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SHELL'S OPTIMAX FUEL ???????


Dav1
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Dav,

Thats as serious as drill gets smashfreakB.gif but there is truth in what he says... kind of..

I have run optimax in both my Audi S3 and mt TVR Tuscan and find that it doesnt increase my fuel efficiency but i do think the engine runs alot smoother and the trottle seems more responsive.. I cant however say that it boosts performance as i have seen nothing (independant) conclusive!

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Sensible?!?! ME!! How DARE you SMOKE6.GIFjump.gif

Anyway Dav, like I said, the only way to really know is to try it. I ran it in my A3 TQ for about a month (about 6 tanks), and while I DID see a mileage improvement, it was just enough to cover the added cost of the Optimax. This was back when it first came out, but IIRC I was only saving around1-2 Pfennig (Pre-Euro German Cent) per liter over the Mid-grade. smashfreakB.gif If cost weren't a factor, I'd probably use it all the time, as it can't hurt, and any octane jump is better than none, right FIREdevil.gif

Better response? grin.gif

jump.gifbeerchug.gifjump.gif

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Dav1 - Get in contact with BMW customer services and ask them if there is any benefit of running on Optimax.

i would be surprised if there were not to be honest!

Running a higher octane fuel, allows for more ignition advance, and therefore more power from your engine! It can however cost a bit more! I never put anything other than Optimax or Super unleaded + Octane booster, in my old TTR. ECLIPSe.gif

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  • 8 months later...

Sorry for the late (!) reply.

Been thinking of changing the S8 for a BM and started browsing this forum.

Here is a previous post I wrote about Optimax:

Having read a few posts about the pros and cons of super unleaded / optimax vs standard (premium 95RON) unleaded fuel, just thought I'd elaborate on why higher octance values should be any better.

First - back to basics:

I'll assume we all know the four stroke cycle (induction, compression, ignition, exhaust) - or 'suck, squeeze, bang, blow'.

When the piston is approaching the top of its stroke during the compression phase, the fuel and air mixture is about to be ignited by the spark plug.

It is important that the mixture does not start to burn, and hence rapidly expand before the piston reaches top dead centre (TDC), otherwise the resultant burn will attempt to force the piston back the wrong way.

It is also important that the piston has not travelled too far past TDC before the burn, since this will result in wasted power.

Burn Speeds:

Contrary to popular belief, high octance fuel does not burn faster, it simply burns in a more uniform fashion. I have deliberately used the word 'burn' here instead of 'explode' - it is highly undesirable for the fuel/air mixture to explode - more on that later.

This uniform burn is very important in achieving maxium power from an engine.

Ignition Timing:

For maximum engine power (actually 'torque' at a given RPM - but same thing), it is critical that the fuel/air mixture has just started its rapid expansion in its burn phase just as the piston passes TDC.

In order to achieve this, the spark must actually initiate the burn process before the piston reaches TDC.

Since the fuel/air mixture will try to burn at a constant rate, whilst the engine will operate at different RPM, it is important to vary the point at which this spark ignition takes place, otherwise the correct piston position : burn rate would only be matched at one particular engine speed.

Engine load is also a factor, since if the piston is more reluctant to be pushed down its bore, it is desirable for the burn process to occur later in the cycle.

Octane:

The octane rating of the fuel is another way of saying 'the fuel's reluctance to explode when mixed with air'. The higher the octane, the higher this reluctance.

This is important for the following reason:

If the fuel/air mixture is ignited just before the piston reaches TDC, and the resultant rapid gas expansion cannot start to push the piston down the bore very quickly, the burn process takes place in a space smaller than a critical volume. If this happens, the burn turns into an explosion, or detonation, and the engine will exhibit a knocking noise, or 'pinking'.

The way round this is to make sure the ignition happens a bit later in the phase, so the burn takes place in a space always slightly larger than the critical volume.

This is exactly what happens when the ECU fitted to Audis detects pinking. Piezo-electric transducers are fitted to the cylinder head, often next to each combustion chamber, and they produce a voltage proportional to tiny vibrations caused by pinking. When this happens the ECU 'retards' the ignition (makes it happen later in the phase).

The Power Relationship:

If we use a fuel with a higher octane rating, we are effectively decreasing its liability to produce an explosion when the burn takes place in a space slightly smaller than the aforementioned critical volume. This is great news, since where we would have to previously retard the ignition, we can now leave it where it is.

The more ignition advance we have, up to a point, the more power we can extract from the engine.

The ECU Problem:

As already mentioned, the ECU's fitted to most Audis (and all turbo / s / rs versions), have the ability to 'listen' for the onset of detonation or pinking, and if detected, will retard the ignition slightly.

This retardation is carried out very quickly, since excessive detonation can cause severe physical damage to an engine.

Once the ignition is retarded, the ECU will 'listen' for further detonation, and if none is detected, will leave things as they are for a while.

If, over time, no detonation is detected, the ECU will tentatively advance the ignition in stages.

(Don't confuse this process with the dynamic advance / retard process which is initiated from the ECU's RPM / throttle position or vacuum map. The advance we are talking about here is the 'benchmark' figure which will move this whole map up or down.)

The ECU's reluctance to advance the ignition gives us our problem...

Good days / bad days:

Suppose we fuel our Audi with premium (95 octane) unleaded and go for a hard drive. If the engine has a resultant basic ignition advance caused by the use of 97/98 octance fuel, it is highly likely the ECU will detect pinking and retard the ignition. This will cause a drop in the power output.

Now if we fuel with optimax and go for a drive... power will NOT be restored (assume we ran the tank almost dry so there are no issues from cross-contamination)

This is because the ignition is still retarded - we could put 100 octane aviation fuel in, but without our previous ignition advance, we will still get no more power. This is the important point.

ECU Wake Up Call:

Over time, if with the previous example we used only higher octane fuel, the ignition setting will again become advanced. However, if we perform an ECU reset, the ECU performs its search for the basic ignition setting more rapidly, and we get full power restored immediately instead of sometime next week...

ECU Reset:

1. Make sure you have your radio code handy

2. Disconnect the battery for 60 secs or more

3. Reconnect the battery and turn on the ignition (don't crank it) but DO NOT touch the throttle pedal for 20 mins (this allows time for the ECU to recalibrate its drive-by-wire throttle position sensor) If you accidently touch the throttle, start again.

4. Cycle all electric windows and sunroof to allow auto calibration

5. Activate the radio

6. Go for a drive - go gently at first and slowly increase the pace up to full bore and maxium RPM

7. If you have a tiptronic then this will take a short journey to re-learn the change points to a certain extent.

(If you know where the ECU fuse is this will save bothering with the windows / radio etc.)

Summary:

High octane fuel produces no benefit unless the ignition is suitably advanced

High octane fuel does not explode faster than other fuel - its reluctance to explode is what allows an engine to yield more power

Audi ECU's retard ignition more quickly than they advance it again - a shortcut is to perform a reset.

beerchug.gif

Jon

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