If you're reading this magazine, you're an octane junkie whether you know it or not. You pay for the highest-grade gasoline available at the pump and have, on occasion, tried out various octane boosters in the hopes of shaving that extra tenth off the 1/4-mile. You've even contemplated paying the $5-plus per gallon of race gas.

Whether boosters work or not isn't the question. If you've used a booster, you already know it works based on the not-so-accurate accelerometer called your behind. The question is which booster offers the highest octane gain. At $3 to $10 per treatment, a bottle of octane booster can almost equal a third of the cost for an average tank of premium gas. At that price we all want the best bang for the buck. To find the answer, we tested three octane boosters, each with different active ingredients, to see how much of a kick in the pants they supply.

So what is octane? Without getting into chemistry, molecular bonding and hydrocarbon chains, octane is simply a measure of fuel-detonation resistance. The higher the number, the less prone the fuel is to detonation.

Detonation is the un-uniform ignition of the air/fuel charge in the combustion chamber. Normally, the combustion flame front originates from the spark center. When detonation occurs, the charge is lit at not only the spark center, but also from hot spots caused by build up from carbon deposits within the combustion chamber. This causes an uneven flame front, resulting in a sudden rise in combustion pressures, which can damage a piston on the power stroke.

A more catastrophic scenario, called pre-ignition, occurs when the charge unintentionally lights off without a spark. This usually means the event occurs toward the end of the compression stroke when charge temperatures and pressures are still rising. With pre-ignition, the sudden change in charger pressure from premature ignition as the piston is still moving up is equivalent to taking a hammer and beating it on top of your pistons. The sound is very similar, just like a ping.

Higher octane fuels are especially helpful to boosted or high-compression performance engines, along with older engines. With boost or a high-compression ratio, the air-fuel charge is compressed to higher pressures, which makes it more susceptible to detonation. Older engines with carbon deposits built up in the combustion chamber also benefit from high-octane fuels as the added space occupied by the deposits also effectively increases the compression ratio.

To avoid detonation, engines with knock sensors retard the spark timing at the onset of knock. Retarding spark timing or richening the air/fuel mixture to reduce knock ultimately robs power. This is why an increase in octane increases horsepower. Since the engine's knock threshold is effectively raised with higher octane fuels, spark timing is not retarded. This allows combustion and charge expansion to occur so that more force is put into the power stroke. Bottom line: Higher octane fuel only helps you make more power if your engine is at the verge of detonation (whether you know it or not).

Most likely, anything you've read in enthusiast magazines involves roundabout methods of measuring the effects of changes in octane. Most methods incorporate a chassis dyno, a 1/4-mile run or, at best, an engine dyno. Each method is flawed because each uses horsepower changes to correlate to octane changes.

The problem with measuring horsepower is its a function of the combustion process, which varies with ambient conditions, spark timing and air/fuel ratio--all of which are continuously varying and often computer controlled. The measurement method also isn't precise enough to establish an accurate estimation of octane gains. Most chassis dynos will have a range of variability of up to 5 whp. This is enough to cover many octane boosters' gains. Though differences are measured, the amount of variability and lack of experimental controls prevent a reliable margin of repeatability.

Instead of trying to evaluate gains in octane through horsepower, it's more appropriate to evaluate actual octane gains. Having to account for how each engine reacts and adapts to more octane adds more pieces to the puzzle.There are two primary ways of measuring actual octane values: 1) The combustion fuel research (CFR) engine and; 2) a newer infrared method. Currently the American Society for Testing and Materials (ASTM) uses the CFR engine method, developed in the 1920s to obtain official octane measurements. The CFR engine measures octane by combusting the fuel and physically measuring the knock that occurs.Infrared technology offers a faster method of non-official octane measurement by spectral analysis of all the different chemical contents in a fuel. It guesses the octane of a sample based on the percentage of various hydrocarbons present and references these contents against a known library of data. Unfortunately, this library of hydrocarbon data doesn't include the active ingredients found in commercial octane boosters. This is why our test was performed on a CFR engine, which also made it certified.

Octane boosters can be broken into three types based on their active ingredients. Methyl cyclopentadienyl manganese tricarbonyl (MMT) and ferosene are used in limited amounts in off-the-shelf boosters. The majority of commercial boosters use MMT. Another type of booster uses alcohols or aromatics as the active ingredient. Many tuners use toluene as a home-style octane booster. Toluene, an aromatic circular hydrocarbon chain, is a regular component of pump gas and is available in various grades at chemical supply stores. Premium street gasoline carries roughly 3- to 5% toluene, which partially helps octane characteristics. Unocal's 100-octane race gas has almost 25% toluene.

The drawback to any of these additive ingredients is the diminishing effect they have on higher-octane fuels. Adding the same booster to 87-octane pump gas will yield a lot more octane gain than adding a bottle to 91-octane premium gas. Excessive concentrations of these additives also damage emissions-control hardware, such as spark plugs, injectors, oxygen sensors and catalytic converters. This is why most off-the-shelf boosters have an emissions-legal street formulation and an off-road formulation that exceeds the government-regulated concentration of MMT or ferosene.For the power-hungry, there really is no point in testing 87-octane gas and street-legal octane boosters. That's why we performed our tests with a base sample of 91-octane premium gasoline taken from a local SoCal station. By law this gasoline must have an octane rating of at least 91 octane. Octane boosters were obtained at local auto parts stores while 99%-pure gasoline-grade toluene was sourced from the laboratory of Rockett Brand Racing Fuel.

We chose Nitrous Oxide Systems' (NOS) Racing Formula octane booster, which uses MMT as its active ingredient, and Outlaw's Super Concentrated Octane Booster, which uses ferosene. Outlaw originally had off-road and street formulations, but just recently combined the two into just one street-legal formulation. The toluene mixture we tested was based on an Internet-sourced home brew.Saybolt LP, a division of Core Laboratories, performed a certified, independent octane test on samples of three different types of octane boosters we prepared. Each sample, along with the base fuel, was tested for research octane number (RON) and motor octane number (MON), as prescribed by ASTM Method D-2699 and D-2700, respectively.

The resulting pump octane number--also referred to as antiknock index (AKI)--was calculated using the (R+M)/2 method, which takes the average of RON and MON. These tests have a repeatability (same operator/same lab) of 0.2 octane for both RON and MON, and a reproducibility (different operators in different labs) of 0.7 for RON and 0.9 for MON. In addition to octane testing, each sample's specific gravity, or density, was tested according to ASTM method D-4052.

The CFR engine is basically a carbureted, single-cylinder, variable-compression engine. The head can be raised and lowered to change the compression ratio and thus increase knock intensity. By reading the knock intensity at a given compression ratio, the operator can determine the octane rating of a sample fuel. Each engine, (one dedicated to RON testing and the other to MON testing) has to be warmed up to maintain a 100- to 130*F oil temperature and a 2- to 3-Hg manifold vacuum. Air/fuel ratio is held at an elevation-corrected constant. Prior to every test, both a toluene and an iso-octane mixture of known octane are run through the engine for reference and calibration checks.

The less-severe RON test is performed at 600 rpm with intake and air/fuel charge temperatures regulated at 125*F. Ignition timing is held at 13-degrees BTDC. The higher load MON test is performed at 900 rpm. Intake air temperature is held at 100*F; air/fuel charge temperatures must be 300*F. Spark timing varies between 19- to 22-degrees BTDC based on the compression ratio. The results of these two tests are averaged for the AKI number that you see at the pump.

Each of our samples were mixed in a ratio equivalent to having added the entire contents of an octane booster bottle to a 15- gal. tank of 91-octane fuel. Each 1-gal. sample was stored in sealed metal containers at room temperature to prevent evaporation or degradation of the fuel or the octane booster. The toluene mix was composed of 12.5 oz of toluene and 3.125 oz of mineral spirits, treating the same 15-gal. fuel tank. As there was too little to make a difference in our 1-gal. test samples, 0.375 oz of transmission fluid (claimed to act as a lubricant) was left out. According to Tim Wusz, the mineral spirits and motor oil would only lower the octane rating if added in sufficient amounts. The results are shown in the tables below.

Since we wanted to determine the efficacy of the home-brew octane booster recipe, we re-verified the toluene results, using an infrared measurement system. Tim Wusz of Rockett Brand Racing Fuel performed the same tests using different equipment. The AKI results of the MMT and ferosene boosters were ignored as they were invalid on this equipment. We re-measured the RON and MON on the base gas and the home-brew mix. Any change was negligible.

To see how much toluene had to be added to 1 gal. of base fuel to make a significant difference in the AKI, Rockett Brand concocted three different mixtures ranging from volumetric 10- to 30% toluene.

As described by Wusz, there is an optimal window of effectiveness for toluene additives. Beyond that, the increased fuel density begins to have detrimental affects on proper fuel carburetion and also retards combustion.

Toluene-laden fuels burn slower and make less power on high-revving engines. So much in fact that much of the air/fuel mixture is still burning as the charge exits through the exhaust valve. This is a sure way to destroy your emissions-control equipment and not pass smog. For these reasons, true race fuels don't just use toluene or other active ingredients to boost the octane. Instead they use better-refined hydrocarbon chains that raise octane while retaining optimal combustion characteristics.

So there it is, octane boosters tested and explained in a nutshell.

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