91 or 98 petrol

just done back to back check on 2 tank fulls 91 vs 95 in ae100 corolla with a 4afe . got 25km more with 95 .Simular traffic conditions , but cant be exact . Overall the difference was more in how the car drove, it just felt better and more responsive on the 95 . cost difference was $4.00 on 40 litres .I will continue using 95 .

We run a chev 6.2l in the jetboat and it runs 91 also.
Seem to get away with it with some v8s

Assertion one:
It's my understanding that the octane rating is a measure of stability (against [pre]ignition) in petrol.

Assertion two:
It's my understanding that the energy (bang, power) in petrol is the same for each octane rating.

I've read a lot about this subject and most of it's shit in my opinion, spouted by those only guessing. I am yet to be convinced otherwise the above.

its not about power

its not about pre ignition

its not about timing

its not about compression

to put it as simply as possible : 91 burns like crap.

the only thing that should run New Zealands 91 is used for cutting grass

my thinking was , 91 burns bad giving less punch so i put my foot down further increasing the amount of fuel i would burn.

I was in ozzie then running a 3.6 V6 Holden calais on their E10 fuel, it was only $1.04 liter and it still went like a scalded cat.

Yep the car may perform better using 95 but does the extra mileage make up for the extra cost of the petrol!

in some cases it has

but you use 95 for the improved running moreso

Bollix.

well some guys have mechanical sensitivity

they are able to tell running differences.

I am yet to test a car thats designed for high octane cost less to run on 91.

What gets me is people act like its a choice. If your modern car is designed for 95 or higher you MUST run it or you risk engine damage, in some cases that damage can be catastrophic (like with GDI engines). Those high octane vehicles that don't get damaged are still having to constantly rely on their knock sensors etc to try and cope with the incorrect fuel, this is never a good thing. That is also why they never turn out cheaper to run on low octane, because the poor old engine is struggling to adjust everything to cope with a fuel it isn't designed to run.

If the car is designed for low octane then and only then do you get a bit of a choice.

I am yet to test a car thats designed for high octane cost less to run on 91.

What gets me is people act like its a choice. If your modern car is designed for 95 or higher you MUST run it or you risk engine damage, in some cases that damage can be catastrophic (like with GDI engines). Those high octane vehicles that don't get damaged are still having to constantly rely on their knock sensors etc to try and cope with the incorrect fuel, this is never a good thing. That is also why they never turn out cheaper to run on low octane, because the poor old engine is struggling to adjust everything to cope with a fuel it isn't designed to run.

If the car is designed for low octane then and only then do you get a bit of a choice.

BTW, as anyone who has built their own ignition system will tell you, knock sensors are far from precise technology. They are not something you want to be relying on constantly to avoid engine damage.

Do enlighten me Jazz!

turbo legacys need 98 we have had issues with running them on 95. i use 98 in my turbo 2.5litre skyline and i go through half-3quarters of a tank a week an only costs me 80-90 bucks to top up. i do a fair amount of driving too. 91 is pretty crap fuel haha

Best thing to do is to read posts 44 and 45 in this thread first.
(Pour yourself some coco and be ready for a lengthy bedtime story)
That will give you some good general background on whats going on.

Then consider you have three general causes of damage:

1. Damage from incomplete combustion, usually cause by a buildup of unburnt material.

This is much worse than it sounds and is a big issue with GDI engines, in fact I would go so far as to say that most of the trouble with GDI engines comes from running low octane fuel. Even more traditional engines can suffer holes in pistons and all kinds of trouble from a buildup of carbon etc. Expensive catalytic converters and even oxygen sensors can be damaged from incomplete combustion too. Much of the time mechanics are just focused on locating and fixing the problem so the owner of the car may not even be told that incorrect fuel was a likely cause.

2. Damage from detonation, yes there are knock sensors to try and prevent this but they only make adjustments after knocking has started and are not that accurate. This out of design parameter adjustment tends to aid incomplete combustion which in turn aids problem number 1 and if you are really really unlucky problem number 3. Detonation can directly damage rings and even the piston, particularly pitting which in turn encourages carbon buildup.

3. Pre-Ignition
This is the worst of all and luckily not so common, not to be confused with detonation true pre-ignition often happens around 180 BTDC. Once this starts you can wave goodbye to your pistons, rings and anything else that gets in the way. This catastrophic damage tends to happen after just a few revolutions of pre-ignition. Its not like detonation where you can get away with it for quite some time, and some engines are even designed to cope with detonation as part of the natural course of running.

The combination of low octane fuel and hot spots from carbon on the piston (also caused by low octane fuel) can create a ripe environment for this to happen.

Best thing to do is to read posts 44 and 45 in this thread first.
(Pour yourself some coco and be ready for a lengthy bedtime story)
That will give you some good general background on whats going on.

Then consider you have three general causes of damage:

1. Damage from incomplete combustion, usually cause by a buildup of unburnt material.

This is much worse than it sounds and is a big issue with GDI engines, in fact I would go so far as to say that most of the trouble with GDI engines comes from running low octane fuel. Even more traditional engines can suffer holes in pistons and all kinds of trouble from a buildup of carbon etc. Expensive catalytic converters and even oxygen sensors can be damaged from incomplete combustion too. Much of the time mechanics are just focused on locating and fixing the problem so the owner of the car may not even be told that incorrect fuel was a likely cause.

2. Damage from detonation, yes there are knock sensors to try and prevent this but they only make adjustments after knocking has started and are not that accurate. This out of design parameter adjustment tends to aid incomplete combustion which in turn aids problem number 1 and if you are really really unlucky problem number 3. Detonation can directly damage rings and even the piston, particularly pitting which in turn encourages carbon buildup.

3. Pre-Ignition
This is the worst of all and luckily not so common, not to be confused with detonation true pre-ignition often happens around 180 BTDC. Once this starts you can wave goodbye to your pistons, rings and anything else that gets in the way. This catastrophic damage tends to happen after just a few revolutions of pre-ignition. Its not like detonation where you can get away with it for quite some time, and some engines are even designed to cope with detonation as part of the natural course of running.

The combination of low octane fuel and hot spots from carbon on the piston (also caused by low octane fuel) can create a ripe environment for this to happen.

You might not see pre-ignition damage so much in cars but its seen quite frequently in smaller engines. Especially where 91 Octane fuel has been stored for a few months causing it reduce in octane rating even further.

Didnt need the coco

LOL, got you to sleep without it, no doubt!

Hmm, what year! My 2004 Legacy is fine on 95

That being said, someone told me that it needs a couple of tanks of 98 to really get the benefit - something to do with the computer needing to recognise the better gas. Anybody know if there's anything to that!

That would be logical, it would make sense that after a period of having to constantly retard the ignition timing because of a fuel it wasn't designed for that it would hold the revised map for a while.

This assumes it was designed for 98, I don't know hardly anything about that age of Subaru.

Jazz your describing general MPI combustion events, nothing specific to the GDI system. Which of the 4 GDI combustion mode are you supposed to be referring to!

You will know carbon unfortunately is a by product of combustion and as GDIs have a 40% EGR tolerance compared to a conventional MPI,s 8% tolerance carboning of the inlet manifold will result regarless of the octane content in fuel.

You are describing "Knock" like its a single damaging event, the truth is all engines knock, and it in itself if kept within reasonable levels is perfectly normal and acceptable.
Yes knock sensors are a reactive sensorthey are also only effective at a very specific frequency hence most sensors ignore noise above and below there tuned range this is when damage tends to occur (think Honda Accord V6)) most modern MPI systems and including MMC GDI have octane flags and knock control logic to determine "octane level" based on knock sensor monitoring and will easily switch to a more conservative timing map due to "learn knock" hence why GDIs will run fine on 91 octane and some are rated for regular instead of premium

Most modern cars have an adaptive keep alive memory in them to adapt to a lower octane fuel as you suggest. Sure it will run ok, but once you chuck 98 back in it and reset the memory you sure notice the difference!

QUESTION
Jazz your describing general MPI combustion events, nothing specific to the GDI system. Which of the 4 GDI combustion mode are you supposed to be referring to!

JAZZ ANSWERS
Yes, I am describing general MPI combustion events. It was not my intention to imply that all those problems are specific to direct injection engines. Sorry if that was not clear, I realise my specific mention of GDI engines in the first paragraph could be taken as to mean the whole post was regarding GDI.

QUESTION
You will know carbon unfortunately is a by product of combustion and as GDIs have a 40% EGR tolerance compared to a conventional MPI,s 8% tolerance carboning of the inlet manifold will result regarless of the octane content in fuel.

JAZZ ANSWERS
The key point here is if the engine is designed for high octane and you run it on low octane carbon buildup usually INCREASES due to less efficient combustion. Its the increase above and beyond what the engine was designed for that is the problem.

QUESTION
You are describing "Knock" like its a single damaging event, the truth is all engines knock, and it in itself if kept within reasonable levels is perfectly normal and acceptable.

JAZZ ANSWERS
No, not at all. If you go back and read what I wrote again you will see I have been very specific here.

I am talking about two very different things
1. Detonation (or as you put it "knock")
2. Pre-ignition

If you read the section on pre-ignition you will see I even point out as you have that engines are often designed to cope with a reasonable level of knock/detonation as part of the standard course of running.

I refer to pre-ignition (not knock) as a single damaging event because thats generally exactly what it is. True pre-ignition can easily cause catastrophic damage in just 3-4 cycles, sometimes just a single cycle will do it.

As explained, its not very often seen in production vehicles but running low octane fuel for a long time in an engine that is not designed for it can indeed setup conditions that will cause this. Also, as explained you see this fairly regularly with small engines i.e chainsaws due to running severely low octane fuel.

QUESTION
most modern MPI systems and including MMC GDI have octane flags and knock control logic to determine "octane level" based on knock sensor monitoring and will easily switch to a more conservative timing map due to "learn knock" hence why GDIs will run fine on 91 octane and some are rated for regular instead of premium

ANSWER
This is far to simplistic an explanation. Without wanting to go into things in rediculose detail or re-explain what I said in posts 44 and 45. There are basically two points I would like to make.

1. Yes, the car adjusts its timing map for low octane fuel. However, this is a band aid approach designed primarily to preserve the engine as much as possible. It cannot adjust its compression ratio or other aspects like piston design to cope with a lower octane fuel that the engine was not designed for.

As a result modern engines designed for high octane don't run "fine" at all on low octane, the combustion process becomes inefficient and thats why you get the excess carbon buildup and all the other problems as pointed out above.

Yes, you can have direct injection engines that are specifically designed for low octane fuel, those will generally be quite O.K on low octane from a damage point of view. However direct injection as a concept does not work to anywhere near its full potential with low octane fuel, even when designed specifically for it. Thats why direct injection engines are predominantly high octane engines.

2. Direct injection engines designed for high octane will hardly ever enter stratified charge mode (where an ultra lean mixture is burned) when run on low octane fuel. While this is not an issue from an engine damage standpoint its the other reason why such engines don't run "fine" on low octane fuel.

QUESTION
Jazz your describing general MPI combustion events, nothing specific to the GDI system. Which of the 4 GDI combustion mode are you supposed to be referring to!

JAZZ ANSWERS
Yes, I am describing general MPI combustion events. It was not my intention to imply that all those problems are specific to direct injection engines. Sorry if that was not clear, I realise my specific mention of GDI engines in the first paragraph could be taken as to mean the whole post was regarding GDI.

QUESTION
You will know carbon unfortunately is a by product of combustion and as GDIs have a 40% EGR tolerance compared to a conventional MPI,s 8% tolerance carboning of the inlet manifold will result regarless of the octane content in fuel.

JAZZ ANSWERS
The key point here is if the engine is designed for high octane and you run it on low octane carbon buildup usually INCREASES due to less efficient combustion. Its the increase above and beyond what the engine was designed for that is the problem.

QUESTION
You are describing "Knock" like its a single damaging event, the truth is all engines knock, and it in itself if kept within reasonable levels is perfectly normal and acceptable.

JAZZ ANSWERS
No, not at all. If you go back and read what I wrote again you will see I have been very specific here.

I am talking about two very different things
1. Detonation (or as you put it "knock")
2. Pre-ignition

If you read the section on pre-ignition you will see I even point out as you have that engines are often designed to cope with a reasonable level of knock/detonation as part of the standard course of running.

I refer to pre-ignition (not knock) as a single damaging event because thats generally exactly what it is. True pre-ignition can easily cause catastrophic damage in just 3-4 cycles, sometimes just a single cycle will do it.

As explained, its not very often seen in production vehicles but running low octane fuel for a long time in an engine that is not designed for it can indeed setup conditions that will cause this. Also, as explained you see this fairly regularly with small engines i.e chainsaws due to running severely low octane fuel.

QUESTION
most modern MPI systems and including MMC GDI have octane flags and knock control logic to determine "octane level" based on knock sensor monitoring and will easily switch to a more conservative timing map due to "learn knock" hence why GDIs will run fine on 91 octane and some are rated for regular instead of premium

JAZZ ANSWERS
This is far to simplistic an explanation. Without wanting to go into things in rediculose detail or re-explain what I said in posts 44 and 45. There are basically two points I would like to make.

1. Yes, the car adjusts its timing map for low octane fuel. However, this is a band aid approach designed primarily to preserve the engine as much as possible. It cannot adjust its compression ratio or other aspects like piston design to cope with a lower octane fuel that the engine was not designed for.

As a result modern engines designed for high octane don't run "fine" at all on low octane, the combustion process becomes inefficient and thats why you get the excess carbon buildup and all the other problems as pointed out above.

Yes, you can have direct injection engines that are specifically designed for low octane fuel, those will generally be quite O.K on low octane from a damage point of view. However direct injection as a concept does not work to anywhere near its full potential with low octane fuel, even when designed specifically for it. Thats why direct injection engines are predominantly high octane engines.

2. Direct injection engines designed for high octane will hardly ever enter stratified charge mode (where an ultra lean mixture is burned) when run on low octane fuel. While this is not an issue from an engine damage standpoint its the other reason why such engines don't run "fine" on low octane fuel.

Just a small extra note,

In the last point above where I refer to direct injection engines not entering stratified charge mode very often when run on low octane fuel. I am basing that on my knowledge of VW's system. I would assume GDI's are much the same as they run a very similar sort of stratified charge mode as far as I am aware and I am picking would face the same issue.

To be fair though, that is a bit of an educated assumption on my part.