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Combustion and Ignition

 - Proper mixing and burning of the mixture is critical to power delivery.
 - Starting and maintaining the burn at the correct speed is essential.




The process of igniting the air/fuel mixture and totally combusting it is not easy to quantify. It is rarely discussed and few people realize how important it is to the power of the engine. Unless we properly ignite the air/fuel mix, the energy potential of the engine will not be realized. The fuel must be ignited at the correct time and with enough intensity to initiate combustion. Then we must maintain the burn moving from the spark plug right through the entire combustion space at the correct speed. If flame travel is too slow, cylinder pressure will rise too slowly to give the crankshaft a maximum push. If combustion is too rapid it can become too violent and eventually destroy the engine.



A number of factors affect the combustion process. The ignition systems function is to initiate the combustion process. It must be robust enough to get the flame started. The EMU signals when the spark plug should fire, but there are many factors which influence how fast the flame spreads through the air/fuel mix. When we understand these factors we can proceed to gain the maximum from the combustion process.
The primary factors are mixture quality, mixture movement, and the design of the combustion space.

Mixture Quality and Turbulence.
 - Proper vaporization of the fuel is important.
 - Location of fuel injection, pistons and chamber designs all affect mixture.


Mixture quality doesn’t mean the mixture ratio. Mixture quality refers to the fuel particle size and the dispersal of the fuel throughout the air. Prior to combustion we have to break the liquid fuel down into vapour. The means choosing the correct injector and the correct fuel type. We also need to inject the fuel at the optimum location and angle. Other factors are the shape of the inlet port and valve and the angle they make with the cylinder wall so as to cause a swirling of the mix.

The shape of the piston crown and the combustion chamber are also major influences on the mixing of the air and fuel. Four valve chambers and flat-top or dish-top piston crowns are the best in this regard because the don’t impede the swirling motion of the mixture. Even minor redesigns of flat-top pistons have caused increases of 2-3% in power output. The older the design of the engine, the more gains that are to be had by replacing the pistons for more modern profiled items.

This motion has several effects on the mixture. It tends to homogenize the mixture, ensuring that there is an evenly spread of air and fuel throughout the cylinder. It also ensures that the fuel is broken down into ever smaller droplets to aid in combustion and it also makes sure that the temperature of the mixture is uniform across the whole cylinder. Cool pockets of mixture burn too slowly and wont be fully burned before the exhaust stroke (although this can aid in turbo spool up). However, the real culprit of uneven temperature in the mixture as pockets of hot mixture to hotspots. This may cause the entire mixture to ignite before the spark resulting in a violent per-ignition. More commonly the hot pocket may self ignite after the spark. The desired spark flame front and the hot pocket flame front then collide within the cylinder leading to uncontrolled and violent combustion. This is referred to as detonation.

The Shape of the Combustion Space.
 - 4 valves are always preferable to two.
 - Radial 4 valve design is the ultimate solution for powerful engines.


As mentioned, the combustion space influences mixture motion prior to ignition and it continues to exert an influence after ignition, helping the flame to spread through the cylinder is an orderly and controlled way.
The 4-valves chamber is always better in this respect. The spark plug is in the middle of the combustion space so the flame reaches the top and bottom of the cylinder at close to the same time. The deepest areas of the chamber are in the middle where the spark is. The outer areas are more shallow, so the mixture here is less likely to promote detonation.

The radial 4-valve chamber is even better in this regard because the chamber is shaped like an upside down saucer. The entire outer perimeter of the chamber is shallow. However, it is a more complex design, the valve actuation arrangement is much harder to manufacture and build so it is only seen in competition engines.

Converting Road Engines for Competition.
 - Combustion chamber is optimized for max power at the cost of poor cruise.

In high performance engines we change the porting and the shape of the combustion chamber to reduce swirl and tumble. To control flame speed we reduce the combustion chamber squish area, open up the clearance between the piston crown and the combustion chamber wall and change the shape of the piston crown. These changes would all contribute to a road engine being less fuel efficient at cruise.
When we factor in forced induction, we have to decide at what throttle openings and rpms we want the engine to work best and plan our mods accordingly.

Ignition Advance.
 - Variable ignition advance across the load and RPM range is essential.
 - Ignition mapping is just as important as fuel mapping.


There are many factors which must be considered when deciding how late into the compression stroke we initiate the spark in the cylinder. Most engines give max horsepower when we fire the spark so that max cylinder pressure is reached at 12 to 14deg after top dead centre. To achieve this we must initiate ignition sometime before top dead centre. This is called the ignition advance angle. At low engine speeds it will be around 10deg before TDC and at higher speeds it will be at around 20deg before TDC and higher still it could be 40 to 50deg before TDC. There variable advance angles are required to give the mixture enough time to burn thoroughly and thus give the max horsepower. Mixture density, air/fuel ratio and fuel quality all influence the ignition advance angle.

Mixture density itself is influenced by a large number of factors and for lower densities we must start the spark earlier to allow enough time for the low oxygen mix to burn.

Very lean and very rich mixtures also burn slowly and require more spark advance. Ideal mix (13:1) burns quickest and requires less advance.

When efficient exhausts are used, the unscavenged exhaust gas in the cylinders is reduced so that there is more fresh mix to burn so advance angle need to be reduced. This also causes less exhaust molecules to interfere with the mix which further reduced the spark advance angle.

The fuel used also affects advance angle. Petrol burns rapidly so requires less time than other fuels. Alcohol id slower to burn and nitro is even slower. But when an accelerator chemical is added to these fuels, spark advance needs to be reduced.

During dyno setup, an ignition map is built up carefully via the laptop at all engines speeds, boost pressures and throttle openings. Compensating factors are also built in such as inlet charge temperatures, engine coolant temperatures and exhaust turbine temperatures.

Points Type Ignition.
 - Old carb system.
 - Outdated since introduction of electronic control.


Before electronic ignition became the standard, engines relied on crude points type distributors to time and distribute the spark. One or two coils provided the high voltage spark.

The distributor had two switches, the rotors and the contact breakers and an ignition advance/retard mechanism to vary timing. The switching of the primary circuit was done by the contact breakers (controlled by the distributor cam). When the points were closed, current flowed through the coil, then through the points to earth. This produced a magnetic field surrounding the secondary winding. When the primary magnetic field collapsed (points opened), a current was induced in the secondary winding creating a high voltage current capable of jumping across the spark plug gap.
This high voltage signal flowed from the coil to the rotor then onto the distributor points and onto the sparkplugs. There are many disadvantages to this setup when tuning an engine and it is impossible to tune an engine in one area without suffering greatly in another. If you tune for top end power and advance etc… then lower end characteristics suffer etc… there are lots of ways to combat this by using weights and so on but they only succeed upto a certain point. Once you go beyond 6cyl the situation gets worse.
This system worked well when there was no alternative.
High Energy Ignition.
 - One of the first electronic ignition systems.
 - Adequate for most road applications.
 - Not optimum for performance engines


The common electronic transistor ignition system also uses a contact breaker system and is also on inductive storage type ignition system. It relies on magnetic pulses to open and close the low voltage circuit.

The distributor shaft turns a pulse generator rotor inside a permanent magnet. This induces a signal in the pick-up coil. The electric signal flows through the electronic module and switches on and off. Current flows through the primary coil when its on and when its off the magnetic field collapses and just like the old points system, the high voltage signal flows through the distributor to the spark plugs. This system doesn’t require tungsten points so a higher current is delivered to the sparkplugs.


Multi Coil and Capacitor Discharge Ignition.
 - The only choice for modern performance engines.
 - High energy spark and long duration.


But for higher engine speeds and higher intensity spark or longer duration, multi coil or CD Ignition is used.
We can opt for a multi coil HEI system or a capacitor discharge system with single or multiple coils.
With multi coils more time is available to saturate each coil fully. But to save money or if the regulations don’t allow it, we can use a single coil.

CD ignition stores the ignition energy in a capacitor. Even the best transistor ignition amplifier and coil combination begin to falter at 7500rpm on a v8. even when these rpm speeds are not reached, the CD system is inherently superior to the transistor HEI method and should therefore be used on competition to fast road cars. The bigger, longer duration spark produced by CD systems will always equal or out perform the HEI based systems.

CD ignition in single coil setups wont run out of ignition energy until 10000rpm levels are reached because a capacitor charges and discharges much faster than a coil.

In CD systems, current in the primary circuit powers a mini-oscillator/transformer which charges a capacitor to 400volts. The distributor has a magnetic or LED trigger switch. The amplified signal breaks the primary circuit and the capacitor instantly dumps its energy in the coils primary winding, which steps up the voltage to 30-40000volts which is directed by a rotor button to the sparkplugs.

Conventional HEI systems have a rise time of 100 microsecs; CD has a rise time of 20microseconds. This allows the system to fire fouled and wet plugs which means the car will quickly restart after a stall or a spin.

Multi Spark CD Ignition.
 - Much longer duration spark for sustained burn.
 - As much as six sparks per sequence at low revs.


The standard CD ignition produces a spark of short duration. In extreme conditions, turbulence within the chamber might blow out the spark before it gets sufficient heat into the mixture is initiate combustion. This is where multi-spark CD ignition comes in.

This system will give as much as six sparks per spark sequence at idle and will revert to one long duration, high intensity spark at higher rpms and loads.

Ignition Voltage Requirements.
An engines voltage requirements depend on the cylinder pressure and the width of the spark plug gap. An engine running 0.028in gap with a compression ratio of 11:1 will require 20kv to get a spark. At 0.04in and 12.5:1 we require 27kv and at 0.06in and 15:1 we require a minimum of 35kv. In the above three example the stored energy required just to get a spark going will be 10mj, 18mj and 31mj respectively.

Therefore, if our ignition system had a max of 20mj across the spark plug gap, we would have to run with narrow gaps just to get a spark in the 35kv situation. In the 27kv situation we would get a spark but only have 2mj left over to keep the spark gap ionized and get the flame started. But in the 20kv situation we would have enough ignition energy to get the spark going and have a duration long enough to initiate combustion.
So the more secondary ignition energy we have left the better our chances of starting and sustaining combustion at high engine speeds. Also, note that as the compression ratio and plug gap increases, more energy is required to initiate and sustain combustion.

If upgrading the ignition system is not an option because of budgetary constraints or regulations, the opposite must be done, the plug gaps must be reduced to the minimum and the compression ratio must be lowered.

Choosing an Ignition System.
 - Most modern stock systems are good.
 - When exceeding the factory RPM or using methanol then upgrade.


Modern ignition systems have to operate reliable without misfiring in order to pass government regulations, therefore, the ignition system on most new cars are quite good and will usually be adequate for a stage one or two upgrade. The main area where standard systems have a problem is at very high rpm. Once you go past the max rpm of your stock engine or if you use methanol or alcohol based fuels then you will probably have to upgrade your ignition system.

Another area to consider is reliability. Most stock systems are quite reliable, but there are a few notable exceptions and these systems should be replaced as soon as any mods are undertaken.

Fitting Ignition Systems.
Excessive heat, vibration and moisture must be kept at bay around the ignition system. Mount the spark amplifiers and coil away from the header and turbo and shield them from all local heat sources. Direct a small airflow over the spark box if possible. When fitting components to heat sinks make sure you use heat sink grease. Mount all electronics on anti shock rubber mounts.

Pressurized water jets must never be directed at the ignition system. A tiny amount of water will cause a high-voltage flashover.

Never disconnect any wires with the engine running. Never crank the engine while the leads are disconnected or while the coil lead is disconnected, without first isolating the ignition system. Disconnect the low voltage positive ignition wire at the coil and then you can crank in safety.
Battery chargers and welders operated with the battery connected will also more than likely damage or destroy the ignition system. The main harness going to the spark box as well as any earth straps going to it should also be disconnected when welding. Also place the welder earth close to the welding point.

Improving Spark Timing Accuracy.
 - Distributor less systems have best accuracy,

Improved spark timing accuracy can be had by switching to a distributor less system. Harmonics in the valve train can play havoc with the distributor which makes highly accurate spark timing impossible. What should be a 32deg before TDC firing angle at 8000rpm can become a 35deg angle on cylinder one, a 29deg angle on cylinder two, a 33deg angle on cylinder three and a 32deg angle on cylinder four. Because the number one cylinder is firing 3deg advance owing to camshaft spring, we have to retard the timing on all of the other cylinders, just to avoid detonation at number one cylinder. This costs us power. Also, the remaining cylinders are overly retarded and will suffer afterburn. This could reduce a turbos life and limit our boost pressure.
When we discard the distributor, we have the option of individual cylinder trim. We can adjust the firing angle to suit each individual cylinder.

Because of engine breathing characteristics, irregular coolant flow (due to head design), fuel and nitrous distribution problems, one cylinder may run hotter or cooler than the others. With ignition trim, we can move power from one cylinder into another.

Without the distributor, another means of informing the EMU when each piston hits TDC must be employed. Rather than using a pulse trigger, we use firing pins embedded in the flywheel. As each pin passes the magnetic pick up sensor a pulse is delivered to the EMU. The EMU then calculated the appropriate advance angle and switched the ignition primary circuit on and off.

On aftermarket systems you need to make sure the pins are placed accurately and that the air gap to the sensor is correct. If firing pin diameters should be the same as the diameter of the sensor pickup. The air gap should be around 0.04in.

Determining Optimum Ignition Timing.
 - Least amount of advance for max power.
 - Must be mapped on the dyno.


Naturally the ignition timing must be set perfectly for your engine. The least amount of advance required for max power should be used. The only way to get the timing right is on the dyno. The advance angle should be increased until max torque is reached or until you are beginning to approach detonation. The engine should never actually detonate. A steady state dyno needs to be used to allow proper heat to be loaded into the cylinders otherwise the angle will be overly advanced. Acceleration runs are good for initial setup, but for fine tuning the load must be held at each of the load points.

With turbos, an additional factor must be considered; the spool up of the turbine may be improved by slightly retarding the spark. If you tune the engine very close to the detonation limit, you must compensate for changes in atmospheric conditions. A 10% increase in relative air density may overlean the mixture and cause piston meltdown, if you don’t richen the mixture accordingly. A decrease in humidity might cause engine detonation unless spark advance or the amount of cam advance is reduced.

Identifying Engine Wreckers.
 - Pre-ignition is the main cause of engine failure.

Pre-ignition is caused by extreme combustion temperatures melting the top of the piston and the ring lands. If there is a hole in the piston crown like a welding hole or the centre electrode of the sparkplug is melted then pre-ignition has occurred. It can usually be traced to combustion chamber or exhaust valve deposits becoming incandescent, bit it may also be caused by blocked water jackets creating a hot spot or a glowing sparkplug with a heat range too hot for the engine. It can also be caused by a hot piston due to lack of lubrication, improper clearance or a broken ring.

Spark Plug Heat Range.
 - The hotter the engine, the colder the plug range.

With any modified engine the correct heat range sparkplug must be used. You cannot keep using the manufacturer recommended plug. A hot plug is used to avoid fouling in engines with low combustion temps. A cold plug is used to avoid overheating when temps are high – racing engines etc The length of the insulator nose and the electrode alloy composition are the primary determinants. Hot plugs have long noses.

Engines in sports tune will need plugs one or two steps colder than standard. More heavily tuned road engines may require two distinct plug types, one for everyday use and another colder set for hard use.
Ideally rally cars should have one set of plugs for warm up and another set for maximum power driving. Otherwise use the hottest retracted-gap racing plug available. Never race while the warm up plugs are fitted.
Examining the nose of the plug will tell you if it is in the collect temperature range for your engine (the AFR must be set correctly first). Always begin testing with overly cold plugs and build up to the warmest plug that is suitable for your state of tune. You must run the engine at maximum load and then abruptly cut the ignition (this wont do the turbo any favours, but as a once off exercise it shouldn’t overly wear the turbo).
Once you have established the heat range, stick with this. If you are moving to another manufacturer of spark plug then you have to do the exercise over again, you cant depend on that manufacturers heat range to be calibrated the same.

This is why tuners stick to certain brands… not only because of the quality of the plug but because the heat range numbers used are spaced well and there is enough spread across the heat ranges to accommodate the engine in different states of tune.

Normal –
Insulator nose white or very light tan.
Little or no cement boil where centre electrodes protrude through insulator nose.
Electrodes not discoloured or eroded.

Too Cold –
Insulator nose dark grey or black
Steel plug shell end covered with dry, black soot deposit that will rub off easily.

Too Hot –
Insulator nose chalky white or may have satin sheen.
Excessive cement boil where centre electrode protrudes through insulator nose.
Cement may be milk white or meringue like.
Centre electrode may be blue and rounded off at edges.
Earth electrode may be badly eroded or have molten appearance.

Pre-Ignition – use colder plugs and remove combustion chamber deposits
Insulator nose blistered.
Centre electrode and side electrode burned or melted away.
Detonation – retard ignition and richen mixture
Fractured insulator nose in sustained or extreme cases.
Insulator nose covered in tiny pepper specks or even tiny beads of aluminium leaving the piston.
Excessive cement boil where centre electrode protrudes through insulator nose.
Specks on plug shell end.

Insulator Glazing – replace with plugs of same heat range. If condition persists fit plugs one grade colder.
Shiny yellow, green or tan deposits on insulator nose, particularly close to centre electrode.

Ash Fouled – clean or replace with plugs of the same heat range.
Thick yellow, white or light brown deposit on insulator, centre and side electrode.

Electrode Material and Gap Style.
 - For forced induction a thick earth electrode is best.
 - There are several competing designs and heat ranges.


For forced induction engines a plug with a thick earth electrode that will conduct heat away quickly must be used. A thin electrode made of poor metal quality will melt or will glow like a glow plug and cause pre-ignition.
When burning methanol, we cant use platinum electrodes because they act as a catalyst.

The projecting nose or projecting core plug is the best one to use, with copper implants in both electrodes to increase heat transfer rates. It has a wide heat range and will resist fouling and pre-ignition. At high rpms the insulator nose is cooled by incoming fuel and at low rpm it runs hot to prevent fouling. However, projecting nose plugs shouldn’t be used on very high boost engines or on engines running 20% or more nitro.
If you change from a regular plug to a projecting nose type you may have to retard the ignition very slightly because the flame travel is reduced.

The next best plug is the conventional gap plug. It is usually available in colder grades than the projecting nose plug. It has a wider heat range and provides better ignition flame propagation.

For highly supercharged engines use the retracted-gap racing plug in the coldest grades available. This plug type must only be used when absolutely necessary because it tends to foul easily and it generates a poor ignition flame front.

The fine wire plug is sometimes suitable for fast road engines. It was originally developed for two stroke engines, but can be used in 4stroke engines when a wide heat range is required. They are expensive but work well when low speed fouling is a problem. They are not suitable for high boost or nitro assisted engines.

Lastly, make sure that there is physical space for the plug in the cylinder chamber. Also make sure the plug reach is not too long or too short. Exposed threads in the combustion chamber can become hot spots. These factors should be checked with the head removed or by using an identical head.

Spark Plug Gap.
 - Increased power and compression will necessitate narrower gaps.
 - Upgrading the ignition system will allow wider gaps to be used.


The coil saturation time, the compression ratio and the engine speed all dictate what the spark plug gap should be. Increasing compression and engine speed, and reducing the saturation time will all lead to the gap being made narrower and vice versa.

All modified engines retaining the stock ignition system and operating at higher than normal revs will require a narrower spark plug gap. A wide gap improves engine idle and low speed performance on lean mixtures. But at high rpms and load the ignition system is not able to provide a sustained spark across the same gap and so it must be narrowed in order to avoid high speed misfires and possible breakdown or the high-voltage system insulation. Rally engines may need a gap as low as 0.03in with conventional plugs or a gap of 0.025in with retracted-gap racing plugs. Engines running methanol will need gaps of 0.005in to initially burn it and keep it burning.

Engines with a capacitor discharge ignition system can use the original manufacturers gap for short special stages, but for longer staged events and for international rallies with restricted service intervals, a maximum gap of 0.045in should be used to avoid complete CD failure during the event. The closer plug gap reduces the load on the spark box. If trouble arises the system should survive to the end of the event.

Coil Polarity.
 - The spark should always jump from the centre to the outside.

Upto 40% of the energy potential is lost when the polarity on the coils reversed. The spark should always jump from the centre electrode to the outside electrode. The polarity can be checked by looking at the coils low voltage connections or by using an oscilloscope.

Spark Plug Maintenance.
The plugs need to be filed, gapped and tested every 5000mls. Retracted gap plugs cannot be filed and fine wire plugs should never be filed. Projecting nose and conventional plugs should have the earth electrode bent back enough to allow filing of the sparking surface. This lowers the voltage potential requires to create a spark because the impedance is reduced and because electricity will jump a sharp edge more easily. Filing removes the dead metal and re-exposes the highly conductive material.

Never use a wire brush and don’t use a ‘plug cleaner’. Don’t bother cleaning the plug. If its overly dirty then use a toothbrush and some non-oily solvent, otherwise just throw it away and use new plugs.

Spark Plug Leads.
 - Use inductive suppression leads.
 - Use diameter of 8mm or more silicon wires.


Most road cars use radio suppression cables with a carbon impregnated rayon cord to conduct the high-voltage current to the spark plugs. With age the electrical resistance of these type cables increases so they should be replaced regularly or preferable discarded altogether. High quality inductive suppression leads should be used instead. These have a metal core to conduct full electrical energy to the plugs, they have a metal induction spiral wound within the cable to provide noise and induction suppression. Inadequate suppression will affect the EMU and other electronic devices. Never use ordinary copper or stainless steel core cables.

Inductive spiral leads also reduce inductive crossfire. Inductive crossfire in its mildest form causes rough running. In its worst form it will result in severe engine damage – same as severe pre-ignition.
This problem should not be taken lightly. It is not a rare occurrence on competition engines running powerful CD systems.

A crossfire on a blown engine will destroy the cylinder walls, pistons, bearings, cranks, heads and if you’re lucky the head gasket. This kind of severe damage due to crossfire is most common in v8s because of the firing order of the cylinders, but it can and does happen on all configurations….
As well as using high quality leads, each lead should be separated by an inch and should never be clipped using metal clips. On v8s, consecutively firing leads(such as No’s 5 and 7) should be crossed once to cut out any inductive cross firing.

Also keep the leads away from other metals by about an inch.

The average race engine needs up to 35000v to operate at optimal levels. To work in this environment use a minimum of 8mm silicone wire with plug gaps not greater than 0.028in and a max of 12psi boost. At gaps of 0.04in and boost levels beyond 15psi only the best grades of ignition wire must be used 9 or 10mm wire must be used. Alternatively, use 8mm Moroso spiral core wire with a dielectric insulation strength of well over 50kv. This combines the highest levels of electro-magnetic suppression with the least electrical resistance to ensure max ignition energy at the spark plug.

Distributor Cap and Rotor Arm.
The distributor cap, rotor arm and coil tower are other areas where high-voltage leakage and flashover can occur. Keep these areas free of moisture, grime and dust. Inspect for cracks and carbon buildup.
However, don’t scrape or sand the distributor cap terminals or the rotor arm contact because you will remove the insulating glaze and cause leakage to earth. Caps and arms should be made of high quality alkyd material.
Also be wary of flashover on 6 and 8cyl engines because the high-voltage posts are very close to on another.

Primary Voltage and Ignition Performance.
 - Consider the entire electrical system and loom.

Power being consumed by the ignition system has an affect on the other electrical systems in the car. Electronic fuel injection, electric fuel pumps, fans and air con all place heavy loads on the electrical system and high energy CD systems can tip the balance past the levels that the alternator, wiring loom etc can handle. This can lead to reliability issues and engines cutting out for no apparent reason. CD systems can have a current draw that’s three times as high as a simple HEI system. Upto 10amps can be required.
The draw from HEI systems is much lower, but the instantaneous amp draw when the module is first switched to charge the coil, is very high. Therefore, a HEI system must be wired directly to the ignition switch using 10 guage wire. Additionally, this surge can cause the EMU to shut down if its not setup correctly.