Dave A
USER UNDER INVESTIGATION - DO NOT TRADE
Registered: 10th Dec 03
Location: County Durham
User status: Offline
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http://www.spark-plugs.co.uk/pages/technical/diagnosis.htm
stick it in tutorials 
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Robin
Premium Member
Registered: 7th Jan 04
Location: Northants Drives: Clio 182 Cup
User status: Offline
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Could be useful 
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DAZ1985
Member
Registered: 3rd Sep 06
Location: Scholar Green, Cheshire
User status: Offline
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Good stuff that, what more do you need to know?
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Dave A
USER UNDER INVESTIGATION - DO NOT TRADE
Registered: 10th Dec 03
Location: County Durham
User status: Offline
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got pages and pages of stuff about detonation and pre ignition as well, its a bit boring tbh (unless you are a geek) and goes into miliseconds of piston travel and frequencies of shockwaves that resonate the cylinder bores at certain Hz.....
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Robin
Premium Member
Registered: 7th Jan 04
Location: Northants Drives: Clio 182 Cup
User status: Offline
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Slim it down Dave, make a guide about the common terms for things, the ones like that which a lot of people don't understand
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Nick-S
Member
Registered: 3rd Mar 04
Location: Leigh. Drives: RS Megane 230 F1 Team R26
User status: Offline
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my car dosent have spark plugs 
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Dave A
USER UNDER INVESTIGATION - DO NOT TRADE
Registered: 10th Dec 03
Location: County Durham
User status: Offline
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SPARK PLUG BASICS:
The spark plug has two primary functions:
Ignite air/fuel mixture
Transfer heat from the combustion chamber
Spark plugs carry electrical energy and turn fuel into working energy. A sufficient amount of voltage must be supplied by the ignition system to spark across the spark plug's gap. This is called "Electrical Performance."
The temperature of the spark plug's firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. This is called "Thermal Performance", and is determined by the heat range selected.
It's important to remember spark plugs do not create heat, they only remove heat. The spark plug works as a heat exchanger
by pulling unwanted thermal energy away from the combustion chamber, and transferring the heat to the engine's cooling system.
Types of Abnormal Combustion:
Pre-ignition
Defined as: ignition of the air/fuel mixture before the pre-set ignition timing mark
Caused by hot spots in the combustion chamber...can be caused
(or amplified) by over advanced timing, too hot a spark plug, low octane fuel, lean air/fuel mixture, too high compression, or insufficient engine cooling
A change to a higher octane fuel, a colder plug, richer fuel mixture,
or lower compression may be in order
You may also need to retard ignition timing, and check vehicle's cooling system
Pre-ignition usually leads to detonation; pre-ignition an detonation are two separate events
Detonation
The spark plug's worst enemy! (Besides fouling)
Can break insulators or break off ground electrodes
Pre-ignition most often leads to detonation
Plug tip temperatures can spike to over 3000°F during the combustion process (in a racing engine)
Most frequently caused by hot spots in the combustion chamber.
Hot spots will allow the air/fuel mixture to pre-ignite. As the piston is being forced upward by mechanical action of the connecting rod, the pre-ignited explosion will try to force the piston downward. If the piston can't go up (because of the force of the premature explosion) and it can't go down (because of the upward mo-tion of the connecting rod), the piston will rattle from side to side. The resulting shock wave causes an audible pinging sound. This is detonation.
Most of the damage than an engine sustains when "detonating" is from excessive heat
The spark plug is damaged by both the elevated temperatures and the accompanying shock wave, or concussion
Misfires
A spark plug is said to have misfired when enough voltage has not been delivered to light off all fuel present in the combustion chamber at the proper moment of the power stroke (a few degrees before top dead center)
A spark plug can deliver a weak spark (or no spark at all) for a variety of reasons...defective coil, too much compression with incorrect
plug gap, dry fouled or wet fouled spark plugs, insufficient ignition timing, etc.
Slight misfires can cause a loss of performance for obvious reasons (if fuel is not lit, no energy is be-ing created)
Severe misfires will cause poor fuel economy, poor driveability, and can lead to engine damage
Fouling
Will occur when spark plug tip temperature is insufficient to burn off carbon, fuel, oil or other deposits
Will cause spark to leach to metal shell...no spark across plug gap will cause a misfire
Wet-fouled spark plugs must be changed...spark plugs will not fire
Dry-fouled spark plugs can sometimes be cleaned by bringing engine up to operating temperature
Before changing fouled spark plugs, be sure to eliminate root
cause of fouling
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SAL
Premium Member
Registered: 19th Dec 05
Location: Radlett, Hertfordshire
User status: Offline
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so what do the ht leads do then  good info there dave 
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powey
Member
Registered: 6th Nov 05
Location: Gwent, South Wales
User status: Offline
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ht lead powers the spark plug?
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Steve X16XE
Member
Registered: 31st Dec 06
Location: Barnsley, South Yorkshire
User status: Offline
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Coil give the power.
HT leads conduct it.
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22B
Member
Registered: 9th Sep 04
User status: Offline
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quote:
Originally posted by powey
ht lead powers the spark plug?
no they just carry the voltage to the plug, the distributor or DIS pack or coil pack(no wires straight to plug voltage) distribute the spark
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