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Super Unleaded fuel gives an improvement in consumption approximately equal to the increase in octane, ie 95 > 98 = 3% better consumption. The downside is that SUL is more than 3% more expensive than unleaded. Does HJ, or any other contributor, know of any Octane Booster which is cost-effective to use?
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SUL must be cheap round your way! 3% = about 2.5p/litre. In my experience it's up to 10p a litre more expensive (about 14%) which is unlikely to returned in improved economy.
I'm sure you realise that octane rating bears no relation to efficiency, energy content etc. SUL has exactly the same energy content/kg as normal UL. Unless the car it's used in has an ignition system able to take advantage of the increase knock resistance and modify the operation of fueling and ignition appropriately SUL will have no benefit.
Regards
John S
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It will only give that improvement if your car has "active knock control" and it has been calibrated with it operating.A lot of cars don't have it,it's not that easy to calibrate because it picks up minute vibrations in the cylinder block caused by detonation and retards the spark when this happens-Slight differences-engine to engine-can cause all sorts of concerns.Most manufacturers calibrate on 95 octane and leave it at that.
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Roger's point is certainly valid if he is talking about older cars, pre electronic days, that needed the old proper leaded four star before they started reducing the lead content. I have found that ordinary unleaded does not have quite the knock resistance as old leaded. In these circumstances, a tendency to pink would certainly by reduced by using super unleaded, with a corresponding improvement in economy, apart from less engine damage.
I use (Triumph 2000, 1964) ordinary unleaded with Millers VSP, which works out cheaper than SUL or LRP.
But on a modern engine I am sure fuel economy doesn't come into it.
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Cliff
Fuel economy is more likely to come into play on a modern engine than an old one. A modern engine with knock sensing can continually adjust the timing to keep the engine at maximum efficiency. However, there will only be a benefit with SUL if the engine is designed to take advantage of that fuel. Check the handbook - it should say if it will provide any benefit. Generally it's the 'sportier' cars which can benefit.
As for older cars, 'Normal' unleaded is an octane point or two lower than the old 4 star, but provided the engine doesn't pink it won't do any damage, neither will it harm economy. Just because the octane rating is lower it doesn't mean the petrol has a lower energy content. However, if the engine needs the timing retarded to prevent pinking on UL, then there will be a performance/economy drop.
If it needs higher octane rating to prevent pinking, then the options are SUL or an additive. Equally the additive may be needed if the valve seats haven't been changed to accept unleaded. I suspect driving the engine gently on normal unleaded to prevent pinking will probably prove more economical than using a higher octane fuel, but probably not a good idea in the long run.
Regards
John S
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You are quite right, I should have said, "modern car but lacking a knock adjuster".
It also depends where you live. In East Anglia I agree driving gently to avoid pinking is probably the best bet. In Wales every trip involves storming up a hill on full throttle, so anti-pinking ability becomes a big factor.
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"In East Anglia I agree driving gently to avoid pinking is probably the best bet. In Wales every trip involves storming up a hill on full throttle"
Careful, don't upset David Woollard. Some of those hills in East Anglia can be over 100 feet high!
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The calorific value of petrol IS proportional to the octane rating, at least for most practical purposes. So SUL does contain more energy, about 3%, than normal unleaded. It takes a degree in chemistry to explain so I just accept what experts in the petro-chemical industry say. As SUL is about 10p/litre more than unleaded in all areas, it is not cost effective to use it just for economy. But if I can find an octane booster which is cost-effective then I would like to use it.
My car is "only" a Vauxhall Astra, but like most modern cars it has 'selective knock control' which optimises the use of SUL.
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Given that aviation fuels still use lead additives to boost octane rating, it's not true that there is a relationship between calorific value and octane rating.
I should perhaps have said that I believed there was no significant difference in cv between UL and SUL. It's true that petrol calorific value can vary slightly depending upon the hydrogen/carbon ratio. Data I have indicate a 5% variation in net cv between 41.9 and 44.0 GJ/t. I believe that covers all petrols, including aviation fuels, and I suspect the variation for motor fuels is lower. Some of this variation will also reflect the blend changes between summer and winter fuels. Therefore although it seems feasible there may be some small cv variation between UL and SUL I'm intrigued it's as great as 3%.
I'd welcome at least the simplified version of the background to this if you have it.
As you say, though, the potential cv improvement alone doesn't justify the cost premium of SUL.
Regards
John S
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