spark plugs?
05-31-2003, 07:55 PM
#1
Burning Brakes
Thread Starter
spark plugs?
i just bought a set of spark plugs, has any used this? bosch super cu-electrode WR 8 DCW. how does this measure up to the split fires? which is better to use with the guru chips. @ what gap should i set this?
06-01-2003, 01:27 AM
#3
Race Director
See these threads on spark plug discussions:
<a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic;f=18;t=005308" target="_blank">Cool Spark Plug Info </a>
<a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic;f=18;t=005260" target="_blank">Which Sprak plugs?</a>
<a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic;f=18;t=005130" target="_blank">What plugs do you reccomend?</a>
The Bosch WR8DCW would be too hot even on a stock Turbo. This would be OK on an early low-compression 94NA. But increasing the boost would tend to favour a colder plug such as a WR6DC. Also no benefits to the Spitfires or +4 or any mutli-electrodes for that matter since the ionization path of least resistance will create only a single spark anyway. Most likely this will be the outside electrode with the closest gap to center one.
A quick discussion on the energy imparted by the spark. The two electrodes on the spark-plug with an insulating air-gap creates a capacitor. Nothing flows between the two electrodes until the gases in the gap is ionized by an electrical potential of a certain voltage. The larger the gap, the larger the voltage needed to ionize the gases. This larger voltage also imparts a higher activation-energy to the start the chemical reaction of combustion.
The density of the gases in the gap also affects the needed voltage. Higher-compression or boosted engines will have higher density mixtures in the gap, thus requiring more voltage to jump across. That's why the spark-plug gap specs shows a larger gap for the 944NA than for the 951.
Now here's the catch. If you get more power from a spark that's forced to jump across a larger gap, why not just make a huge gap to begin with? It depends upon the power of your ignition system. The coil must output enough voltage to jump the larger gap. And the voltage is not constant. At higher-RPMs with less dwell time to charge the coil, the voltage will be lower. That's where the high-RPM misfires come from; not enough voltage to jump the spark-gap and you get dropped cylinders. The solution here is to narrow the plug gap to require less energy to start a spark across (but the energy transfered to the combustion mixture will be lower as well).
The current-flow also has a time value in how much can flow into the coil at what voltage. The inductive coils used in our cars can only suck up so much current from the battery in any given amount of time. It's not really an issue with the low redline on these cars, but if we had a 9000rpm car, then capacitative-discharge (CDI) system can force more power through the coil more quickly.
So the solution is to run as large a gap as possible for the density of your mixture and the particular ignition system in the car. Fortunately, we've got a very good system in our Porsches and you can actually increase the gap to about 0.045-0.050" on the Turbos and still get reliable firing at high-RPM even with high-boost.
<a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic;f=18;t=005308" target="_blank">Cool Spark Plug Info </a>
<a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic;f=18;t=005260" target="_blank">Which Sprak plugs?</a>
<a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic;f=18;t=005130" target="_blank">What plugs do you reccomend?</a>
The Bosch WR8DCW would be too hot even on a stock Turbo. This would be OK on an early low-compression 94NA. But increasing the boost would tend to favour a colder plug such as a WR6DC. Also no benefits to the Spitfires or +4 or any mutli-electrodes for that matter since the ionization path of least resistance will create only a single spark anyway. Most likely this will be the outside electrode with the closest gap to center one.
A quick discussion on the energy imparted by the spark. The two electrodes on the spark-plug with an insulating air-gap creates a capacitor. Nothing flows between the two electrodes until the gases in the gap is ionized by an electrical potential of a certain voltage. The larger the gap, the larger the voltage needed to ionize the gases. This larger voltage also imparts a higher activation-energy to the start the chemical reaction of combustion.
The density of the gases in the gap also affects the needed voltage. Higher-compression or boosted engines will have higher density mixtures in the gap, thus requiring more voltage to jump across. That's why the spark-plug gap specs shows a larger gap for the 944NA than for the 951.
Now here's the catch. If you get more power from a spark that's forced to jump across a larger gap, why not just make a huge gap to begin with? It depends upon the power of your ignition system. The coil must output enough voltage to jump the larger gap. And the voltage is not constant. At higher-RPMs with less dwell time to charge the coil, the voltage will be lower. That's where the high-RPM misfires come from; not enough voltage to jump the spark-gap and you get dropped cylinders. The solution here is to narrow the plug gap to require less energy to start a spark across (but the energy transfered to the combustion mixture will be lower as well).
The current-flow also has a time value in how much can flow into the coil at what voltage. The inductive coils used in our cars can only suck up so much current from the battery in any given amount of time. It's not really an issue with the low redline on these cars, but if we had a 9000rpm car, then capacitative-discharge (CDI) system can force more power through the coil more quickly.
So the solution is to run as large a gap as possible for the density of your mixture and the particular ignition system in the car. Fortunately, we've got a very good system in our Porsches and you can actually increase the gap to about 0.045-0.050" on the Turbos and still get reliable firing at high-RPM even with high-boost.
