Tech Corner

Sunoco Tech Corner


Race fuels v pump fuels

We are all excited about the “high octane” properties in race fuels, but if we can buy a super unleaded and add a bottle of octane booster, will that make a pump fuel a race fuel?

The simple answer is unfortunately not.

The simple reason for this is that a race fuel is built from the ground up using very stable and narrow refinery streams and selected high-quality chemicals. This makes sure that not only does a race fuel burn cleaner with the potential for more power but it also provides consistency.

Pump fuels use hundreds of hydrocarbons from the refinery tower and are designed to meet European standards for pump fuels (EN228) and to be as cost-effective as possible. Consistency is not high on the agenda, as all modern cars are designed to cope with fuel inconsistency. Using pump fuel in a highly-strung race engine is like tuning the race engine precisely on a race fuel and then randomly changing the A/F (air/fuel) ratio and ignition timing and expecting the engine to perform the same….

So to summarise:

Race fuels are consistent, burn cleanly and are designed to maximise the power potential within race fuel regulations. The consistency of the race fuels means that engines can be mapped precisely so that you can get the maximum out of each fuel.

Price per litre is not at the top of the list and the race fuels are designed to be at their best at high RPM and wide-open throttle. Pump fuels, on the other hand, are designed to meet European standards (these standards are very wide) while being as cost-effective as possible. The application they are designed for is for journeys between A and B at low RPM at maybe an average of 10-20% throttle opening.

Octane in Europe v USA

At fuelling stations in Europe, octane is displayed at the RON (Research Octane Number) and the minimum octane to meet European legislation EN228 is 95 RON. Some fuel stations opt to sell a Super Unleaded with octane ratings between 97 and 99. Please note that in the US, all fuel stations publish the AKI index (Anti Knock Index), which is the average of RON and MON (Motor Octane Number). The RON measures the fuel’s ability to withstand detonation at low RPM at half throttle opening, whereas MON is measured at a higher RPM and at full throttle. In Europe, a 95 RON fuel has a typical MON value of 85, resulting in an AKI rating of 90 (95+85)/2.

Storage stability, pump fuels v race fuels

Pump fuels are designed to be used in road cars used on a daily basis, so no efforts are made to make them stable. Pump fuels are low-octane fuels with price point as one of the most important aspects, plus of course that they meet current European specifications for pump fuels EN228.

Typically, race fuels have higher octane due to the use of pure chemicals and much more refined refinery streams which are much more stable than lower quality refinery streams used in pump fuels.

These lower quality refinery streams are less stable, meaning that pump fuels tend to go stale much faster than race fuels. One of the reasons for this is that Butane is typically used to give the pump fuel a vapour pressure to enable engines to start up when cold, and Butane boils at 0°C.

This means that in open containers such as fuel tanks, pump fuels start to go off as soon as they are put in the tanks (unless the temperature is below freezing). Race fuels (good quality race fuels such as the Sunoco race fuels) do not use Butane to adjust the vapour pressure, but instead use chemicals that boil at around 27°C to control the vapour pressure, resulting in the fuels retaining properties much longer.

Sunoco Optima is an excellent example of a fuel that will stay stable for up to 3 years in vented tanks (unless exposed to excessive heat) but if stored in a sealed, airtight tank will keep fresh for as long as ten years, if not longer.

Another factor that will affect all types of fuels is sunlight causing oxidation of certain hydrocarbons. We therefore strongly recommend not to store fuels in clear containers and to keep them in airtight steel drums.

Another tip is to take the fuel out of the tank after a race meeting and put it into a sealed can to make sure that it stays fresh for longer periods. However, do not put the fuel back into a drum that has fresh fuel in the can, as you can then potentially degrade the fresh fuel. New fuel drum – into tank – out of tank – into drum with used fuel. Next time, use the fuel from the used fuel drum and top up with fresh fuel to ensure maximum consistency.

Calculating octane when mixing fuel

All fuels can be mixed with each other, and using a high-octane fuel to bring up the octane in a low-octane fuel is a widely-used practise and much better than using octane boosters. Calculating the octane is relatively simple and, when mixing unleaded with unleaded fuel, quite straightforward. Use this calculation:

(% fuel A x octane fuel A) + (% fuel B x octane fuel B) = Octane of mixed fuel

If you, for example, mix 30% Premium Unleaded (A) with 70% Sunoco GTX (B), the calculation will look like this: (0.3 x 95 RON) + (0.7 x 103 RON) = 100.6 RON

Note 1: High-ethanol-content fuels do not mix linear with non-ethanol fuels. Ethanol boosts octane quickly in low-octane fuels but less so with high-octane fuels.

Note 2: Leaded fuels boost octane faster than a linear mix would calculate. If you mix 20% of Sunoco Supreme with 80% of Premium Unleaded, the above calculation indicates that the following would happen: (0.2 x 115 RON) + (0.8 x 95 RON) = 99 RON.

However, when tested in an octane machine it comes out as 100.5 RON.

What makes leaded racing fuels leaded?

Although very few engines need leaded race fuels today, it still has a vital role in motorsport – but it is not the lead (Pb) that we know from the periodic table from school. Tetraethyl Lead (TEL) is what is used in fuels and racing fuels as it is an excellent octane booster, increasing the efficiency of engines when used in high-compression engines.

Another advantage with lead is that unleaded hydrocarbons of high octane are typically double-bonded and, as such, burn slower than lower-octane, single-bonded hydrocarbons. By adding TEL to single-bonded hydrocarbons, you will get a high-octane, fast-burning race fuel.

A further advantage of TEL is that it provides a cushion between the valve and the valve seat. If you are running stainless valve seats in a cylinder head that is designed for unleaded fuels, you basically do not need leaded fuels.

However, if you have stainless steel inserts in a cylinder head which is not designed for unleaded fuels, combined with harder valve springs and a more aggressive cam profile, the stainless-steel valve seat could work itself into the cylinder head (due to the soft cylinder head material) and possibly come loose as a side effect.

Specific gravity and oxygenated fuels

For fuels, specific gravity (SG) is measured as a ratio between the weight of the fuel compared to water, which is the standard for liquids.

As an example, Sunoco 260 GT Plus weighs 0.761 kg per litre at 15°C compared to water, which weighs 1 kg per litre. That means that the specific gravity for the Sunoco 260 GT Plus is 0.761.

What will the SG tell us about the fuel? Well, pump fuels must have an SG of a minimum of 0.72 to meet EN228 specification for pump fuels but there is no maximum SG applied. This tells us that there is room for variation, and today you can find unleaded pump fuels in the UK commonly ranging from 0.725 to 0.755.

The higher the SG, the greater the likelihood that the fuel will contain more double-bonded hydrocarbons – i.e. burn slower. It also tells us that you need to lean out the A/F ratio if the oxygen content is the same if the engine is tuned for a lower SG fuel.

The rule of thumb is that for every 2nd digit of the SG increase, you reduce the fuelling by 1%, and for every 2nd digit of SG decrease, you increase the fuelling by 1%.

Likewise, if the fuel has an oxygen content of 3.7% instead of 2.7%, you need to increase the fuel by 1%, and if the fuel has an oxygen content of 1.7%, you need to reduce the fuel by 1% if the SG remains the same.
Taking all aspects into consideration, the upside with pump fuels is that they are cheap. The downside is that the variation throughout the country means that the race engine could run lean or rich. Race fuels, regardless of specification, will have their unique oxygen content and SG throughout the year, whereas pump fuels also have summer and winter blends to throw into the mix of inconsistency.

If you are unlucky and map the engine on a pump fuel with a high SG, high octane rating and a low oxygen content and later, from the very same pump, get a fuel with a lower SG, a higher oxygen content and a lower octane, the engine is going to run lean and detonate – this is when it starts to be expensive.

Methanol and Ethanol and Octane numbers

Methyl Alcohol (methanol) and Ethyl Alcohol (Ethanol) are great fuels due to their high oxygen contents.

Methanol has in the region of 50% oxygen and Ethanol around 33%, making them cool-burning but low-energy fuels.

Saying that, thanks to their cool burning and “built-in” oxygen, their lower energy can be compensated for by flowing a much greater volume of fuel.

Whereas Methanol is mostly used straight, Ethanol is mostly used as E85, meaning that 15% of the E85 will be single- or double-bonded hydrocarbons and the remaining 85% Ethanol (as long as we are talking racing E85R).

It is very important to understand that E85 fuels sourced at fuel stations are very rarely E85. In the winter they are more likely to be E72 and in the summer E82. The reason for less Ethanol in the winter is that it would be very difficult to start a car on 85% Ethanol when ambient temperatures are below freezing. However, due to the wide variation in pump fuel E85, it is not at all recommended to use pump E85 as a race fuel.

There are significant variations in the octane stated on racing E85, mainly due to the fact that the octane testing machine was never designed for a fuel with that level of oxygen content and also not designed to measure above 100 RON and 100 MON.

To determine the blends of fuels containing Methanol and Ethanol, the common practice is to compare the octane of the blend with the same fuel but without the Methanol or Ethanol.

By then doing some linear mathematical calculations, a “Blending Octane Value” is determined for Ethanol and Methanol, but please note that it is not the same as petrol octane rating.

The issue is that if you use the BOV (Blending Octane Value) of the Methanol or Ethanol in a blend, it basically works fine for small concentrations of the alcohol and tends to bump the octane of lower petroleum products more than of higher-octane products.

By using 3 times the amount of Ethanol, the octane is not going to increase by a factor of three. Furthermore, the octane-testing machines are not designed for the amount of oxygen in an E85, so we have to estimate. Some racing fuel companies estimate quite liberally, while Sunoco’s policy is to estimate on the very conservative side. That’s why the Sunoco E85R is rated at 104RON whereas most competitors are rated at 109RON.