Which RON fuel for my 2 litre petrol is best?

[Dandock]

But, as the timing automatically compensates, I suspect it impacts on the performance and/or economy.

Possibly technically correct. But what are we talking about, 1%, 2% under ideal conditions?

I guess the % loss depends on at what level one is prepared to compromise. Two years ago I was obliged to top up at a budget supermarket in France. Performance difference is of course unknown as we can’t accurately measure acceleration etc but I thought that consumption reduced by at least 10%.

To avoid any confusion I drive an S. I was also assuming that the conversation was based on the recommendation in the 2L handbook and was therefore 98. Apologies for any misunderstanding or misinterpretation.

[steve w]

Personally I use V-Power in my various PORSCHE,presently a 911,finest fuel for the finest vehicle.
SteveW

[Ferdie]

Personally I use V-Power in my various PORSCHE,presently a 911,finest fuel for the finest vehicle.
SteveW

Don't you have to declare a vested interest or summut with a comment like that ??

i.e SteveW, VP Sales and Marketing Shell (company car Porsche 911)

[Greddyuk]

I use Tesco Momentum 80% of time which is 99 RON

[Paul]

I use Tesco Momentum 80% of time which is 99 RON

“supermarket fuel”.....

[Rab J]

I use Tesco Momentum 80% of time which is 99 RON

“supermarket fuel”.....

I used to run a Boxster S on Sainsburys super unleaded. All went well for a while untill one day when what I filled the car with was 99% H2o. Haven't used it so much from then.

[thewanted]

The handbook recommends 95 for the 2.0. I've only recently bought my 2.0 Macan. First full tank was what ever the OPC put in it. Next tank I put in was 95 as recommended in the handbook. I think I'll try v power next to see if there is any difference in mpg and performance. I'm getting 29mpg on 95 on my commute which is a mix of everything!

Apologies for the bump, but I'm curious if you ever did the super unleaded test and what the results were?

[jaffacake]

I know I'm likely to write too many words here, but feel it's a good place to share some knowledge I've gained over time. I used to help tune engines in modified RX-7 twin turbo (FD) cars - they were sensitive beasts. I'm no expert, but I worked with one who explained a lot in a way I could understand. It's simplified and experts may nitpick a little, but it might help a bit.

We all basically know how an engine works. You throw an air/fuel mix into an engine and set fire to it to make a bang and move the pistons. But how much fuel, how much air and which fuel?

One basic analogy is a saucepan on the hob with a lid on it. If you turn the heat up too far, the pot boils hard and the lid rattles. Turn the heat down too far and your food may not cook as intended. You adjust carefully to get it just right - or you get lazy and shove a wooden spoon in the gap to let the steam out. But this lets out the heat and isn't very efficient. You burn more fuel than you need to when cooking your peas just because you can't be bothered to tune the gas flame.

Similar in an engine. When you set fire to that fuel/air mix, you want it to bang - but you want it to burn and not explode. If the mix is wrong, you will wear components out faster, may cause permanent damage or even blow your engine up.

When you "tune" an engine, you adjust the air/fuel ratio to get the optimum setting. Too much air and it will explode. Too much fuel and you lose economy. Even at a 2k rpm idle, those 6 cylinders are all banging 33 times a second, it pays to get it right.

The best performance you can get is one click before it explodes, many engines have been lost to find those right settings. The problem is that this optimum setting varies depending on the fuel used, the temperature of the air, even the moisture content of the air. It's why your car feels great on a cool Autumn morning but, if your car isn't ready for it, you'd blow your engine up on that same cool day.

With old carburettor based engines, it was primitive and you'd pull the choke out on a cold day to reduce the amount of air until the engine warmed through. With modern injection engines, these things are all controlled automatically by electronic sensors measuring things like air intake and exhaust gas temperatures.

If the air is colder, it bangs more, so the engine adjusts by reducing air - or effectively adding fuel. So you burn a bit more fuel in winter too. But on a hot summer's day everything runs hotter, the air is hotter, so you need to keep it cool then too. So it's hard work.

One thing that can help is octane. Adding it helps cool the combustion down and make it smoother - race fuels are around 120 octane for a reason.

Go back to our saucepan on the hob analogy. You set the ring to 1, not much happens. 2, 3, 4, 5 - bubbles appear in the water. 6, cooking happens. 7, the lid rattles, 8, the pan boils over.

In a normal car, we want to be at 6. In a performance car, maybe 6.5. If the lid starts to rattle, the "knock sensor" in the engine will hear it. You get slight imperfections in the combustion and the sensor can literally hear it.

A lambda sensor, which measures the oxygen levels, is also involved in this process - https://www.autobutler.co.uk/wiki/lambd ... what-is-it - this particularly helps with varying intake temperatures. Because science, a litre of air contains more oxygen when cold than it does when warm. Measuring how much oxygen there is helps a lot.

You'll have heard terms like "lean" and "rich" - these basically mean that you lean out the engine by reducing the fuel in the mix. If you add fuel, it's a rich mix. Rich mixes typically burn cooler, but waste fuel.

So, ideally, what you want is a leaner, more powerful, mix that burns cooler. You do this by adding octane. Adding octane allows you to turn your pan up to 7 without the lid rattling. You can get to 8 without it boiling over.

If you add higher octane fuel, you engine automatically adjusts. If you add lower octane, again it adjusts, automatically. If you have a mix in the tank, it finds the best setting. It leans out the engine until it hears knocking and then makes it a bit richer.

I should add that "timing" is involved here. Think cams and tuning forks. Timing, at its basic level, is at which point during the cycle you spark the fuel mix. The more compressed the fuel/air mix, the bigger the bang - but the higher the risk of it going boom - what we call detonation.

I remember talking to a Mazda engineer during the development cycle of the RX-8 and he explained that for it to go on sale in the EU, it needed to automatically adjust to over 100 different grades of fuel.

These sensors measure every bang in every piston for every revolution of the crank. As a tuning friend of mine puts it:

"It’s a combination of several sensors but octane is entirely sensed by knock sensors not lambda. There are short term and long term corrections but short term is a few crank revs and long term is minutes of full load. Both much less than a tank full depending on how you drive.

"Porsche are traditionally quite agressive in knock calibration so if you go from 95 to Tescos 100 on a hot summer day, I would expect you would notice an immediate difference as the ignotion will advance til it finds knock."

So basically your Porsche engine will automatically find the most of power it can - especially for those with sports chrono and a Sports Plus setting. On standard mode, timing will be less aggressive and more optimised for economy and emissions. Hit Sport and the fuel is leaned out giving more power. Hit sport plus and cams are lifted further and you get the biggest kick. It's strongly advised never to use high revs or Sport Plus with low octane fuels. The Macan will run on 93 octane but you should never rev it high - the chances of detonation and engine damage increase significantly.

You'll see VW/Audi/Porsche share engine blocks a lot these days. But just because it's the same engine, doesn't mean it's the same settings, the same timing, the same fuel mix. It might be the same engine found in a Golf or a Q5, but they are not tuned the same. The saucepan handle might not rattle at 95 octane in the Golf, because it has less power. In the Porsche, it may need super unleaded to get the proper results.

Similarly the Gen 2 Macan Turbo has the same 2.9 Audi lump that so many other cars do. What fuel they use it utterly irrelevant, it's tuned by Porsche and you should only use the fuels they tuned the car for.

Hope this helps somebody

[thewanted]

Fantastic post jaffacake! Thanks so much

[Paul]

+1

Great insight (and just confirms really that what it says on the fuel filler cap / in the handbook is what you should be doing )