Article by: Bradley Denton
What is waste spark and why do I want it?
Both the m20 m30 engines use a single inductive ignition coil and a distributor cap/rotor to fire the spark plugs. The coil is triggered by Motronic via a transistor. This is a simple and effective method to providing spark. It does however, have its limitations in a higher performance application.
In a forced induction (and high compression NA engine) cylinder pressures are significantly elevated over that of a normally aspirated engine. This makes it much more difficult for the ignition system to ionize the spark plug gap (initiate the spark). Also, with the elevated pressure comes the problem of insufficient dwell time with a single ignition coil. An ignition coil has to charge to be able to fire spark plug. When the transistor in the DME switches on, the coil begins to charge. To fire the coil, the transistor is switched off.
As engine speed increases the available time to charge the coil decreases, and it is possible that the coil will not fully charge, compromising an already struggling system. The solution is typically to reduce the spark plug gap to something around 0.025″ (remember, larger gap can be beneficial to efficient combustion) to make it easier to fire the plug. However sometimes this isn’t even enough. An ignition box such as an MSD unit can be installed but requires that the stock wires and distributor carry the increased voltage, which will shorten the life of the dizzy and potentially causing arcing problems with the factory wires.
Another solution is to run more than one ignition coil. In this case three coils, each firing a pair of plugs. It fires the necessary plug on the compression stroke as well as its paired cylinder on the exhaust stroke “wasting” that spark. Since there is more than one coil in the system, this gives each coil more time to charge allowing a saturated charge even at higher rpm.
The other benefit is that the dizzy is no longer needed, removing its limitations from the system. It will never need to be replaced again.
Note – The biggest caveat here is that you must be running some form of standalone, such as Megasquirt, to implement wasted spark. Unfortunately Motronic cannot be used to run this type of system.
This system chosen for my turbo 535is consists of a GM DIS coil base plate and upgraded MSD coil packs. The new coils have different secondary terminals requiring new spark plug leads. I used Mallory Sprint universal 8mm wires.
Since the base plate, which can be sourced from a large selection of GM V6 powered vehicles contains its own ignition drivers and we will be using the ones in Megasquirt, the unit was gutted of its electronics. Jumper wires will installed to make the proper connections inside the unit.
With this completed it is necessary to make some changes and connections to the Megasquirt unit. Since MS only comes with a single IGBT ignition driver, we must add two more. The easiest solution is to use the Glen’s Garage Idle Board which while providing an excellent means to use the stock PWM idle valve, contains two additional pads for the two additional drivers. It already provides power and ground to the drivers, only requiring a trigger and the output to the coil. The connections for this can be seen below.
An additional DB9 connector was added to that additional pins would be available to connect to the coils. On my setup, Spark A, the existing one on the MS board, was jumpered to to the DB9 connector retaining the original signal to the coil. Since I wanted to be able to be able to go back to a stock coil if I chose to do so, I added a Weatherpak connector to the factory coil power and signal wire.
This can be unplugged and plugged directly into the Waste Spark Connector, making changing back extremely easy.
The firewall was chosen as the best place to locate the coil assembly. A backing plate was made out of some sheet aluminum, drilled and tapped to hold down the coils, painted and them mounted to the firewall (the silver piece is the original GM backing plate).
Here is a shot of all of the coils in place:
With the coils mounted it was possible to determine the correct length for all of the spark plug wires. Since I am not a fan of colorful wires and wanted a stock look, I chose black wires and wanted to route them in the original plastic loom. Since the wires not enter the loom from the rear, some plastic had to be removed with a Dremel. I highly recommend you buy the MSD spark plug wire stripping/crimping tool as it makes stripping the wire much easier and allows for a strong, proper crimp on the connector. The complete wire set can be seen below. New spark plugs, a set of BKR7E were gapped to 0.030″ and installed as well.
The coils firing order is shown below:
A few small changes must be made in Megasquirt so that it knows there are two additional coils it must fire. “Wasted Spark” must be selected from Number of Coils drop down menu. Megasquirt automatically assumes that coil B and C are connected to the outputs that I show in the drawings above. Connect the spark leads to the proper coils according the the drawing above. Also, I increased the dwell to 2.5 ms, as I called MSD and asked them what the optimal dwell for these coils was.
The settings can be seen below:
With everything completed and installed, this is what the finished product looks like. Very pleasing if I do say so myself.
Upon turning the key, the car fired to life immediately with no hiccups and settled into the smoothest idle that the car has ever had. Anyone who has ridden in or driven my car will attest to how smoothly the car idles and drives. Well, it has improved with no other changes. I have to breakup under boost running 0.030″ gap at 8.5psi and throttle response feels a tad bit sharper. I cannot currently comment on any improvements in fuel economy but I will evaluate this over the next few weeks. Also, I will never have to buy another expensive cap and rotor again. For less than $200 this is a significant upgrade that will benefit the car even more once the boost is increased even more. For those running MS or standalone, I highly recommend converting your car to wasted spark ignition.
All new parts were ordered from Summit Racing.
MSD GM DIS Coils – MSD part #8224
Spark Plug Wires = Mallory part #774.
Crimping Tool – MSD Crimp Tool part # 3503.
Spark Plugs – NGK BKR7E
Coil Base is from a mid ’90s Pontiac with a V6 (yes, my car now has a Pontiac part on it).