-----------------------------------------------------------------------------------
Higher octane fuel is needed to prevent detonation (i.e. explosion of the charge vs the regular, much slower, burning of the charge) in the combustion chamber after ignition, when the increasing compression and pressure of the charge, together with the relatively slow burning of the charge after the spark plug ignites the charge, causes the heat to increase to such a level that the remainder of the unburnt charge suddenly explodes in a very highly explosive shock called detonation.
Remember that in a properly controlled, normal combustion burn, the compressed intake fuel/air charge in the chamber burns relatively slowly. It is not an "explosion" as such. The actual normal "burn" takes time, and swirls away from the sparkplug where the ignition started, towards the far side of the cylinder bore in a "flame front" - often in the form of an ever-increasing circle of burn outwards from the plug, and sometimes in well-defined patterns caused by the shape and definition of the combustion chamber itself, as well as from the "squishing" of the charge that is being compressed by the piston near the top of its stroke. Suzuki has its popular 4-valve "Twin Swirl Combustion Chamber" or "TSCC" which can be seen in the GSXR and Bandit motors as the two distinct shapes of the chamber across each set of valves of the 4-valve head. This head was designed to burn very quickly in twin "swirls" of flame, to consume the charge before the unburnt charge has chance to detonate.
The faster the actual burn occurs, the better for controlling combustion and reducing the chance of the bad detonation - because if the charge burns very quickly, it reduces the time that the unburned charge is exposed to ever-increasing heat (heat soak) and pressure and thus being susceptible to that dreaded detonation. The well-designed modern, quick-burn motors need much less ignition advance because their burn speed is nice and fast, so the spark plug does not need to ignite so far in advance of the piston reaching the top of its stroke (TDC).
Remember, all high-performance spark-ignition motors need SOME ignition advance - because it takes time for the burn to complete after the spark occurs, and the best ignition advance setting is when the pressure buildup due to the burning charge is at its very highest pressure just AFTER the piston reaches TDC, and as it starts to descend on the downward stroke.
It's also why nearly all motors have spark timing that advances with increasing RPM up to a certain point - to allow enough TIME for the burning to complete before the piston gets to TDC!
Detonation REDUCES with INCREASING RPM! Yup, in general, motors that are spinning fast allow less time for heat soak during the charge compression and burn phase before the piston starts dropping in its stroke and pressure is bled off - so the faster an engine is running, the LOWER the octane gas it needs! Honda did many tests years ago on the affect of RPM on octane requirements, and concluded that this time component in the burn cycle results in a dramatic difference to the fuel octane required. If I remember correctly, their weird 500cc 4-stroke oval-pistoned racer they tried to compete against the Yamaha and Suzuki 2-strokes of the time, needed something like only 87 octane fuel while it was spinning up well over 16000 RPM. It needed much higher octane fuel when running at lower RPM, with the same spark advance, etc. (I could be wrong with the specific numbers as it's a long time ago, but you get the picture Wink
A good indicator of a very well designed cylinder head is how LITTLE ignition advance it requires! The early 5-valve Yamaha FZ750 motor had a TERRIBLE burn speed, and it required something like 55 degrees of ignition advance because its cylinder head was so badly shaped and the charge burned in such a bad pattern in the head that it required a LONG time to develop peak pressure after the spark. This is also partly why the early 4-valve Suzuki TSCC heads (fashioned after the principles of the Cosworth F1 motors of the 70's, which started the trend towards fast burn) in the GSX1100 and later Katana 1100 produced so much more effective pressure (MEP) and power than the Honda CBX1000 6-cylinder motor, which had a very old fashioned combustion chambers, and did not produce anywhere near the BMEP and torque of the Suzuki of the same period.
Ok, the difference is not entirely due to efficient burning of the charge, as the effective filling of the cylinders by good ports/flow design is also very important, but all things being equal, a well-designed head can allow higher compression and make more power on the same grade fuel.
If you think about it, the best shape for a combustion chamber, would be to have a perfectly round sphere shape, with the spark-plug right in the dead center, to minimize the distance (and thus time) that it takes the burning flame front to reach the outer edge of the chamber before it can detonate! Of course, a perfectly round sphere shape is not possible as you have the moving piston on one side, and the need to have a number of valves in a an inverted V-pattern on the other side, but you will notice that modern high-performance heads have perfectly centered spark plugs to reduce the distance from the ignition point to the outer reaches of the chamber. They also have the outer edges of the combustion chamber closed off to nearly meet the piston and cause the charge to be "squished" into the center of the chamber and away from the outer edges of the chamber. This flat area is called the "squish" area, and it is there primarily to squeeze the charge into the middle of the chamber and reduce the distance that the expanding flame front has to travel outwards before all the charge is burnt (again, to minimize detonation).
BTW, all 5-valve Yamahas (eve their recent R1s) have had this burn issue, and their combustion chamber shape is so flat that raising the compression ratio on these motors by having pistons with raised domes caused the chamber shape to be so flat and poor for burning, that it actually reduced top-end power!
They struggled for years with the compromise between increased compression and low RPM torque versus lower compression and more top-end power but reduced torque - all because their combustion chamber shape for a quick burn was not so great! The primary reason Yamaha have reverted back to a 4-valve head on the latest R1, is because of this very reason - to have a head with a better burn design, that can take higher compression and that burns efficiently and quickly and produces more power with more compression and lower pollution.
Anyway, back to the burn process.....so we know that the idea is to burn the charge as quickly as possible, with as little spark advance as possible. This allows the use of a lower octane than a slower-burning head design of the same power, so the very broad generalizations that only motors of certain "power" or only motors over a certain "compression ratio" need high-octane premium gas - are really not valid. You could have a very good, quick-burning motor that needs only 87 octane to produce 200 BHP and a poorer designed head that requires Premium gas to produce only 180 BHP
Now, increasing the mechanical compression ratio of the motor results in more pressure buildup in the chamber. Pressure creates heat. Pressure also prodcues power in a motor, which is good. But, heat is bad for detonation. Remember, detonation is when the burning charge has reached a point where the heat and pressure is so high, that the fuel suddenly erupts into an explosion. So, raising the compression ratio is generally good for torque and power, but also creates more heat and increases the risk of detonation. (although it does not mean there WILL be detonation, depending on how much "safety margin" there was before detonation would occur with that same fuel).
The octane grade of a fuel indicates the extent of tolerance it has to heat and pressure before it suddenly explodes into detonation. A high-octane (premium grade) fuel simply has more resistance to detonation than a lower-grade fuel, before it explodes during the combustion burn in the chamber. A high-octane fuel has almost identical BTU (British Thermal Units) of energy as a standard grade fuel. They burn at the same rate, and at the same termperature, and at the same speed (well, as much the same in all respects as to be virtually indistinguishable in measurable difference!). So all talk of premium grade fuel running "cooler" or "burning quicker" or "making more power" is in 99% of cases, BS - at least while the engine is not actually detonating!
Note that there ARE different KINDS of fuel that have different octane levels. Some race fuels have oxygenates in them, that result in an internal super-charging affect, almost like adding Nitrous, and thus make more power than other, lower octane fuels. But this is comparing apples with oranges. For all intents and purposes, at any one gas station, the premium grade fuel burns and runs exactly the same as the standard grade fuel......EXCEPT that some motors have burn conditions in their cylinder heads which REQUIRE that high-octane premium gas resistance to detonation, otherwise they would detonate on regular fuel.
Now, have said the above, that the fuel octane grades in themselves DON'T provide different levels of energy and power for the motor, you should realize that if the motor was DESIGNED to perform its best with the high-octane fuel (to utilize it's resistance to detonation by having a high compression ratio, or advanced ignition timing, or very high dynamic dynamic BMEP with the use of large cams, free-flow exhaust, low-restriction intakes, etc, etc), then that motor WILL make more power with the high-octane fuel than with standard grade fuel!
If one ran standard fuel in these motors that are designed for high-octane fuel, and they are modern adaptable motors with knock sensors and dynamic fuel injection and dynamic spark timing mechanisms (as nearly ALL modern car/truck gas motors are these days), then the motor would likely start to detonate, and the knock (detonation) sensors would hear that detonation, and start to adjust the spark and/or fuel injection to reduce that detonation. The ECU does this so quickly that no damage occurs, but the spark is generally retarded and the fuel mixture is enriched from the optimum settings to avoid damage, but this also reduces the motor's power and torque, and fuel economy.
BUT, realize that old-tech motors WITHOUT these clever adjustable spark and fuel systems and knock sensors, such as most motorcycle engines (yes, we are WAY behind car engines in this respect), and especially motors like the Bandit with its carbs and "fixed" spark timing (i.e. no knock sensors to retard the timing) - are all designed to run fuel of a minimum octane grade, and if you run fuel of a LOWER grade than the minimum required, there is no way for the motor to "adapt" to this fuel and prevent detonation from occuring.
It is extremely important with these older tech motors, to use the minimum grade fuel to prevent detonation! It also means that there is ZERO benefit in using a fuel grade that is HIGHER than the minimum grade that does not actually detonate. You will NOT get any more power, or any better economy, or run any cooler using a higher octane fuel in these motors than the minimum grade fuel required! You're just waisting your money. On the other hand, remember that is is FAR better to err on the side of caution and use a higher grade fuel, than to use a grade of fuel that results in ANY detonation! It only takes a few minutes of running with a bad dose of detonation, before it can punch a hole in the piston, or break a valve, etc - such is the force of that internal detonation explosion!
What does detonation actually SOUND like? Well, it's often heard as a "tinkling" or "metallic" or "ringing" or "squeaking" sound from the motor. The metallic sound of detonation is the sound of the actual explosion inside the cylinder head, which is like taking a sledge-hammer to your head and valves and pistons and spark plugs! You are hearing those metal components ringing from that explosion! They are crying for relief! Wink
Detonation occurs most often when you are working the motor hard - for example, with wide throttle, at low RPM, up a hill, in a high gear, on a hot day! All those conditions increase the chance of detonation
- wide throttle because that is when you have the most charge in the head and thus pressure and heat buildup
- low RPM because that allows the burning charge to be exposed to heat soak over a longer TIME period in the cycle
- up a hill or in a high gear - because that increases the load required on the motor (more throttle for longer)
- hot day because detonation increases with increases in heat. The hotter the motor, the great chance it will detonate.
What to do if you hear detonation? Simple - immediately downshift to a lower gear, and use less throttle! This will normally immediately stop detonation, because you have less throttle and charge and compression and heat, and also higher RPM with less heat soak of the charge.
Then, find the nearest gas station, and fill-up with premium gas...until you can get home and figure out what the problem is. It may be a bad batch of gas, or spark too advanced, or simply too high a compression ratio for that gas grade.
This is the site that I found this on, original author unknown.
http://www.zzrbikes.com/modules.php?nam ... ic&t=22484
Neil
) octane muck. Using higher octane fuel introduces a flat spot which is pretty annoying. It goes at least 15km (usually more) extra per tank on low octane fuel.
