The 103 engine has what can be descibed as a hemispherical (actually semi-hemispherical with a small quench area) combustion chamber. Such designs do not have good detonation resistance and have always been known as having an "octane appetite, but they are amenable to large valves and the angled valve layout yields good high rev volumetric efficiency for high specific output.

Also, there is no knock sensor as in modern cars. You can experiment around with lower grade fuels. If you hear knock, you might be able to reduce it by changing your driving habits. Since higher inlet air temperatures increase the octane requirement, you might find that lower octane fuel is satisfactory is cold weather, but not in hot weather.

I note that the newly announced C6 Corvette sports a 10.9:1 compression ratio; 93 PON fuel is recommended, but NOT REQUIRED. Knock on lower octane fuels will cause the spark to be retarded to below the knock point, so it won't make full power, particularly at lower revs where knock is usually most prevalent.

Duke