Guide to performance ignition systems – 137

March 3rd, 2010 by NZPC

Resident tech-head Rob Dawson discusses the ins and the outs of performance ignition systems

By now you’ll be pretty well versed on the three essential ingredients necessary to make an engine work. I won’t go through them again, but in this chapter of our series of little tech features we’re going to look at the ignition side of the equation. I know that a lot of people realise what the basic components of an ignition system are, but what is the actual job of the system itself? In basic terms, the main function of an ignition system is to generate an electrical pulse that has a charge sufficiently high enough to send an arc across the spark plug gap. The second aspect, which is critical to the efficient operation of the system, is the timing. There’s not a lot of point in that arc travelling across the spark plug gap if it happens when the piston is at bottom dead centre now, is there?
On your average car, especially older cars as far as the distributor is concerned, the stock ignition system consists of two circuits. There is the primary circuit, which is made up of the battery, the ignition switch, a ballast resistor, the coil itself (primary windings only read on), the points inside the distributor, and low voltage ignition leads. The main purpose of this section of the circuit is to allow the low voltage charge from the battery to pass on through the coil, where the charge is bumped up a notch or two from the standard 12 volts to around 10,000 or 15,000 volts. Within the coil there are two individual circuits as well.

The primary circuit consists of up to 150 windings of heavy-gauge copper wire, which is insulated to avoid short circuits. The secondary winding within the coil is made up of narrower gauge copper wire but of many more turns up to 30,000. When the current from the battery flows through the primary circuit of the coil, a large magnetic field builds up. When the current is shut off during each break in the firing of the ignition, the magnetic field collapses, inducing a high voltage within the second winding which is released through the centre terminal of the coil.

The secondary circuit of the ignition system consists of the secondary windings of the coil, plus the distributor cap and rotor, the ignition leads and the spark plugs. It’s here where the high voltage pulse which was developed inside the secondary windings of the coil and sent to the centre terminal of the coil is passed on from the coil to the centre terminal of the distributor cap. Inside the distributor cap the rotor swings around and electrically connects the charge from the centre terminal to the individual cylinder terminals, which in turn send the charge through the ignition leads and on to the spark plugs.

Many of you will have heard about the importance of having low-resistance ignition leads, and now you might realise why a low level of resistance is important. Power losses can occur while the charge is travelling through the ignition lead due to the resistance, which means the arc between the spark plug gap will not carry as much power as it could have. This in turn leads to a less efficient ignition of the fuel/air mixture and a less powerful burn. Newer ignition systems have had the mechanical aspect the distributor replaced with electronic parts that have reduced the inefficiencies inherent with the older systems, which were prone to wear and tear and were sometimes unreliable everyone has had a cracked dizzy cap. The individual contact points inside the distributor have had their job replaced by optical sensors, whereby a light beam is broken by a spinning rotor within the crank angle sensor. This determines the timing and thus the spark, as the crank angle sensor informs the ignition system when the piston is at top dead centre. Technically speaking, the spark should fire just slightly before the piston reaches TDC, since the combustion process takes a little longer than it does for the piston to reach TDC and begin moving down the bore. If the spark fired when the piston was exactly at TDC, part of the combustion pressure would be wasted, as the piston would already be on its way back down before it was all over. But if the spark fires just a fraction before the piston reaches TDC, the mixture will already be burning and developing its maximum pressure just as the piston arrives. Got it?

Regardless of the age of the engine and ignition design, one of the most efficient ways of modifying your ignition system is to install a high performance coil. Instead of a stock coil, which might have an amplification of 100:1 primary to secondary voltage (where the power from the battery is turned into ignition power), an aftermarket Bosch or MSD coil might have an amplification power of 150:1 or even higher sometimes up to and over 200:1. This means the electrical charge travelling through the ignition leads is carrying far more energy and will result in a stronger spark inside the combustion chamber. If you install a higher-power coil, the likelihood is that the rest of the components in the ignition system will come under more strain especially the ignition leads and spark plugs. It’s always an idea to upgrade the ignition leads to match the output of the coil. Even more importantly, make sure your spark plugs are gapped properly. If the gap is too narrow the spark may not have enough time to ignite the mixture properly; and if the gap is too wide, there’s a chance the spark will be blown out especially in the case of the turbocharged engine where the incoming compressed air has serious velocity. It’s also important that you use the correct spark plugs. If you use spark plugs that burn too hot, you could run into issues like pre-ignition, where the mixture is ignited by super-heated components within the combustion chamber, be they carbon deposits or a spark plug electrode. That’s not a good thing because the power stroke would come on a little too early and could cause serious damage to the piston, conrod or crankshaft.

So those are the basic components of an ignition system that might be found on your average car, and a couple of basic mods that could bring about a little more horsepower and a cleaner, more efficient combustion of the mixture. But what if you want to go a little further and look at aftermarket engine management systems? Aside from having to shell out a good couple of grand on a decent system, plus another grand on installation and tuning, you need to understand that this kind of modification requires all the supporting components to be in top condition. That means the battery, the coil, the ignition leads and the spark plugs. Aside from those components, the fuel delivery system also has to be up to the task. If your aim is to improve power, then the fuel pump, fuel lines and fuel injectors have to be suited to the level of power you’re after. The ECU will control the ignition and the fuelling, so both sets of supporting parts have to be looked at closely. Working in tandem with the modified fuel curve, which can be determined by an aftermarket ECU, a new ignition curve can be set. This means the firing of the sparks can be controlled to achieve the optimum (or stoichiometric) combustion. While this may not sound like a lot on its own, when there is more fuel being sent to the combustion chamber, as well as more air from an upgraded turbocharger, it’s extremely important that the mixture is ignited at precisely the right moment. With increased loading within the combustion chamber comes a higher potential for serious damage, so it’s worth spending the money on some good quality tuning at the hands of someone who knows what they’re doing and has access to a dynamometer. Another key component required to ensure optimum efficiency is the oxygen sensor, which is usually found mounted on the front of the exhaust manifold. This feeds vital information back to the ECU about the make-up of the burnt mixture, and ultimately assists to control the delivery of the incoming mixture.

As you can see, modifying your ignition system is not so much about achieving big power gains. It’s more about making the most of the extra power that has been developed through other areas of the engine, such as the fuelling and induction.

If your ignition system is not up to the task, the end result is that the extra fuel and air you’ve squeezed into your engine will not combust properly, and a lot of potential power will go to waste. The key is to ensure you have high-quality, high power-generating electrical components where it matters like the battery and the coil and have high-capacity, high power-handling components like the ignition leads and spark plugs as well. The ignition system is a true supporting system; if it’s not running to its maximum potential then the power-generating areas of the engine will not be able to deliver maximum benefits, and no one likes wasted coin!

Words: Rob Dawson

This article is from Performance Car issue 137. Click here to check it out.