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Fuel Octane Ratings and how they affect your car - by Paul Chadwick March 2006

Some Definitions (Petrol & Octane)
Gasoline (or petrol) is a petroleum-derived liquid mixture consisting primarily of hydrocarbons, used as fuel in internal combustion engines.
Many Commonwealth countries use the term petrol (abbreviated from petroleum spirit), or more rarely, motor spirit. The term gasoline is commonly used in North America, and within the oil industry generally, even within companies that are not American. The word is commonly shortened in colloquial usage to "gas"
Octane ratings are ratings used to represent the anti-knock performance of petroleum-based fuels.

Anti-knock, what does it mean?
Every internal combustion engine pre-compresses the charge of air and fuel before the spark-plug ignites and the miniature explosion forces the piston back down. Compressing the charge makes the air fuel mixture suceptible to "self" ignition..... before the spark plug is timed to create the explosion, known as"knocking" this is BAD, it can damage the engine internals, burns away valve seats and causes undue shock/stress to other components.
Car manufacturers have designed their engine management systems to detect knock and adjust the ignition timing automatically to prevent pre-ignition. When you fill-up with a low-octane (95) rated fuel, your engine recognises that the fuel air mix has a relatively low resistance to "knocking" and the timing changed to ensure that the ignition is triggered before the mixture is fully compressed, the engine is protected but there is a downside. Internal combustion engines produce more power from highly compressed charges, the mini-explosion is stronger and forces the piston back down with increased vigour. High Performance cars tend to run higher compression engines and have engine management systems that are adept at reacting to different fuels to extract the highest output. A car that can adapt to higher octane fuels can deliver better fuel economy, less fuel is needed to deliver the same journey, this can offset the extra cost of the fuel purchase over cheaper low octane fuels. Simple cars built with low purchase cost and high reliability in mind tend to be built to tolerate low quality fuels and not to exploit the potential of higher quality blends.

Fuel - What should you buy?
At your petrol station they now sell a bewildering range of fuels, low sulphur, lead replacement fuel, blends that clean, high performance mixes. Many are characterised by their octane rating, its common to see 95 or 97 rating these are often labelled as unleaded & super respectively. Higher rated fuels cost more becuase the additives to the standard blend are expensive. It is difficult to discover the exact octane ratings of these performance brands, usually % improvements are promoted along with the engine cleaning characteristics. It's probably fair to say that they vary between 97 & 99 figures according to individual brand.

High Performance Fuels in the UK
Shell Optimax (V-Power)
BP Ultimate
Tesco 99

If your car is a high performance model then you should be able to feel an immediate improvement in performance and perhaps fuel economy too. If you run a simple or "value" car then it is unlikely that you will benefit from the premium fuels, there's not likely to be any noticable improvement. See results (below) from a TV programme "scientific" test which compared to leading premium brands with a cheap supermarket 95 octance fuel.

Renault Clio - Fuel type makes no difference to this "shopping cart" basic vehicle

Manufacturers Figures
2006 Model Year Renault Clio Expression 1.4 16V
Fuel Type Unleaded Petrol
Injection Type Multipoint Injection
Maximum Power - hp DIN (kw ISO) 100 (72)
Maximum Power (revs/min) 5700
Maximum Torque - Nm ISO (mkg DIN) 127
Maximum Torque (revs/min) 4250

VW Golf GTI - Fuel type does affect this turbo-charged hot hatch, Optimax will deliver noticable improvements.

Manufacturers Figures
2006 Model Year Golf GTI FSI 2.0Turbo
Maximum output PS 200 at RPM 5100
Maximum torque 207 lbs.ft / 280 Nm at RPM 1800

Subaru Impreza STI - Fuel type makes it's most significant effect with this high performance rally car, a staggering 26 BHP improvement in this test.

Manufacturers Figures
2005 Model Year Impreza WRX STi
Type Horizontally opposed Subaru Boxer, 4 cylinder, DOHC/Bank
4 valves/cylinder, Turbo/Intercooler, V V T
Bore mm 92.0
Stroke mm 75.0
Compression 8.0±0.2
Engine Capacity cm3 1994
Net Power 265 PS@ 6000
Torque 343 Nm @ 400


HomePage Engine Install Electronic Multimedia Gallery Motorhome Thermoblok Credit/Links For Sale Dyno Alt Fuel Octane drivesafely	Fuel Octane Ratings and how they affect your car - by Paul Chadwick March 2006 Some Definitions (Petrol & Octane) Gasoline (or petrol) is a petroleum-derived liquid mixture consisting primarily of hydrocarbons, used as fuel in internal combustion engines. Many Commonwealth countries use the term petrol (abbreviated from petroleum spirit), or more rarely, motor spirit. The term gasoline is commonly used in North America, and within the oil industry generally, even within companies that are not American. The word is commonly shortened in colloquial usage to "gas" Octane ratings are ratings used to represent the anti-knock performance of petroleum-based fuels. Anti-knock, what does it mean? Every internal combustion engine pre-compresses the charge of air and fuel before the spark-plug ignites and the miniature explosion forces the piston back down. Compressing the charge makes the air fuel mixture suceptible to "self" ignition..... before the spark plug is timed to create the explosion, known as"knocking" this is BAD, it can damage the engine internals, burns away valve seats and causes undue shock/stress to other components. Car manufacturers have designed their engine management systems to detect knock and adjust the ignition timing automatically to prevent pre-ignition. When you fill-up with a low-octane (95) rated fuel, your engine recognises that the fuel air mix has a relatively low resistance to "knocking" and the timing changed to ensure that the ignition is triggered before the mixture is fully compressed, the engine is protected but there is a downside. Internal combustion engines produce more power from highly compressed charges, the mini-explosion is stronger and forces the piston back down with increased vigour. High Performance cars tend to run higher compression engines and have engine management systems that are adept at reacting to different fuels to extract the highest output. A car that can adapt to higher octane fuels can deliver better fuel economy, less fuel is needed to deliver the same journey, this can offset the extra cost of the fuel purchase over cheaper low octane fuels. Simple cars built with low purchase cost and high reliability in mind tend to be built to tolerate low quality fuels and not to exploit the potential of higher quality blends. Fuel - What should you buy? At your petrol station they now sell a bewildering range of fuels, low sulphur, lead replacement fuel, blends that clean, high performance mixes. Many are characterised by their octane rating, its common to see 95 or 97 rating these are often labelled as unleaded & super respectively. Higher rated fuels cost more becuase the additives to the standard blend are expensive. It is difficult to discover the exact octane ratings of these performance brands, usually % improvements are promoted along with the engine cleaning characteristics. It's probably fair to say that they vary between 97 & 99 figures according to individual brand. High Performance Fuels in the UK Shell Optimax (V-Power) BP Ultimate Tesco 99 If your car is a high performance model then you should be able to feel an immediate improvement in performance and perhaps fuel economy too. If you run a simple or "value" car then it is unlikely that you will benefit from the premium fuels, there's not likely to be any noticable improvement. See results (below) from a TV programme "scientific" test which compared to leading premium brands with a cheap supermarket 95 octance fuel. Renault Clio - Fuel type makes no difference to this "shopping cart" basic vehicle Manufacturers Figures 2006 Model Year Renault Clio Expression 1.4 16V Fuel Type Unleaded Petrol Injection Type Multipoint Injection Maximum Power - hp DIN (kw ISO) 100 (72) Maximum Power (revs/min) 5700 Maximum Torque - Nm ISO (mkg DIN) 127 Maximum Torque (revs/min) 4250 VW Golf GTI - Fuel type does affect this turbo-charged hot hatch, Optimax will deliver noticable improvements. Manufacturers Figures 2006 Model Year Golf GTI FSI 2.0Turbo Maximum output PS 200 at RPM 5100 Maximum torque 207 lbs.ft / 280 Nm at RPM 1800 Subaru Impreza STI - Fuel type makes it's most significant effect with this high performance rally car, a staggering 26 BHP improvement in this test. Manufacturers Figures 2005 Model Year Impreza WRX STi Type Horizontally opposed Subaru Boxer, 4 cylinder, DOHC/Bank 4 valves/cylinder, Turbo/Intercooler, V V T Bore mm 92.0 Stroke mm 75.0 Compression 8.0±0.2 Engine Capacity cm3 1994 Net Power 265 PS@ 6000 Torque 343 Nm @ 400
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