Drive and RestoreAlfa Romeo Twin Spark: why two spark plugs per cylinder?
“Twin Spark” is printed on the cam covers of hundreds of thousands of Alfas from the late 1980s to the 2000s. The name describes exactly what it is: two spark plugs per cylinder. But the idea is much older than the badge, and there are two very different engine families hiding under the same name.
Why two spark plugs?
When a single plug fires, the flame has to travel all the way across the combustion chamber. With two plugs firing at the same time, the flame front has less distance to cover, so the mixture burns faster and more completely. That means:
- less ignition advance is needed,
- the engine tolerates leaner mixtures, which helps economy,
- and combustion is more stable, especially at light loads.
In the 1980s that last point became important for a new reason: stricter emissions rules.
A racing idea first
Alfa used twin ignition long before it was a marketing name. The 1914 Grand Prix car designed by Giuseppe Merosi already had two plugs per cylinder. In the 1960s Autodelta fitted a twin-plug head to the 1600 engine of the Giulia Sprint GTA racing cars, and twin ignition also featured on the TZ.
The 8-valve Twin Spark: from the 75 onwards
The road-going Twin Spark arrived in 1987 in the Alfa Romeo 75 Twin Spark. It was a development of Alfa’s classic Twin Cam (“Nord”) four-cylinder: aluminium block and head, wet cast-iron cylinder liners, a duplex timing chain, and two identical plugs per cylinder firing together. The 2.0 litre gained power and torque over the single-plug version it replaced.
The 8-valve Twin Spark was built in 1.7, 1.8 and 2.0-litre forms and powered the 75, 164 and 155. In the 155 the 2.0 got a longer 90 mm stroke.
The 16-valve Twin Spark: a different engine
In 1995 the GTV and Spider introduced the 16-valve Twin Spark. Despite the name, it is a completely different engine, part of Fiat’s modular Pratola Serra family: cast-iron block, aluminium 16-valve head and belt-driven camshafts. Here the second plug is smaller and sits at the edge of the combustion chamber, where it helps combustion at light loads.
It came in 1.4, 1.6, 1.8 and 2.0-litre versions and was used in the 145, 146, 155, 156, 147, 166, GTV, Spider and GT. Later versions gained a variable-length intake manifold and, from 2001, electronic throttle control and extra catalysts for Euro 3.
The end of the Twin Spark
Direct injection made the second plug unnecessary. From 2001 the JTS engines replaced the Twin Spark in most models, and the last Twin Spark engines were built at the end of the decade, in the 1.8 GT.
What to check on a 16-valve Twin Spark
- Timing belt: the camshafts are belt-driven and a snapped belt is very expensive. Know when it was last changed, and if in doubt, change it, together with the tensioners.
- Cam variator: a rattle from the front of the head, especially at idle when warm, often points to a worn variator on the intake camshaft.
- Two sets of plugs: remember there are two plugs per cylinder, of different types, when servicing the ignition.
For the 8-valve engines the duplex chain and the condition of the wet liners and their seals matter more. Either way, these engines respond well to correct parts and correct settings. Original Alfa Romeo parts for them are still available from specialists such as Italricambi.
Sources
- Wikipedia: Alfa Romeo Twin Spark engine
- Italicar: history of Alfa Romeo Twin Spark engines