95 vs 98 RON petrol

[sjr202]

Another quicky, on a Boxster S, is there a noticeable difference between running 98 RON and say 95RON Optimax? Economy and performance??

[domster]

I would say not. I have never been able to tell the difference in normal circumstances. You may get some more mpg on a whole tank, which is calculable, and the engine may feel ever so slightly smoother, but I'm not sure how much is psychosomatic. In a tuned turbo engine where detonation is a problem, the anti-knock sensors may retard the ignition noticeably with 95RON fuel, but on a normally aspirated engine it won't be that big a difference. Remember, RON isn't a power thing, it's really just referring to how cool the fuel burns. 'Cooler' high octane fuels detonate higher up the range, so you can tune the engine more.

The only time I have seen a marked difference with Optimax is in my Lotus Carlton when it had a detonation problem/misfire. When it came on boost the misfire would occur slightly later in the rev range on Optimax, and the misfire seemed to be milder.

Life's too short to get wound up about this issue. I would recommend Optimax simply because running a higher octane will generally be a better idea, and they chuck more cleaning additives in which can prevent coking. Also, it should earn back the price difference in fuel savings.

When super u/l was 20p more, it's worth debating. Now it is just 5p more, always use Optimax or another super u/l - but don't get too stressed if you have to fill up with 95RON for a tank because you couldn't find a Shell station.

[neon_fox]

I notice a marked difference in my 964C2 (and previously in the 968CS), but Daz could be right, perhaps it's all in my mind!

Fox
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[domster]

I think a lot of it is psychosomatic. I kid myself I am getting another 50bhp, but then I fill up with supermarket juice and thrash the nuts of the car and it feels just as quick