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Topic # 112482 10-Dec-2012 17:04 Send private message

Having recently just purchased a new bike, a comment from the dealer when picking it up worried me, and seems to be in line with a lot of other people's perception about what the octane rating of petrol means (including my own until recently).

There seems to be a perception (lets blame the oil companies marketing) that a higher octane (and therefore more expensive) fuel is a) better for your engine and b) will lead to more power or fuel efficiency.

In many cases, the above are simply not true. 98 RON gas, for example, has no more 'energy' in it than 91 RON. The higher the octane rating, the less likely the engine is to 'knock'. The higher the RON, the higher the 'auto ignition' temperature of the gas. Same energy, same burn time, different ignition points.

The reason some cars require a higher octane gas is that they are tuned for it, often because the engine has a higher compression ratio and therefore a lower octane fuel is more likely to ignite early and knock.

Many modern engine management systems will compensate if you put in a lower RON fuel but modifying engine timing at a cost of some power. So in actual fact, if you put 91 RON fuel in a car tuned for higher, you will get a reduced power output (in a modern car). The marketing around 'more power, smoother acceleration' etc. is all based around the fact that this will happen if you are using a lower octane fuel than your car is designed for.

Unless you drive a high performance vehicle, or a euro designed for it - Your car is likely no better off with a higher octane fuel - nor is your wallet.

My bike dealer told me to put in the 'highest octane fuel possible' and that the bike will 'cough and splutter' if I put in dirty 91. In fact the bike manual suggests 91, which means that the engine won't be designed for 95+, and therefore I would just be wasting my money.

91 RON fuel is no 'dirtier' than 95. Only difference being some oil companies will add 'cleaning' additives to some of their 'premium' fuels, but their effectiveness has not been proven. There has also been suggestions that running a higher octane fuel than required will lead to deposit build ups within the engine, but that is beyond my understanding and possibly incorrect.

I didn't know about how the Octane rating worked until a few months ago when I read about it, and before then I too believed the marketing hype around premium, higher octane fuels.

I'm keen to know if anyone has been running a different kind of fuel than what the manufacturer says to in your vehicle, and what you have noticed.

Don't waste your money on fuel you don't need people!




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  Reply # 729876 10-Dec-2012 17:10 Send private message

I have a Mazda MPV V6, and it certinanly seems to me that the engine runs better with 95/98 rather than 91. I have no idea if it designed for this or not. When my wife used 91 to save money the engine stuttered quite a bit.

Luckily my other car has a big sticker on the inside of the petrol cap that says ?use 91 octane fuel? so for a nobive that was quite helpful :D

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  Reply # 729877 10-Dec-2012 17:11 Send private message

I run 98 in my Evo 1 which is tuned with a Link G4

Running 95 in the Swift Sport, from memory the manual suggests 95

I agree that higher rating is only effective if your car is tuned for it, otherwise you're just wasting your money.

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  Reply # 729895 10-Dec-2012 17:27 Send private message

A lot of Jap imports, especially if they're (even somewhat) performance oriented, are tuned for high octane gas and will not perform properly on 91 gas.  

In some cases, the engine will go into "anti-knock" mode, retarding timing and adding extra fuel until the ECU is reset.  It will be drivable, but down on power and use a lot of extra gas.  This usually doesn't cause the check engine light to come on so you may not even know.

In other cases the engine will start to knock when it gets up to temperature, which will feel like a loss of power especially at lower revs/part throttle and has potential to cause serious engine damage.

But as you have pointed out, if your car is designed/tuned for 91 octane gas, then 95 octane or 98 octane won't make a lick of difference.

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  Reply # 729902 10-Dec-2012 17:41 Send private message

Did a comparison last year for about 12 weeks comparing 91 v 98, ran 6 weeks on each.

I tried to make it as even as possible. I was filling up around 3 times every 2 weeks and was typically travelling the same route every day, although sometimes the accelerator was depressed a little more aggressively than others.

Overall there was a perception of greater performance with 98, but not so that I cared and cost wise it ended up being around 1c per kilometer more. I still fill up with 91.

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  Reply # 729904 10-Dec-2012 17:42 Send private message

I found that throwing 98 into my Primera designed to run on 91 did slightly increase the milage I got per tank, however the extra KM/L did not justify the added cost of 98.

I guess it's another case of RTFM



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  Reply # 729908 10-Dec-2012 17:49 Send private message

insane: I found that throwing 98 into my Primera designed to run on 91 did slightly increase the milage I got per tank, however the extra KM/L did not justify the added cost of 98.

I guess it's another case of RTFM


I just can't see how that is possible. Unless your car was 'knocking' on 91, there is no way you could get better mileage.

It's like saying a car painted with 'racing stripes' goes faster.




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  Reply # 729918 10-Dec-2012 18:19 Send private message

I can consistently get another 40-50KMs out of a tank if I use 95 instead of 91, so about 8% further for the extra money.

Late model VW. It does also have a nice little 95 symbol under the gas flap to make you feel guilty about anything less though.




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  Reply # 729919 10-Dec-2012 18:27 Send private message

Chippo: I can consistently get another 40-50KMs out of a tank if I use 95 instead of 91, so about 8% further for the extra money.

Late model VW. It does also have a nice little 95 symbol under the gas flap to make you feel guilty about anything less though.


Yeah, so in your case you don't get 'more' power by using 95, but you get less power by using 91 as your engine management system will be compensating and re adjusting the timing.




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  Reply # 729946 10-Dec-2012 19:41 Send private message

ajobbins:
Chippo: I can consistently get another 40-50KMs out of a tank if I use 95 instead of 91, so about 8% further for the extra money.

Late model VW. It does also have a nice little 95 symbol under the gas flap to make you feel guilty about anything less though.


Yeah, so in your case you don't get 'more' power by using 95, but you get less power by using 91 as your engine management system will be compensating and re adjusting the timing.


I just got a 2004 Golf with a 2.0 FSI engine. VW recommends the highest octane fuel for it (98), for improved efficiency.

Thanks to wikipedia, apparently an Octane rating relates to the amount of pressure a fuel can sustain before self-ignition (creating a "knock"). There are a couple of interesting points in the article about efficiency relating to this.

1. There is a most efficient point in the engine cycle to ignite fuel (depending on a number of factors including load).
2. Newer cars have higher compression ratios than older cars as this is more efficient (direct injection etc).
3. If fuel self-ingnites there is knocking in the engine which is defined as an incomplete/inefficient fuel burn (ie. the fuel isn't burned in most efficient part of the engine cycle).

If you have an older car, with a lower compression ratio, your fuel is less likely to self-ignite due to compression, therefore higher octane fuel which has a higher self-ignition point has no benefit in your car.

If you have a newer car with a higher compression ratio, a higher octane fuel means less chance of self-ignition a result of which is a more efficient burn. However a lower octane fuel is more likely to self-ignite which results in a less efficient burn.

Presumably as your new car gets older you can use lower octane fuel, as the seals wear it won't sustain the same compression ratio.

I would be interested to know in newer cars whether this makes an appreciable difference to fuel use. To me air-conditioning and driving type/location probably makes a bigger difference. More interesting was going for a taxi ride last week I heard from the taxi driver in a late model Prius he gets 4.1L/100km...

Hope this helps,

Jon

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  Reply # 730058 11-Dec-2012 00:24 Send private message

ajobbins: 
Yeah, so in your case you don't get 'more' power by using 95, but you get less power by using 91 as your engine management system will be compensating and re adjusting the timing.


That doesn't even make sense. If you get 'less' power by using 91 over 95, then conversely you get 'more' by running 95 over 91.

Whether or not you will in fact get more power using a higher rated fuel is determined by your car and its engine management system.



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  Reply # 730063 11-Dec-2012 00:44 Send private message

blair003:
ajobbins: 
Yeah, so in your case you don't get 'more' power by using 95, but you get less power by using 91 as your engine management system will be compensating and re adjusting the timing.


That doesn't even make sense. If you get 'less' power by using 91 over 95, then conversely you get 'more' by running 95 over 91.

Whether or not you will in fact get more power using a higher rated fuel is determined by your car and its engine management system.


Sure it does. Becuase the fuel isn't giving you 'more' power, it is giving you the normal amount of power for your engine. Using 91 when your car isn't designed for it forces your car to be less efficient, and less power.




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  Reply # 730071 11-Dec-2012 06:06 Send private message

I've always used 98 in my turbo cars (They were tuned for it). My current car (Tuboed) using 98 but i do have another program that i could run 95 but at a large KW\HP loss.

Got to get to that 100kph as fast as possible :-)

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  Reply # 730119 11-Dec-2012 08:06 Send private message

ajobbins:
blair003:
ajobbins: 
Yeah, so in your case you don't get 'more' power by using 95, but you get less power by using 91 as your engine management system will be compensating and re adjusting the timing.


That doesn't even make sense. If you get 'less' power by using 91 over 95, then conversely you get 'more' by running 95 over 91.

Whether or not you will in fact get more power using a higher rated fuel is determined by your car and its engine management system.


Sure it does. Becuase the fuel isn't giving you 'more' power, it is giving you the normal amount of power for your engine. Using 91 when your car isn't designed for it forces your car to be less efficient, and less power.


Logically, if using 91 gives you less power than using 95, then it must also be the case that using 95 gives you more power than 91. It cannot be any other way. That is just how maths works.

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  Reply # 730123 11-Dec-2012 08:21 Send private message

I have two cars - both have different requirements and I've tested the limits with both:

Mazda SP20 (Japanese 4wd version) - MUST use 95+ or it knocks It also loses significant power on lower octane gas.

Subaru Traviq (rebadged Opel Zafira) - recommended 91...have used this most of the time but tried 98 to see if the marketing hype was to be believed. I was wasting my money. 91 gives me the optimal performance and it doesn't change when using "premium" fuel.

The Mazda is a lot thirstier, which makes it doubly annoying...not only do I have to pay more for my fuel, but the car uses more of it, therefore costing me even more.

Needless to say, the Subaru is the car of chouice for most regular use.





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  Reply # 730128 11-Dec-2012 08:30 Send private message

My Focus hates 91, but runs nicely on 95. With the higher octane ratings in Europe (98 is very common) it seems that most European engines are the same and the management systems choke on the lower octane fuel.

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