TCDS
The
Type Certificate Data Sheet for the certificated version of the engine we use has the Octane the engine was certificated on. If you look at the Superior Vantage 360 engine (certificated clone of the Lycoming 360 with same bore, stroke, and compression ratio)
TCDS you will find that some versions were certificated on 100LL and also on auto fuel. (ASTM D910, Grade 100, 100LL or 100VLL ASTM D7547; [Motor Gasoline (R+M/2) (See Note 7)] ASTM D4814, Min Octane 91 (no alcohol)) [My comments: The See Note 7 is a copy paste and I believe that it is a typo and should be Note 6 as I could not find a Note 7 in the TCDS.]
If you check parts manuals for the Lycoming parallel valve 360 and 540, you will find they use the same cylinder assemblies.
The Type Certificate Data Sheet
E-274 covers many of the 160 HP Lycoming 320 engines (8.5:1 compression). The carb versions of most of these engines were certificated with Aviation Gasoline 91/96.
When the FAA gets around to approving an unleaded replacement for 100LL, many of us flying RVs will not have an issue using it with the stock or clone Lycoming engines that Van's Aircraft Recommends. Those that ventured off the beaten path will need to do some work on their own to figure out how to get back on the beaten path were most of us are who follow the recommendations of Van's Aircraft are located.
My understand of the unleaded aviation gasoline replacement problem is that the turbocharged and supercharged engines are the ones that have the highest octane need. Finding a fuel that will satisfy the needs of the turbocharged and supercharged engines has been the biggest problem.
The bore on the Lycoming parallel valve 320, 360, and 540 are the same. The 8.5:1 pistons and rings are the same part number on most of the engines. The 320-H2AD engine uses 9:1 pistons. The 150 HP 320 uses a 7:1 piston that ran on the now discontinued 80/87 aviation fuel. IF fuel octane becomes an issue in the future, most of our parallel valve 320, 360, and 540 engines could be retrofitted with 7:1 pistons and run on alcohol free auto fuel. All we would need to do is pull the cylinders, hone cylinders, install new piston and rings then reinstall with new gaskets. (o-ring) The lower compression 7:1 piston and engines are not as efficient as 8.5:1 engines and will burn more pounds of fuel per hour per horsepower produced so it is not an attractive option for me.
