1. A higher RON that what the engine is designed for will NOT produce more power, the effects we feel is largely due to different additive packages for 95 and 97 at our pumps

2. No a car cannot adjust compression at all. Higher compression ratio requires higher RON because before the piston reaches top dead centre the air fuel mixture is already hotter than a lower compression engine. Take a very simplified example, two pistons traveling over the same distance 10cm in the cylinder. One has compression ratio of 10:1 and the other 14:1.

After 10cm travel the 10:1 compressed the air 8 times of nominal pressure, but the 14:1 would have compressed the mixture to 10 times of nominal pressure despite travelling same distance.

High compression = higher temperature = more likely to self ignite before spark from spark plug. Hence higher RON needed to withstand self ignition until the spark fires.

So why doesn't RON 97 give better performance in an engine requiring only RON 95?

simply because of the reason above, the engine needs to have the mixture NOT ignite BEFORE the spark, so 95 is enough to prevent that, putting 97 also achieves same results, hence 0 benefit.

not exactly right

compression is completely fixed, it's the physical ratio of the bore of the cylinder and travel of the piston.

holding valves open will reduce the pressure build up BUT the compression ratio remains becuase the bore and piston travel does not physically change.

secondly , valve timing is not to change compression ratio, is to change the breathing of the engine, and VVTi only works when the piston is at TDC, the entire process of the mixture in and exhuast out is completey driven by the natural remaining pressure from combustion in the cylinder and scavenging of the exhaust system

the piston does nothing to change the suction, both valves MUST be fully closed once the compression stroke begins the upward stroke.

This post has been edited by rcracer: Dec 4 2014, 10:33 AM