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  1. #1
    Senior Member george12's Avatar
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    Default 91 vs 95 octane petrol

    Hi

    I was looking at some car auctions on Trademe today, and came across an answer to a question about whether the car would run on 91 petrol. The seller (who is a car dealer who sells tens of cars a week) said that "It will run on 91, but I wouldnt run ANY car on 91. It is a false economy to put crap petrol in a car. You are better off with 96. The car will like it and it will go further.".

    Does anybody know if this is true? My car is a Japanese make (Honda Civic) and I have always used 91, as this is what the car was designed for. Is there any reason I would be better off putting 96 in it?

    I understand the reasons why you wouldn't want to put lower grade petrol than recommended in, but I have no idea what the positive effects are, if any, or putting higher than recommend grade petrol in.
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  2. #2
    Seasoned Member allblack's Avatar
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    Default Re: 91 vs 95 octane petrol

    Not really answering your question.....but I've been eyeing up BP's 98 Ultimate fuel....I like the idea that it cleans.

    The car was running on fumes this arvo, so swung by and threw in $30 of the stuff....think I got about a cupful of fuel for that ......figured if the car spits the dummy I can just fill it with the usual "crap" fuel and all's good in the world.

    I have a (Dec) 02 Maxima....I figure it should go fine. If it does, I'll probably run it on 98 from BP for a while and see if I can notice a difference. If not, will go back to the usual 91...

    My understanding of octane is....simply the higher the number, the more explosive the fuel is....ergo more "bang for ya buck"!

    However, I'm sure there's more to it than that.....

  3. #3
    Member timaru2007's Avatar
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    Default Re: 91 vs 95 octane petrol

    I have been keeping a log of fuel consumption for over three years now for a '95 Impreza.
    For the first year it ran only on 91 (cheaper) and after that for 2 years on 95 only. On 91 the car ran a long term average of 10l/100km. Now running on 95 it runs a long term average of 9.1l/100km. And that is with a roof rack permanently attached to the roof rails. It is an oval rack though.

    Makes sense to me to use 95 octane...

    F

  4. #4
    tweakedgeek tweak'e's Avatar
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    Default Re: 91 vs 95 octane petrol

    wheres surfer joe got too......

    with exspection of some cars which can use both its best to use whatever one the engine is diesined to use. ie if its a 91 car then use 91. useing 96 on a 91 car generally dosn't make it go any better to make it worth the extra cost. you really need to tune the car to suit the fuel it runs. ideally you want to increase compession for 96, something thats not easy or economic to do.

    some cars can run on either (it retunes itself) so a bit of trail and error to find which fuel it likes best.

    My understanding of octane is....simply the higher the number, the more explosive the fuel is....ergo more "bang for ya buck"!
    octane is basicly a rating of how well the fuel withstands knocking, its not a measure of how much power or "bang" you get.
    Tweak it till it breaks

  5. #5
    Senior Member george12's Avatar
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    Default Re: 91 vs 95 octane petrol

    I'm pretty sure my car won't retune itself. It's 1993, but it has a carb, not fuel injection, and I don't think it has a computer at all - ie, there's nothing TO automatically retune it .

    I'll try 95 for two fill ups and see what happens to my fuel consumption. I get very bad fuel economy with my car, but I suspect it's more related to my driving style than the car. I use about 12-14L per 100km around town, but on a long trip I can get 6.7L per 100km.

    I certainly don't drive like a maniac, but I could probably do a lot better......
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  6. #6
    Large Member plod's Avatar
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    Default Re: 91 vs 95 octane petrol

    Running your high performance car(evo, wrx etc) on 91 when it should run on 95/96 will in fact retard the timing producing less HP. Running your average car(corolla) on 95/96 when it is designed to run on 91 will only cost you at the pump with no performance increase.
    Top gear did a test on this very thing
    Last edited by plod; 25-06-2007 at 10:19 PM.

  7. #7
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    Default Re: 91 vs 95 octane petrol

    Quote Originally Posted by george12 View Post
    I get very bad fuel economy with my car, but I suspect it's more related to my driving style than the car. I use about 12-14L per 100km around town, but on a long trip I can get 6.7L per 100km.

    I certainly don't drive like a maniac, but I could probably do a lot better......
    If it's still the Honda, 6.7/100km is OK on a trip according to a net roadtest but you should also get 8-9/100km city so stop driving up and down Kent & Cambridge Tces or you'll lose your licence before you get it

  8. #8
    Ifindoubtgiveitaclout
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    Default Re: 91 vs 95 octane petrol

    All modern cars have knock sensors in the engine management system. They detect detonation, this causes engines without knock sensors to "pink". In other words the explosion in the combustion chamber happened too quickly & therefore early, causing the combustion force to try & push the piston straight back down the bore while it's on it's way up, this causes the piston to go sideways & smack the bore. The knock sensor retards the ignition timing until the detonation happens too late to make the engine "pink". Low octane fuels burn less controllably & much faster than higher octane fuels. So if you use high octane fuel, which most engines are designed to use, you will get more power, better fuel economy & less stress in general on your engine. The only reason you get away with running an engine on 91 tapwater, is the fact that the ignition systems of todays cars are able to cope with it. Thats why your average full of ***t car salesman says " The cars designed to run on 91"

  9. #9
    Smiling Down On Youse SurferJoe46's Avatar
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    Default Re: 91 vs 95 octane petrol

    Quote Originally Posted by tweak'e View Post
    wheres surfer joe got too......

    with exspection of some cars which can use both its best to use whatever one the engine is diesined to use. ie if its a 91 car then use 91. useing 96 on a 91 car generally dosn't make it go any better to make it worth the extra cost. you really need to tune the car to suit the fuel it runs. ideally you want to increase compession for 96, something thats not easy or economic to do.

    some cars can run on either (it retunes itself) so a bit of trail and error to find which fuel it likes best.

    octane is basicly a rating of how well the fuel withstands knocking, its not a measure of how much power or "bang" you get.
    Here I am!

    Today's lesson is on ...Octane... mostly misunderstood (and there are so many wive's tales about it) ... so that it needs a little clarification.


    Octane is the rated value at which a fuel resists self ignition. Simple. Effective, but still misunderstood.

    If an engine requires a certain value in the fuel...and NZ standards for measuring of that property are not unlike US values with just a different set of numbers, then this will be easy.

    Again..IF an engine is designed with a compression ratio of a certain amount, it will require a known octane rating. If we start at about a 7:1 (swept volume to squished area) in a combustion chamber, we can probably get away with some really low octane values as the stoichemetric temperatures aren't really being exceeded.

    As we decrease the squish area (that's the small chamber that's at the top of the piston stroke, usually in the head but may also be partially in the crown of the piston in the form of a dish), by making the chamber smaller, we will require a higher octane fuel to resist self ignition of the fuel, causing damage and excessive wear to the rings, rod bearings, wrist pin and performance. We could even see a hole form in the top of the piston if the pre-ignition is too severe.

    This begs the question: "If we would need higher octane fuel to run an engine with a higher compression ratio (more like 9.5:1 or higher), then why build such engines?" Easy..the higher the compression ratio, the higher energy yield can be achieved if the fuel is up to the task. It's a horsepower thing..if we raise the ratio of swept volume in the cylinder to the value of the final small space that the air/fuel will occupy at the time of ignition, we get more bang and resulting more power from the fuel.

    Many factors..MANY factors can change these gains and losses in power and efficiency. Newer technology allows relatively high (by current standards anyway) compression ratios by exact control of the amount of fuel admitted (injected into) the engine, the time of the spark plug firing and control of such obtuse factors as individual cylinder air inlet temperatures and piston cooling all coming into effect. The new stuff is amazing and we ain't seen the end of the tunnel yet.

    Now..on to using a higher octane fuel in an engine that does not REQUIRE it....DON'T DO IT!!!!!

    Allow me to emphasize that again: DON'T DO IT!!!!

    Putting fuel of higher octane into an engine that does not require it..is damaging to so many other devices and in ways that may not seem obvious at first but are gonna bite you in the fleshy places later on.


    Higher octane than required fuels cause damage to:

    The exhaust valves and seats...they are getting burning plasma-hot gasses flowing over/thru them as the fuel is STILL BURNING WHILE GOING OUT THE EXHAUST.

    The valve actuation system..cam, lifters/followers, rockers, valve stems..etc. They get hit pretty hard having to open against the excess pressure that should be lowered by the time they are moving..but instead have to push against serious pressures since the fuel is STILL BURNING AND CREATING PRESSURE.

    The cooling system....needs to remove the excess heat from around the pocket at the exhaust valve seat and where the exhaust exits the head. This area is pretty robust, but it is working with a small safety margin when the fuel is burning correctly and not still under ignition as it passes through that/those areas. If you have a single cylinder engine and put fuel into it that is too high in octane, one can see the exhaust pipe near the heat glowing cherry red from all that heat. The same is true in the heavier metal of water cooled engine, only the heat in the cast iron or aluminum may not be as obvious on them .Exhaust manifolds have been known to run cherry red and crack though.

    Generally there is a degradation in power and performance to an engine running higher octane than it is designed to run. You lose gas mileage, lose horsepower and cause untoward heat damage to expensive components too.

    Reality states that using something better (here, the "better" is not a higher octane fuel) is the right thing to do. Just make sure that the "better" is not adding fuel to wrong ideas and old wive's tales.
    The problem with going to the stars is only the first few hundred miles.

  10. #10
    They call me David... The_End_Of_Reality's Avatar
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    Default Re: 91 vs 95 octane petrol

    LOL, another of SJs lectures

    My car has a Shell sticker saying use only Ultra Hi Unleaded 96 in the fuel lid (I have always used 91 but the previous owner did say he had used 96 with no issues...) and it is a 98 Corolla... So what would the best fuel for that be? (It a 4A-FE)
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