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Article by Campbell Boat Owner Ron Fasola, owner of "Strange Brew"
Octane numbers in gasoline seem to be another subject us boaters are interested in that is shrouded in many myths. To fully explore this subject would take way more than a short article, but let's give it a go in the hopes that most of the common questions can be addressed.
OCTANE
Octane is simply the resistance of a gasoline to detonation, (more commonly referred to as "knock" or "ping"). Nothing more, nothing less. A gallon of premium gas has the same number of BTU's as a gallon of regular gas. BTU's are British Thermal Units; a measure of the total energy in a fluid like gasoline. Actually, one BTU is the energy required to raise the temperature of one pound of water one degree F at atmospheric pressure. Higher octane gasoline will not make an engine run cleaner, "smoother", start easier, put out more horsepower, or make your motor rev higher. It won't do squat except prevent detonation.
100 octane is defined as the octane number of 100% or pure iso octane. Zero octane is defined as the octane number of 100% or pure normal heptane. A blend of these two chemicals yields a fuel with an octane relative to the percentage of mixture; i.e. a mixture of 75% iso octane and 25 % heptane will have an octane number of 75.
An industry-standard, one cylinder engine, (CFR engine), which has a variable compression ratio, (the volume in its combustion chamber can be adjusted while it's running), is used to rate or measure the octane of fuels. The compression ratio is adjusted to provide a given level of audible knock while running on a test fuel, and from the compression ratio needed for this level of knock, the octane of the fuel being tested is determined. There are many variables that affect knock, the biggest factors being compression ratio, spark advance, intake air temperature, and rpm. All these variables are held constant or controlled when running the CFR engine.
There are two kinds of octane numbers, RON (Research Octane Number), and MON (Motor Octane Number). The same one cylinder CFR engine with the variable compression ratio is used to measure or rate both the RON and MON of a gasoline, but different operating conditions are used for RON or MON. The RON method uses a lower rpm and a lower intake air temperature. Since it is an "easier" method on the fuel, the RON octane number of a given gasoline is always a little higher than its MON (run at higher rpm and intake air temp) octane number. The two octane numbers are actually needed because engines variables respond to the two octane numbers in different ways; for example RON is more important for starting and low severity operation, and MON is more important for severe or high load operation. This is all way too much information for Joe Consumer, so it was decided that the pumps will list octane by simply averaging the RON and MON and calling it an "Anti knock Index", AKI. This is the (R+M)/2 rating shown on pumps. This averaging of the RON and MON does not relate to anything regarding engine performance; it's just an attempt to give the public something to relate to, to compare, or to buy.
DETONATION (KNOCK)
Now let's discuss knock. For our purposes here, knock is defined as any kind of irregular combustion in a combustion chamber other than combustion initiated by the spark plug. There is pre-ignition, (a term which can arguably be used to describe just about all types of aberrant combustion), rumble, surface ignition, part throttle knock, and a few other types of knock. Some knock can even occur AFTER the plug fires; because as the flame front propagates in the chamber, the pressure in the farthest reaches of the combustion chamber increases and finally the fuel in this "squish area" spontaneously ignites before the flame front reaches it. All "knock" types share in common initiation of combustion in a combustion chamber in a fashion not consistent with "normal" combustion, and they all result in power loss and possibly engine damage, if the knock is extreme enough. A trained ear can detect slight amounts of knock in an engine in a car or on a dyno, and slight amounts usually are not harmful to an engine. But in a boat, there is so much noise from the exhaust, water, and wind, that knock is very hard to hear when underway. It's possible that damaging detonation can take place in a boat motor without being heard. Additionally, boat motors are almost always under much greater loads and higher rpm service than automotive motors.
Every engine has its own unique "octane requirement". Because of differences in compression ratio, amount of carbon build up in the chamber, head design, humidity, intake air temperature, spark advance, and a slew of other variables, no two engines are exactly alike in their need for a specific octane number to avoid detonation. Even a given motor will have a different octane requirement on different days because of air temp and humidity. High air temp and low humidity make for higher octane requirement; (where are those cold and wet summer days in Havasu?!). Paying for octane higher than your engine actually requires is simply a waste of money.
GASOLINE &
FUEL
Race gas, a gasoline with very high octane over 100, is used in high performance racing engines because race engines have high compression ratios and lots of spark advance in order to make big horsepower. These engines, just like low performance motors, just need a gasoline that will not knock when the engine is in its intended use. Race gas is a gasoline, and is not to be confused with true "fuels" like Nitromethane, ("Nitro", the short term for the fuel, does not refer to the explosive chemical nitroglycerine!). Nitromethane does not need nearly as much oxygen as gasoline to burn, so about 8.7 times more Nitro can be fed to a drag racing motor than gasoline, reaping huge energy release. Obviously, way different feed systems and storage needs are required when using nitro. Again, there is no extra horsepower hiding in Race gas if your motor does not need it.
The dirty little secret about different brands of gasoline is that octane numbers DO vary from brand to brand. Not too much, but they do vary. Sometimes it's due to quality control, and sometimes it's intentional on the part of the maker. They all do have to meet the R+M/2 rating, but some are a little higher in order to provide a little "safety factor". What separates the major brands from "cheap junk gas" is usually a combination of additives and marketing. No-name gas doesn't usually have the expensive additive packages, (detergents, etc.) that the major brands have. That's primarily why they cost less. In fact, (dirty little secret #2), at gasoline distribution locations, (where the big rigs fill up prior to delivering to street gas stations or marinas), the base gasolines, (premium, mid grade, and regular), for all the major brands AND the no-names, all come from the same tank farm system of huge storage tanks the additive packages that give the finished gasolines their brand name identity are added right at the filling rack where the big rigs fill their trucks for delivery to name brand gas stations. The drivers put a card into the pump, and the specific additive package for the specific delivery is automatically added as the truck & trailer is filled.
What all this means to
The key is to determine the octane requirement of your particular motor. If it does not knock on regular, lucky you can run on regular and save a few bucks. If it does knock on regular, you simply just have to buy premium. Mixing regular and premium presents no problems. If you are not skilled at recognizing the sound of knock, you can always simply throw money at the issue and stay on the safe side by just always using premium gasoline. But realize that premium gas cannot give you more power; it can only just eliminate the possibility of harmful engine damage from detonation. It can be argued that with premium, one can increase the timing a little and thereby produce a little more horsepower. This is true. This is what is basically being referred to when words like "tuned to run" on regular or premium are used. We're splitting hairs, here. Also realize that some engines will not be satisfied with even premium gas, (i.e. some engines will knock even when run on premium gas).
A fine point: If an engine detonates so badly on regular gas that performance is being grossly affected, and using premium gas eliminates the knocking and therefore allows the engine to perform normally, it can be argued that premium gas "provided more power" in this case. It really didn't; it simply allowed the engine to perform as designed.
If you have a low performance, low compression ratio big block, it will probably run just fine on regular gas. Using premium will only help lighten your wallet and therefore be "directionally correct" in making your boat go faster because it's carrying a lighter load.
There are, however, some basic guidelines for premium gas usage; the biggest factor being compression ratio. In general; over 9.5 to one, use premium. 11 and over to one probably will not be satisfied with even premium and race gas might be needed. Running a little rich on the carb jets will help reduce the octane requirement. Hot summer days don't help; octane requirement goes up with increased intake air temp. Old gas loses octane, so start with higher octane to begin with if you feel your engine is marginal with regular and the boat sits a lot. If your motor is turbo'd, your static compression ratio is low but you will need high octane when your turbo's spool up. Same for blower motors.
Spark timing is a big factor affecting knock. If you hear objectionable knock under load, a quick fix can be to simply shut down and back off the basic timing by about 4 degrees. You will lose a little power until you can get higher octane fuel in the system, but you may save an engine.
Newer injected motors, (both marine and automotive), sport control systems that can detect knock and back off the ignition timing to compensate. They also have high tech combustion chamber designs that allow higher compression ratios without the historical proportional increase in octane requirement.
Be aware that in a boat underway, an exhaust leak at the head-exhaust manifold can sound very much like knock. If you think you hear your motor knocking while running, run a wrench over all the exhaust manifold nuts when you stop and see if that was the problem. If you were able to tighten the nuts a turn or so, (that's a lot!), and the sound still persisted, the leak may have been bad enough to blow out a section of the exhaust manifold gasket; pull the exhaust manifolds and check the gaskets. No amount of tightening can seal missing gasket material. An exhaust leak will be heard every time just the leaking cylinder fires, so consider if the sound you hear is about the frequency of just one cylinder firing. Obviously, neither knock nor an exhaust leak will usually be heard at idle speeds; full load brings out all the problems.
Aviation fuel, (Av gas), is not a great gasoline to run in
a boat or a car. It has the octane
all right, but its volatility characteristics are significantly different from
the needs of a car or boat motor.
This is because planes operate at much higher altitudes than most
Any old-time mechanics out there? Remember that gray colored, grease-like coating that used to build up in the intake system? That was lead, from the leaded gasoline we used to run.