Originally Posted by
martysmartie
Back in the day when you had 4/5 star etc, fuels had a higher octane, when these cars were run on unleaded, which has a lower RON (95) it is a rule of the thumb to retard the timing by a couple of degrees to prevent knock, thus engine damage. Many did not do this.
The higher the Octane or RON, the greater the resistance there is to detonate under compression, hence high performance engines and FI engines generally run on SUL and it can cause damage using regular UL.
Likewise SUL, has a higher octane (97), so this is why they run better.
Of course you will not get any advantage in an engine that is not mapped/tuned for it, however modern engines have knock sensors, so use this to 'back off' the timing if the same happens, so I would say they actually are able to take advantage of it better.
Martin
Thanks for the info Martin.
Well I've now ran a full tank of BP ultimate through my car since I've had it set up (mixture and idle) and I've got to say it's been running miles better! Quieter tick over, instant start up and smoother idle. Also as you said the "knock" had disappeared although my car didn't always suffer from this just the odd occasion.
However now I feel the car is over fueling a little, maybe because the new fuel has cleared all my fuel lines and injectors of any debris. I plan to run another half to a full tank and have the mixture set up again and timing adjusted.
Proof that it does make a difference
