had > a tendency to scratch or lightly score the cylinders after 100 hours or > so. They wouldn't do that running on mogas, but since mogas isn't > available at most airports, we weren't able to provide them with a diet of > straight mogas. The conclusion we reached is that the deposits on the > pistons from running 100LL are a bit thicker than what was left by > mogas. The deposits continuously build and slough away, but the pistons in > the VW engines apparently didn't have enough clearance around the sides > above the top ring for the 100LL deposits to build and slough. We were > able to cure this by taking a flycutter and triming away .020 of the > outside of the piston from the top to the top ring land. The proof was in > David Roe's half VW powered Hummelbird, which had the two cylinders > replaced twice in the first 200 hours due to light scoring. After the > piston mod it ran for 900 hours until the case cracked. When removed, the > cylinders were in excellent condition and could easily have been honed and > used again. We also used a local Dragonfly and my VW powered Avid Flyer as > test platforms for the same mods. The dragonfly is still going strong, > although I don't know how many hours. I sold my Avid Flyer with roughly 60 > hours on it and it hasn't flown since, so that one is inconclusive other > than to say that there was no sign of any problems in the first 60 > hours. The credit for the VW research should go primarily to Doug Reid of > Santa Fe for doing so many engine teardowns and close examinations to > understand the problems we were seeing. I was acting as a tech counselor > and sounding board for Doug. > > > Jeff Scott > >
