Knock Knock...
Most premium unleaded fuels sold in Australia are rated around 95 RON (depending on manufacturer and state controls). Shell's new Optimax is rated at 98 RON (98.4 actually), which gives it the potential for more powerful combustion. Cars which benefit most from higher RON fuel are those fitted with knock sensors (such as the WRX). Knocking (or detonation) is caused by uneven spreading of the flamefront within the combustion chamber, leading to very high pressure spikes. Most knocking in a standard engine is caused by bad fuel, overly advanced ignition timing or too high a turbo boost. Knock sensors allow the ECU to retard the timing to just before the knocking occurs. This effectively optimises the combustion process, maximising the output of the engine.
Dynomite
The car used for the testing was a standard MY99 model Impreza WRX with 18,000km under its belt. The car was run on Caltex premium unleaded petrol (PULP) for 400km and then tested on a four wheel drive chassis dynamometer. Optimax was then run through the system for a further 300km and the car re-tested. The graph below shows the dyno results for each fuel.
Eyeballing the graphs reveals an average of about 3 per cent difference at the wheels across the rev range, with up to 7 per cent difference in some places. You'll notice that the power curve is also a bit smoother on Optimax. This is a function of the "cleaner" burn during the combustion phase, resulting in a much smoother delivery of power. As would be expected with the increased power across a wide range of rpm, the tractive effort curve also shows an improvement with Optimax on board. It's consistently about 10Nm higher across the rev range.
But could the difference in power be felt? It should be noted that the car was running on Optimax before testing began, and I hadn't really noticed a difference in power after the initial change to normal premium. However, when I ran the Caltex premium in the car again, I could honestly say the car felt slower. It was definitely not as "perky" on the conventional PULP as when running on Optimax. These suspicions were confirmed when I refilled with the good stuff - again the car felt more lively.
Economic Cents...
Shell claims you will also get better fuel economy when using Optimax. This is due to the higher density of the fuel effectively giving you more fuel in the same volume. Based on a fellow MY99 club member's fuel records, it would appear that Shell's claim is true. He has gained about 70km more per tank under Optimax than when he was running regular premium. And - remarkably - during this time the new fuel has averaged only 0.5 cents more per litre than the normal premium stuff! Drawbacks? The only one is that as the new fuel is denser, it is also heavier. This will slow your car down effectively neutralising any performance gains (he says in jest with a grin!).
Conclusion
There is no doubt the new Optimax fuel provided more peak power, smoother delivery of that power and better fuel consumption in the test car. And all at a fractional cost over regular premium unleaded! Bang for buck, it's a real winner.
Acknowledgement: Thanks to APS for allowing the use of their dyno and to Patrick Felstead for fuel consumption figures and technical support.
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Optimax and the Audi S4
When I was in Melbourne recently on AutoSpeed business, I eagerly looked for an Optimax pump. In fact I was filling up within an hour of arrival! Did it make a difference on the high compression ratio, 15 psi boost, individual cylinder knock-sensed, standard Audi S4? Yes! Second gear performance was noticeably stronger, and the power delivery (which at times on normal PULP has some subtle hesitations - ignition timing retards?) was also smoother. I could see no improvement in fuel economy, though.
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