As enthusiasts I’m sure we’ve all started a sentence with ‘What’s really interesting is . . .’ only to realise that our audience really doesn’t think it is. Rolling eyes, stifled yawns, we’ve seen the signs . . .

So here goes:

The petrol you fill up with in December is different from the petrol you pump in June.

It makes sense really, petrol is a complex mix of not only crude oil, but liquid, gaseous and solid hydrocarbons, and additives designed to affect the way it behaves in an engine. There’s a balance of ingredients with different volatilities to improve, for example cold starting. So fuel bought in the winter will be blended to provide a volatility suitable for cold temperatures, whilst that available in the summer will have a different composition.

But the big issue with the composition of fuel for classic car owners is that presented by the addition of ethanol to the fuel available on the forecourt. E5, or 5% ethanol, fuel is common, and E10 is very much on the way.

Understanding what this means for classic car owners is important. Running our cars on these fuels is not all bad, but there are factors to bear in mind. Over around 3000rpm performance will be good, emissions low and all’s well. In traffic and at low rpm the higher volatility of these can lead to less than ideal running.

Why? The atomisation of fuel as it passes through a carburettor has a cooling effect, similar to what goes on in a fridge – liquid passing through a constriction and vapourising causes a drop in temperature. At high RPM and with larger amounts of fuel passing through, the greater the cooling effect. But if the engine is running at low RPM and engine bay temperatures rise, and the more volatile fuel can starts evaporating before the carb jets, in fuel lines and filters for example. The leads to disruption of the intended atomisation at the jets, with droplets of fuel entering the inlet manifold and thence to the cylinders, adulterating the mixture, changing the burning profile and giving the same effect as lean running. Cue white smoke and unburnt fuel smells from the exhaust. Difficulty restarting the engine can often be a result of the same effect.

Ethanol blended fuel contains more oxygen than unblended – this can also give the effect of lean running.

And what’s even more interesting is . . .?

The higher volatility components of modern fuel evaporate at ambient temperature. Without these more volatile elements it will be harder to start the engine and the increased density of the fuel will cause poor running. Without the correct mixture being delivered to the cylinder, again the engine could seem to be running lean. The temptation is to try to correct this with the choke, and the situation becomes increasingly less ideal.

One test found 15% of a volume of fuel disappearing in just 5 weeks – although this was in Australia with potentially higher ambient temperatures than we experience.

And here’s something that is just plain alarming . . .

Ethanol is hygroscopic – Ethanol blended fuel absorbs water vapour from the atmosphere.

Vehicles stored in conditions where there is a humid environment and a range of ambient temperatures may well experience condensation in the fuel tank itself, leading to water droplets falling to the bottom of the tank and absorbing ethanol.

Also should water get into the tank through a leaky cap gasket or other reason, once a certain concentration is reached the ethanol/water mix will separate and fall to the bottom of the fuel tank, carburettor float chamber or low point in the fuel delivery system.

Water containing ethanol is highly corrosive.

Ethanol blended fuel itself can also rot rubber hoses and seal, diaphragms and plastic petrol floats. It dissolves the plasticisers use in these materials making them brittle. By now most old cars will have had their fuel systems overhauled and the components at risk may well have been replaced with those made with resistant material, but it is worth bearing this potential problem in mind.

All these issues, and a good deal more, are covered in an excellent, and yes, INTERESTING book, ‘Classic Engines, Modern Fuel, The Problems, the Solutions’ by Paul Ireland. We cannot recommend buying this book highly enough, and a read through the material on the website is also highly informative.

So what’s the conclusion for us, interested in keeping cars in top condition whilst not in use?.

Put simply, try to avoid ethanol blended fuel when you’re using your classic, but don’t worry too much if you’re not crawling about at low rpm – or unless you aren’t sure about the age of the components in your fuel system. In fact E10, or better still a Super grade with Ethanol for a track day or spirited run out will perform well, giving power and clean emissions.

Just hope you haven’t got any residual water in the system from past storage in unsuitable conditions, or that time a previous owner filled up in the rain or washed the car without tightening the fuel cap!

When it comes to storage for anything over a couple of months, especially with carburettor cars, you should be looking at either using a fuel stabiliser additive, or better still emptying the tank and replacing the old fuel with a dedicated storage fuel.

We use a Sunoco 99RON ethanol free product that is ideal. It has proved transformational in terms of keeping cars healthy and ready to start. And if you’re going for a spin, you’ve got some race fuel derived juice to enjoy!

It’s not cheap, but priceless in terms of the potential problems it solves.

We recommend that customers deliver their cars to us with as little fuel as they dare in, we can then extract the remainder, add the storage fuel and run it through the system. Our storage fuel is good for 3 years, so you’ll have some high octane goodness to look forward to when you get on the road!