Ignition Timing

Ignition timing, in a spark ignition internal combustion engine, is the process of setting the time that a spark will occur in the combustion chamber (during the compression stroke) relative to piston position and crankshaft angular velocity.

Setting the correct ignition timing is crucial in the performance of an engine. The ignition timing affects many variables including engine longevity, fuel economy, and engine power. Modern engines that are controlled by an engine control unit use a computer to control the timing throughout the engine's RPM range. Older engines that use mechanical spark distributors rely on inertia (by using rotating weights and springs) and manifold vacuum in order to set the ignition timing throughout the engine's RPM range. There are many factors that influence ignition timing. These include which type of ignition system is used, engine speed and load, which components are used in the ignition system, and the settings of the ignition system components. Usually, any major engine changes or upgrades will require a change to the ignition timing settings of the engine.


Source: Wikipedia.

Advancing vs Retarding

The two most common terms you'll hear regarding ignition timing are advance and retard. The ignition timing is carefully set so that the spark plug is fired at a specific point in the engine's rotation. In most cases, the spark plug is fired just before the piston reaches top dead center (TDC)--the highest point it can reach in the compression stroke. This is generally expressed in angular degrees, as the piston's position is directly associated with the angular rotation of the crankshaft. When the piston is at TDC, the crankshaft is at 0°.
Advancing the timing means to adjust it so that the plug fires before a specified time. For example, if the factory setting is to fire at 15°BTDC (before top dead center), adjusting it to fire at 20°BTDC would be an advanced setting. Advancing the ignition yields more power, but increases the risk of detonation.
Retarding the timing is the exact opposite: setting the ignition to fire later than the specified time. If the factory setting is to fire at 15°BTDC, adjusting it to fire at 10°BTDC would be a retarded condition. Retarding the ignition usually reduces the chances for detonation, but can cause excessive combustion chamber temperatures, as well as resulting in reduced power output from the engine.

Detonation and Pre-ignition

The most important aspect in concern to ignition timing is detonation. Also known as knocking, pinging, or spark knock, detonation occurs when the combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition from the spark plug, but one or more pockets of air/fuel explode outside the envelope of normal combustion time for the engine. This creates a shock wave in the combustion chamber, which creates a characteristic pinging sound (often equated to dropping rocks or BBs into a Folger's can), and dramatically increases combustion pressure. Consequences range from inconsequential to complete engine destruction.
Detonation should not be confused with pre-ignition.
Detonation is most often caused by running too-low an octane of fuel for the given air/fuel mixture, or advancing the ignition timing too far for safe conditions.
Pre-ignition is a different phenomenon, though similar in behavior and sound (pinging noises). Pre-ignition occurs when the air/fuel mixture ignites before the spark plug fires. Pre-ignition is initiated by hot-spots in the combustion chamber, running a spark plug too hot for the application, or carbon deposits becoming heated from previous combustion events. Pre-ignition is heavily destructive, and in boosted applications can lead to melted and burnt pistons, and can lead to complete engine destruction.

Detonation Sensors

The Z32 comes equipped with a detonation sensor. It consists of a simple peizo microphone that enables the ECU to literally listen for the pinging sound detonation causes. When the ECU detects detonation, it switches to a "Low Octane" map that uses more retarded ignition timing than the regular maps. In a TT ECU, the wastegate solenoids are also activated, which lowers maximum boost pressure to around 7 PSI.

Setting Ignition Timing

Setting the ignition timing on the Z32 is simple but VERY vital to good performance and longevity for the engine.
Tools Required:

• Inductive timing light
• 10mm wrench or ratchet.
• 12mm wrench or ratchet.
• Short high-tension wire (Honda spark plug wires work great)
• A Consult interface (Nissan DataScan, Conzult, ECUTalk, etc) is greatly preferred, but not totally required.

Procedure

Image credit: Ash's fantastic writeup, The 6 P's.

1. Warm the car up to operating temperature. It must be totally warmed up and should be idling at a normal RPM (around 750 RPM). Then shut it off.
2. Disconnect Cylinder 1's coilpack connector.
3. Connect a high-tension wire to the coilpack, and the other end to the spark plug. This effectively creates an extension for Cylinder 1's coilpack.
4. Loosen the three 10mm bolts on the Crank Angle Sensor, but do not remove them. It should be loose enough to rotate by hand, but not so loose that it will spin by accident.
   
5. Start the engine, and again ensure it's idling normally and is fully warmed up.
6. If possible, use a computer to verify that the ECU is timing for 15° (BTDC). When fully warmed up and idling, the ECU will time for 15°BTDC, but in many cases, other factors can cause the ECU to set the timing elsewhere (for example, an improperly adjusted TPS will cause the ECU to modify the timing).
   • You can actually still set the timing if the ECU is sitting at, say 20°. It shouldn't be sitting at 20°, but the important thing is that the physical timing matches what the ECU is expecting.
7. Connect the inductive pickup of the timing light to the high-tension lead installed on Cylinder 1's coilpack.
   
   

   Note: This image shows the inductive pickup being attached to the coilpack connector harness. Most timing lights have trouble picking up a pulse from this, and often gives an incorrect reading when they do.
8. Ensure that the timing light is set for 0° advance (if applicable), and aim it towards the crank pulley from the 10-11 'o'clock position.
9. When the trigger of the timing light is depressed, it will flash each time the cylinder fires. As the engine rotates, the mark on the crank pulley passes by the markings on the cover above it. The timing light's flash allows you to see, for a brief instant, when the cylinder is firing in relation to the rotation of the engine, as the mark on the crankshaft aligns briefly with a number on the cover above it.
10. Gently rotate the CAS so that the timing mark on the crank pulley "lands" at the 15° mark on the cover above it.
    
11. Shut off the engine.
12. Tighten down the 3 10mm bolts holding the CAS to the bracket. It needs to be snug, but keep in mind that the bracket is aluminum.
13. Remove the high tension wire, and reinstall the coilpack like normal.

Other Methods to Read Timing

There are a few other methods often used and recommended for setting the timing.

• Using the coilpack connector as an inductive pickup. Many timing lights have difficulty picking up a signal from the coilpack connector, as they're designed for high-tension wires. Many have experienced inaccurate readings from this method, and this author has seen a 10 degree discrepancy.
• Using a PTU wire/loop as a connector. This often causes the same problems as the above method. The black loop on the PTU subharness is a ground and will cause the light to flash for EVERY cylinder firing.
• Using Consult alone to check timing (no!). A common misconception is that this eliminates the need to check timing with a timing light. The reality is, checking the timing with a computer is a great way to see what the ECU is timing for, but that doesn't affect the physical timing. The ECU adjusts ignition timing constantly depending on various conditions, but it does so assuming that it's getting an accurate reading on the position of the engine. Using a timing light is the only way to ensure that the ECU is seeing an accurate reading on the engine's position, but it's good practice to do this in addition to checking the timing with Consult to ensure the ECU is timing for 15°.

Difficulty Setting Timing

The most common difficulty when setting timing is getting the timing light to detect the firing signal through the inductive pickup. This is best resolved with a high tension extension, as noted above.
Another, less common problem, is the inability to set the timing to the specified range. That is, the CAS physically cannot rotate enough to get it to 15 degrees. There are three possible causes for this:

1. The car is not fully warmed up, or is not idling properly. In short, the ECU is not timing for 15 degrees, and the user is trying to set the timing to 15 degrees. The physical timing should match the ECU, not be set in spite of the ECU.
2. The timing belt is off one or more teeth, causing the engine's rotational position to be incorrectly read by the CAS.
3. The drive pin on the exhaust cam is worn or broken, either from the machining problem found on early '90 models, improper CAS installation, or another outside force.

An Explanation of the Z32's Ignition Timing (A War & Peace Essay)

There seems to be a lot of confusion regarding how the Z32 sets its timing, and how to "adjust" the timing.
In a traditional setup, a car has a distributor which rotates. As it rotates, it bridges a connection to points on the distributor cap--one for each cylinder, and that sends high voltage to the respective spark plugs. These setups allow you to change the ignition timing slightly by rotating the distributor cap, which advances or retards the ignition timing. You can check this setting with a timing light, in a similar fashion as described above.

However, the Z32 doesn't have a distributor. It uses electronic, computer-controlled ignition. While classic, distributor-based ignition systems used elaborate vacuum-variated and centrifugal-weight-based devices to "tweak" the ignition timing at different engine loads/RPM speeds, the Z32's electronic ignition is completely computer controlled. The ECU contains maps detailing where to set the ignition timing for different engine loads and RPMs. When you have your car tuned, a big part of what the tuner is doing is tweaking the ignition timing maps on the ECU for maximum power and safety.
At idle, under normal circumstances, the ECU sets the timing for 15° BTDC. This is why when you check the timing for the Z32, you're looking for 15° on the crank, and this is correct. Many wrongly assume that this number never changes, and that if it's not flashing at 15°, the CAS needs to be adjusted. However, a lot of times, the ECU isn't timing for 15°.

If, for example, the TPS is set incorrectly, the ECU might not be in "idle mode," and time for, say, 20° BTDC, meaning the ECU is trying to fire at 20° BTDC. When the owner checks the timing and sees it's flashing at 20° instead of 15°, he incorrectly assumes that the timing is off by 5° and adjusts the CAS so that the timing light's flash is occurring at 15°. Now when the ECU thinks it's firing at 20°, it's actually firing at 15°, effectively retarding the ignition timing by 5°.

This creates further confusion when owners believe they can check their timing by simply looking at Consult. Consult often reports 15°, so they think their timing is correctly set. The ECU might be timing for 15°, but if the CAS isn't in the right spot, the timing will be off. Checking with Consult only tells you what the ECU is timing for, not what's actually happening under the hood.

The CAS is not a distributor, it's just a sensor, and adjusting the CAS is not the proper method to "modify" your ignition timing--its only job is to give the ECU an accurate reading on the speed and position of the engine. The adjustment factor is only to ensure that this reading is accurate. The ECU is what adjusts ignition timing, so looking at just one of these factors won't tell you if your timing is right--you have to look at what the ECU is doing in addition to using a timing light to make sure what the ECU is trying to do is actually happening. If the ECU is (for whatever reason) firing at 25°BTDC, the timing light should be flashing at 25°.