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Ignition Timing

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Making changes to the ignition timing is one of the easiest ways to increase the power and efficiency of a four-stroke internal combustion engine and it has therefore become one of the first things an engine tuner will address.  Ignition timing is a term that defines when the spark plug fires in relation to the piston's position within the cylinder.  Without all of the knowledge that I'm about to give you, one would naturally assume that the spark plug should fire and ignite the air/fuel mixture when the piston is at the top of the cylinder and the air/fuel mixture is compressed as much as possible.  At this point (referred to as Top Dead Center or TDC) the igniting air/fuel mixture will rapidly expand and push the piston back down, powering the engine.  Unfortunately for all of us trying to tune our engines, there is one thing that prevents us from doing something so simple - it takes some time for the flame front to ignite the air/fuel mixture once the spark plug fires, therefore the spark plug must ignite a short time before TDC to achieve the desired result.  It only takes a matter of milliseconds to burn the mixture in a cylinder under any circumstances, but when pistons are flying up and down at the rates they do in an engine, those milliseconds become critical.  If the spark plug fires when the piston is at TDC, the piston may be well on its way down the cylinder by the time the air/fuel mixture is completely burned.  To make matters even more difficult, there are a number of factors that can greatly affect the speed at which the mixture burns, including cylinder shape, mixture strength (lean or rich), type of fuel, compression ratio, how much air/fuel is in the cylinder, pressure, temperature, and humidity.  Since cylinder shape, compression ratio, and the type of fuel are going to remain constant for an engine while it is being tuned, this article will concentrate on the remaining factors.  Compression ratio is something that can be changed between tuning sessions, so one should know that an increase in compression ratio can be treated as a general increase in the cylinder pressure, which will be discussed in detail.  The type of fuel used also greatly affects ignition timing, but it will only be briefly discussed here as a full discussion would double the size of this article.  To learn more, see the link at the bottom.

 

The units most commonly used for ignition timing are degrees BTDC (Before TDC), with zero degrees BTDC meaning that the piston is at TDC when the spark plug fires.  If the ignition timing is at 10o BTDC, then the spark plug fires when the crankshaft is rotated 10o before the piston is at TDC.  If there is any number of positive degrees BTDC and the spark plug fires before the piston has reached TDC, then the timing is considered to be "advanced."  If the degrees are negative and the spark plug fires after the piston has reached TDC, then the timing is considered to be "retarded."  These two terms are also commonly used when making changes to the timing, as increasing the degrees BTDC is referred to as "advancing" the timing, and decreasing the degrees BTDC is referred to as "retarding" the timing.  Engines usually have marks on the balancer or pulley that is attached to the end of the crankshaft and a mark(s) on the engine block.  In order to determine an engine's timing, a timing light is used.  A typical timing light has an inductive pickup that clamps over the number one spark plug wire.  When current passes through the wire and the spark plug fires, a signal is sent to the timing light, and the light flashes at the same time as the number one spark plug.  The flashing light will appear to freeze the motion of the crankshaft, and the timing can be read with the marks.  Normally, zeroo BTDC is when a mark on the balancer lines up with a mark on the block.  Some engines have many marks on the balancer or the block indicating degree increments, but others have only one.  When there is only one mark on each, an "advance" timing light must be used.  This kind of timing light has a dial on it marked in degrees.  One operates it by turning the dial until the two marks line up and then reading the degrees from the dial.

 

Now we get to the good stuff.  In order to make the most power, the spark plug must fire at the right time so that the air/fuel mixture is completely burned at about the time when the piston reaches TDC so that the expanding gases can shove the piston back down.  If the ignition happens too late, the expanding gases are shoving against something that has already moved away on its own and full power is not realized.  If the ignition happens too early, the expanding gases are shoving against a piston that is moving towards them, and they will actually slow the piston down.  If this condition occurs when one is starting the engine, one may experience "kick-back," as the engine doesn't yet have enough momentum and the starter motor isn't strong enough to overcome the gases trying to push the pistons backwards.  Once the engine is running, this overly-advanced condition may become evident with a "knocking" or "pinging" sound.  Parts of the air/fuel mixture will autoignite (ignite on their own, with no spark plug firing) if enough pressure and/or heat is applied.  If the spark happens too early in the cycle, the air/fuel mixture parts that are sensitive to autoignition can react (ignite) from the pressure created by sandwiching them between the rising piston below them and the flame front and corresponding shock wave from the burning air/fuel above them.  This autoignition can create quite a shockwave of its own that is commonly heard as the "knock."  Unfortunately, this shockwave can damage and eventually destroy the engine if it happens too often.  This condition is more prone to occur in high heat and the higher pressures caused by higher compression engines and by forced induction.  Higher octane gasolines produce mixtures with air that are less likely to autoignite, so their use will lessen the likelihood of knocking and allow the engine tuner to advance the timing further.

 

"Pre-ignition" is when the air/fuel mixture autoignites from excess heat and/or pressure before the spark plug has fired, and has little to do with the ignition timing.  Unfortunately for the tuner and the customer, it can sound exactly the same as an autoignition from timing that is too advanced.  Lowering the pressure and/or the heat in the combustion chamber will reduce the chances of pre-ignition.  Lowering the pressure on a forced-induction engine is as simple as lowering the boost, but on a normally aspirated engine it may be as daunting as changing a head gasket to lower the compression ratio.  Fortunately, pre-ignition may be caused by something as simple as a spark plug that is too "hot."  This heat range rating on the spark plug refers to its thermal conductivity and its ability to dissipate heat.  A "hotter" plug will retain more heat in its tip and may stay hot enough to ignite the air/fuel mixture at an unwanted time.  Therefore, "colder" plugs are desired when the pressure in a combustion chamber is increased.  Care must be taken when choosing a heat range for spark plugs, as plugs that are too "cold" will result in poor starting and drivability.

 

As an engine speeds up, the spark plug should fire earlier (timing should be advanced) for the simple reason that there is less time for the combustion to happen as the piston speeds increase.  The engine tuner's job is to make that spark happen at just the right time throughout the rpm range.  For those that want to get technical, it turns out that the spark should occur at a point so that the flame front travels through the mixture and burns it completely, and the heated, expanding combustion products reach a maximum pressure when the piston is about 15-20 degrees after TDC.  With a dynamometer at his disposal, that job would be very easy if it weren't for all of the variables mentioned earlier.  Fortunately, the effect those variables have is very well understood and they can be accounted for if the engine has the right management system.

 

Under normal circumstances, pressure has the most significant effect on the ignition timing.  When the pressure of the air/fuel mixture increases, the flame front travels through it much faster as the front has less space to jump when traveling from molecule to molecule since the molecules are closer together.  The same can be said if more of the mixture is in the cylinder or if the temperature is lower, as cold air is more dense than warm air.  An increase in humidity will also act like an increase in pressure as the extra water molecules will help bridge the gaps between the air molecules, increasing the speed the flame front can travel.  Since an increase in pressure results in a decreased combustion time, the ignition timing must be retarded as the pressure increases to avoid knocking.  As far as what makes the pressure change - the cylinder pressure increases as the load on the engine increases and/or if forced induction (turbocharger, supercharger) is used.  When an engine is idling or under light load and there is little pressure, the ignition timing may be advanced.  Once the engine experiences a load or forced induction kicks in and the pressure builds, the timing must be retarded.

 

Since measuring the pressure inside the actual cylinder would be highly impractical, engine management systems use the intake manifold pressure.  Newer vehicles have what is known as a Manifold Absolute Pressure (MAP) gauge or sensor.  The absolute pressure measurement has the normal atmospheric pressure (14.7 psi or 1 bar) factored out of it, so a full vacuum would read zero and normal, sea-level atmospheric pressure would read 14.7 psi on an absolute pressure gauge.  In the engineering world, this differs from gauge pressure which really measures the difference between atmospheric pressure and the thing that one is measuring.  For example, your fuel pressure would be read in gauge pressure, and the units would technically be "psig" as opposed to "psia."  If the fuel pressure was 30 psi (psig), it would actually be 30 psi over atmospheric pressure.  An absolute pressure measurement is used instead of gauge pressure so that the vehicle's management system may include the surrounding barometric pressure and know what the "real" pressure inside the manifold is.

 

Newer vehicles have sophisticated electronic management systems that control everything that happens within the engine and even some things that happen outside of it.  These systems are commonly referred to as "Engine Control Units" or ECU's.  For those with such a system, tuning the engine becomes a matter of electronics and computer software.  The distributor has disappeared, and multiple coils, sometimes one for each spark plug, have taken its place.  Through the use of sensors such as the MAP sensor, the Mass Air Flow (MAF) sensor, and the crankshaft position sensor, the ECU can monitor all of the variables within the engine along with the environmental conditions, and tell each spark plug exactly when to fire.  If one has such a vehicle with an ECU that is not programmable and can not be changed with the addition of a chip, then one is going to be very limited when it comes to tuning.  If one is fortunate enough to have a fully programmable ECU, then one may change the ignition timing and possibly many other things as well until one has gone absolutely insane.  Many non-programmable ECU's can be reprogrammed, or "re-flashed" with new programs, and others can accept aftermarket add-on chips that change the programs.  One may have to perform a little research to determine what type of ECU is in the vehicle.

 

Older vehicles with distributors can be much simpler to tune (sometimes.)  Besides the timing light and a wrench, no fancy equipment or computer knowledge is necessary.  Way back in the day before crank position sensors, ECU's, and individual coil-packs, the distributor was used to determine when each spark plug should fire.  The distributor can be a very simple device, consisting of a rotor and a cap.  Through the use of chains and gears, the rotor spins at the same speed as the crankshaft.  Inside the cap are "points," which are small metal tabs.  There is one point for each spark plug, and the rotor periodically touches the points as it spins.  The rotor is connected through a high voltage coil to the battery, and the points are connected via spark plug wires to the spark plugs.  As the rotor spins, it contacts the points, completing the electrical circuit and sending short-duration, high-voltage currents to each spark plug, hopefully in the correct order.  Changing the ignition timing can be done by simply twisting the cap in relation to the rotor so that the points are touched a little sooner or a little later.  Unfortunately, this changes only the "static" timing, which is the ignition timing that the engine will see throughout its entire rpm range if no other timing devices are used.  If no other timing devices are used, then the timing can not change along with the pressure, rpm, and all of those other variables, and it will be impossible for the engine to operate at peak power and efficiency over the broad range that it must during every day driving.  For racers that operate in a very narrow range, static-only timing may be sufficient.

 

Fortunately for everyone else, the automotive engineers way back in the day really knew what they were doing, and they came up with two devices that allow the engine to operate with greater power and efficiency over a very broad range.  The two variables that have the greatest impact on timing, rpm and pressure, are taken into account with the "mechanical (or centrifugal) advance" and the "vacuum advance" devices.  Using the same principal that keeps water in a bucket when one swings it around in a circle, the mechanical advance consists of weights on springs that move away from a spinning shaft.  The weights are attached to some other moveable parts and levers that will create the same twisting action between the rotor and the cap as changing the static timing by hand.  As the engine rpm increases, the weights move further out, changing the rotor-cap relation further.  As stated earlier, an rpm increase should advance the timing, so the mechanical advance device is used to advance the timing as the engine's rpm increases.  The device used to factor in the pressure in the cylinder is the vacuum advance.  It is a device that creates the same result as the mechanical advance, but it does so in response to a low-pressure situation in the intake manifold.  Under low-load conditions the air rushing through the manifold creates a partial vacuum (negative readings on a psig gauge, readings below normal atmospheric pressure on a psia or MAP gauge) and the device advances the timing.  As the load increases and the pressure increases, the vacuum advance will allow the rotor-cap relation to spring back, retarding the timing.  When one wants to tune using these devices, one can simply adjust the static timing and allow the devices to perform their functions.  If one wants to change the amount of timing that is advanced and the points at which the advances are made, one can replace the springs and/or weights within the mechanical advance and use an adjustable vacuum advance and/or change the location where it senses the vacuum in the manifold.

If you're looking for me to tell you where to set the timing - this articles is too long as it is.  Every engine is different, and timing is going to vary from as little as 8 degrees BTDC to over 40.  With a little research, one can find out a good starting point for the ignition timing.  Fine tuning should be done on a dyno or under controlled conditions at the track.

To summarize:

1.  Ignition timing is a way to describe when the spark plug fires in relation to the piston's position and is measured in degrees BTDC.

2.  The ignition timing must take into account the fact that it takes time for the air/fuel mixture to burn.

3.  The ignition timing should advance as the engine rpm increases.

4.  The ignition timing should retard as pressure in the cylinder (as measured in the intake manifold) increases.

5.  Every engine is different, and it's the engine tuner's job to take these factors into account (and a few others) when setting the ignition timing.

Gasoline has a significant effect on all of this stuff I just described.  If you want to learn just about everything there is to know about gasoline in one place in a language that's easy to understand, please read:  http://www.faqs.org/faqs/autos/gasoline-faq/part1/