Thread: Hi Octane fuel

Our Subaru's ej20 motor likes 98

My disco ignores the stuff, no difference between normal ulp and premium.

So we fuel them accordingly

Originally Posted by PhilipA

Just to throw another spanner.
Ethanol has only 60% of the calorific value of petrol, so 10% ethanol should yield 4% poorer fuel economy than straight petrol.
So the price difference of 4 cents is really nil.

BUT just to add to the subjective nature of the argument, I cannot say I have noticed any reduction in fuel economy using E10, which I have tried a couple of times.

Isn't this because Ethanol is partially oxygenated? Which means that while blends like E10 have a slightly lower (theoretical) calorific value, in practice they combust more completely....

Thanks to all for your very interesting & informative replies. Never knew there was so much involved in filling up!

Going to use up this tank of fuel & go back to regular & see if there's any difference. Think I'll take her up the shed & put the light on & see what the timings set at. Will ring George first, to see what he set it to. (If he remembers!).

Thanks for all your advice, learnt heaps.

Best regards,

Panda

ps I'll keep you posted as to the outcome

Originally Posted by Slunnie

Firing ATDC sounds like a waste as the horse has already bolted so to speak. That the fuel is probably too good for the engine and a waste. If they are the specs you're probably better off running it on the lowest octane fuel (reg ULP) and setting the timing to suit. .....

......

As with most Series ignition settings, these are the static settings with the engine stopped. With the engine running the automatic advance mechanisms (centrifugal and vacuum) will have advanced the spark well beyond this, according to the designed curves relative to rpm and manifold pressure.

So the engine will never actually fire after TDC except at cranking speed - when it probably IS a good idea to prevent shock loads on the starter or kickback when starting by hand. Even when idling (although the centrifugal advance will not have started to change timing), the vacuum advance will ensure it is well advanced, while retarding it to allow smooth full throttle response from idling without preignition.

John

John

"Octane level" is indeed a measure of how a fuel resists exploding suddenly, instead of burning smoothly without suddenly all exploding at once. Pure iso-octane (a hydrocarbon with 8 carbons in it and a particular molecular shape) was deemed to have an octane rating of 100. (There was some other hydrocarbon which had a very low number, but I forgot what one was used 30 yrs ago). Hence 98 octane resisting exploding 98% as well as does pure oso-octane. Using pure iso-octane in an engine would be terribly expensive to make, and it would also have problems vapourising on cold days, seals would explode and the fuel would provide no lubricity.

My D3 V8 gets about 5% better fuel consumption on 98 octane than it does on 95 octane (but the D3 has more electronic engine sensors than does NASA). Hence if the price difference is <5%, I go with 98. The higher octane fuels also have some more expensive additives, which do things like keep the fuel system cleaner. HOWEVER, if you put fuel (or crankcase oil, for that matter) which has all kinds of cleansing properties in an old, dirty engine, there's a risk of just moving dirt/deposits from some safe spot and plugging something expensive. There is a slight (ie, insignificant) increase in fuel density in premium petrol. It has to do with the fact that in making it higher octane, the hydrocarbon chains in the fuel are longer and straighter, which lets about 1% more pack into a litre. With no tetra-ethyl lead available to raise the octane, longer and straighter hydrocarbon chains are the alternative way of getting the octane up.

Ethanol has other properties which owners need to be aware of. As any beer drinker can understand, ethanol and water mix delightfully. In a fuel system, the ethanol can pick up old droplets of water and flow it into the engine fuel injection/carbies. The problem is that water in metal causes rust/corrosion, so putting in ethanol can cause injectors or carbies to suddenly suffer plugging from corrosion. Ethanol is also a good solvent, so it can dissolve the gum lying in the bottom of the fuel tank and deposit it in the carbie/injector.

Sorry about being a bit long-winded and technical. Something about being a Chemical Engineer who worked in a refinery a long time ago. I do recall that there were something like 34 different specifications that petrol had to meet or it had to be re-run through the refinery processing units. Changing any one component or property can cause all sorts of problems, which is why industry is not too keen to change too much too quickly.

Thanks for that post Jim,

I spoke to my mechanic (who has worked on Series since God was a boy), & he said basically the same thing.

Reckons it will run better on the premium stuff, but not to put in the additives (as I did yesterday!), as not a good idea & can stuff the engine. He said it'll be ok as a one off, but not to put it in in future.

For a few extra cents a litre I'm definitely going to stick with the good stuff.

Man I've learnt a lot from all these posts! Never realised there was so much involved in petrol!

Which reminds me, did anyone see the Top Gear episode (I think it was Top Gear?) where some bloke put used restaurant cooking oil in a diesel & it ran? He added something else to it, can't remember what it was, but they had an "official" who drained the diesel prior to the experiment, primed it etc, & it ran sweet as. Apparently only cost something in the order of 5 pence a gallon. If that's the case & it catches on, won't take the government long to suss it out & put heaps of taxes on it!

Originally Posted by Panda

Which reminds me, did anyone see the Top Gear episode (I think it was Top Gear?) where some bloke put used restaurant cooking oil in a diesel & it ran? He added something else to it, can't remember what it was, but they had an "official" who drained the diesel prior to the experiment, primed it etc, & it ran sweet as. Apparently only cost something in the order of 5 pence a gallon. If that's the case & it catches on, won't take the government long to suss it out & put heaps of taxes on it!

Mate that is diesel not petrol. Very different beasts. Even then the diesel TD5 specifies no bio diesel.

Yea, I know it is diesel not petrol. I think it was a diesel Volvo. & yea, even I know they are entirely different! Just thought it was interesting.

Originally Posted by Captain_Rightfoot

Mate that is diesel not petrol. Very different beasts. Even then the diesel TD5 specifies no bio diesel.