In a message dated 3/27/2002  at 7:34:17 AM Pacific Standard Time, wdodson@bak.rr.com writes:

<<<...In todays' mail I recieved Lycoming Service Instruction No. 1070M.  It deals
with aviation gasoline fuel and octane ratings adequate for Lycoming
engines.  It is not a pretty sight when one reads between the lines.  What I
see is the demise of aviation gasoline as we know it...>>>

Walter:
I was browsing through the LML postings I have saved, and ran across this one from a month ago, to which I had intended to respond, but just forgot.

Your observations about the imminent demise of 100LL and the problems it will cause for operators of high-output air-cooled engines are indeed correct, and could become a significant problem for domestic operators sooner than we might wish.

The requirement for fuel with lead content of 100 "LowLead" (which contains four times the TEL of the highest-octane autogas ever available, Sunoco-260) is established by the certification requirements for detonation margin in the worst case (kind of like a sustained hot-day climb out of Death Valley {or Phoenix} at Vx).

Detonation margin is affected by a variety of factors, including inlet air temperature, peak cylinder pressure, rate of temperature rise, peak and mean combustion chamber surface temperatures, etc.

One of the major factors is chamber surface temperature, which, for a given heat input (power) is closely related to the heat rejection capability of the cylinder head system. Clearly, the heat rejection is most severely limited during max power operation in min-cooling situations (the hot-day climb at Vx).

The detonation margins can be improved by a variety of methods, one of which is to use fuel having a very high effective octane rating (detonation resistance). But if it is necessary to operate a given engine on a fuel with reduced detonation resistance (93 autogas compared to 100LL, for example), one of the most straightforward methods to avoid engine damage is to limit the maximum allowable manifold pressure (ie: derate the engine). That maximum MAP is established by careful testing in a tightly-controlled environment.

That derating method could be used to recertify existing aircraft engines for use with a lower-octane fuel. It MIGHT not be a terrible problem for your aircraft, but consider the implications on a typical cabin-class "light" twin (C-340, C-421, Piper P-Navajo, Aero-Commander 685, for example) in which:

(a) the air-cooled engines are turbocharged to very high power outputs (severely challenging the structural as well as heat-rejection capacities of the engine) and
(b) the ability of those aircraft to achieve the required minimum single-engine climb rate at gross weight depends on the engines producing the power at which they were certified.

A reduction of that certified maximum power (by mandated derating) could easily transfer many of those (and other) aircraft into the (limited usefulness) category of "full-fuel, zero-person" aircraft.

<<<...In North America we are probably safe for some years to come ...>>>

Maybe, maybe not. There's more info on this subject at:

      http://www.epi-eng.com/ac-v8.htm

Jack Kane
EPI, inc.