Wrong laaa... retarding ignition timing is not to lower compression ratio...
to lower the compression ratio the VVTi or iVtec or variable valve technology will keep the intake valve open much longer than its required so the mixture will reverse back into the fuel injector chamber thus lowering the compression ratio

this also works with retarding ignition timing to compensate for the low RON or premature ignition (Engine knock)...

but if your engine is design for Lower RON like RON91/2/5 no point pump 97 just waste of money only...

most of CKD cars are made for RON91 due to thailand fuel... (Thai sell RON91) so inorder for your car to travel to thailand and not dying there engine is made to lowes regional requirement hence in South east Asia is RON91... this is also why before RON95 petrol pump sell minyak merah (RON92)

Some europe car uses min 95 cause based on europe region where min RON is 95...

Japanese performance car usually go around RON97-RON100 eg. Type-R is RON98... but RON97 can laaa any below will kill your engine...

Beep beep..fellow engineer passing through..just checking out how many mechanical engineers in /k..

I have to admit I'm an electrical and not a mech lol.

I have some questions, I'm no engineer in mechanical field and I only have a general idea on how an engine works so correct me if I'm wrong

Question: does higher RON allows for higher performance in normal car? Eg myvi saga

Fact: A higher RON allows for a higher compression ratio without knocking

Doubt: to mechanical engineers, does a normal car engine adjust it's own compression ratio to the fuel octane ratio? If it's not variable then wouldn't it compress the fuel back to the same ratio? Then wouldn't the performance be the same between RON 95 and RON97 if the compression ratio is the same?

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.

Ask johnkor, 1st class upper Lowyat uni phd in automobile

The difference in octane generally indicates the resistance to detonation. It has nothing to do with compression ratio. So throw the idea of the compression ratio out the window.

A car mostly cannot change its compression ratio, but it can advance or retard the ignition. It does this in concert with the O2 sensor for AFR and knock sensor.

eg; If a car senses it is using a lower RON than what is recommended due to the detection of knock and pinging via the knock sensor it starts to retard timing, which lowers the power. And if it detects that knock is 0 and AFR meets the target it gets more aggressive with the timing advance.

But, no. On normal cars, the car will go no more aggressive on the timing than it's stated RON target. If your car maker recommends a RON91 you gain nothing by going to 98 as the ECU will go no higher to preserve the engine from blowing holes in your piston.

I only know how to tuang Tecron Concentrate plus every 20K mileage to cuci engine, thats about it.

er.... u mechanical engineer? from where wan?

From university of lowyat la...

the moving piston can change compression ratio wan