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I read on the forum here that 98ROZ fuel can result in better performance. In the UK - is this super unleaded? I haven't tried that yet - I've always thought it wouldn't really make much difference. But that was when I had a big french barge. Any thoughts?
 

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I'm not a believer. Octane is a measure of how well a fuel will protect against knocking, not how much power the fuel produces. The car is designed to run without knocking on 95 RON (equivalent to 91 R+M in the US and the 'regular' grade in the UK) and as long as it isn't detecting knock, will produce full power. Filling it with 98 wouldn't add anything - you're just going to be spending more.

In the US, lower octane rated fuel is available and - whilst you can run the car on this - it may end up retarding the timing to avoid knocking and produce less power. In the UK, this shouldn't be the case as 95RON is going to be the lowest grade available.
 

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Are there no mpg related benefits from using higher octane? I thought a higher octane fuel allowed you to go further (rather than faster) gallon for gallon?
 

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It is quite likely that (after a couple of tanks) a high pressure turbo engine is going to produce a bit more power on 98RON than 95RON. It will also probably use slightly less gas.
 

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68wooley has it right, octane measures the resistance to pre-detonation, pinking or knocking, and does not measure energy content. In the US, 91 is the posted octane number (PON), the average of Research and Motor Octane Number (RON & MON) where RON might be 96 and MON 86. On my re-mapped Fiat Coupe 20v turbo, the ignition timing is changed to take advantage of higher octane fuel (99 RON, so equivalent to around 94 PON) but on all my other cars including the 2017 124 Spider I fill up with cheap supermarket fuel, since it meets the standard EN 228 just the same as branded gasoline, whichever refinery it may have come from. People who say their engine runs better are probably only seeing a difference from detergent additives, mostly it is a placebo effect. But if you believe it and are happy to pay the extra, go for it.
 

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I’m not suggesting that there would be a massive increase in power, but it is quite likely that a 124 would manage to eek out a few more horses on 98RON than 95RON It’s not going to push out another 10%, but it’s not inconceivable that it could push out another 5hp.
 

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I’m not suggesting that there would be a massive increase in power, but it is quite likely that a 124 would manage to eek out a few more horses on 98RON than 95RON It’s not going to push out another 10%, but it’s not inconceivable that it could push out another 5hp.
Wouldn't this only be the case if the ECU was retarding the ignition to avoid knocking running on 95, which it shouldn't be doing given it is the recommended grade? What other aspect of 98RON would allow it to produce the extra power?
 

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Wouldn't this only be the case if the ECU was retarding the ignition to avoid knocking running on 95, which it shouldn't be doing given it is the recommended grade? What other aspect of 98RON would allow it to produce the extra power?
The fact that ECUs adapt to fuel quality. It will probably push the engine slightly harder once it’s got used to 98. I’m not talking a massive increase here...but I wouldn’t be surprised if it picked up a couple of HP here and there across the rev range.

I read a test once for the Volvo C30 T5 (optimised for 95RON) where they ran it for two tanks on 95RON and then dyno’d it and then two tanks of 98RON and dyno’d it (being careful to take out other factors like ambient temperature and humidity etc) and the 98RON smoothed out some of the dips across the rev range and pushed out as much as 8hp more at some points in the rev range and 6hp more at peak. Both fuels delivered slightly more than the manufacturers peak claims for the engine.
 

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Please, buy an OBD tool and look at the ignition timings - you def. can see a different. With standard fuel the ECU starts to pull back the ignition pritty early.
Btw. the ECU learns over time.

US carguys are really funny. Gasoline is cheap as fua*k compared to europe an they fill up the cheapest gas they can find. Makes sense ;)
 

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US carguys are really funny. Gasoline is cheap as fua*k compared to europe an they fill up the cheapest gas they can find. Makes sense ;)
You hit the nail square on the head! Not just gasoline, its the same general attitude for just about every consumable liquid we put in our cars/vehicles.

Steve
 

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US carguys are really funny. Gasoline is cheap as fua*k compared to europe an they fill up the cheapest gas they can find. Makes sense ;)
Gotta save money somewhere to pay for healthcare and university...

In any case putting 91 R+M (~95RON) isn't being cheap - it is the highest octane grade readily available in much of the western USA. 91 is what's recommended and there isn't a readily available higher octane alternative out here. In Colorado, the cheap stuff is 85R+M because someone in the distant past convinced everyone cars running at altitude didn't need gas with as as high an octane as running at sea level. That's since been shown to be questionable, but we still get screwed over paying a premium for what everyone else gets as 'regular'.
 

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The fact that ECUs adapt to fuel quality. It will probably push the engine slightly harder once it’s got used to 98. I’m not talking a massive increase here...but I wouldn’t be surprised if it picked up a couple of HP here and there across the rev range.

I read a test once for the Volvo C30 T5 (optimised for 95RON) where they ran it for two tanks on 95RON and then dyno’d it and then two tanks of 98RON and dyno’d it (being careful to take out other factors like ambient temperature and humidity etc) and the 98RON smoothed out some of the dips across the rev range and pushed out as much as 8hp more at some points in the rev range and 6hp more at peak. Both fuels delivered slightly more than the manufacturers peak claims for the engine.
Again - octane is not a measure of fuel 'quality'.

Most cars cannot advance the timing beyond their nominal optimal level which, unless there's other factors at play, they should be able to do on the recommended fuel grade, which in the UK is 95RON. If the ECU isn't detecting knock on 95RON, it'll advance the timing as far as it can. Put 98RON in there and there'll be no difference - it can't advance the timing any further.

So, unless the timing's being pulled on 95, there's nowhere for it to go with a higher octane fuel. I guess it could be getting pulled on 95, but it'd seem strange to recommend a fuel grade on which the car can't produce full power. They only way to really check would be controlled dyno runs using both grades.
 

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As I said, I wouldn’t be surprised if you got a tiny bit more out of 98 than 95 but it’s only likely to be a tiny amount and it may make no difference.
 

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@68whooley:

The 124Spider ECU can detect the better gas quality and you can see how it pulls back the ignition.
Buy an OBD tool and the torque App and you can see it.
You also can feel it.
 

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The ECU responds to the presence or absence of knock. There is no octane sensor.
 
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The ECU responds to the presence or absence of knock. There is no octane sensor.
Which could mean it responds to having a higher octane in it.
 

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Which could mean it responds to having a higher octane in it.
If it was knocking at the previous octane, yes.
 

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