there was a hundreds of Novas engines and we will nevr find out how many blew up, but even if it was 20 out 100 it is only 20% failure, but 2 Sirios out of 6 is about 33 %. I think no need extra explanation.
To change head clearance from 0.0175 to 0.029 is realy huge difference ( about 0.025 cc head volume increase), I agree with that. But I am very certain that detonation came mainly not from compression ratio. If I will be asked what to do first, I will sugest to glue sleeve to crancase. I explain many times what happening with AAc sets and will do it again. Sleeve sitting in all Sirio engine pretty free and during the work sleeve has micro vibration and create a lot of microscopic debry. That debry goes to combustion chamber and light up during the combustion process and create a lot of temperature, which is creating abnormal detonation. I will be agree to slow down detonation if you change head volume on 0.012, but if it was reqired to rasise further, for me meaning-it wasn't compression ratio. It is my assuming and I will start from it. I also can sugest to use Ferrocene in this kind of case, it will remove detonation completely, but it will keep combustion speed at the same rate. Again if it is detonatin cause by compression ratio-it is easy solve by couple different ways, but if detonation create by other issues-it will not help.