99 RON Tesco Petrol

Hi All, What does everyone think of the recently introduced 99 RON Tesco fuel? I use Optimax but I have used the Tesco stuff a few times and didn't feel a difference.....

I have switched to 99 RON Tesco, no difference on performance... the only difference is in value for money. It would be really nice to have 100 RON Optimax at Tesco prices....[8D]

A few of us had a go at Shell, but we all received the same standard reply ... No

It would be nice to have 100 RON (Shell call it V-Power) to start off with, let alone the price [ ] Have been using Tesco 99 regularly for some 5 months now with occassionally switching back to Optimax and I have concluded that I get more city miles out of Optimax than Tesco 99 RON but it is the opposite on motorway with tesco giving slightly more (very slightly) mpg. Of course the tests are not carried out under any scientific condition what-so-ever...

I couldn't pick a difference between the two for road use... but I've only had about 3-4 tanks of each since I thought I'd give it a go. The lower price is very tempting and may help keep the other players down in price with their offers.

I have been using Tesco 99 for over 6 months with the occasional tank of Optimax (which I used initially in the car). I can't say that I notice any real difference between the two except that the Tesco petrol is cheaper and, for me, the Tesco service station is more convenient.

If you change the type and octane rating of the fuel you use, you will need to reset the engine management computer to derive any benefit. It can then adapt to the new fuel's higher rating. Ian W

...as far as I understood the ECU adapts eventually so after 3-4 tank full, it should be taking full advantage of the higher octane. Waiting to be corrected though. Also the engine adapts immediately if you go down in RON...

You are right about the engine management adapting immediately to lower octane fuel - the knock sensor will detect any detonation and back the timing off instantly. If this happens consistently, the computer will eventually calculate and store an adaptation value (in degrees of ignition retardation) for each cylinder that prevents detonation re-ocurring. This in effect modifies the base ignition map by the amount of the adaptation value - i.e if the adaptation value is, say, -2.6, it will normalise the timing for that cylinder at 2.6 degrees less than the base map for all engine operating conditions. What I can't see is how a mechanism exists to reduce these adaptation values if higher octane fuel is used. The system would firstly need to know there had been a potential change 'upwards' in fuel quality and then 'test' the timing by advancing it a bit to see if detonation occurred etc. etc. I have a feeling this doesn't happen and it may be that the only way to adapt to higher octane petrol is to reset the computer. I'm more than happy to be proven wrong here though. Ian W

If this is the case then it would seem to me to explain the reports of how the ECU seems to freshen up if the battery is disconnected or if you have had a service. My non scientific surmise is that if you either have to use 95 octane or you get a bad batch of TescoMax the ECU retards the ignition to protect the motor. If there is no procedure or mechanism for the ECU to advance the ignition back to what it should be then the car will be running below ultimate until it is reset. So, the conclusion seems simple - always use OptiMesco. But if you have found yourself in the position where there is none about and have to use central heating oil to get home, then you might want to to the disconnect battery to reset. I believe there is a procedure to make sure that this can be done without setting off the alarms and losing various codes - is it disconnect only if the key is in the ignition turned to a particular position? Anyone know?

Apparently there is a easier way i.e. remove a fuse to reset the ECU (?), love to know which one, anyone? ...but to be honest I find it hard to believe that a car is non optimal after using 95 octane until the ECU is reset. Also, I have read on this forum and others that the ECU also adapts to individual driving styles i.e. it will only work at optimal performance setting if you are really pushing the car to the max. I remember someone posting that his Porsche didn't feel right after his wife had driven it and got to its full potential after he would rev it to near red a couple of times...

To reset the ECU on the 996 Carrera it is fuses B8, C1 and C2. Remove all 3, leave for a couple of minutes and replace. I'm not sure if the fuse assignments are the same for all models though. You need to check (there is a handy little guide inside the fuse box cover) and remove any fuses that refer to engine electronics, engine management and fuel injection. Hope this helps Ian W PS - Edit - Obvious maybe, but your engine must be switched off!! Removing the fuses whilst the engine is running could wreck DME and fuel injection components.

Just checked the little guide inside the fusebox cover and it is exactly the same for 986, but I'm sure no one here cares [ ]... In fact the booklet is truly universal i.e. it is in several languages with anything not common with different models (911, Cabrio & Boxster or terriorties like Japan & Canada) is clearly mentioned in parentheses. Though I am not brave enough to try it out on my own for the first time, so if any of you are planning to do an Ace Cafe London visit this month then I might get tempted to try it out...