Petrol

[lombardi]

I believe this is not my imagination,since owning my gtv6 1986 mod have used shell and caltex 98 octane with good results,the other filled up with mobil 98 and i swear the engine has been transformed,seems to idle a lot smoother and engine feels a lot more elastic,wonder if anyone else has had this experience.ciao

[John Hanslow]

Yep, I had an old Mazda 323 wagon.  Used lots of oil and and a bit of a wreck.  However when I filled the tank up with say BP 98 Octane, it performed much better.  It accelerated smoother and seemed to have a bit more power.  Must have been the solvents cleaning the the dirty engine- it worked.

Having said that, have not noticed much difference in the 156.

[Andrew156]

Sometimes I think this petrol caper is mind over matter, BUT,

In my 156, can't feel the difference between any 95 and 98 RON fuels
Our Mazda 323 SP20, again no difference[/li][/list]
LS1 Commodores X 3 (V8 to the uneducated ) no difference between 91, 95 or 98 RON fuels
SLR/5000 Torana circuit car - massive difference, loved Mobile and BP 98, pinged on Shell 98


Go figure

[dehne]

the mobile 98 is far serperior than the others thats why they charge a little more for it the bad news is caltex is trying to buy them out and we will lose a good fuel i only use the mobile in my race car but the good people of bendigo mobil decided to sell out to caltex so the next best is bp ultimate unless you can get shells 102 at ur servo the is one in lygon st that has it

[John Hanslow]

So Andrew156, it looks like the older cars have a more positive response to 98 Octane.
Cheers.

[lombardi]

My book for the 1986 gtv6 says to use nothing lower than 98 octane.ciao

[Andrew156]

So Andrew156, it looks like the older cars have a more positive response to 98 Octane.
Cheers.

In my limited experience yes. In the Torana it may have been contributed to by the 11:1 compression, but sertainly in my later models, no change. In my 156 I can't feel the difference.

I'm not aware of any tuning for 156s but with say the LS1 Holdens's of which I know a bit about, after tuning, there is evidence that suggests Mobil and BP 98 actually perform better in dyno tests versus other PULP brands.

The one notable difference I did find though is that despite no percieved performance improvement, they did go a bit further to the tank on all 98 RON fuels. As I understand it is due to the greater 'mass' of PULD, but now I've reached the end of my knowledge on the subject.

[Darryl]

You need to tune (see below) the car for the fuel. If you chuck a tank of 100 octane in something tuned for 91 it is unlikely to transform its performance but is unlikely to do any harm.

If you on the other hand try to run something tuned for 100 on 91 it isn't likely to be happy - and if the "tune" includes high compression it may be outright damaging.

I said see below for "tune" because a modern efi engine will to varying extents fine tune itself. This is most extreme on turbo cars which have the ability to adjust effective compression by adjusting boost - backing it off in response to knock.

Regardless, you have to suspect that a fair chunk of the self-tune ability of a modern ECU is aimed at tuning out mfg variations and long term changes in conditions not dealing with changes in fuel quality "on the fly" (beyond some basic self preservation behaviour). Hence, unless you are going to reset the ECU, then let it tune up on eg BP ultimate, and keep using it all the time, it seem unlikely you will see a significant improvement (except turbo cars with good knock based boost control) vs consistently using some other fuel.

However, you also need to consider the difference you will get from using a consistent product (regardless of octane rating). One hopes that the bigger brands premium fuel is at least consistent. This should mean that the ECU will be able to gradually adjust/tune to somethig close to "optimal" and stay there vs. always being a bit out of tune trying to track fuel quality changes from whatever you chucked in it this week. That probably can't be said so much for even a consistent brand of "lesser" fuel but certainly won't be true if you just buy 91 from whatever outlet is handy when the gauge is low... It is also why I really don't get the "stick a tank of ultimate through it every now and then" idea - seems like a waste of money and a way of messing up the tune that is somewhere near right for whatever crud you run normally, by having the ECU try to adapt to ultimate only to have to adjust back to 91 again next fill.
The exception is that if you end up with a tank of bad fuel and you top up with decent fuel it might bring it back to a level that is nearer to what you normally run - but then you would probably do better by sticking a bottle of octane booster in in that case...

Which brings me to a specific example - I have a 1988 Jag XJ40 that the seems to have come from the factory with a fairly aggressively lean mixture and advanced timing (and 88 vintage ECU isn't clever enough to adjust itself). It is supposed to run on regular unleaded crud but if I don't run it on something decent it pings going up hills/accelerating. So I carry a few bottles of octane booster for "emergencies" especially on long trips. It does help that it has a 90 litre tank and that the aggressive tune gets really good economy for a 2 ton car when cruising (like 1000km/tank) so it is usually possible to find decent fuel anyway...

So - imho - consistency matters more than octane rating unless you have a car that is, by virtue of its compression ratio or a dumb and poorly calibrated for local conditions ECU, unsuited to lower octane fuel. If you have an old car, get it tuned for the fuel you actually use. In my experience, stock older alfa engines aren't too fussed and can be tuned to run on 91 just fine - not so if you start bringing up the compression etc, and not saying you can't get more power on better fuel with the right tune (you definitely can).

Std disclaimer - all just opinions an observations - and every car is different - ymmmv etc and I'm certainly not an expert on any of this...

[MD]

Excellent points Darryl,

My quick summation is that good modern ECUs simply dump more fuel in for low octane rated fuels based on knock sensing inputs. That's why often you cannot tell the difference between higher/lower rated fuels. It will depend more on the ECU smarts than the fuel whether there is noticable difference or not. Theoretically, there could/should be a higher fuel consumption for lower grades of fuel based on that premise. This could be very marginal too.

Older cars with dumb ECUs or carb induction will soon tell you they are not happy if the octane rating strays off the original tuned octane rating as Darryl has pointed out so consitency is the name of the game. Stick with the fuel it was tuned on.

[Doug Gould]

Fuel 101.

There are 4 refineries in Australia that refine 98 octane: Shell, BP, Mobil & Caltex. Everyone else is buying from them or importing. 91 Octane comes from one of those refineries or is imported. 95 Octane is a mystery. I suspect that some 95 octane fuels are made by shandying 98 & 91. Some have ethanol. I think 95 is a lottery. Some of the companies that buy on the spot market, often from Singaporean refineries, have questionable fuel standards.

If a car has a knock sensor, it will adapt to lower octane fuel, usually with spark timing (not fuel quantity). These cars respond significantly to higher octane fuel. Otherwise, if a car's compression ratio does not require 98 octane, then there will b e no benefit. I have a suspicion that 98 octane fuel gets better detergents added, but when I've asked oil company fuel technologists, they have denied this.

It is worth noting, that Australia adopted National fuel standards only very recently (2006 or 2007?). Generally our fuel is dramatically better now than 5 years ago.