Higher Octane Petrol

[paul27848]

I'm wondering if anyone uses the more expensive fuel, if so do they see performance gains and are there any downside effects other than extra cost of the higher octane fuel?

The question is prompted by comparing the effects/needs of different octane fuels on two of my other cars:

1) Porsche Boxster, 3.2l and 252 bhp = 78.75 bhp/l. If normal unleaded fuel is used the ECU automatically retards the ignition timing and reduces performace.

2) Audi TT, 1.8l and 265 bhp = 147.2 bhp/l. No automatic retardation with normal unleaded, but performance is noticably better and smoother with higher octane fuel.

3) S2 with Everest chip, 2.25 exhaust with main cat removed and bhp believed to be approx 200 = 125 bhp/l

If we assume that bhp/l is an indicator of engine stress level then the Porsche is the least stressed, but still requires higher octane fuel to produce max performance. The Audi is the most stressed with almost double the specific bhp of the Porsche, and suffers badly when using normal unleaded fuel. The Elan is near the Audi bhp/l stress level but seems content with normal unleaded, but would better fuel give better performance with no detrimental effects?

Paul

[Rambo]

I use Tesco higher octane (99) every other tank full on both my cars. I feel that the engine is slightly more responsive and I get slightly more MPG but this is very marginal (approx 0.5 MPG)

[lotusflasherman]

I have an Everest Chip so only use Shell V Power Unleaded or Tesco Momentum, both 99 Octane. Prefer Shell as their Clubcard gives me back £5 or £6 every so often so offsets the extra cost. If I can't find 99 Octane when I need fuel I'll only put enough in to get me to some 99 and then fill up and won't use full boost until I do.

Are you telling us you've got an S2 with an Everest Chip and put crap petrol in it ? Your knock sensor must be working overtime when you're on full boost at full throttle if you do
Many TV programmes did tests on it years ago .. here's one that gets interesting around 4 minutes in. (Shell Optimax was 98 then V POwer is 99 now). Conclusion is it pays for itself on a performance engine.

5th Gear Fuel tests.

[Giniw]

I put some Sans plomb 95 in my Elan (sans plomb=unleaded) and it's written in the manual that it's what it's calibrated for (I don't know for the recommended petrol in UK — I think the octane rating here is not the same as yours?)
I don't think that putting a higher octane petrol would give me more performance since the ECU maps are calibrated for a 95 octane value.
And I didn't have any detected knocks when I checked, so ...
As for the Porsche engine, I suppose they obtained their claimed power output only with the highest octane, hence the reduction with a lower octane petrol.

If I can't find 99 Octane when I need fuel I'll only put enough in to get me to some 99 and then fill up and won't use full boost until I do.

Why not? The engine knock counter would reduce the advance if it detects a few knocks so it would be safe anyway, wouldn't it?

PS: the 98 octane petrol here is supposed to have more cleaning (?) additives in it so I sometimes put a tank of it, but otherwise I don't think it's any better on our cars.

[Kathryn]

I can't say I've noticed any difference between 97 and 99 octane! My local garage is a Shell so I often use V-power, but if I'm in town I'll pop into the Apple Green for their 97 super which is usually the same price or only a couple of pence more than Shell's standard unleaded. I did notice a difference when I put a little Asda fuel in as an emergency though! Rough as hell when I started her up the next morning! I'm waiting til I pull into a garage and they've run out of super unleaded to try standard unleaded again, but it's not happened yet!

[dapinky]

I just use 'standard' Sainsburys unleaded - dunno what the octane rating is, but it works!

[lotusflasherman]

We are talking about Everest Chips and higher than standard boost. Everest Chip runs boost up to 0.9 bar so a mechanical compression ratio of 8.5:1 will have an effective compression ratio on full boost and full throttle of 8.5 x 1.9 (less a bit for induction inefficiency) so about 15:1 so I suggest the higher the octane is, the better if you are going to get the benefit from your expenditure on the chip ... it will run perfectly on lower octane fuel at part throttle and standard boost but at full throttle, full boost the knock sensor will have some work to do. The knock sensor is similar to a rev-limiter, a damage limitation device that you don't really want to use much, if at all .... If it's operating you're not running the engine at it's optimum.

I'm now going out to play football and these guys explain it better than me .. https://www.turbobygarrett.com/turbobygarrett/compression_ratio_with_boost

[Giniw]

We are talking about Everest Chips and higher than standard boost.

All right, I was more referring to a stock engine, sorry.
So the everest chips maps are designed with a higher octane in mind? I haven't had read that before.
Again I understand that it allows it to go further, but I just didn't know it was designed with that in mind.

If it's operating you're not running the engine at it's optimum.

Of course, I agree with that, it's just that it won't (shouldn't) damage the engine if the advance is reduced.