Fuel Chemistry

The gasoline that we buy is really a mixture of many chemicals. This mixture is constantly being changed by the refineries due to season, laws, and of course bottom line profit.


Gasoline is a mix of mostly petroleum distillate(s). It contains: anti fungus, detergent, stabilizers, MTBE (oxygenated or reformulated gasoline), and anti knock agents. Recently some states have mandated that all gasoline sold have additional oxygenates, or as is commonly known, reformulated. The oil companies and government assure us that for most cars there is no loss of performance and only 3%-5% loss in fuel mileage. Apparently, once again our Merkurs are the exception, there seems to be at least 5% loss in performance with oxygenated fuel. :(


Failure Mode

Old gas can decompose chemically and the higher vapor pressure molecules can evaporate off. It is also susceptible to contamination from: water, rust, other liquids, and particles. 



Fuel additives are available to: increase octane, clean the fuel delivery components, and clean combustion chamber. 


Engineering Data

Octane is a chemical that once upon a time was used to decrease knock in engines. Recently (in the 70s) the government mandated that "octane ratings" advertised by the oil companies be the average of "motor" and "research" (r+m)/2=octane rating. Higher octane has a higher threshold of energy level required to start combustion, therefore there is less chance of detonation. If a engine is not detonating, higher octane WILL MAKE LESS POWER. Use the lowest octane without detonation. Remember that the knock sensor on our cars are pretty sensitive and the EEC will RETARD the timing when knock is detected. 

 There are two types of detonation, part throttle and full throttle. For part throttle detonation, a higher research octane is needed. For full throttle detonation, higher motor octane is needed. These numbers are not published on the gas pump, and each oil company has slightly different numbers. These mixes also change during the season, different temperature weather requires different mix gasoline. Changing brands of gasoline during the year can minimize detonation in engines that operate "near the edge".

Octane rating can also be changed by mixing gasolines. Unleaded, reformulated, 100 octane is available; it costs over $3.50 per gallon therefore most people mix only what is required (see the chart below). Remember, XR's have 15 gallon tanks.*


The rate ofthe reaction is controlled by the distribution, molecular proximity, andrelative amounts of the reactants, i.e. how even and how rich the mixture.If the fuel and oxygen are well mixed (why airflow and turbulence in the head and cylinder are important) it will burn quickly and evenly.Detonation is not a problem with burn rate, but with spontaneous combustion initiating somewhere other than at the spark plug gap. Raising the threshold energy for reaction insures that it only initiates from the firing of the spark plug. Higher octane fuel does NOT mean higher power (essentially the same amountof energy inall gasoline). However, higher percentage of octane molecules does. However octane rating is obtained by various means, and seldom by the more expensive method of increasing the percentage of actual octane molecules. Octane rating is simply a measure of detonation resistance, which can be achieved by adding chemicals (like lead) that inhibit combustion. Actual high levels of octane molecules raised the threshold energy by raising the top of the energy release curve, making the drop, and hence the energy release, greater.

Higher REAL octane would increase power, but it's doubtful you could get it, and if you did find a source,you'd be hard pressed to verify it was the real thing. If someone offered me gasoline with a higher percentage of octane, I'd have him take a sample to a friend in the labs at Lubrizol and verify it, and I'd also want to talk to a chemist at the refinery it came from, and then MAYBE I'd think about using it.  

Rate of reaction(along with initialization energy) is a function of the type of molecules that are being combusted and it will vary dependent on the type of hydrocarbon used. The reduction in detonation does indeed result from an increased initiation energy, but the fact remains that you need to increase spark advance when using higher octane fuel to compensate for a slower rate of reaction. I somewhat mistated in my previous post that higher octaneallows larger advance, whereas you rather than allowing, you actually needto increase advance to compensate for a slower rate of reaction. The real advantage of higher octane fuel is that you can cram more oxygen into the cylinders without detrimental preigntion effects (oxygen is the limiting reagent, so this is what we want more of). 



-Octane (rating or 2,4,4-trimethyl pentane content) does not affect flame speed. 

-Octane RATING does not impact energy content. However iso-octane (2,4,4-trimethyl pentane) CONTENT does positively correlate withenergy content. 

If you have a stock motor (XR) with the KS connected running stock or near stock boost and the timing is in the 10-13deg. range you are wasting money on higher than recommended octane fuel.  


Historical Performance Tricks 

In the old days, it was trick to blend the leaded with the unleaded premium, thereby obtaining an octane higher than the premium by itself. Higher octane obtained on the account that the unleaded premiumoctane was obtained through MTBE, and the fact that the octane achieved through tetraethyl lead is of a non linear relationship. In any case, I am glad that lead was outlawed. Think about the figures, I think it was several grams of tetraethyl lead per gallon and I am suret hat adds up to several tons of lead dumped across america's highways each day. 



To meet NOx regs in the US, auto manufacturers retarded the timing in cars in the early '70s. NOx forms at higher combustion temperatures, and also has a relation to pressure as well (scales with P^0.5 as I recall). In spark ignition engines, as you retard timing from TDC, both peak combustiont emperatures and peak cylinder pressures go down (as does NOx). On a simple level, this is because the gases are expanding at the same time as the combustion chamber. On the other hand, if you spark as you are compressing (ignition BTDC), the expanding gases (due to combustion) are acting against a compressing combustion chamber. Thus, peak cylinder pressures and temperatures are higher. In terms of efficiency, if combustion were instantaneous, you would expectthe engine to be at its peak power AND efficiency while sparking at TDC. However, combustion does take some time. So, in practice, peak power normally occurs at timing somewhere BTDC. A quick look at tune-up specifications for Euro and US alfas will reveal that the rest of the world tunes their cars for more advance than in theUS. Expect your power to increase as you advance your timing from USspecs.... to a point. At some point of advance, power goes down. Simplistically, you can imagine the expanding gases working against the piston moving up. At some point, you'll be firing too early and the pressure produced from combustion works against the piston (on the up stroke) more than it works with the piston (on the down stroke). Again, if combustion were instantaneous, this effectwould not be there. If this were the only issue with spark advance, the method of maximizing acceleration would be a fine way of determining proper spark advance. Of course, life is not so simple due to knock. Knock is synonymous with detonation. Strictly speaking, a detonation is different from a deflagration (typical "burn") in that the flame front is supersonic. Detonations typically produce much higher pressures than deflagrations. Spark ignition engines are designed to operate with deflagrations. As the flame front proceeds, the gases behind cool andcontract a bit. When a detonation occurs, combustion occurs much more rapidly, and peak pressures increase due to there being less time for the combustion gases to cool (and contract) behind the flame front. Thus,when you detonate the fuel-air mixture in an SI engine, you produce cylinder pressures (and temperatures) much higher than the design conditions. This leads to melted/damaged pistons and other "bad" things. Detonation is something to avoid in SI engines. Diesels, on the other hand, are designedto operate in this mode. For a given fuel air mixture, the tendency for knock increases with spark advance. For a given spark advance, the tendency for knock increases with fuel-air mixture initial temperature, and reduces with octane number of the fuel (empirically determined, by the way, using a standard engine whose compression ratio is increased to the point of knock.... not much "science" in that number). The tendency for knock is also a function of fuel-airmixture stoichiometry (actually Lewis number.... but for this discussion, the practical quantity is fuel/air ratio). I don't know on which side of stoichiometric gasoline falls with respect to minimum ignition energy, but I'd expect it to be on the rich side. That is, the point of minimum ignition energy is with a slightly rich mixture. Of course, in the realworld, the fuel-air mixture is not truly premixed, and the theory is abithard to apply.... So, in practice, to make your car knock, advance the timing, drive it on a hot day, and use low octane fuel. I've also observed that the tendencyf or knock is supressed with richening the fuel-air mixture. (When I bought my'74 gtv, the owner had the timing advanced to >30 degrees BTDC!, but the TA was shot and the car did not knock at all).


Saftey Issues Relating to FUEL 

People scare me when they reference mixing up fuel batches in theirgarage and driving around in cars with all kinds of gas leaks and oilspewing out all over the place. I thought I'd post this general info onauto chemical hazards, with the hope that I'll prevent someone fromgetting sick. Professional mechanics and you chemical engineering wizards probably know this stuff, but others may not.

All petroleum compounds are toxic to the human body at certain concentrations, and we need to prevent breathing and touching this stuff. Spent motor oil is nasty too, as it contains all sorts of metal particulates. Gasoline is comprised mainly of benzene, toluene, ethylbenzene,xylene(s) and (in some areas) oxygenated compounds (things like methyltertiary butyl ether and tertiary butyl alcohol). Benzene is the nastiest of these compounds, and is an OSHA listed human carcinogen (itcauses cancer). Toluene and Xylene, are very aromatic (read: stinky),and are hazardous substances. However, they are not as toxic, even at concentrations typically found in gasoline. All petroleum based compounds should be handled intelligently. Buy a cartridge respirator to protect your lungs if you're messing with fuelmixtures. Work in a well ventilated area, wear gloves and a respirator when using parts cleaning fluids, many which contain very nasty carcinogenic chlorinated compounds, like trichloroethylene. In addition, all those oil leaks that everybody has from time to timeare dangerous. Carbon monoxide gas is given off when oil drips on yourhot engine parts and vaporizes. Carbon monoxide is colorless andodorless, and you must avoid breathing this stuff. A few years ago, I felt dizzy and weak (not a hangover). A visit to the doctor and a blood test revealed minor carbon monoxide poisoning. Upon inspection of my car, I discovered a valve cover oil leak down the back of the engine,and with a portable gas monitor, I measured intermittent toxic concentrations of carbon monoxide inside the cockpit. I replaced the gasket immediately, steam cleaned the engine, and I felt better in a few days. 

Get those leaks fixed !  


I was trained as a chemist and althoug I haven't practiced that profession for a long time, I would warn anyone against the adverse affects of some chemicals especially something like Xylene or Toluene on rubber, plastics, the human skin.  I worked in a lab for 8 years and was constantly having problems with these compounds and anything plastic or rubber..like internal parts of car components.  Ethanol is probably the least problem. Small amounts of Xylene or Toluene shouldn't be a problem ( just small amounts though ). Xylene and Toluene are extremely toxic and can be easily absorbed through the skin.  So be careful. I have run both ethanol, xylene, and nitromethanol in a motorcycle and can attribute to the amount of power it produces and it's affect on fiberglass tanks.  


Sources of FUEL Additives

A number of members have inquired about a source of Ethanol, Xylene and Toulene. Perhaps some of the listees associated with the chemical industry, would like to share their knowledge of where to obtain such products at low cost. 


1) Changing fuels, can sometimes affect the diaphram in your pressure regulator. This is due to the rubber used, absorbing oils from the fuel, and taking a 'set' . A different fuel causes the rubber to expand(or contract) from original, and may cause a leak. This is very unlikely to happen with original (Bosch) equipment, but has been known to happen with after-market adjustable pressure regulators.

2) If Ethanol or Methanol is used, add a little ATF which will coat the inside of your fuel system, and prevent rust. 


Further Research

Octane rating is discussed under FAQ part 3 of 4.


Listed below are some chemicals that can be mixed (by volume) with the pump gas to increase performance, AT YOUR OWN RISK. These assume starting with pump automobile gasoline, 92 octane rating. These increases in octane can be usefull in diagnosing stumble in peak BMEP range (about 3000 RPM).

XYLENE ($3 per gal)

resultant octane rating






TOLULENE ($3 per gal)

(available from paint stores)

resultant octane rating






ETHANOL ($1 per gal)

(grain alcohol) Hi-octane "gasohol"

resultant octane rating

10% MAX


ADDITIVES (thanks Erik)

I work for ValvTect Petroleum Products. Headquarters are in Northbrook,

The additive you are looking for is Octane Performance Improver. We
actually make two versions, Marine and Automotive. Do not let the
marine label fool you, it is perfectly suitable and registered for on
road use (the label does not say anything about marine). This is the
product I use in my cars. We call it marine because it is the basis for
our Certified Marine Fuel Program (Marina's bulk treat their gasoline
with this product and sell specific marine grade fuel that is different
form the typical road fuel). In addition to bulk treat packages, this
product is also packaged in 12 and 16 ounce bottles. The 12 oz treats
40 gallons and the 16 oz treats 125 gallons. Again, we primarily market
this product to the marine industry.

Our automotive product is designed only for bulk treatment to be used
at service stations. Gasoline treated with Octane PI may be coming to a
gas station near you soon. We are just rolling this program out, so it
is just developing legs. There are some stations in the Chicago area
that are just starting the program. If you want this product at the
best price-- ask your local gas station to treat their gasoline. There
is nothing like customers asking for it to get it done. Automotive
Octane PI will appeal most to private brand chains.

So what is the difference between the automotive and marine formulas.
Mostly that the marine formula provides exhaust valve seat protection.
There are a lot of older boats that require exhaust valve seat
protection that are still in service.

Currently the automotive formula is only available in bulk treat
formula (1 gallon of additive to 2500 gallons of gasoline, usually sold
in 55 gallon drums).

So how do you, the Merkur buying public, purchase Octane Performance
Improver. The best way is to go to our website at www.valvtect.com and
email a request asking where your nearest dealer/supplier is. It may
well be a marine warehouse distributor like Westmarine, or it may be a
fuel jobber. You could also call the 800-728-8258 customer service
number and ask where to get product. I recommend the email rout. If you
get no satisfaction, e-mail me and I will see what I can do to get you
product. You will probably have to buy in cases of 12 units.

When visiting our website, remember our typical customer is a fuel
jobber. For that reason there is greater focus on our marketing
programs than on heavy duty technical information. It is my job to pass
on the tech information to our/my customers (we don't want to make it
too easy for our competition). I am the district sales manager for MN,
IA, ND, SD eastern NE and western WI.

MSRP on OPI is $8.00 per 12 oz bottle and $15 per 16 oz. bottle. On the
12 oz bottle your cost to treat a gallon of gasoline is 2 cents. The 16
oz. treat cost is 1.2 cents. This is very economical considering the
performance. It will be like going up a grade in gasoline for 2 cents.

You get good cleaning in a few tank fulls, but additional cleaning will
occur to about 5000 miles. The product is designed for continuos use.
When you stop using OPI the deposits will start to come back (but it
will not be over night). When you first start using OPI continue to use
the grade of gasoline you are running well on, then try dropping a
grade. Try to avoid gross over treatment (3 or 4 times over treat). I
usually add 6 oz per tank full on my Scorpio. I am usually filling 16
to 17 gallons. at a fill up.

Feel free to ask any additional questions.

Erik 88 Scorpio 212,000 miles. Might be sold.


I read with interest the section on passing emmisions by blending up to 3% Meth into the Fuel. I live in Western Canada, and we have a fuel company up here that offers 3 octane levels of fuel, standard, mid and premium grade. The mid grade is 5% blend by volume and the premium fuel is 10% blend of Meth by volume. I have run these blends many times, favouring the damp and cold winters and Smog testing as the times to use it, as it does cost a bit more. Having checked the owners manual, and talking to Ford, the 10% fuel mix was allowable and would not cause any harm to the systems. My own findings are that along with drop in NoX, the preformance improved over none Meth added fuel.