timing

[graham7773]

My Ford Telstar is supposed to run on 91ron unleaded, no E10. Now that fewer servos are carrying 91ron, I am thinking to run it on 95ron ULP but I think the timng has to be changed a degree or 2. 12 degrees before tdc instead of 10? Any of you mechanics like to help with accurate info? All fuel company execs are going straight to hell.

[JimStone]

Can run with stock timing if you want to, but increase advance to get best benfit from higher RON fuel.

Very simply - the higher RON means the fuel is less volatile. Due to the issue of less volatility if you leave enginge at standard timing the slower travel of the flame front will mean primary ignition will occur a few degrees past optimum point. To get the best from the higher RON fuel you need to advance the timing. Each engine is different depending on basic combustion chamber design, engine managment system and overall condition. Most modern engines can accomodate changes in fuel automatically via knock sensors and such, what model is the Telstar - ie does it run a knock sensor? if not, most cars can 'safely' run a few extra degrees, but you will need to be careful and monitor for any knocking.