Octane Ratings

[Alan Cherry]

Anyone got any thoughts on the following-
Usually I run Shell Optimax (97 Octane) in my Astra GSI ( GrpN + less restrictive exhaust manifold + taper throttle bodies + mbe management system - but could do with remapping ).
I tried a blend of Optimax and a new super BP petrol that a reasonably local garage sells (only sold at 6 garages in the country) BP is 102 Octane, but costs £2.42 a litre - I think the engine felt a little stronger, but can't be sure as I was running at Castle Combe, and I've never run there before.
Does Higher Octane neccesarily mean more power ?
Reason for asking is that is local Tesco's super unleaded is 99 Octane rather than the usual 97 - Am I better off sticking to optimax ?

Any thoughts welcome

[John Turner]

I believe that higher octane does improve performance and, in my experience, fuel economy too. So although BP Ultimate, which I believe is 98 octane, is expensive, I reckon I recover the additional cost in better fuel consumption. Can't comment on this 'new' BP 102 octane fuel, but I certainly am happy to use Shell Optimax as an alternative.

[Tim Falce]

I normally use Tesco 99 but decided to be a tightwad at my last couple of races and used Sainsburys 4 star and ordinary unleaded, I didn't notice any difference...... except I finished nearly last in one race.

[Al Weyman]

I use the Tesco 99 probably one of the cheapest petrol areas in the country where I live, Watford and currently a further 5p a litre off if you spend more than £50 in the store first. it certainly does not work any worse but I am more worried about detonation than any marginal performance increase so will buy the 99 for no other reason than the stated higher octane, maybe being naive of course for reading what it says on the tin!

[Locost47]

I spent a rainy afternoon listening to an engine expert explaining all this. Much of it went over my head and there was a long list of "it depends" things but the general gist was as follows.

Higher octane fuel burns slower (counter-intuitively) and allows you to run more advance which may liberate extra power but you run a greater risk of pre-ignition (pinking). If your engine has both knock and lambda sensors the ECU should keep advancing the ignition until it senses pinking and then back it off a little so that it runs smoothly.

Some engines are more sensitive to changes in fuel than others. There was an article on this in Evo magazine sometime last year, i think. From personal experience i can say that my old Impreza ran like a dog on UG95 but felt like it had a rocket up it's @rse after a couple of tanks of Optimax. Strangely though it didn't seem to take to the BP equivalent. Don't know why.

[Chris Y]

As a rule of thumb, if your car has been set up on 95 octane fuel, then it won't run any better on 99 octane fuel.

Rule of thumb though!

[Notso Swift]

It can all get a little hard, Locust47 your summary is correct... but then you get fuels rated the same Octane (RON) but they may be different under another Test, MON, for instance.
The tests are standardised, but they are not that relevant to modern motors. They take a big single cylinder motor that runs at X temp, with Y air temp and Z rpm and then they vary the compression to determine the octane… the maximum octane rating possible in 100, by the way because it is base on a percentage against a standard fuel. Numbers higher than that are extrapolations. Based on it being “better “ than the standard (ie Avgas 100/130, 130 is based on the equivalent figure at sea level, 100 is the figure at a certain altitude)
Anyway the RPM is like 500. Very different to what your motor runs at
As a result of the variables fuels behave differently. I have back to back dyno tested to local brews both 98 RON, and one continually allows an 2-4 deg advance through the rev range… and hence more power (We also tested Elf Turbo Max, wow, 102 octane but 40 hp more at the wheels, but 6 times the price, eek!)
Some fuels may be more stable at higher temperatures, others have a bigger bang, they may have suspended oxygen in them (FIA allows up to… 2.3% or something like that) so the octane may not effectively describe how much “energy” is stored in the fuel. (FYI LPG is 115 octane, but has about 25% less energy compared to regular Petrol) Then the oil companies give Summer and Winter mixes, because the lighter components burn off quicker in storage (evaporate)

So it is all hard, and there will be a chemical engineer out there who will explain it better and fix my errors.

My advice is to pick one fuel, tune you car to that, and then stay with it.
And agreewith Chris Y as well, there are limits without physically changing you engine (which is where fuel with suspended oxygen comes in )

[MikeBz]

I remember reading a long time ago that as the octane rating goes up the calorific value actually goes down, but not enough to make any measurable difference to performance. It's a common misconception that 'high octane' means 'high energy'.

Mike

[Al Weyman]

If higher octane means you can run more compression and more spark advance then yes it does mean more power.

[MikeBz]

A general truism, but not necessarily always true... A lot of engines don't need the ignition to be advanced to the point of knocking to make best power (at least at some parts of the rev range).

I suspect that nat. asp. engines equipped with knock sensors have an 'optimum amount of advance' map which they will back off from at the onset of knock, rather than simply advancing and advancing until knock is detected.

Mike

[ian_w]

All engines will have an optimum spark advance at any given rpm and load, for the technical amongst you this is typically referred to as MBT ( Minimum spark advance for Best Torque ). However on some engines it is not possible to advance the ignition all the way to MBT before it starts to detonate. Advancing the ignition beyond MBT will result in a drop in performance and can ultimately lead to catastrophic engine failure due to pre-ignition.

The octane of the fuel is a measure of its resistance to detonation and as several people have pointed out this is a very complex process and not all engines will respond the same and two fuels of the same nominal octane can give different results.

Unless your car has a knock sensor then the engine management system will always run a fixed map of spark advance. Switching to a higher octane fuel will not give any performance improvement unless you remap to suit the new fuel. Engines typically tend to be detonation limited at lower speeds and can normally run at or close to MBT at maximum power. Therefore, the benefits of the higher octane fuel will mostly be seen at the lower speeds and it will not necessarily give you any more peak power.

Knock sensors are basically accelerometers that measure the vibration of the engine. When the engine detonates the vibration increases sharply ( which is the 'tinkling' noise you hear ) and this is detected by the knock sensor. The engine management system will have a map of MBT spark advance and will try and run at this value. When detonation is detected it will retard the individual cylinder that detonated by a fixed amount. It will then start to ramp the spark advance back towards MBT until it detects detonation again. The spark advance on each individual cylinder is therefore constantly varying as the engine management system endeavours to run as close to MBT as possible in order to give maximum performance.

[Alan Cherry]

I have discounted the 102 octane on terms of cost - £2.42 a litre isn't really in my budget ( whats a budget ? )
So it's between Tescos 99 or Optimax. Although we get 2 races per day, conditions etc change, so direct comparisons won't work (lap times differ between chasing or being chased) looks like I'll toss a coin, fill it with whatever and get it mapped using that and stick to it.

[phoenix]

Originally Posted by Alan Cherry

I have discounted the 102 octane on terms of cost - £2.42 a litre isn't really in my budget ( whats a budget ? )
So it's between Tescos 99 or Optimax. Although we get 2 races per day, conditions etc change, so direct comparisons won't work (lap times differ between chasing or being chased) looks like I'll toss a coin, fill it with whatever and get it mapped using that and stick to it.

There is always the option of a dyno, which will give you the true answer to your dilema.

[Alan Cherry]

Originally Posted by phoenix

There is always the option of a dyno, which will give you the true answer to your dilema.

Can probably only afford 1 session getting it mapped once - could probably gain more by going on a diet ( both me and the car ! )

[R59]

That £2.42 a litre for 102 octane isn't far away from some of the proper control fuels supplied by the likes of Anglo American Oil (76 or Sunoco), or Carless, etc... And given the choice, I'd buy a proper control fuel rather than a mix of "superdooper102" and water/dirt/etc... from the local filling station's old tank that they've just put back into use for the "new" fuel.

To the point, which has been answered - higher octane burns slower.
E85 - the green(er) fuel that the new Saab 95 green version runs on, is a 85% bio-ethanol, 15% unleaded mix. It has an octane racing of 102, and the Saab has a cunning management that makes best use of it, and produces 20% more power, and 15% more torque compared to regular unleaded (Saab's figures - and backed up by some road tests).

Makes me wonder if this "BP-Super102" is really E85 through the side door....
We'll soon see when everyone's fuel hoses rot out after a year!!

Rob.

[MikeBz]

Originally Posted by racing59

...Saab has a cunning management that makes best use of it...

I guess we're talking ECUs and not GM bean counters...

Mike

[Mad Dog]

I recently put the Mini on a rolling road and did a back to back with optimax and tesco 99.There was no power difference between the two and the ignition timing ended up being set the same.The good thing with Tesco is that you get points towards your xmas turkey. xxxxxx

[andypipe]

Originally Posted by Mad Dog

I recently put the Mini on a rolling road and did a back to back with optimax and tesco 99.There was no power difference between the two and the ignition timing ended up being set the same.The good thing with Tesco is that you get points towards your xmas turkey. xxxxxx

You tight sod......................
I put mine to beer

[JohnMiller]

[QUOTE=racing59]That £2.42 a litre for 102 octane isn't far away from some of the proper control fuels supplied by the likes of Anglo American Oil [QUOTE]

Actually, the BP is significantly cheaper. Does anybody know the MON rating for it (I can't be arsed to look).

[REALIST]

[QUOTE=JohnMiller][QUOTE=racing59]That £2.42 a litre for 102 octane isn't far away from some of the proper control fuels supplied by the likes of Anglo American Oil

Actually, the BP is significantly cheaper. Does anybody know the MON rating for it (I can't be arsed to look).

90

[REALIST]

BP102 is fully described on the BP website

[JohnMiller]

Thanks!

[maddogf3]

Originally Posted by ian_w

All engines will have an optimum spark advance at any given rpm and load, for the technical amongst you this is typically referred to as MBT ( Minimum spark advance for Best Torque ). However on some engines it is not possible to advance the ignition all the way to MBT before it starts to detonate. Advancing the ignition beyond MBT will result in a drop in performance and can ultimately lead to catastrophic engine failure due to pre-ignition.

The octane of the fuel is a measure of its resistance to detonation and as several people have pointed out this is a very complex process and not all engines will respond the same and two fuels of the same nominal octane can give different results.

Unless your car has a knock sensor then the engine management system will always run a fixed map of spark advance. Switching to a higher octane fuel will not give any performance improvement unless you remap to suit the new fuel. Engines typically tend to be detonation limited at lower speeds and can normally run at or close to MBT at maximum power. Therefore, the benefits of the higher octane fuel will mostly be seen at the lower speeds and it will not necessarily give you any more peak power.

Knock sensors are basically accelerometers that measure the vibration of the engine. When the engine detonates the vibration increases sharply ( which is the 'tinkling' noise you hear ) and this is detected by the knock sensor. The engine management system will have a map of MBT spark advance and will try and run at this value. When detonation is detected it will retard the individual cylinder that detonated by a fixed amount. It will then start to ramp the spark advance back towards MBT until it detects detonation again. The spark advance on each individual cylinder is therefore constantly varying as the engine management system endeavours to run as close to MBT as possible in order to give maximum performance.

Well Ian ,do you feel ignored or what ??????????

[ian_w]

Just a little. Seems like people are more interested in hearsay and gossip rather than listening of somebody who might actually know just a little bit about the subject. I think I'll go off and sulk for a while!!!

[REALIST]

Originally Posted by ian_w

Just a little. Seems like people are more interested in hearsay and gossip rather than listening of somebody who might actually know just a little bit about the subject. I think I'll go off and sulk for a while!!!

No no don't do that!! That was one of the most clearly explained technical arguments I have read, especially on 10/10!

Now it seems to me after reading that, that there is no point running BP102 if we cannot alter the mapping. On the same basis, will BP Ultimate 97, or Optimax give us any benefit over regular 95, in what is basically a road going engine? (FR2000, Clio16V)