I've calculated that I need to increase my head gasket thickness by roughly 0.7mm to get down to 8.5:1 compression. Then I'll be able to run 91RON fuel, still have credible mileage, and a nice woolly running motor. A bit of extra heat will not be a worry, neither will abt 10% less BHP. I'll guess and say the OEM head gasket is abt 1mm thick, so 2 of them should more than do the job. Ayes/nays anyone???

Did you try the 91 gas yet without ripping off the head? Is it fuel injected or carbs? If it's fuel injected, you could probably run anything and it wouldn't matter much. 8.5 to 1 sounds pretty weak. What's the current compression?

LH 2.2 motor, head on, 91RON is in use, but I was hoping that I could retune the motor into a better burn. I fear that the existing 9.2:1 compression requires too much retard for the motor to run well. I am aware that the squish will be upset. Perhaps 9.0:1 is safe, But I don't know that. 96 RON is just too expensive to use lots of it on my budget. I believe that decompressing the motor will allow more advance and actuallly smooth the burn considerably. It may increase performance on this fuel too, by providing a longer, more effective thrust on the piston and a better long burn. My moddle diesel engine experience tells me an over compressed, over advanced motor revs less than a correctly tuned one, and even if you retard the ignition, it still doesn't run as well as a correctly compressed and timed motor. 0.025" is a practical extra gasket thickness, made in Aluminium will be best. I may have to buy a Dremel or similar to make it. I haven't seen the OEM, so I'm not sure what I getting into yet. There may even be a custom gasket making service somewhere I can afford.