Unleaded or Super Unleaded

[Edwardo1]

So now I will be making more frequent trips to the pump and having been driving my van and a few SUV for the last 4 years I now have the choice of different petrol.

What is everyone's choice when it comes to filling up and do you think there is much difference?

[doodlebug]

Super.

[Leo-RS]

So now I will be making more frequent trips to the pump and having been driving my van and a few SUV for the last 4 years I now have the choice of different petrol.

What is everyone's choice when it comes to filling up and do you think there is much difference?

You must be joking right?

Why have you bought an Audi RS6 as by even asking that question you have shown you have little to no knowledge of the performance car market.

I could go into the whole 10% power loss thing, it's only 5ppl more or the cost of a ham sandwich on a fill but yeah, go fill your £90k highly tuned performance car on piss water, just don't be alarmed at the stares you will get at the pump from proper car enthusiasts

[Chesterfield]

Sticker inside the filler flap will show you it prefers 98ron. But if caught short and there is no super available, it states 95 minimum will do.

[wildbore]

I have been using Super, which is no problem in most garages. Some garages, though (especially Texaco and especially in Wales) charge for Super as if it is the essence of Unicorn milk they are dispensing, and then - when it's about 12ppl more than standard, I do sometimes think about using standard unleaded. So far,though, I have resisted the urge.

[wildbore]

Why have you bought an Audi RS6 as by even asking that question you have shown you have little to no knowledge of the performance car market.

I could go into the whole 10% power loss thing,

I don't really track this issue for cars, but I have been interested in this subject as a result of riding bikes.

Interestingly, last year (or the year before) one of the mainstream bike magazines did an in-depth scientifically rigorous investigation into this (multiple petrol suppliers, multiple bikes, full dyno testing, etc.) including using modern bikes that were rated as requiring 98 (I emphasis "modern" as ten year old and older machines definitely have an issue using standard fuel when the manufacturer specified Super, my mate's 2002 Aprilia barely runs on standard).

The results were surprising and unequivocal. Super made no difference whatsoever as the engine management system compensated for the lower rated RON, and the engines all had mapped performance outputs, anyway, that the ECU was quite capable of generating using the supplied fuel, whatever it may be (95 or 98).

I guess that in a remapped engine then the performance that can be attained from Super is higher than from Standard because the specified higher power output is closer to the edge of the envelope in which the ECU is working, but for a standard factory ECU, there seems to be no issue with using standard fuel.

Maybe the ECU has to use more boost or more fuel to get the same performance from 95 instead of 98? Perhaps component stress is higher using lower grade fuel. Certainly, if Audi recommend 98 I am not going to ignore that advice. But, in terms of power output, I would be surprised if using Super made much difference.

[W8PMC]

As above, it will state on the manual & a Sticker on the fuel filler cap that the car should be run on S/UL, probably a minimum of 98ron. Most owners of such cars will opt for Shell V-Power or Tesco 99 as that's like giving the car Champagne rather than forcing it to drink Asti.

The world will not end if you splash & dash with regular Unleaded, however that's really only meant for those times when you may be caught short & unless you live somewhere remote that should be very rare.

[Edwardo1]

So now I will be making more frequent trips to the pump and having been driving my van and a few SUV for the last 4 years I now have the choice of different petrol.

What is everyone's choice when it comes to filling up and do you think there is much difference?

You must be joking right?

Why have you bought an Audi RS6 as by even asking that question you have shown you have little to no knowledge of the performance car market.

I could go into the whole 10% power loss thing, it's only 5ppl more or the cost of a ham sandwich on a fill but yeah, go fill your £90k highly tuned performance car on piss water, just don't be alarmed at the stares you will get at the pump from proper car enthusiasts

Ah Leo!! Glad to see someone is out there vetting everyone before they dare buy a super car! "Proper enthusiast" didn't know that was the criteria when I handed over all the cash to Audi! I hope when they find out they don't confiscate it and put me back in a diesel as punishment.

People like you " Mr I know everything" are the only ones who get laughed at pal with the only fun in your life coming from a car.

From the latest posts and through previous reading the whole debate comes up as there are different views on which fuel to use. Does it actually give you any performance increase or is it just another way off oil companies squeezing another £££ out your pocket.

[SuprSi]

Why have you bought an Audi RS6 as by even asking that question you have shown you have little to no knowledge of the performance car market.

I could go into the whole 10% power loss thing,

I don't really track this issue for cars, but I have been interested in this subject as a result of riding bikes.

Interestingly, last year (or the year before) one of the mainstream bike magazines did an in-depth scientifically rigorous investigation into this (multiple petrol suppliers, multiple bikes, full dyno testing, etc.) including using modern bikes that were rated as requiring 98 (I emphasis "modern" as ten year old and older machines definitely have an issue using standard fuel when the manufacturer specified Super, my mate's 2002 Aprilia barely runs on standard).

The results were surprising and unequivocal. Super made no difference whatsoever as the engine management system compensated for the lower rated RON, and the engines all had mapped performance outputs, anyway, that the ECU was quite capable of generating using the supplied fuel, whatever it may be (95 or 98).

I guess that in a remapped engine then the performance that can be attained from Super is higher than from Standard because the specified higher power output is closer to the edge of the envelope in which the ECU is working, but for a standard factory ECU, there seems to be no issue with using standard fuel.

Maybe the ECU has to use more boost or more fuel to get the same performance from 95 instead of 98? Perhaps component stress is higher using lower grade fuel. Certainly, if Audi recommend 98 I am not going to ignore that advice. But, in terms of power output, I would be surprised if using Super made much difference.

In one of these engines if you floor it with 95 RON the ECU will detect knock in each cylinder and dial back all the ignition timings and reduce boost pressure. This will cost you a lot in performance. Keep in mind with a turbo charged engine the intake air temperatures are quite high so they are usually much more prone to pre-ignition than naturally aspirated engines. If you gave the car a frequent thrashing on 95 I'm pretty sure you'd end up having a catastrophic engine failure within 5000 miles. The ECU can only compensate so much and knock/pre-ignition is very harmful.

My car's mapped so will probably highlight the issue much more keenly as the engine is running with much smaller margins for error, but if I fill with BP ultimate (97ron) instead of v-power/momentum (both 99) I notice it immediately, and the car feels a lot more sluggish. I try to avoid anything not 99 octane or above at all costs, but if I don't have a choice but to use 95 I'll fill with the bare minimum and just drive slowly being careful not to let the car get on the boost until I can put 99 back in the tank.

[Edwardo1]

The sort of info I was looking for cheers Si

[wildbore]

In one of these engines if you floor it with 95 RON the ECU will detect knock in each cylinder and dial back all the ignition timings and reduce boost pressure. This will cost you a lot in performance. Keep in mind with a turbo charged engine the intake air temperatures are quite high so they are usually much more prone to pre-ignition than naturally aspirated engines. If you gave the car a frequent thrashing on 95 I'm pretty sure you'd end up having a catastrophic engine failure within 5000 miles. The ECU can only compensate so much and knock/pre-ignition is very harmful.

My car's mapped so will probably highlight the issue much more keenly as the engine is running with much smaller margins for error, but if I fill with BP ultimate (97ron) instead of v-power/momentum (both 99) I notice it immediately, and the car feels a lot more sluggish. I try to avoid anything not 99 octane or above at all costs, but if I don't have a choice but to use 95 I'll fill with the bare minimum and just drive slowly being careful not to let the car get on the boost until I can put 99 back in the tank.

Whilst I agree in principle with what you are saying about knocking and retardation, boost, etc., the key question is: does this apply to the RS6 in standard spec?

I am suggesting that the ECU might be perfectly able to deliver the specified output (552 BHP) on standard petrol without resorting to engine-damaging retardation because the engine is actually not working close to the envelope edge in standard tune (as evidenced by some of the amazing BHP figures people are achieving with a remap). That seems to be the conclusion of the motorbike testing I mentioned. Your car has a remap and so it is not surprising that you see significant issues if you fuel up with 95 RON fuel.

I think that only a car, several tanks of fuel and a dyno will establish the answer.

[SuprSi]

That's fair enough.

I think that only a car, several tanks of fuel and a dyno will establish the answer.

I'd be very interested to see that! The only thing I'd suggest is the engine doesn't have a 'target' power output so I'd expect there to be more HP regardless of tuned/standard, although there should be less of an impact as you said. What would be more interesting would be looking at the data logging on the engine to look at how safe the car would be to run on the lower grade fuel on the standard map.

[wildbore]

That's fair enough.

I think that only a car, several tanks of fuel and a dyno will establish the answer.

The only thing I'd suggest is the engine doesn't have a 'target' power output...

But isn't that exactly what a remap is? A higher target power output? I think that an ECU does work to a specified power output and will work with the currently available fuel and the currently available air to achieve it where possible. Anti-knock retardation only kicks in when the fuel is of unacceptably poor quality.

We could be talking at cross-purposes - or in violent agreement

[HPsauce]

Obviously will depend on what the car is designed for, age etc. etc.
What I can say is that MY experience on D2 S8's, backed up by many others I know, is that it runs better, faster and more economically on V-power or Momentum.
BP whatever-its-called 97 is a waste of time.

Any old 95 is fine too but overall you probably won't save money as you use more to achieve similar outcomes and lose out at the very top.

Mind you cars of such vintage will run fairly happily on much worse than 95, though not spectacularly well .......

A final thought. I would expect normally-aspirated engines to behave differently to turbo- or super-charged ones where there is more scope for the software to adjust for shortcomings.

[wildbore]

Re the D2 S8: a car of that age I would expect to behave like this. As I mentioned, a friend's 2002 Aprilia runs very poorly on standard fuel. For him Super is mandatory, not optional.

I think ECUs are a lot cleverer nowadays, and engines are probably in a lower state of tune as standard than we perhaps realise, hence the ready availability of the 700 BHP remap!