Ignition Timing basics on a Classic Car

Ignition Timing Basics
Ignition Timing Basics

This guide captures ‘bygone’ advice on Classic Car ignition timing basics. It is part of a series I am building that describes what to do on a Classic Car Basic Service.

Useful information for Classic Car and Retro Car enthusiasts, as many of these tasks are no longer required on modern day cars.

Remember folks these are ‘bygone guides’ … useful but the safety and personal protective equipment measures are reflective of bygone awareness. Stay safe.





Ignition Timing basics

Checking and adjusting your ignition timing can produce great results and solve many problems. This particular post is written to set out the basics of what its for and how it works. For advice on how to check and adjust this can be found in my Basic Service List

Incorrect ignition timing costs money. It is possible to spend a great deal more than is necessary over a given period of motoring time, say 8000 km, simply because fuel is being wasted by ignition timing that is not at the
optimum setting. It is also possible that a severe case of incorrect ignition timing., if allowed to continue over a long period, may cause damage to moving parts of the engine. So regular adjustment of the timing is an essential step in servicing your car.

The petrol engine is powered by heat generated in its cylinders by the combustion of a petrol/air mixture, the means of ig:aition of this mixture being an electric spark. The exact moment of combustion is critical, because the piston must be in the best possible position when the ignited mixture begins to force it down its cylinder.

If combustion occurs too soon, the pressure tries to force the upwards moving piston downwards before it is ready to change direction. This means the engine is working against itself, causing the temperature inside the com­bustion chamber to rise unnecessarily.

Subjected to this overheating for too long, the crown of the piston becomes burned, as do its valves, and the engine loses compression and power.


When you come to check and adjust your ignition timing you will need an ignition timing light. Here are the most watched on eBay. Don’t be tempted to spend a fortune a Power Spark or Accuspark will be ideal for a simple classic car.


What is Pre-Ignition

This above explained situation is known as pre-ignition, and can be identified by a knocking sound from the engine and a tendency to run on after it has been switched off.

If combustion occurs too late, the piston has already begun its journey towards the bottom of the cylinder. The energy in the fuel is not properly used, fuel is wasted and overheating occurs. The engine sounds flat and lacks power.

Combustion is not instantaneous. It takes perhaps 0.003sec. So it is timed to begin a fraction of a second before the piston reaches the top of its travel, known as top dead centre or TDC. This fraction of time is converted for practical purposes into a number of degrees of crank­shaft rotation.

Ignition timing is carried out to ensure that ignition occurs at precisely the correct fraction of a second. It is done by adjusting the position of the distributor (which controls the flow of sparks to each cylinder in turn) in relation to the distributor cam on the engine (which is automatically synchronized with the positions of the pistons).

If the distributor is turned one way in relation to its cam, ignition will be advanced-that is, will occur earlier. If it is turned the other way, ignition will be retarded.

Since all the pistons in an engine are linked to the same crankshaft, they can move in relation to each other only in a predetermined pattern. So once the timing has been set correctly for one piston, the others are also correct.

There is however one complication: to make the most economical use of the fuel and to gain maximum power, ignition must occur ever more in advance of TDC as the speed of the engine increases. This is because the time that the fuel takes to burn is constant but-in that same 0.003 sec-both piston and crankshaft are covering more ‘ground’.

To cope with the timing alterations necessary when the engine speed is increasing, or when the throttle is only partly open, the distributor is provided with two adjusting mechanisms. These are the centrifugal advance mech­anism and the vacuum advance mechanism, both of which I will explain in later posts and link in one day.





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