High octane or standard?

[Braxted]

I have just got a cracking 1995 S2 from a lovely chap on this Forum called Martyn. The car as been de-catted and has a SS sport exhaust. I very dumb arse question, should I use High octane or standard fuel?

[MPx]

Its ECU controlled so will adjust performance to suit whatever you want to put in. Better fuel will give better performance but it will be extremely marginal especially on the public roads. I always use V-power. Its ~8% higher cost than standard fuel so most think a total waste of money, but in my view:
You get about 3% more MPG so some clear return
Its got the best detergent elements meaning it will keep the engine and fuel components in much better condition (and I keep my cars for many years so its me that benefits).
I've spent money on a performance car. I've spent extra on a chip/exhaust/brake upgrades. Why wouldn't I spend extra on better performance fuel too for the extra fraction that that gives me??

To me it simply isn't logical to buy into having a toy like an Elan and then try to "save" a hundred quid a year by putting snake p**s in instead of the best fuel.

[Braxted]

You completely nailed my thoughts, the reason for the question is that I had my +2 mod’ed to run on unleaded quite a few years back and was told at the time not to use premium fuel as it was too rich, so I thought I should check on this Elan first! Mind you there is a massive difference between a 50 year old car never designed to run on unleaded fuel and a 23 year old car which is!

[Simon_P]

First off unleaded in the UK refers also to a switch to 2 star petrol (95) as opposed to High octane (98) 4star.

You should use high octane fuel in your +2 it was designed for it and is relatively high compression. If tuned correctly it will run better on 97/98/99/100 fuel preferably one without ethanol. It being converted to unleaded has nothing to do with it. There is nothing "rich" about high octane fuel other than the price the worst that can happen is that you don't get any of the benefit.

The M100 was I believe developed to run on 95. It has an ECU to handle knock so will get the best out of the fuel you put in it. There is a small advantage to using 98 but it isn't offset by the cost.

I run my 60s elan on98 and the M100 on 95
Note
Octane ratings are UK and I have avoided the misleading and meaningless Americanism "Premium".

[theelanman]

I put the highest octane available in.......not 95 tho......97/98/99
To be fair I just always went for shell optimax.....or whatever it's called now

[DeanG]

Quite different in the USA. 93 RON is the highest you can find in most places. California suffers with 91 RON. In the hinterlands it is not uncommon to find 85 or 87 as the only grade available. On adventure trips I carry octane booster. I was caught without it in the wilds of Vermont. The M100 doesn't like tractor gas.

[Simon_P]

I believe that US pump octane ratings are PON = RON+MON/2 although not directly comparable 93 would be about 97 RON. That said I have also heard that other ratings are used I think universally it is written on the pump which rating in small letters.

... Hence my comment which should have mentioned that UK numbers are RON.

Also speaking to someone that knows what they are talking about it appears that 95 RON tends to give the optimum from an emissions perspective. I was also reminded that higher turbo pressures would benefit from higher octane fuels, worth considering if you are using a performance chip.

[Simon_P]

Just reading about US pump gas 91 PON is about 95 RON, so whatever the rating 85 or 87 really is tractor fuel. I last saw 87 RON in Europe 15 years ago.

[Giniw]

As the ECU maps are calibrated for a 95 octane petrol I don't think there are any benefits to put anything that has a higher octane with the original chip?