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Fuel Types

 - 95 octane is normally used, anything above this can yield more power.
 - A great way to get more power for short term use.
 - Must be accompanied by a remap, otherwise, less power may result.


Fuel options in this country are a bit limited. In most areas there are no options outside of 95octane pump fuel. Some stations provide 97octane. However, for rallying and other formulas such as drag racing or for trackdays using road cars, there are a wealth of options available from octane's from 98 upto 120octane and above. Most competitors and enthusiasts have probably heard of using high octane petrol, methanol and nitromethane (nitro), but this is only a small sample of the wide variety of fuels available for occasional use.

Fuel Octane Rating.
 - A measure of a fuels anti-knock capabilities.
 - Is not a measure of power potential.


People sometimes think that a fuels octane rating is a measure of its power output. This is not true. In fact changing from 95 octane to 98 octane in a normal road car, without remapping the ignition map, will probably result in a power loss !! without a remap or without a knock management system, there is no way for an engine to take advantage of increased octane ratings.
Octane rating is in fact a measure of a fuels anti-knock capabilities. The higher the octane rating, the more resistant the fuel is to knocking. The likelihood of damaging detonation and pre-ignition occurring is lessened by using higher octane fuel. When the engine is remapped to take advantage of this anti-knock characteristic then more power is produced.
History of Octane Standards.
At the start of WW1, there was no such thing as an octane rating for fuel. However, it was found that highly tuned engines in cars and planes reacted differently to different batches of fuel that was thought to be identical.

Two tins of fuel, both from the same refinery and both with identical weights etc.. would react differently in the engine. The engine would run well on one tank of fuel and when it was refilled it would blow up while running on the other batch of fuel.

The labs weren’t able to distinguish between a good and bad batch of fuel. To overcome this problem, special single piston fuel research engines were made and distributed to different labs around the world. The labs would test the quality of the fuel by running the engine on the fuel and raising the compression ratio until knocking occurred. This was called the HUCR rating (Highest Usable Compression Ratio).

To calibrate the engines before testing, two pure fuel substitute substances were used to establish a high and low reference point. The high reference fuel used was isooctane (2-2-4 trimethylpentane) and the low reference fuel was heptane (n-heptane).

The following method was then used to determine a fuels quality. Firstly, its HUCR was determined as mentioned previously. Then various runs would be carried out using different mixes of the high and low reference fuels, iso-octane and n-heptane until a blend was found that had a knock behavior identical to the knock behavior of the fuel under test. Then the fuel under test would be rated according to the percentage of iso-octane and n-heptane used. So a fuel that behaved the same as 95%_iso-cotane and 5%_n-heptane would be rated at 95 octane petrol. This is how octane ratings came into widespread use.

Research and Motor Tests.
 - Research test, or RON measures the fuels low RPM performance.
 - Motor test, or MON measures the fuel high RPM performance.
 - MON is more relevant for forced induced engines.


Newer research and motor tests have come into widespread use. The same basic methods and test engines are still used, but a new test call the motor octane test has more relevance than the older (but more widespresd) research octane test. The motor method uses the same engine, but runs it at a higher rpm and intake temperature and gets the octane figure based on these conditions – therefore, this test is more relevant to forced induction engines.

To repeat, the MON (Motor Octane Number) is more relevant for highly stressed engines than the RON (Research Octane Number) but it is the RON number often gets quoted at. This is because, for the same fuel, the RON number will always be higher than the MON number, and it sounds better to quote the RON number.   Use the MON number to indicate the knock resistance at high load and RPM. Use the RON number to indicate the knock resistance at part load and lower RPM.

The difference between the RON and MON is called the fuels sensitivity. A typical batch of fuel could have a RON of 95 and a MON of 85. This means that the fuel has an octane rating of 95 at low load and an octane rating of 85 at high load. The next batch of fuel could have the same RON of 95 with a MON of 88. Obviously this fuel will be better at high engine load even though the RON is the same.

High Octane Test.
- High octane test, or SON tests for octane's over 100 octane.

Any fuels which have an octane rating that is higher than 100 have a different test called the Supercharge test (obviously the traditional tests won’t work on fuels greater than 100 octane). The Supercharge Octane Numbers (SON) are got by extending the old system in a linear fashion beyond 100. In this test the high reference fuel used is iso-octane with lead additives. Two tests are carried out, the F3 and F4 tests. The F3 test is done at cruise and the F4 test is done using full load. This is why high octane fuels have two numbers eg. 100/130.

Avgas Race Fuel.
 - Used for aviation and competition.

Most of the older racing fuels are actually Avgas fuels rebadged as racing fuel. ‘Racing 115’ fuel is in fact ‘Avgas 115/145’. Leaded ‘Racing 100’ is the same as ‘Avgas 100/130’. The old green Avgas was used as leaded 108 racing fuel and the newer blue Avgas was used as unleaded 108 racing fuel. Recently, Avgas 112/160 has been launched as ‘Racing 108’ leaded fuel.

Dedicated Motorsport Fuels.
 - Aviation fuel taylored for motorsport use or originally derived for motorsport.

A lot of the big companies produce fuels specifically for motorsport use. These fuels are designed to give the anti-knock capabilities of Avgas with improved throttle response and power. Typically, 4-5% at the top end and more at the midrange. In the USA VP Motorsport 103 is very popular(99MON, 107RON, 3%oxygen). One of the top fuels in USA is Power-Mist RFG(104MON, 112RON, 6%oxygen). Formula 1 fuel is also top class, but it is somewhat limited by the regulations. F1 fuel has a maximum RON or 102 and max oxygen content of 2.7% and is made up of toluol, xylene, sopentane, iso-octane, hexene-1, n-butane, 2-methylpentane.

Octane Boosters.
 - Selected brands are effective, but some brands have little effect.

As mentioned, toluol and xylene are the main anti-knock agents in F1. In the past, a good way to increase the anti-knock properties of petrol was to add toluol to the petrol (upto 33%). However, it can’t be used on unleaded fuel as a reliable octane booster. The most effective octane booster is MMT(methyl cyclopentandienyl manganese tricarbonyl). It is used in NF Racing Formula and Nulon Pro Strength. They will both add 3ron to 95ron fuel and 1.5ron to 98ron fuel.   Most other octane boosters are less effective. You have to check the contents carefully to see if it contains any usable chemicals. Also, most of the claims made on the package are usually not true.

Methanol.
 - Very high latent heat of vaporisation allows big power gains up to 20%.
 - Requires revised fuel delivery.


A fuels octane rating is not the whole story. There are many factors that contribute to a fuels power potential. Methanol has a MON of  90 yet can produce power increases of 20% over petrol. Shell ‘A’ racing fuel is 96% methanol and 3% acetone. The key to methanol is its very high latent heat of vaporisation. It takes a lot of heat to be converted from liquid to vapour.

Petrol has 135Btu/lb whereas methanol has 472Btu/lb. The heat required for proper atomisation is drawn from the piston crown, inlet tract, combustion chamber and inlet and exhaust valves. This results in an internally cooler engine which puts less heat into the inlet charge, so the charge density goes up and the horsepower goes up. Another factor acting in methanols favour is the amount of energy available in the burned fuel. Using petrol, the best power ratio is around 13:1. With methanol the air/fuel ratio for max power is about 5:1. Because were consuming much more methanol, were also absorbing more heat from the engine. (Note: this means we need to flow double the amount of methanol – bigger fuel lines, bigger fuel pumps, bigger injectors etc…).

The major downside to using methanol is that it is extremely poisonous. It can slowly build up inside of you and cause blindness and insanity. It can be absorbed through the skin and lungs. It is also present in the burnt exhaust gas especially in rich burning mix.

It can also be damaging to your engine. It will eat through fiberglass resin. It has a scouring effect on tanks and fuel lines. It will absorb high amounts of water so it must be kept airtight. After use the whole system has to be flushed out with petrol. It promotes water induced rust and corrosion inside the engine. It is particularly damaging to aluminum and zinc. It regularly leads to blocked injectors. Methanol does not lubricate like petrol, so  flat-slide and barrel-type throttles tend to stick on methanol. In colder climates starting problems will be experienced. Methanol burns much more slowly than petrol so the ignition needs to be advanced.

Nitro methane.
 - Can double the power output.
 - Used on drag cars.


Nitro methane doesn’t even have an octane rating because it is too variable, yet it can more than double a petrol engines power. This is because extremely rich mixtures can be used and because of its unique chemical makeup. Nitro methane contains 53% oxygen so it permits large quantities of fuel for conversion to heat energy. Because it burns so slowly, it keeps pushing the piston to the bottom of the stroke.

Nitro used to be mixed with methanol for use in drag racing, but nowadays nitro is used with almost no methanol blended in. It is necessary to reduce the compression ratio in engines to protect against detonation. The air/fuel mix must always be set very rich, sometimes 2:1. Igniting nitro is always a problem. Dragsters use 1.2 amps in each plug. 50deg advance is used.

Nitro methane is equally or more dangerous than methanol. After combustion, the vapor contains large amounts of nitric acid, which causes muscle reaction and prevents a person from breathing, so gas masks are essential to survive. Nitro on itself is not explosive.

Other Fuels.
Petrol, methanol and nitro methane can have various compounds added to them.

Propylene oxide (epoxy propane) is used with nitro to increase combustion flame speed. It can also be used with petrol and methanol to give 2-3% power gain. It must be stored in plastic or aluminium containers because it becomes volatile when in contact with copper or rust. Nitro propane can be added to any fuel to increase the oxygen content. Obviously, serious remapping must be done afterwards.

All of the above additives must be used in specific mixes and there are a host of precautions which must be practiced when mixing them….

There is not enough scope here to go through the details, but like any chemicals, you shouldn’t use them without discussing it with an experienced tuner or chemist.

Real World Effects of Octane Increase.
On a highly tuned two litre turbo (Scooby, Evo, GT4) the following can be expected. If the engine is running at 400bhp on 98RON fuel then the use of 101 Shell clubman fuel will bring the engine to 450bhp due to the added boost and advance allowed because of the octane increase. With a further increase to 102 WRC fuel and the use of toluol additive to bring the MON to 113octane, the engine will produce just over 500bhp(at 9psi boost higher than the 98octane level).