Ignition

here is one of those areas that people think they understand but they really don't have the full idea of it.

Its also an area where companies can make money out of the (generally stupid) public. Spark plugs, HT leads and so on that can give up to 20bhp, what you don't normally see is the fact they were tested on a 1000bhp chevvy V8 which means the gain was only a few percent.

So does that mean the ignition system will not give you more power and conversly if its dodgy it won't lose you any power? Of course not, that would be silly to assume that.

So in case you don't know a 4 stroke engine works like this, suck, squeeze, bang, blow. It suck air and fuel in, compresses them, ignites them and then exhausts the left over gasses.

Seems pretty simple. It is, except the engine runs at various speeds and loads.

This means differening amounts of fuel and air are taken in and also the ignition has to fire at different times.

So what does the igntion system consist of?

Lets start at the end and work our way back.

Spark plug, this is actually fitted into the cylinder head and it forms part of the combustion chamber roof. This is where the actual spark that causes the mixture to fire happens. The spark jumps from the middle electrode to the other electrode that is welded to the plugs body. Spark plugs come in a variety of heat ranges, thread dia's, earth electrode numbers, resistors, thread lengths and core set ups.

Then theres the HT leads, these are wires that carry the energy to give the spark at the plug. these come in a variety of constructions, pretty colours and diameters.

Then the distributor cap, this is where the rotor arm lives and the HT leads plug into, like the name suggests it distributes the spark to whichever cylinder needs it.

Newer cars run a coil pack which does away with a seperate coil and distributor cap and rotor arm.

The coil, this is what the spark originates from, it goes in at 12 volts and comes out at roughly 25k Volts (some are higher some are lower).

The igntion module, this is where the rev counter feed comes from and this is also what gives the signal for the coil to fire.

Crank angle sensor, this is mounted on the gearbox bellhousing. Its an electro magnetic device that 'reads' a toothed ring and tells the module what position the cranks at.

You may have heard of advance and retard. What this means is the spark happens before (advance) or after (retard) the piston is at TDC (top dead centre) for the cylinder thats firing. 12 degrees advance means the combustion is started 12 degrees before the piston reaches TDC.

Why does the spark need to be advanced or retarded?

Well like I said earlier the engine runs at a variety of loads and speeds. This means theres a varying amount of time available for the mixture to burn. So this should tell you the spark has to be initiated at varying times to allow the bang to actually push the piston down on the power stroke.

If the timing is to far advanced the sparks going to be to soon and basically the flame front will try and push the piston thats coming up to go back down, this causes damage and a loss of power. It also makes itself heard by a nice clattering noise which is detonation.

If the timings to retarded the piston will be getting the shove to late and a loss of power is the result. Damage will also occur as exhaust valves and seats will be seeing much higher temps, which may eventually glow and cause the mixture to fire before the plug actually fires (pre-ignition which brings about the same noise and damage as detonation).

The aim is for the flame front to give the piston the biggest, hardest and longest shove it can. This basically means more power.

Vacuum timing change, on the carbed engines theres a pipe that goes from the inlet manifold to the ignition module. This is the vacuum advance (or retard if you like). This alters the timing dependant on the load of the engine. At cruise the inlet vacuum will be high the mixture will be leaned off to near stoich (stoichiometric mixture for petrol is 14:1 that is 14parts air to 1 part fuel, full power rich is around 13:1 and it can be leaned off to around 17:1 but some GDI engines can run at 25:1) or less so the engine can tolerate a bit of advance.

However say you decide to boot it, the vacuum in the inlet drops to closer to atmospheric, now the mixture is being a tad enriched, this means it burns quicker so the timings retarded to prevent damage to the engine.

Injected cars work the same except the inputs are throttle position and the MAP sensor instead of a mere rubber pipe so it can be more exact.

Advancing the timing from stock can give more power because the manufacturer is conservitive with the timing, they have to take into account engine safety, crap servicing, bad fuel etc etc, so advancing the timing may indeed give a noticeable and worth while power increase.

Spark plugs, now these apparently are the difference between a nova and a nova that kills ferraris.

Heres a FACT a single electrode plugs is superior to a multi electrode plug.

So before buying those super 4's etc stop. Best place for these is the sea or a car thats not serviced for years at a time.

There sole purpose in life is to extend service intervals, nothing else.

Ok whats wrong with that you ask? Nothing, if you don't look after your car get them. Would you buy them if you knew they cost you power and fuel?

Didn't think so.

The spark can only jump to 1 electrode at a time. Also the spark chooses the easiest route (simple physics). What does this mean? Well a smaller spark for a start.

Those electrodes also shroud the spark. The spark is effectively hidden from the mixture. You want the spark to see as much of the mixture as possible. This reduces power and prevents a clean burn.

A sparks a spark you say, very true and if there was plenty of time for the mixture to burn and it wasn't moving I would agree, wouldn't matter what caused the spark it will still burn. theres a very small window for combustion to occur and the mixtures tumbling into the cylinder so the more thats exposed to the spark at the start the quicker it will burn.

Theres some very intresting videos involving high speed cameras and pyrex heads to show this.

So basically if you can get a single electrode plug every time. Also I prefer to use NGK plugs, they last the longest are the most stable and have good even heat ranges plug to plug.

V shaped earth electrdoes fall into the same catagory as multi electrodes.

Spark gap is also very important, to small you get a small short spark, to large and the spark may not actually arrive at the plug.

Leads, these need to be clean and in good condition. If they are not then the spark that should be goign to the cylinder to burn the mixture is just going to earth somewhere on the way and cause a missfire.

A missfire costs power, lets say its a 4 cyl engine with 100bhp, thats 25bhp per cylinder, if 1 cyl fails to fire thats 25bhp gone.

The standard bouigord HT leads are extremely good. A good lead is well insulated, has a good solid constuction and doesn't fall apart. best leads aside from the standard leads are magnecors.

Don't run leads to close together as they can crosstalk and bring about a charge in the lead thats running alongside it (this is how the coil works!). So keep them seperate and don't let them run along the engines metal wwork.

Dizzy cap and rotor arm, these need to be clean. You can drill the cap to vent ionised air to prevent crossfires (the spark may jump to another terminal due to lower resistance) which costs power and causes damage.

So this should help clear up some things and show you will not get 20bhp from a new lead or plug but it should also tell you that anything below par will cost you power.