Can premium unleaded petrol replace standard unleaded fuel?

By: Andrew Norton

Our outboard expert explains how premium unleaded petrol can replace standard unleaded fuel.

Can premium unleaded petrol replace standard unleaded fuel?
The correct additives in marine fuel tanks can extend petrol life by preventing it from going 'stale'.

I recently discussed why E10 petrol is unsuitable for marine engines. But with standard unleaded fuel being phased out, can premium unleaded be used in an outboard?



The answer is YES, provided the right oil is used with the petrol. Several years ago, power equipment manufacturer Stihl researched why its premix chainsaws and brushcutters were seizing when premium unleaded was used. The lab testing showed that the aromatic chemicals used in premium (95 RON) to raise the octane from standard unleaded (91 RON) were creating a chemical reaction with the lubricating oil, resulting in the oil not staying in suspension with the petrol, and not reaching the crankshaft and wrist-pin bearings and piston-rings.

However, oils have come a long way since then and according to marine fuels experts like Ken Evans of Mercury Marine, and Dave Denny of Lakeside Marine (the national Tohatsu Outboards distributor), filling up with premium is not a problem when semi-synthetic oils are used. Ken also says that mineral-based oils are okay, but he recommends spending that little bit extra for a semi-synthetic oil. Examples of semi-synthetic oils are Valvoline TC-W3 Outboard 2-Stroke (which Lakeside recommends for all two stroke Tohatsus), Bombardier XD50, and Quicksilver Premium Outboard Oil.

Ken recommends switching to premium unleaded even if standard unleaded is available. He says that although standard unleaded used to have stabilisers to increase the shelf life, currently it doesn’t, and from the refinery its shelf life is just three to four weeks.



After this period the petrol starts to separate into "light" and "heavy" components. The light components include benzene, to raise the octane, and these evaporate, leaving the heavy components or base oil stock. The heavy components won’t atomise easily in a carburettor or fuel injection system, leading to engine misfiring through pre-ignition and rapid carboning-up of the combustion chamber. Another problem is that these heavy components bypass the piston rings, diluting the sump oil in four-strokes. This is indicated by the engine "making oil" and the sump oil-level actually rising between oil change periods.

Ken says that being a more refined fuel, premium unleaded has slower and cooler "burn" in the combustion chamber, resulting in more thorough air/fuel combustion, and this is why even a carburetted engine designed to run on standard unleaded will perform better on premium. Tests I’ve conducted in carburetted four-stroke engines designed for standard unleaded (both marine and automotive) show increased power and less susceptibility to pre-ignition when the engines are using premium. However, Ken says that using RON 98 premium is a waste of money in engines designed for standard unleaded, although of course it will benefit supercharged four-strokes such as Mercury Marine’s Verado outboard range. Remember, if in doubt about the fuel octane needed for an engine, always opt for a higher octane fuel to prevent pre-ignition and possible piston crown-damage through uncontrolled detonation.

Even though premium has a longer shelf life than standard unleaded, Ken still recommends not filling up the fuel tank until the next run, ensuring the fuel is as fresh as it possibly can be. He also recommends buying fuel from high volume outlets such as the major oil companies and supermarket chains, rather than small servos where the fuel may have been in the storage tanks for a couple of months.

Another precaution to take, particularly with portable fuel tanks, is to always close the fuel vent when the engine is not running. Direct sunlight on a fuel tank can quickly evaporate the light fuel components, again leaving the base stock. Fortunately, plastic fuel tanks don’t warm as quickly in the sun as the old steel tanks, although the latter could be used to store fuel between uses.



Provided non-ethanol petrol is used, there are ways of preserving the fuel by preventing it from going "stale."
An example is Bombardier 2 + 4 Fuel Conditioner which contains Isopropyl Alcohol or rubbing alcohol. Mixed with non-ethanol petrol, even premix fuel at 25:1, it absorbs any moisture in the fuel for up to a year, keeping the fuel fresh. It’s mixed at 500:1 for everyday usage and 250:1 for storage. I’ve used it for 16 years with great success in two and four-stroke engines.

However, 2 + 4 has two main drawbacks. First the cost: at $14 for a 236ml bottle it adds almost 12 cents a litre at 500:1 and almost 24 cents at 250:1. In my opinion this is still cheaper than ditching the petrol.

The other drawback is that as it absorbs moisture in the fuel it keeps the sparkplugs slightly wet. This is okay with sparkplugs having at least 0.8mm electrode gaps, but some small current model outboards with smaller gaps have ignition systems that will not handle the wetness. I also recommend against using 2 + 4 in two-strokes having points ignition, since the voltage at the sparkplug is simply not enough. It’s a shame that boaters will have to opt for premium unleaded and pay more for their enjoyment afloat. But as always, through government inaction, the oil companies have the upper hand, so we have to adapt to maintain our marine engine investments in peak running condition.


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