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Everything You Never Knew About Gasoline

By   /   August 20, 2012  /   Comments Off

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With gasoline once again passing four-dollars a gallon it may be interesting to explore the characteristics and misconceptions of this currently overpriced commodity, and maybe save some money. After reading this article you will know more about gasoline than 99.99 percent of the people on the planet.

Octane Rating

The majority of motorists will judge the power and quality of a gasoline by the octane rating. It’s a common misconception that the higher octane fuel will provide more power and economy. In reality, the lower octane fuels will provide the same energy per gallon as the higher octane fuels. Octane rating is nothing more than the fuels ability to resist engine “knock” or that “pinging” sound heard beneath the hood. The basic rule is that the higher the compression ratio the higher the octane rating must be. While a lower compression engine may run fine on regular fuel, a high compression engine may ping and rattle and require a higher octane fuel.

The knocking, pinging and rattling is caused by the air/fuel mixture in the cylinder detonating on it’s own during the compression stroke without the need of an electric spark. It’s basically an uncontrolled explosion. The rattling noise you hear is the engine block vibrating from the contained explosion. The air/fuel mixture within the cylinder is supposed to burn progressively after being ignited by the spark plug. That is, a nice smooth traveling flame front that progressively creates pressure inside the cylinder and gently pushes down on the piston to move you down the road.

Detonation inside a gasoline engine must be avoided at all costs. The instant explosions inside the cylinder can destroy spark plugs, flatten rod bearings, blow head gaskets, crack pistons and cause poor performance and economy.

Octane Requirements

There are hundreds of variables when it comes to the octane requirements of your engine. Cylinder head design, piston design, camshaft design, atmospheric conditions, ambient and underhood temperatures, ignition timing, and a host of others that combine to create octane requirements. The biggest determining factor in octane requirement is the compression ratio of the engine. A high compression engine, say 10 or 11 to one, will squeeze the air/fuel mixture into a tighter area and will detonate with a low octane fuel because of the extra heat created during the compression stroke. In this respect you could say that high octane fuel produces more power because it allows you to run a higher compression engine, which will create more power without detonation.

With today’s lower octane fuels a static compression ratio of 9.2 to 1 is about the limit for 92 octane premium fuel, but then there are the variables. You could build an older engine with a 10 to 1 compression ratio and by installing a high performance, high overlap, long duration camshaft, it may run fine on 92 octane pump gas. With the high performance cam the intake valve will close late and bleed off some of the cylinder pressure.

Your engine may run fine during the winter on regular 87 octane fuel, but sound like a percussion band during the summer, especially while climbing a hill. Engine heat will drastically increase octane requirements. The bottom line is this: If your engine doesn’t ping on low octane regular fuel, don’t waste your money on the more expensive high octane fuel. There is no benefit whatsoever. If your engine detonates on regular then move to a higher octane fuel. Start by consulting the factory recommendations for fuel requirements.

92 Octane verses 100 Octane

Those of us who have been around awhile remember the days when you could fill your high compression muscle car with 100 octane fuel. Chevron Custom Supreme boasted an octane rating of 103. Back in the day it was simple and inexpensive to boost the octane rating by adding the required amount of tetraethyl lead (TEL) that also lubricated the valves. Leaded fuel was phased out in the mid-seventies with the introduction of the catalytic converter that changed unburned hydrocarbons and carbon monoxide into water vapor. Lead destroyed the composition of the converter.

Methyltertiary butyl ether (MTBE) replaced TEL as an octane booster. MTBE was an excellent octane booster, but was outlawed by the EPA because it was too stable, it would never break down, and was therefore a threat to groundwater should the underground fuel tanks leak. To increase octane rating without additives required extra refining steps that greatly increased the price of gasoline. In many parts of the country including California, ethanol which is alcohol made from corn is used as an octane booster.

The drastic drop in octane rating with modern fuels is not as bleak as it may seem. There are two methods to determine the octane rating of a motor fuel. The Research Octane Number (RON) employs a single cylinder gasoline engine, turning at 600 RPM, with an air inlet temperature of 125 degrees F. The compression ratio is adjustable. The compression ratio is increased until detonation occurs, which determines the octane rating. This was the method used back in the day. The other method is the Motor Octane Number (MON) that also uses a variable compression engine, but spins at 900 RPM with an inlet temperature of 300 degrees F. This method requires a higher octane fuel to prevent detonation. As a result the octane rating will be lower than that of the RON method. A fuel rated at 95 octane using the RON method may only be 91 octane using the MON method. The octane rating of today’s fuel is a combination of the RON and MON methods. You’ll see this posted on the gasoline dispensers on the octane selection button. Today’s 92 octane premium fuels are roughly comparable to yesteryears 97 octane leaded regular.

Octane Boosters

Lurking on automotive shelves is a product called octane booster. It’s very expensive and claims to increase octane rating three points. Many who own sixties muscle cars are tricked into buying this stuff to prevent detonation. While the claims may be true, octane rating is graduated in tenths, not whole numbers. Rather than get the expected 95 octane from 92 octane pump gas, the octane rating will only be increased to 92.3.

Lead Substitutes

Leaded fuel served as an excellent valve lubricant. If you own an older car built during the mid seventies or older it would be wise to use a lead substitute or upper cylinder lubricant, such as Marvel Mystery Oil, to lubricate the valves. When leaded fuel was being phased out the car manufacturers began flame hardening the valve seats to prevent the valves, especially the exhaust valves, from wearing themselves into the cylinder head due to lack of lubrication.

There are three things you can do if you own an older car, add an upper cylinder lubricant or lead substitute to the fuel, have hard exhaust valve seats machined into the cylinder head, avoid sustained high speed driving. Tests on taxi cab fleets indicated that wear occurs most rapidly during sustained driving above 55 MPH. It’s not unusual to find exhaust valves worn so far into cylinder head that there is no longer tension on the valve spring. We are talking big bucks to repair this.

Gasoline Energy Content

When shopping for gasoline the most important thing you can know is the energy content of that overpriced gallon of fuel. It will affect the power and fuel economy of your engine. Unfortunately, the oil companies don’t post or advertise the energy content because there is nothing much they can do about it. It is what it is. The energy in a U.S. gallon of gasoline is measured in British Thermal Units, and ranges between 111,000 and 125,000 BTU’s per gallon. The energy content is dependent upon the quality of crude oil that the gasoline was refined from. Some batches of crude produce higher energy levels than others. Have you noticed how every now and then your engine runs stronger than usual and gets really good fuel economy? That’s because you lucked out and filled up with a high energy fuel.

Can you go back to the same gas station and expect the same results? Yes, provided you fill up before the tank truck gets there with a new batch of fuel. Price, brand and octane rating have nothing to do with energy content. The cheapest cut rate gas you can find will have the same energy content as high priced major fuels. Exxon-Mobil refines the majority of fuel in the United States. Chances are excellent that the fuel sitting in your tank was refined by Exxon-Mobil, regardless of brand name. The raw refined gasoline is piped to distribution centers where proprietary additives are added before delivery to your gas station. It is the additive package that makes ARCO gasoline ARCO, and Chevron gasoline Chevron.

Burn Rate

Automobile fuels are formulated to have a burn rate that provides maximum performance and fuel economy at 2,500 to 3,000 RPM, which is what the majority of engines will be turning at highway speeds. A burn rate formulated to this RPM will completely consume the air/fuel mixture as the piston reaches the bottom of its’ power stroke and provide maximum power and economy. If the burn rate were slower, much of the burning air/fuel mixture would still be burning as the piston ends the downward power stroke and begins the upward exhaust stroke. Pressure that could have been used to push the piston down is now flowing out the exhaust valve.

Dedicated racing fuel has a much faster burn rate and can produce considerable more power at high RPM’s. At 6,000 RPM, the faster burning fuel will be pushing down on the piston during the entire power stroke rather than be pushed out the exhaust valve while it’s still burning.

Many racers mistakenly regard Aviation fuel as race fuel because of its’ high octane rating of around 100. This can be extremely beneficial to prevent detonation if you are running a very high compression engine. The downside is that Av-Gas has a much slower burn rate than automobile pump gas because aircraft engines turn at a slower RPM. You’ll lose considerable power at the higher RPM’s. Dedicated racing fuel will provide the best elapsed times at the drag strip.


Volatility is a gasoline’s ability to change from a liquid to a vapor. Liquid gasoline won’t burn. It needs to be hacked-up into a vapor by a carburetor or fuel injector and mixed with oxygen before it will burn inside a cylinder. Refineries adjust volatility based on seasonal requirements. In the cold winter months gasoline is hesitant to change from a liquid to a vapor so the volatility is increased for easier starting and cold engine operation. In the hot summer months a gasoline with a high volatility will change quickly from a liquid to a vapor and may cause vapor lock. This means your carburetor or fuel injectors will be fed vapors instead of liquid fuel. Therefore, volatility is reduced during the summer months.

Running Your Car On Moonshine – Ethanol & E-85

Grain alcohol makes an excellent motor fuel, but like everything else has its’ ups and downs. The upside is its horrendous octane rating of around 110 to 112 without the use of octane boosters. It burns cleaner than gasoline and produces less pollutants because it contains oxygen molecules that allows more complete combustion. You don’t have to hunt or drill for it, all you need is a cornfield, and it’s a renewable energy source.

The downside is its’ low energy level compared to gasoline. The BTU rating for ethanol is only 75,000 BTU’s per gallon compared to an average of 114,000 for gasoline. Ethanol will destroy seals in carburetors, especially those in motorcycles and small engines. E-85 is capable of eating up metal parts, even stainless steel. Food prices are rising because we are burning our food supply in our engines. That doesn’t sound like a bright idea.

The use of ethanol is expressed in percentages. E-85 means that 85 percent of the fuel is ethanol and the remainder is gasoline to help boost the energy level. E-10 means that 10 percent of the fuel is ethanol and the remainder is gasoline.

Thanks to its’ high octane level it would be simple to build a 400 horsepower engine from a 350 cubic inch small block Chevrolet. You could run an ultra-high compression ratio of 12 to 1 without detonation and more than offset the lower energy rating. The problem with building a dedicated E-85 engine is that you will have to run E-85 exclusively. If you’re on a road trip where E-85 is not available, and you are forced to run on 92 octane pump gas, your engine will probably rattle itself to death.

Can any gasoline engine run on E-85? Yes, and one thing you will never have to worry about is octane rating and detonation. Running E-85 in an engine built for gasoline will result in a noticeable drop in power and fuel economy due to its lower energy content.

Ethanol is many times added to gasoline as an octane booster. In states like California it’s required during the summer smog season to reduce emissions. California limits the use of ethanol to 10-percent, or E-10. At this percentage you will probably not notice any significant decrease in power or fuel economy.

If your running an older car with a carburetor it would be wise to rebuild it with alcohol resistant seals and diaphragms, even if your area is selling E-10. Make sure and use alcohol resistant fuel lines. If running E-85 a slight jet tweak will provide a better running engine.

Even E-10 can destroy a carburetor on a small engine. Never allow alcohol fuel to sit in the fuel bowl. Turn off the fuel valve and run the engine until it runs out of fuel.

Congratulations! The only people that know more about gasoline than you do are the chemistry wizards! Perhaps knowing everything a motor fuel has to do in order to push you down the highway will ease the pain at the pump.

Ron Brault is a 34 year resident of Twentynine Palms, and graduated from the Union Oil Marketing Academy in 1968.


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