S2 on 99 RON - thought is was a good idea !

Frazerb · Jan 19, 2008

After reading one of the posts pre Christmas I have made the effort for the last three fills to put Tesco 99 RON petrol in the S2 cab. By all accounts this was a good thing and performance / efficiency would both get slightly better. I think there was even a comments that the car would run smoother. I only use the car at weekends on generally dry days.

Anyway a couple of weeks back I started to notice the car running slightly off. It always pulled great but an idle and steady cruising something was just not right. I then proceeded to spend close to £100 on a new distributor cap and plugs. - the problem did not go away.

Only today when out in the car did I think would the 99 RON pertol be causing this slight miss-timing. don't get me wrong this is not major spluttering but the engine is just not as happy as it used to be and I can feel it even at idle.

So has anyone any experience of this or have I missed something about 99 RON in an S2.

Comments / suggestions always welcome.

Thanks in advance....FB

Diver944 · Jan 19, 2008

If you have the standard chip in place then it is written for 95 RON fuel. If it detects poorer quality fuel then the knock sensor will pick it up and the ECU will retard the ignition to a safe level. If you use higher than 95 RON fuel then the ECU will simply leave everything set at 95 and you will get no extra benefit (except maybe better cleaning detergents compared with other cheap fuels)

However if you have a chip mapped to use Super Unleaded then yes you will get a benefit as the timing can take advantage of the higher rated fuel. If you use 95 RON fuel now, then the ECU will simply retard it back to a safe level

Frazerb · Jan 19, 2008

Cheers Paul - problem is I am not sure if the car is chipped or not. I have to guess at no but will contact Nelson Porsche on Monday - the previous car mechanics - and ask.

Meanhile I think I will put 95 back in and see how it goes.

Cheers..FB

sawood12 · Jan 20, 2008

generally if you are using high RON fuel your mapping is not an issue. You shouldn't have any probs running 99 RON if your ECU is mapped for 95 RON. If your car was running fine before and you were using 95 RON before then it is safe to assume that whatever chips you have are mapped for 95RON.

Is the Tesco stuff proper petrol and not this petrol/bioethanol blend? I've read somewhere that one of the petrol suppliers high octane fuel is a petrol/bioethanol blend which is fine for modern day cars but can cause problems in older cars.

Mark_Bennett · Jan 21, 2008

Tesco 99 is a 5% Bioethanol blend

http://www.greenergy.com/tesco_99_octane/index.html

zcacogp · Jan 21, 2008

I bow to the greater wisdom of Mr Smith and the mapping on chips, but in my experience my S2 has always run better on higher octane fuel. I use Tesco 99 for preference whenever I can.

Subjectively, it is smoother and pulls better. Objectively, I am sad enough to keep detailed records of fuel consumption and see typically 35-45 (10%) more miles from a tank of Tesco 99 than from a tank of Tesco normal (although this may not be the best fuel out there, admittedly.) That extra milage brings the £/mile to a lower figure for the higher-octane fuel.

FrazerB, in the light of my experience I would therefore doubt that your problems are caused by the fuel. Additionally, you say that you are now on your third tank of Tesco 99, and the problems have only just arisen. This would make me doubt it even more.

New dizzy cap and plugs are all well and good, but what about the rotor arm and leads, for instance? Or something else on the engine? I have not changed any of these (apart from the plugs) in two years (20k miles) of ownership, and the car runs fine. A good blast on the motorway improves things no end tho', as does a service and a clean-up of the earth points on the engine and bodywork.

I'd be asking the question "Why would an S2 stop running smoothly?"

All the best in getting to the bottom of it.

Oli.

333pg333 · Jan 21, 2008

Sounds like a 100oct we have here that contains 5% ethanol. Some people get spooked by it but at only 5% it shouldn't have any real downsides as far as deterioration of internals. Ethanol is hygroscopic and attracts water, but again at low levels and as long as you burn it up, then you shouldn't have anything to worry about. Interestingly Motec ran some classes here and showed that they were able to advance a test car by 7 degrees with this fuel which increased power and tq quite considerably. I am going to be using E85 later this year. The benefits are too big to ignore.