Optimax's Octane?

[Tom]

I filled up with SuperUnleaded at Sainsbury's this morning (OK, I just wanted to buy one of those breakfast packs!) and noticed that it's 97 octane. My [says quietly] Integra Type R has a sticker on the inside of the fuel filler cap saying that it needs/prefers 98 octane.

 

I was wondering whether I should stick to Optimax (I usually do) but wonder what octane rating it is?

 

Can anyone remember?

 

And, by the way, yes I do hope to buy another Se7en soon!!!

[Graeme Smith]

98

[Tom]

Thanks, thought it might be

[Joe 90]

I believe it's actually 100. A friend (pedant) phoned Shell because he couldn't find out any other way.

 

SEP field working, not spotted in 101,800 miles. Some photos on webshots, updated 10 June

[Peter Carmichael]

I found this

 

Interesting that Optimax is only certified to meet BS EN 228 (normal unleaded), but not BS7800 (super unleaded), even though it is claimed that it exceeds both standards and typically has a RON of 98.3-98.6

 

Either way, your Integra has knock sensors and will not suffer mechanical damage from being run on a lower grade fuel. You might lose some performance and fuel economy however as it may back off the ignition to a safe value.

 

[Tom]

Thanks Peter, that's interesting stuff.

 

Thinking about all this 'RON' stuff, led to me recalling something I read in a book about D-Day, where a pilot (I think) was saying that the Aviation fuel they were using was only 97 octane back in WWII. I may have misheard, but it certainly surprised me whatever the figure actually was. It seemed very low.

 

Is that likely?

[pete east]

Tom - Avgas is 100 Octane and was developed in line with a government spec for use with high performance aero engines being built just before and during the second world war. We still use it now although it is a bit over specced for most light aircraft engines. One key aspect of it however is that it does not suffer from vapour locking which can occur where a fuel destined for motor vehicles is used in aircraft. Our insurance company told us that if we used motor fuel in our plane our insurance would be void. We therefore stuck to avgas despite the massive hike in cost between it and car fuel.

[V7 SLR]

BP's Ultimate is at least 99 apparently.

[Tom]

Strange that AVGAS is so expensive then, especially as there is cheap diesel for boats and agricultural vehicles.

 

What do formula 1 cars use then? Isn't that very high octane stuff?

[Joe 90]

In F1 pump super plus (97 octane) is mandatory.

 

SEP field working, not spotted in 101,800 miles. Some photos on webshots, updated 10 June

[pete east]

I have used avgas in a car but it didnt seem to make much difference. The cost of it makes leaded 4 star seem quite attractive!

[Graham Perry]

I only know a little about the WW2 fuels but I think the Anglo-Iranian Oil Company developed (literally a few yards from where I am sitting) a special 100 Octane (MON) fuel that enabled the Mk9 Spitfires to run more boost and hence produce 300hp more, or something like that. Later in the war confusion reigned because people started seeing the phrase 100/150 Octane and assumed it was 50% higher. However, I 'think' this was due to a trick used on certain aircraft that ran Mega Boost i.e. the Tempest and later Spitfires to deliberately run super rich mixtures to cool the combustion chamber prior to the flame arriving which gave even more power (The wonderful Napier Sabre for example was tested to 170hp/litre) I don't think the 100/150 fuel was any more than 100 octane but it had some additives to encourage the cooling effect. We probably have it all in the library here somewhere but I don't have time to go and look it up at the mo.

 

I am sure someone will correct any of my inaccuracies

 

 

 

Edited by - Graham Perry on 15 Jul 2004 18:45:57

[mav]

so if 97 is madatory in F1, how come Shell / Ferrari use "perfect" optimax?

[Bare]

Strange... My R type, sez 91 octane 'recomended' .. Same 205 hp rating ?? inna UK or is it 'officially' bigger due to higher octane fuel availbility there?

PS: it runs acceptably on mid grade 89.. gotta love those knock sensors.

[Joe 90]

The fuel regs in F1 have an interesting history. Until the late 50s fuel was free and the teams were running more and more exotic brews. They were adding nitro compounds, and they weren't very nice. One of the mechanics recounted walking through a puddle of spilt fuel and it dissolved his shoes

 

The fuel companies didn't like this state of affairs because they were the major sponsors, but they couldn't advertise to Joe Public that their fuel won races because everyone knew it was special brew. The problem was that no one could define "pump" fuel with enough precision to prevent cheating, and the teams weren't prepared to put the local stuff in their eexpensive engines, so the FIA mandated AVGAS, which was guaranteed quality everywhere. Of course the fuel companies weren't happy, because they still couldn't use it for advertising.

 

The next change came when the fuel regs specified a maximum octane rating, and that only the components normally found in pump fuel could be used. This worked well until the turbo cars came along. The chemists discovered that turbo engines love toluene, a pump fuel component normally found in only small percentages. The regs allowed them to blend a fuel of perhaps 70% toluene, and they didn't exceed the octane limit because it is measured at the entirely irrelevant speed of 1500 rpm.

 

The current regs set limits on the components allowed in the fuel and are designed to accomodate 97 octane super plus unleaded, but no more. The teams have to brew up something and submit it for chemical analysis. With modern scientific instruments it's possible for the scrutineers to check that the fuel used is the same as the "homologated" fuel without having to do a full chemical analysis.

 

What this means is that your road car would probably run quite happily on most F1 fuels, but not vice versa. The F1 engines are so finely tuned, they are designed for a specific fuel only.

 

SEP field working, not spotted in 101,800 miles. Some photos on webshots, updated 10 June

[Peter Carmichael]

Bare,

 

US fuels are sold according to the average of RON and MON figures. UK are sold on the basis of RON.

 

A US 91 "octane" is the same fuel as a UK 97 "octane", more or less. The link I gave earlier had MON and RON figures for Optimax. The average for these would give a US "octane" of 92.6.

 

Sahf London;

every 1st Wednesday from 19:30 at The Duck just around the corner from Clapham Junction station

[Roger Ford]

BP Ultimate is 97 (same as any SUL), not 99, according to extensive argument/research on Scoobynet. Early rumours had it as 100, and Scoobynetters were furious with BP for not living up to the rumours (they get upset easily over there).

[AMMO]

I did some fuel testing in the mid '90's. we ended up using BP Super unleaded which was 98 octane at the time. No expert on the subject but this is what I recall: Because of the fact that certain ingredients evaporate out of the fuel to guarantee the 98 octane rating BP SUL was in the region of, or in excess of, 100 octane. No reason to believe that the situation is still not the same with 97 SUL and indeed regular 95 octane. The ratings may be higher depending on how fresh the fuel is.

 

I also tested some evil ELF 109 octane that made me puke when I took too much of a whiff of it. It was almost brownish in colour a had a really weird smell from the tailpipe when the engine was running. Made no more power. We avoided that stuff like the plague. Makes me feel ill almost ten years later just thinking about it. £100.00 for 50 litres at the time. Also tested some AGIP 100 octane WSBK control fuel. £50.00 for 50 litres. At least it didn't make me puke! Same as above. No more power.

 

The simple reason for this is that there is no point using a higher octane fuel than the engine requires. The engine will not make more power.

 

I regularly run road engines at 11:1 on 95 octane and on one occasion even 12:1 with no problems.

 

Recently at Dyno / Curry 3 one car actually made more power on 95 octane than Ultimax. The reasons for this can be put down to two things, 95 octane has a higher calorific value than 97 (I don't pretend to fully understand this, PC will explain 😬) and that there is no point running a higher octane fuel than you require unless the ignition map is optimised to run on that fuel.

 

For the road I run 95 octane. When I do a trackday I run Optimax only to keep the engine cool due to the prolonged periods at high revs.

 

Once my car is back on the road I would like to do some more fuel testing. Top of the list is BP. I would like to compare it with Optimax. BP always made a good product. Be interesting to see if this is still the case.

 

AMMO

[Nifty]

That evil Elf sounds like it came from Pepee Le Poo *tongue* 😬

 

Keep off the straight and narrow 😬

[riggsbie]

AMMO is correct, no power benefits are usually seen by running higher octane fuels unless the calorific value of the fuel is higher and unless the ignition timing is modified to get the benefit from the higher octane.

 

There are many fools out there putting expensive Optimax in their "sports cars" and getting no real benefit from them. Good marketing Shell !

 

Riggsbie

 

I work in automotive R&D at Ford)

[Peter Carmichael]

no power benefits are usually seen by running higher octane fuels unless the calorific value of the fuel is higher

Agree and I presume you are pointing out that in practice this doesn't happen.

unless the ignition timing is modified to get the benefit from the higher octane

Disagree, conditionally. Ignition timing will be a non-issue as long as the engine can be timed to minimum best timing without encountering knock, which I understand is typical for 4-valve designs.

 

The limit state is when the minimum best timing (MBT) is just about to trigger the onset of knock (OOK). If you degrade the fuel octane to the point where MBT meets OOK by reducing the additives and so increasing the average calorie content of the fuel, you will end up with more power than you started with.

 

The other key factor affecting onset of knock is the compression ratio. The other way of looking at it is to set your CR appropriately for a chosen fuel grade, again so that MBT and OOK are conincident. The higher compression ratio in itself will result in more thermodynamic efficiency (although subject to diminishing returns) and hence more power.

 

What I am really saying is that the optimum engine for a higher octane fuel has higher compression. If the compression isn't a limiting factor, twiddling with the ignition timing will not find you more power than you could otherwise achieve with the lower octane fuel. I suppose also there is a secondary effect related to cam timing, which could mean that variable valve timed engines could be more vulnerable to knock conditions - I don't have experience of this because most of my mucking about is with tuned engines with long cam durations rather than production specs.

[Tom]

*confused*Crumbs.....

 

So, really, the answer to my question is to run whatever octane fuel the car's engine was designed to use: in this case 98. 🤔

 

[riggsbie]

Wow Peter,

 

A full technical reply, hands up who understood all that ? 😬

 

It is funny reading pistonheads forum where the average Scooby owner says that he can feel an improvement in power when he uses Optimax..... I guess if you pay extra for your fuel you have to convince yourself that there is a benefit and you are getting value for money

 

A lot of OEM management systems only seem to be able to adapt their ignition timing (using a knock sensor) only one way. We had an Audi TT in work and during our usual competitor benchmarking it was noticed that the ignition timing retarded a little when using a lower octane fuel and then would not correct itself back to it's original ignition timing when it was run again on a higher octane fuel..... I guess it just tries to "protect" the engine and not optimise the engine's performance ????

 

Riggsbie

[Roger Ford]

As I understand it, Scoobies (and not doubt others) gradually advance the ignition until they detect the onset of knock, then retard it a bit. In other words, they WILL benefit from higher octane fuel, but it takes a while (suggested to be about half a tank) to see the benefit.

 

Not sure I can tell any difference, but then my Scooby rarely goes anywhere without a trailer full of Caterham behind it anyway, which kind of knocks the edge of the performance anyhow.

[edmandsd]

I've been trying different fuels on the dyno with my BD engine recently.

 

In a straight back to back using the same nominal ignition settings leaded 4 star was best, with optimax second (no additive) and 76 118 Octane race fuel last. Even by cranking the ignition up to 8 degrees more advance made no difference on the race fuel regardless of y engine's 12.6 to 1 compression ratio.

 

I'd therefore decided to run the 4 star but when we came to fine tuning the BD on the dyno it was flat, unresponsive and c 10 bhp down since we'd run the fuel comparison test a couple of month's earlier. Reason was that i'd bought the 4 star from a petrol station on the coast (whereas previously it had been bought from somewhere else) and had obviously been left in their tanks for ever and a day and gone off. As a result I now run the engine on optimax (without additive).

 

The Hart engine's going on the dyno shortly and i'm planning on running Carless Classic which is rated at 102 octane, but only if it out performs Optimax.

 

One other test we did on the dyno was to try iridium tipped spark plugs compared to the standard NGK's. I'll let you guess the outcome on that one.............

 

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