ignition timing .....

all this timing banter has quite intrested me. does anyone wnat to explin the amazingness of the ignition timing to me?? (bruce M could prolly explain it me thinks)

If the fuel-air mix ignited instantly, then the correct timing would be top dead centre. Thats where the pressure in the combustion chamber would be put to best use; its not slowing the piston on its way up and then gets maximum pressure on the way down.
The fuel-air mix actually burns slower than that, so in an attempt to get get maximum pressure at the desired TDC, you need to set fire to it some time earlier (a few milliseconds or so), and thus get as much power out of the fuel that you can. This burn time is sort of constant, so the faster the engine is going, the earlier (interms of engine revolutions) the ignition spark needs to be. To achieve this, you want about 5-10 degrees advance at idle, and about 28-32 degrees at 3500rpm. Thus the need for an ignition advance curve. Its possible to advance the ignition by mechanical means (distributors) or electronically (ignition mapping) both are trying to do the same thing - set fire to the petrol at the best time.
This is a simplification of the ideal set-up but hopefully you get the idea.
I’m sure some of our other contributors can explain some more, but hopefully you get the gist.

How we optimise all this is a whole new chapter.

Cam timing is a different story.

cool mate cheers. so if the ignition timing is a little off can it do any harm to the car on a long drive???

Okay here goes…

As above the timing of the spark maximises the power output of a dose of air/fuel mix.
If the timing is spot on the explosion matches the piston starting it’s path down the cylinder and then the energy from the bang is converted into kinetic energy (movement) with some heat and sound energy as well.
If the piston is still heading up the cylinder then the bang “knocks” it on the top trying to reverse its path. This happens if the timing is too early or “advanced”. This shocks the piston/conrod/crank and is very bad. Also there is a lot of extra heat generated in this situation as the energy is not converted into movement( energy cannot be lost only converted into different forms). If the spark is late (or retarded) then the bang happens when the piston is already on its way down the cylinder and the force of the bang is wasted and not converted into movement. to illustrate: imagine trying to spin a playground roundabout which is already turning quickly, as you try to spin it quicker it moves away from your hand and you are unable to exert a force upon it. In this situation ,in the engine, the energy from the bang is wasted but it has to go somewhere, so it is converted into extra heat. This can cause the engine to be underpowered and overheat!

Of course the bang event is not instant and the pressure created grows to a peak over a (short) period of time.

As the engine turns quicker (higher revs) the distance it moves in the period between the spark and peak pressure is further. So the spark must be at an earlier point in the pistons cycle.

As the amount of fuel/air mixture is varied (by your right foot and the opening of the carb/throttle body) then the time is takes to burn changes too. When the pedal is on the floor, lots of air and petrol rush into the engine. This is then squeezed into a small space and when the spark hits this dense mixture it burns very quickly. However, when cruising on an A road at 60mph. You are hardly pushing the pedal so much less fuel/air is let into the engine. This mixture is less dense when compressed and takes longer to burn.
So… when less mixture is going in then the spark must be advanced to make best use of the petrol/air. If you don’t then the engine will be less efficient and more likely to overheat. A prime example of this is a tuned aircooled engine. A 009 ditributer does not adjust the spark based on “load” (the measurement to estimate mixture density). Therefore when cuising at 60-70 with a light throttle the spark is not advanced enough and the engine will be more likely to be very thirsty and creating extra heat. Simply, at 3,500 rpm at light throttle or maximum throttle the 009 gives the engine the same spark advance.

“mapped” or “3D” ignition is when the ignition is controlled by a minicomputer. the computer looks up a table based on the current engine revs and load. From these two values the advance is selected. The load is normally measured in a production car by the vacuum created at the throttle body/carb but in aftermarket kits this can be measured by throttle position sensor. The throttle sensor is useful for highly tuned engines as the vacum signal around idle can be very weak.

I’ll stop now. Definition of pinking,knock and detonation to follow if anyone is foolish enough to ask :slight_smile:


Don’t stop Bruce - I was just getting into that.

How long before this stuff appears in “Speed Dork” or “Pimp up my Ashtray” ?

Assuming you weren’t being sarcastic :lol: Alan.

I’ll start a new topic to continue , covering pinking, knock and detonation.
An often misunderstood concept.

To follow in the next couple of days.


If only Bruce’s mental ablities had turned into Kinetic energy his car would have been completed years ago :smiley: