Denny, The Continental C-75 engines were originally certified on 73 octane unleaded auto gasoline....and I believe will perform as well on auto fuel as any. There is the question of vapor pressure problems --at higher altitudes, tho EAA tests show no problems, if the fuel meets ASTM standards as specified in the STC. Lead supposedly lubricates the valve faces, preventing valve face
errosion. It also increases the octane rating of the fuel --(makes it burn ar a controled rate as opposed to exploding). With the low compression engines, the octane increase in not of concern, and some would suggest may be a problem from "longer burn rates". I think this is a misconception about hi-octane vs low-octane. Neither burns hotter, nor has different BTU/lb of fuel. The hi octane fuel simply doesn't explode (detonate), but burns in a controlled manner at high cylinder pressures (high compression ratios and high temperatures), where lower octane fuels will detonate (explode). If the fuel detonates, there will be resulting damage to the engine internmal parts....like blowing off a cylinder head, "hole-ing" a piston, etc...not good. Continental Overhaul Manual directs one to run the overhauled engine with leaded fuel in the test cell, but if storing the engine, finish the run with unleaded fuel to prevent corrosion. Page 75, Section 17-6 Preservation. "If the engine is not to be installed in an aircraft and placed into service immediately, the last 15 minutes of operation should be used to circulate a corrosion-preventive oil mixture ( suitable for light operation). This will be an additional period, since the engine must be stoppped to change oil. During the same period, unleaded gasoline should be supplied to the carburetor." Here are some points to consider: Potential Pluses for Auto Fuel: *Lower cost *no problem with lead foulding of plugs and resulting sticking valves, etc... *less air pollution (Lead salts) Potential Negatives for using Auto Fuel: *Difficult to store, handle, and transfer fuel - contaminated containers. *Potentially more dangerous as to spills and fire --also environmental spill danger *Auto fuel has a a shorter shelf life than av gas.... more likely to grow organisms. *FBO concerns If you fly frequently, then having fresh auto gas in the tank is no problem. If you fly infrequently, this may become a problem ---just like the fuel in your lawnmower, after a long cold winter, it seems to age fairly quickly. I used Amoco unleaded for years, with excellent results -- then the coupe was not flown for a long period. I developed a foul odor and an algae growth in the tanks. Even tho I frequently drained the sumps, water still collects from condensation. I began to drain "green stuff" --looks like algae to me. I havent had it tested for algae, but algae does grow in fuel and especially in jet fuel. An algacide like "Prist" is added to jet fuels to prevent the growth. Maybe I got a contaminated dose of fuel....Amoco was not interested in evaluating the stuff, even tho I sent it to them. The response was ---Hummmmm, interesting. They did return the pictures and sample. After cleaning the tanks, and refueling with av gas, I have had no further problems. ( I think the Algae likes to eat up copper. I utilized the old fuel in my autos, and lost the fuel gauge almost immediately in both cars.....Co-incidence ???) Recently, there was an article about auto fuel in one of the aviation mags---pointing out theat the shelf life of auto fuel was about as year; while av fuel was maybe twice that time.... I don't know if that is correct, or not.....but.... So Fly the coupe every week. Use auto fuel for less "excessive lead" problems, at an inconvenience of refueling and lower fuel costs. If flying infrequently, one might want to choose av gas - or a mixture of av gas and auto fuel. Some pilots Swear by Auto fuel, some Swear at Auto fuel..... The STC is available from EAA for ? about $45.00. Check the EAA site: <A HREF="http://www.eaa.org";>Experimantal Aviation Association</A> Hope this is helpful. Fly Safe - Have Fun Harry Francis N-93530 Blacksburg, VA
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