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just in case anyone was in doubt, I have been running my 9k Aero (JT 3", big IC, ITG intake) with Standard ECU and 95RON fuel.

I thought going back to standard was bad, but this is worse;
Compared to Optimax on Standard ECU:
Throttle response worse, 2+mpg less (easily) and less boost under full throttle.

We all knew this - but going back to 95 has really driven it home how big the effect is.

Flip side is the old dear can still wind the speedo right round to the peg quite easily (first time I have ever tried and was quite surprised to say the least)

Roll on stage 4 and a tank of super. All talk an no ££ action
 

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I never really rated Optimax until I went back to standard unleaded - and then the difference was apparent. Now I stick to Optimax
 

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I've been using Shell V-Power (100RON) since last weekend at the Ring. Have about a quarter of a tank left

All I can say is


Zahid
 

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wow thats awsome all i ever get is 91 octane over here... and it costs a pretty penny...
but i too notice say from 76 to chevron.. 76 feels a lot cleaner...
 

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Originally posted by Eric (9-3 Hirsch):
[qb]Remember that their octane ratings are different.  Our 91 is equivalent to their 97 or so. [/qb][/b]
Not quite.....

Your 91 is the equivilent of our 95.
Your 93 is the equivilent of our 97.
Your 94 (when available) is the equivilent of our 98.
 

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so, our 105 octane 'race fuel' (which cost 2x-3x the amount of anything else) would be the equivalent of... 107? or something close?
 

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wow thats awsome all i ever get is 91 octane over here... and it costs a pretty penny...[/b]
You think it's expensive over there, Shell Optimax is now the equivalant of $6 per gallon here
 

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There are two octane figures commonly used:

Research Octane Number (RON)
Motor Octane Number (MON)

In Europe, the RON is the figure which is almost exclusively used, although you will see both numbers on pumps in France.

In the US, the usually quoted octane number is an average of the RON and MON, i.e. (RON + MON)/2.

For motor fuels, the RON is typically 8 to 10 higher than the MON for a given fuel.

European standard "super unleaded" is 95 RON, and has a MON of 85, so would be called "90 Octane" in the US.
 

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Originally posted by Mark B:
[qb]In the US, the usually quoted octane number is an average of the RON and MON, i.e. (RON + MON)/2.[/qb][/b]
Isn't this sometimes referred to as AON, American Octane Number?

octane number

A rating of a gasoline in terms of its propensity to cause knocking.

Gasoline is not a chemical compound; it is a mixture of many different compounds.

By 1882, experimenters noted that spark ignition internal combustion engines knocked more on some gasolines than on others. Ideally, when the spark ignites the fuel-air mixture in the cylinder of a gasoline engine, the flame front spreads out smoothly from the spark into the unburned part of the mixture, giving a gradually increasing push to the piston. However, as the flame front spreads, the hot products of combustion behind the flame front compress the unburned part of the fuel-air mixture. Compressing a gas raises its temperature. Radiation from the burning fuel can also raise the temperature of the unburned fuel. The unburned fuel-air mixture can be heated so much that some of the hydrocarbons in it reach their ignition temperature and ignite all at once–explosively, causing knock. Knock can destroy engines.

To select a way of rating the propensity of a gasoline to cause knocking, a Cooperative Fuel Research Committee was set up in 1927 comprising representatives of the American Petroleum Institute, the American Manufacturers Assn., the National Bureau of Standards, and the Society of Automotive Engineers. A single-cylinder engine with a variable compression ratio had been built by John Campbell at General Motors. Graham Edgar at the Ethyl Corporation prepared samples of various pure hydrocarbons. including normal heptane distilled from the sap of the Jeffrey Pine. The engine enabled researchers to burn mixtures of Edgar's pure hydrocarbons while varying the compression, to see at what point knock occurred.

In 1929, T. A. Boyd proposed to the committee that a variable-compression engine be the basis for rating gasolines. Some committee members felt that such an engine would be too complicated for routine use, but the Waukesha Engine Company volunteered to build a prototype. By 1931 Waukesha was able to display its engine at a meeting of the American Petroleum Institute; skeptics were persuaded and thousands of the engines were subsequently built. (In fact, in 1980 the American Society of Mechanical Engineers designated the engine an “engineering landmark.”)

In the committee’s opinion, no one test was able to give a rating useful over the whole range of operating conditions, and so two methods were defined: the Motor Method (ASTM d 357) and the Research Method (ASTM d 908). Both methods are based on comparing the performance of the gasoline being tested with the performance of a mixture of 2,2,4, trimethyl pentane (also called iso-octane) and normal heptane. The octane number is the percentage of iso-octane in that mixture whose performance (in regard to knocking) is the same as that of the gasoline under test. For example, if the performance of the gasoline under test is the same as that of a mixture of 80% 2,2,4,trimethyl pentane and 20% normal heptane, the gasoline is 80 octane. Octane numbers above 100 are found by extrapolation.

The two test methods give different results, and the difference in the results differs from gasoline to gasoline. As a broad generalization, the motor method captures the gasoline's performance at high engine speeds and loads, and the research method at low speeds. The octane rating on American gasoline pumps is usually the average of the research and motor octane numbers, which is sometimes called the anti-knock index. In Europe, pumps have traditionally displayed the research octane number.

Fuel is just one of many factors affecting whether an engine will knock. Consequently in any particular engine gasolines with the same octane number but from different blenders may perform differently: one may cause knock and the other may not. Similarly a gasoline that causes knock in one engine model may not in another. This is not proof that the octane rating was inaccurate.
source
 

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My car runs much better on 95 than on 97 or 98, and returns nearly 10% better mpg too. Strange huh?

I still don't know why this is, but I was told something really interesting on TSN:

"I can tell you that lower octane fuel does give better fuel economy. Some will argue that because they think more expensive (and higher number)means better. Higher octane simply means it resists pre-ignition, allowing the engine to extract more power without knocking. But higher octane fuel has lower energy per volume, so it gets consumed faster to create the same power."

A few other people chimed in to say they noticed the same affect on fuel economy.

Does what he said make sense?
 

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Originally posted by JohnCC:
[qb]My car runs much better on 95 than on 97 or 98, and returns nearly 10% better mpg too.  Strange huh?
[/qb][/b]
Not really.

I think it all depends on what model Saab you have.
It may be that the LPT's (and N/A's) as they don't really need the 97/98 grade would not show better mpg.
The HOT's and the older FPT's which do prefer the 97/98, I feel will show worse mpg when run on 95. That's certainly my experience.

Being a sad anorak, I've done fuel experiments with my 9-5 Aero, run over virtually identical weekly routes, when running 97 and Optimax, and I have even noticed a difference in mpg between 97 and Optimax of about 0.5-1.0mpg in summer time. It was only between 0.0-0.5mpg in winter time, no doubt due to less performance drop off in cooler temps.
I thus use cheaper supermarket 97 fuel from autumn to spring and Optimax through the warmer months.
 

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Yep.. mine is a '97 FPT. I think you're right Bill.. that I get better mpg from 95 is not strange.. that I get worse performance, is!
 

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Originally posted by BillJ:
[qb]My 9000 handbook mentions that higher octane will tend to give better performance, while lower octane will tend to give better fuel economy (i.e. more MPG). [/qb][/b]
I'm inclined to agree with this too. 95 gives me much better MPG. Optimax seems to disappear down the drain returning about 2mpg less.

I haven't tried this on a long motorway run yet to see if this makes any difference, but will do when I next do the 500 mile trip back to the highlands.
 

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I only ever use 97 or 98 in my SS Aero (Stage2)so I can't comment what it feels like on 95.

My old VX220T had a few flat-spots which were really noticeable. When I first got it I was using 95 but after a few tanks of 98 the flat-spots disappeared and really improved throttle response. Fuel economy dipped slightly. Then I got a Stage 2 on that and used nothing but super again.

I must admit to not noticing a difference between Optimax and BP Ultimate despite the RON difference. I tend to buy BP (even though I work for Shell!) because I get Nectar points...
 

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Initially I used 95 RON in my 9-3 Aero i then tried Optimax and couldn't really feel any difference as i used to when i fuelled my Mondeo.

Then i got a BSR upgrade and i once again tried Optimax but this it really did light the blue touch paper


And yes i did seem to get slightly better fuel consumption, which is nice.
 

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hi there,
95 octane is ****, any of you with a 9.3:1 compression ratio 2.3 turbo (95/93) are running right on the edge of knock all the time on boost, trionic puts in more fuel and puts back the ignition timming when knock occurs, both of which reduce power and increase fuel consumption.
Please use "good" fuel for your engines sake.
bye
Ps. this also applies to engines with tiny turbos (gt17) as these cause high back pressure in the exhaust manifold which contaminates the charge air again increasing knock.
 
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