Well, if you car is actually 8.7:1 compression ratio then higher octane gas won't help. The problem I have is that if the car has a stock B18 engine then the compression was somthing like 11:1, the later B20B's had 10.8:1 compression ratio.
I had never heard of any early Volvo's with a low compression motor until the B20F came on the scene for the'71 model year where it dipped to ~8:7:1

I always put in 103 oct Sunoco CAM2 fuel or 110 oct Airplane fuel mixed 1:4 with the best grade auto gasoline I could find. That was a modified '68 144S with a 11.5:1 CR B20B. Sounded like a piper cub airplane, and it REALLY made a difference!
Advanced timing can place the intake stroke of the piston farther ahead of the opening of the cam lobe to allow more immediate entry of air/fuel mix to enter the combustion chamber. This is optimized by the cam lobe profile, duration and overlap with the exhaust valves.
I ran my '68 at 23.5 degrees BTD, and R grind cam, and it flew!
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'89 245 Sportwagon, '04 V70 2.5T Sportwagon