Originally Posted by Daphne01

Any problems switching between V power and Tesco momentum. I think they both have the same 99 rating so wouldn't expect there to be any issues but...

Originally Posted by Markyscoob

Blah Blah, This thread is getting caught up in Ego bashing and ***** waving right now.

Let's stop this and just summarise all the crap with some cold simple facts.

ANY modern car comes with an ECU that will use the sensors and particularly the Oxygen sensors to determine mixture, A/F ratios and ignition timing. They start with a base map and modify from there to get the best A/F ratio they can.

However. Many manufacturers base map is based on 95RON petrol for UK or Euro spec cars and the car may retard ignition or richen fuel ratios from there, but will not advance beyond the base setting, i.e. there will be no benefit to using 97RON fuel.

The cars have knock sensors to detect detenation and retard to protec the engine.

Nissan Micra's do this, Kia's, Vauxhalls and Mercedes. Our Subaru's are no different in this respect except for the base map on a Subaru imported to the UK is set to 98RON. Subaru chose to do this.

Your ECU cannot know what fuel you have put in, it will aim for the correct A/F and ignition for it's map which should give you the book performance. However, it needs to detect the knock to correct for it. You therefore WILL GET DETENATION if you use 95RON fuel.

Of course, once this happens a few times, the ECU will then retard the ignition and remember the new settings. Now you get less power and no more detentaion. However, switching back to 98RON fuel will now not give you the original performance until the ECU is reset. Even a ful STI can therefore run happily on 95RON forever more. It will however have tax and insurance for an STi and run like a WRX.

A Mapped car will have that base level set for the particular engine, removing some of Subaru's cautiousness and generalisation, as each engine is slightly different.

However, the programming for the Knock detection and correction is still there in the ECU and it will retard as before when it detects knock. Of course, if it knocks with much higher temps and pressures than before, it may cause damage before the ECU can react.


There is another way.......to avoid knock and detenation on 95RON, don't rev it and don't use full boost. Disconnect the 3 way (with car turned off) and you get lower softer boost and avoid the knock until you can refill with 98RON fuel.

i.e. DRIVE GENTLY!!!!!

Fuel burns at a rate of 60ft per second. From the plug, it will burn progressively across the piston crown building pressure on the piston and building pressure as a result of increased temperature of the gas. This progressive pressure rise allows for the piston to rise over TDC and start to come down to a point where the pressure can be applied eficiently to the rod and crank. Detenation is where the high pressure and temperature of the air/fuel mixture is ignited early be a hot spot in the combustion chamber and literally explodes causing a sudden pressure/temperature rise too soon. As ignition may be 30 or so degrees prior to TDC, the result will apply pressure down to the crank too early, and the piston will not react meaning it is stuck at TDC with higher temperature and pressure than it was designed for....result is localised heating and shock loading that can cause failure.

This then causes Tony Burns to say I told you so and jump up and down smiling......

Originally Posted by CharlySkunkWeed

The only bit i can correct you on is on what it says in the handbook (it says you can use 95 if 98 isnt availabe with only a loss of performance mentioned) but the rest i wouldnt know.

Originally Posted by CharlySkunkWeed

Or 97 if in Northern Ireland ! (mine is mapped for it BTW)