Higher octane petrol

  • Does any one know from experience if using 98/99 RON fuel makes any noticable differance to how a K8 performs or MPG wise?

  • In that order, none at all and a bit.

  • Our engines were designed to be run on the normal unleaded gas unlike the 3SGTE engines. (MR2Turbo)

  • ok thanks, i was just curious

  • havent noticed any diff performance wise, but without a dyno youll be struggling anyway, have about 4mpg extra using premium fuel, may or may not be worth it but premium costs only 1p more that standard at my local esso, so make no sense not to, but Id still use it anyway as it contains good additives to clean and maintain your internals, so if youve ever used redex, well, you wont need to, so saves you money that way too, so for me its a double win even if it costs a few pence more. I think an interesting topic or poll at some point would be to see who uses what fuels religously, ie standard/premium always, and who has had emissions problems at mot time, wouldnt be conclusive obviously, but interesting non the less!

    The ecu is designed to detect different octane levels and run on a different mapping due to that though, so its not like youll be wasting money as your ecu will make it work for it, also depends where you spend your time on the rev range, if you drive fast and find yourself at the high end of the rpm guage quite often, then a higher octane fuel will benefit you as it allows the ecu to adjust the timing further and get more power out of the engine, but only at higher revs.

  • Octane is the eight carbon hydrocabon as opposed to heptane(7) and hexane (6). Petrol as we know it is mostly Hexane in chain hydrocarbon form and the carcinogenic benzine ring form. The exothermic (heat releasing) oxidisation of hydrocarbons (burning) is what provides the motive power of internal combustion engines. The shorter a hydrocarbon chain, the shorter its reaction time, and the longer the chain, the longer the reaction time. By increacing the proportion of Octane, the burn time is increaced and power and torque also benefit.

    Think of gun powder as a propellant vs C4. The gunpowder gives more "shove" because it burns relatively slowly giving continued reaction as the space in the bore increaces. A higher speed reaction is all spent before the cannonball/piston gets moving.

    Back to engines. In real terms, you can buy a few bhp with 98/99 RON fuel but unless you are racing in a class with stringent tuning rules its not really cost effective, it will just feel a bit sweeter. It is a good idea to run a tank of "super" before an MOT beacause the slower burning octane will yeild more CO2 (carbon dioxide) and thereby reducing CO (carbon monoxide) content of the emmissions. CO is formed by the incomplete oxidation of hydrocarbons due to the speed of reaction in engines, and it is this incomplete reaction which makes CO so poisonous, it is desperate to bond with another Oxygen atom, and is so powerfully reactive in this respect that when dissolved in blood (by breathing it) it steals the oxygen from your heamaglobin-suffocating you. Lovely!

    But, where the increase in Octane does show a significant benefit is when forced induction (turbos / superchargers) or NA tuning (skimming the head/high comp pistons) push the compresion ratio much above 11:1 where detonation starts to rear its head. The longer hydrocarbon chains have higher detonation points. So if you are considering serious tuning, then higher RON fuels are more of a necessity than mere option. I know a guy who had a large intercooler and big turbo on a Ford focus RS - about 300bhp, and he had to run that on 99 RON or AVGAS 110 for track days.

    This is very much an abridged chemistry of fuel but it should hopefully illustrate the principals :)

  • Thats not exactly right about the gun powder vs C4, while something may react more quickly, it doesnt necessarily mean thats its reaction is less violent or propulsive. Infact its the opposite, if I flik a BB pellet at you, it wont hurt, if I shoot it at you, it will, because its moving faster which gives it more kinetic energy. Much like a propellant, if you used C4 as a propellant its reaction is so quick and violent that few the things could withstand its force, c4 has a detonation wave around 20,000 feet/ps. That would surely make it the more desireable choice of propelant, if it didnt rip barrels apart, the only practical ability of high explosives in munitions is for igniting the powder.

    A faster burning compound would be better since it can transfer more kinetic energy in less time, the longer it takes to transfer the energy the less energy is transfered, as the projectile leaves the breach and travels the barrel it is rapidly moving away from the source of propulsion, which as the distance becomes greater the force of that propulsion has less effect. If a compound gives 5000 fps of expansion and another makes 10,000, then the projectile will only be accelerated as fast as the compound used can expand. While it would seem an easy choice which to use, the problem in their application comes largely from the projectile material and the G forces that it is placed under. Think of the difference between throwing a handful of jelly and a solid lump steel, the jelly wont cope with the forces placed upon it and just disintegrate before its left your hand, the steel however would maintain its shape and therefore be fine. When subjected to high powered propulsion you can make metal act the same as the exampled jelly with it distorting massively, tearing up barrels and becoming wildly inaccurate due to deformation.

    On another note though, the quantities, either volumetric or weight, if the same, would provide very different results due to them having different energy quantities contained within them, if you had 1gram/5ml of powder and 1 gram/5ml of c4, which would be the more powerful, in either gram or ml, the c4 is more powerful.

  • My analogy was supposed to be taken in the context of driving a piston down a barrel, and you touched on the reason: C4 would burn so rapidly that it would destroy the weakest componant in order to vent the massive pressure, rather than utilise the barrel to propell the piston. Whereas gunpowder leaves everything intact-piston included. By no means am I calling C4 a sissy.

    This is, however, far from the point. Higher RON burns slightly slower producing slightly more power and torque, surviving higher compression ratios.

  • If they made the barrels and breaches bigger and more reinforced, then it wouldnt be a problem, and then imagine the anarchy you could have lobbing solid slugs out of battery guns. While it is far from the point, explosives and blowing stuff up is far more interesting than octane anyway, its been asked a million and one times on the internet already, and the answer remains the same. Application. :)

    Who wants to talk about rail guns?! :P


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