When developing the ignition system for the Enablieri Potenza Strada engine, a lot of options were discussed: Wasted Spark, Coilpacks, Coil On Plug, GTA style mechanical distributor… the list went on.
Traditionally when retro-fitting a TwinSpark engine to a 105 chassis, the upper distributor is removed, and a replacement is fashioned for the lower oil pump driven unit – usually a standard mechanical advance Marelli or Bosch unit from the period 105 box of tricks, re-curved to suit the desired max spark advance (usually about 28 degrees for the TS, as opposed to 34 or higher for a Nord engine). With a fabricated baseplate to allow an 8 terminal nissan distributor cap to be fitted, this solution is simple to use, particularly with a carburetted setup, and gives the engine a period ‘GTA’ look with 8 high tension leads wending their way around the engine bay fed by two ignition coils. A few of the Alfa Ricambi crew around the world also manufacture aftermarket – GTA style distributors that have a more polished (manufacturing and appearance wise) finish.
Once you move to computer controlled ignition, a few more doors open. In it’s basic setup, the MegaSquirt controller can take the place of the mechanical advance in the distributor – you lock this mechanism, and let the MS advance and retard the spark from the distributors statically timed position – giving you a 3 dimensional table of engine load vs rpm to tune your ignition, while still using the same period correct spark delivery mechanism of distributor and HT leads.
The next cab offthe rank is the use of Coil Packs. By deleting the distributor from the engine, you can use the MS outputs to drive a pair of 4 outlet ‘Wasted Spark’ Coil Packs. These resemble a transformer unit with 4 spark plug leads connected. By pairing two of these together, you can drive 8 spark plugs. They’re called Wasted Spark due to the simple firing logic they require. A little background here – in basic configuration, the MS controller is watching the toothed trigger wheel on the crankshaft pulley of the TS engine. This wheel has a couple of teeth missing to tell the ECU where TDC of the engine is. The drawback of course, is that TDC for piston 1, is also TDC for piston 4. How does the ECU identify which spark plug to fire without knowing where the engine is in it’s compression / exhaust cycle ? With a distributor delivering spark, you have a reference for this cycle, but once the distributor is deleted – the ECU has no way to know if it should ignite cylinder 1 or 4 when the gap in the trigger wheel goes past. The only way for the ECU to know which part of the cycle the engine is on is for it to have a sensor watching either a camshaft, or the distributor drive. By using a Wasted Spark configuration – you fire the spark plugs for cylinders 1,4 at the same time, and then cylinders 2,3 at the same time. This works because of the paired cylinders – when one is on compression stroke, it’s partner is on exhaust. A spark during the exhaust stroke does pretty much nothing, hence the term ‘wasted’. The benefit here is that you simplify the setup significantly, and reduce the number of electrical components the ECU relies on to run the engine. By connecting them up so each cylinder has a plug fed from both Coil Packs – you enjoy the same system redundancy as the Distributor configuration. As the Coil Packs have no moving parts – they’re also more robust. You still have 8 spark plug leads to deal with – but that preserves the period look if that’s your goal.
Option number three is to use Coil On Plug units. This is how most mass-produced cars are setup – instead of one or two large coils feeding multiple spark plugs, each plug gets it’s very own miniature coil mounted on top. This has a great benefit of removing all the HT spark plug leads floating around the engine bay making electrical noise and wearing out. Instead low voltage wires from the ECU trigger the coil – which then delivers the spark direct to the top of the spark plug. This can make for a very neat installation – and personally I think it can look great too, but if you’re aiming for a period-look engine bay, this one is not for you. These discrete Coil On Plug units can be wired in pairs to effectively run Wasted Spark with all the advantages that brings. This is how we’ve chosen to configure the Strada engine.
By coincidence, the Denso 129700-4150 ‘Pencil’ Coil On Plug units from the same Suzuki GSXR-600 that provides the Mikuni throttle bodies were an almost perfect fit for the TS engine – they fit perfectly in the spark plug wells, supporting themselves without adding any additional bracketry, and only protrude about 5mm too high to allow the TS engine cover plate to be fitted. Typically we don’t use the cover plate anyway as it lends the engine more of an 80’s look – but a few spacers would allow it to be used if you wished.
The Denso COP units have been fitted to Winky the development Giulia Super for nearly two years now – performing daily driver duty 80km every weekday, and double duty at the track during club events. While there are two spare units in the box of tricks – touch wood, we’ve never had to dig one out.