Pinking / Pinging / Predetonation in Audi 4.2s

[sutherland]

More than once, and in more than one Audi 4.2 (S8 and S6) I've had pinking. This seems to occur if I've been using 99 Octane fuel in the UK or 102 octane fuel in Germany and then only been able to get 95 octane. I've found (depending on the car) that it can come at 2000-3000 rpm under medium load or between 5,000-6,000 rpm under hard load.

I thought that the ECU was supposed to adjust back for the lower grade fuel to avoid this, but I've read elsewhere of folk having the same problem on standard 4.2 Audis and Audi saying there's nothing wrong etc. I don't much like my car doing this so wonder if anything can be done about it. Is it worth disconnecting the battery to force an ECU reset when one is forced to fill up with 95 octane fuel?

[PsYcHe]

Not sure.. They are supposed to detect it and adjust the timing accordingly, but it's possible that the mix you have is confusing it.

I've mixed 98 and 95 quite a bit and never noticed it happening on mine.

[Tobes49]

More than once, and in more than one Audi 4.2 (S8 and S6) I've had pinking. This seems to occur if I've been using 99 Octane fuel in the UK or 102 octane fuel in Germany and then only been able to get 95 octane. I've found (depending on the car) that it can come at 2000-3000 rpm under medium load or between 5,000-6,000 rpm under hard load.

The Audi base map is designed for 97 octane fuel, It the knock sensor pick up pre-ignition it will retard the timing map,

The car will keep trying to advance the timing until either
- The 97 octane base map is reached
- Or per-ignition is detected, Which it retards the map again and repeats
(so when running on 95 octane fuel it just goes round in a circle)

Maxium ignition timing is light throttle about cruising RPM and high RPM at full throttle hence why you hear it at these 2 points.

I thought that the ECU was supposed to adjust back for the lower grade fuel to avoid this, but I've read elsewhere of folk having the same problem on standard 4.2 Audis and Audi saying there's nothing wrong etc. I don't much like my car doing this so wonder if anything can be done about it. Is it worth disconnecting the battery to force an ECU reset when one is forced to fill up with 95 octane fuel?

The Audi is designed for 97 octane fuel, The simple answer is you shouldn't run it on 95 octane.
If you do get stuck at a station that only sells 95, just half fill the tank and then stop again at the next station and mix it with the 97 octane fuel.

[Tobes49]

Originally Posted by PsYcHe

Not sure.. They are supposed to detect it and adjust the timing accordingly, but it's possible that the mix you have is confusing it.

I've mixed 98 and 95 quite a bit and never noticed it happening on mine.

The S8 runs a lot high compression ratio than a normal A8 hence why it not so much of a problem on your engine

[Muppet_man]

Audible minor pinking at the engine speeds you refer to is no cause for alarm. The engine wants to run as advanced as possible for the best results. Pinking at high speed that does significant damage is inaudible to you as it happens so fast it would be impossible to hear.
Just trust that your engine management is preventing this as it should do.

The higher compression your engine is, the more susceptible it is to pinking as it becomes close to causing a diesel effect where the fuel detonates due to pressure and heat alone without the need for ignition (spark).

Older design cars had lots of problems with pinking when we moved over to lead free petrol. Vauxhalls in particular had a cure of fitting a thicker head gasket (and some do it yourselfers just fitted two normal ones!) as this lowered the engines compression and made it less prone to pinking.

Because of this phenomenon a coked up engine pinks more readily than a clean one, so it's a great excuse for revving the car to full pelt on occasion just to give it a clean out - but don't say I said so to the man in the car behind you with the flashing light on the top, it's not an ice cream van!

[sutherland]

Thanks for the response. I'll not worry about and stick to higher octane fuels as much as possible.

Originally Posted by Tobes49

The S8 runs a lot high compression ratio than a normal A8 hence why it not so much of a problem on your engine

This makes sense. The two cars I've had this occur on are a 340bhp S6 and a 360bhp S8.