The Octane Misconception

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#1638699 22-Sep-2016 11:50

timmmay:

Paul1977:

Timely that someone should bring this thread back to life. My father has recently bought a car with a high compression engine (2005 Toyota Mark X). I have the same car so had done a little research when I got mine, and the general consensus online was that it should be run on 98, but could likely get away with 95 (but with reduced power as computer would adjust timing to prevent knock). Everyone seemed to be in agreement that you shouldn't run it on 91.

I explained this to my father when he got the car, but just discovered last night that he has since filled it up with 91 "because the dealer never mentioned needing 95 or 98". I, of course, had words with him about why he would just ignore my advice!

But my question is, just how much can the anti-knock sensors compensate for? Surely it can only adjust the timing so much? Could he be damaging his engine?

At this point, with half a tank of 91 left, should he top up with 98? Will this bring the overall octane up as the fuel mixes together? Or is he better to wait until the tank is near empty and fill it completely with 98?

If the manual says use 98, use 98. If the manual says use 91, use 91. If you need to switch then I'd probably let it get pretty low first.

Yeah, I'd made it very clear to him he should be using 98 - I don't know what possessed him to put 91 in it.

tdgeek

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#1638710 22-Sep-2016 12:18

NonprayingMantis:
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.

No. What AJ is saying, is that both give the same power, but if you use 91 in an engine that requires 95, the ECM system will throttle it, i.e. tone it down to avoid knocking.

tdgeek

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#1638713 22-Sep-2016 12:25

Paul1977:

timmmay:

Paul1977:

Timely that someone should bring this thread back to life. My father has recently bought a car with a high compression engine (2005 Toyota Mark X). I have the same car so had done a little research when I got mine, and the general consensus online was that it should be run on 98, but could likely get away with 95 (but with reduced power as computer would adjust timing to prevent knock). Everyone seemed to be in agreement that you shouldn't run it on 91.

I explained this to my father when he got the car, but just discovered last night that he has since filled it up with 91 "because the dealer never mentioned needing 95 or 98". I, of course, had words with him about why he would just ignore my advice!

But my question is, just how much can the anti-knock sensors compensate for? Surely it can only adjust the timing so much? Could he be damaging his engine?

At this point, with half a tank of 91 left, should he top up with 98? Will this bring the overall octane up as the fuel mixes together? Or is he better to wait until the tank is near empty and fill it completely with 98?

If the manual says use 98, use 98. If the manual says use 91, use 91. If you need to switch then I'd probably let it get pretty low first.

Yeah, I'd made it very clear to him he should be using 98 - I don't know what possessed him to put 91 in it.

c per litre. Its cheaper, save money.

Paul1977

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#1638719 22-Sep-2016 12:36

tdgeek:

Paul1977:

timmmay:

Paul1977:

Timely that someone should bring this thread back to life. My father has recently bought a car with a high compression engine (2005 Toyota Mark X). I have the same car so had done a little research when I got mine, and the general consensus online was that it should be run on 98, but could likely get away with 95 (but with reduced power as computer would adjust timing to prevent knock). Everyone seemed to be in agreement that you shouldn't run it on 91.

I explained this to my father when he got the car, but just discovered last night that he has since filled it up with 91 "because the dealer never mentioned needing 95 or 98". I, of course, had words with him about why he would just ignore my advice!

But my question is, just how much can the anti-knock sensors compensate for? Surely it can only adjust the timing so much? Could he be damaging his engine?

At this point, with half a tank of 91 left, should he top up with 98? Will this bring the overall octane up as the fuel mixes together? Or is he better to wait until the tank is near empty and fill it completely with 98?

If the manual says use 98, use 98. If the manual says use 91, use 91. If you need to switch then I'd probably let it get pretty low first.

Yeah, I'd made it very clear to him he should be using 98 - I don't know what possessed him to put 91 in it.

c per litre. Its cheaper, save money.

But on 98 he'll get more kms per litre.

Satch

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#1638720 22-Sep-2016 12:37

"The Octane Misconception" - is it just me or does this sound like the title of a Big Bang Theory episode?

gzt

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#1638723 22-Sep-2016 12:43

Compression is not the only factor in a modern engine. There are 'high' compression engines designed to run on 91.

If it's an import and it has a Japanese manual, flip through the manual until you find the tyre specification and chassis and engine type. On the same page is fuel type, in Japanese.

Then, use Google translate on the fuel type.

Paul1977

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#1638725 22-Sep-2016 12:48

gzt: Compression is not the only factor in a modern engine. There are 'high' compression engines designed to run on 91.

If it's an import and it has a Japanese manual, flip through the manual until you find the tyre specification and chassis and engine type. On the same page is fuel type, in Japanese.

Then, use Google translate on the fuel type.

I realize engine compression isn't the only factor. I've done my research on the car and it should absolutely be run on 98. My question was just about whether it could adjust timing enough to prevent knocking and potential damage since the tank now has 91 in it. And whether it was best to top up now with higher octane or wait until he tank is near empty.

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mrdrifter

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#1638740 22-Sep-2016 13:19

But on 98 he'll get more kms per litre.

The AA just had an article in their latest magazine where they tested this specific scenario and showed the the extra KM per Litre didn't make up for the difference in price. They recommended the same as the US AAA and to only run the higher octane if the vehicle specifically requires it.

tdgeek

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#1638742 22-Sep-2016 13:21

Paul1977:

tdgeek:

Paul1977:

timmmay:

Paul1977:

Timely that someone should bring this thread back to life. My father has recently bought a car with a high compression engine (2005 Toyota Mark X). I have the same car so had done a little research when I got mine, and the general consensus online was that it should be run on 98, but could likely get away with 95 (but with reduced power as computer would adjust timing to prevent knock). Everyone seemed to be in agreement that you shouldn't run it on 91.

I explained this to my father when he got the car, but just discovered last night that he has since filled it up with 91 "because the dealer never mentioned needing 95 or 98". I, of course, had words with him about why he would just ignore my advice!

But my question is, just how much can the anti-knock sensors compensate for? Surely it can only adjust the timing so much? Could he be damaging his engine?

At this point, with half a tank of 91 left, should he top up with 98? Will this bring the overall octane up as the fuel mixes together? Or is he better to wait until the tank is near empty and fill it completely with 98?

If the manual says use 98, use 98. If the manual says use 91, use 91. If you need to switch then I'd probably let it get pretty low first.

Yeah, I'd made it very clear to him he should be using 98 - I don't know what possessed him to put 91 in it.

c per litre. Its cheaper, save money.

But on 98 he'll get more kms per litre.

How? As the OP said, octane is the flashpoint of the fuel, energy produced is the same. Or do you mean that the mileage is the same on 91 and 98, except the anti knock will reduce efficiency of the 91?

MikeB4

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#1638783 22-Sep-2016 14:47

91 is rubbish. Never use in an engine that has periods of down time example a boat. The damn stuff goes off and becomes impossible to start.

All our vehicles run on 95 or 98.

richms

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#1638799 22-Sep-2016 15:17

My old skyline with the NA 2.0 would suck on 91, had to rev like crazy going up hills or it had no power, and chewed thru lots more. It was hardly a perfromance car at all, but I think it was quite high compression to get the power needed from putting a stupidly small engine in a heavy car.

The grandpa 4l falcon doesn't really seem to give much of a crap either way when others have borrowed it and put 91 in it instead. Fuel usage on it is all over the place since I dont really have a set driving pattern to test it with and a lot of time it has a trailer on the back which seems to mess up the ECU for a while afterwards even after you take the trailer off it is heavyer on the gas. I put 95 in it anyway since I use it on the other car and in the mowers etc and am usually filling a jerrycan or 2 at the same time as the car since its the one that I take them in to the servo to fill.

Its only a couple of bucks a tank difference so who really cares? I spend more than that on the overpriced cans of redbull in the service station, and if there is more 95 sold then they are less likley to change the storage and run out of it like a servo near here has done a few times. That is annoying when you want to fill up a car that needs it.

My MR-S hates 91. Again, lent it, someone filled it with that and then bought it back saying they thought something was wrong with it. Totally different car with that stuff in it.

Richard rich.ms

ajobbins

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#1638811 22-Sep-2016 15:28

MikeB4: 91 is rubbish. Never use in an engine that has periods of down time example a boat. The damn stuff goes off and becomes impossible to start.

All our vehicles run on 95 or 98.

What characteristic of 91 octane fuel makes it 'rubbish'? How do you figure? And why would 91 'go off' but not a higher octane fuel?

Some of the higher octane fuels have added detergents (Which independent studies have shown have negligible benefit at best, and can possibly do harm to some engines). This is more of a marketing thing than an actual benefit of the fuel.

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MikeB4

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#1638820 22-Sep-2016 15:49

91 will start to deteriorate after about 6 months but earlier if not in a sealed container.

ajobbins

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#1638830 22-Sep-2016 16:06

MikeB4:

91 will start to deteriorate after about 6 months but earlier if not in a sealed container.

I can find no info online to suggest that Octane rating has any effect on the deterioration of fuel

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richms

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#1638833 22-Sep-2016 16:10

I have only had that happen when in the tank and carb of something small. When I left 91 in the generator it crudded up the carb in about 3 months of disuse. 95 had not done that at all even with 6+ months of sitting around. May have been a bad batch but have heard from many others about issues so now only get 95 for the power tools and generators.

Richard rich.ms

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