This article is about why we need to check the rotor phasing, and how to set the mechanical location of the rotor button inside the distributor when EFI is being used. For training purposes Elite ECU terminology has been used but the same process can be applied to any ECU.
What Is A Distributor?
The job of a Distributor is, as it's name suggests, to distribute the spark from the ignition coil to the correct cylinder. In many applications it also contains the pickup sensor and triggering teeth to provide a signal when to fire the ignition coil. When a Distributor is used with a fuel injection system where ignition timing is to be controlled by the ECU, the signal is instead sent to the ECU.
A common Small Block Chev V8 MSD Distributor
How An ECU Performs Ignition Timing Control
For the ECU to be able to control ignition timing it requires the signal from the triggering system. The ECU then calculates the amount of ignition timing based on the tuners settings, and then varies the delay after the distributor signal has occured before sending a signal to the ignition coil to fire.
Quite simply if the trigger tooth occurs at 50 degree BTDC, and we want the spark to occur at 30BTDC the ECU simply waits 20degrees and then sends the signal to the coil.
Locking the Distributor Advance Mechanism
If you have ever inspected an older distributor with a mechanical advance system you will notive that the rotor button is spring loaded and moves. This is how the mechanical advance system works by having a set of spring loaded weights that move with the distributor RPM. As they move outwards they shift the sensor position relative to the triggering tooth which changes the ignition timing. When an ECU is preforming the ignition control the mechanical advance system needs to be removed and locked to one position.
If we look at how the ECU performs the timing control we can recognise that the trigger tooth needs to occur before the spark. You need to consider what is the most ignition timing you will ever command with your tuning and then set the position of the Distributor so that the trigger even occurs before this. So when an advance mechanism it to be locked it should be done so in the most advanced position it can be moved to before being locked.
Why We Need To Check Rotor Phasing
When the job of a Distributor is to distribute the spark to the correct cylinder, and when an ECU is controlling the timing of the spark, the two can sometimes not synchronise well with each other. It is possible for the ECU to send the spark out for a given cylinder but the mechanical position of the rotor button could cause the spark to either not go to the correct cylinder, or if the rotor button is not correctly positioned it could intermittently be sent to the correct cylinder for some events and to an adjacent incorrect cylinder for other events. This occurs when the Distributor was originally designed to directly fire a coil or ignition system, so the pickup teeth are aligned together with the rotor button.
In normal operation the position of the rotor button relative to each cylinders distributor cap terminal moves depending on the ignition timing. When the rotor button is in the correct position it will be a little to one side of the terminal for that cylinder for retarded timing, midway through the timing range it will point dead-centre to the terminal, and when there is a large amount of timing advance it will point slightly to the other side of the terminal.
The following diagram shows this correct movement range when the spark is being sent to Cylinder #3.
When the rotor button is not phased correctly the range of movement can have the rotor button pointing to a different cylinder terminal under some amounts of ignition timing. In the following diagram it is shown that under high advance the spark goes correctly to Cylinder #3, but with retarded timing it could instead go to Cylinder #6. This is what is called Cross-Firing and the rotor phasing needs to be adjusted.
An easy check to do to tell if rotor phasing is a problem is to set the Ignition Lock Mode to Always On, then try to command a very low amount of timing and see if this causes misfiring. If not try to command a high amount of ignition timing and check again. If there are no issues at these extremes then the problem may not be rotor phasing.
How To Set The Rotor Phasing
The rotor phasing should be checked any time the distributor has been removed or moved to an unknown position. To set the rotor phasing we need to do the following steps:
- Consider the real world ignition timing range we would like to tune the engine over. For example we may need 0deg (TDC) for engine starting, and as high as 40deg BTDC for light load conditions at high RPM. Find the mid point in this range, which for this example is 20BTDC
- Physically rotate the crankshaft so that Cylinder #1 is at 20BTDC. It does not need to be degree-perfect, just position as well as you can to this amount.
- Grab a paint pen of any type and mark on the distributor body directly under the middle of where spark plug lead for Cylinder #1 attaches.
- Remove the distributor cap.
- Rotate the distributor so that the rotor button points directly to the mark you placed on the distributor body.
- Bolt down the distributor body and do not move this again.
- Go online with your Elite ECU.
- Set the Ignition Lock Mode to Always On and set Firing Angle to a value that corresponds with a timing mark you have on your engine. This may be a simply TDC mark so the timing can be locked at 0.0 degrees.
- Connect a timing light to the spark plug lead for Cylinder #1
- Start the engine and check the ignition timing. In our example it should be firing the spark at TDC.
- If the timing is not firing at TDC adjust the Trigger TDC Offset Angle until it does fire at TDC. Do not move the distributor body to do this.
- Set the Lock Mode back to Disabled. The task is complete!
Now if we command 0deg or 40deg ignition timing the rotor button is always pointing to the correct cylinder terminal and cannot cross fire to the wrong terminal.
Adjustable Rotor Buttons
Sometimes it is not possible to set the rotor phasing and also have an appropriate TDC Offset Angle. If this occurs you may need to consider an adjustable rotor button. These allow the rotor button discharge terminal to be rotated to the correct position.
An MSD Adjustable Rotor
The process for doing this is a little different from the previous method. To set an adjustable rotor button:
1. Go online with your Elite ECU.
2. Set the TDC Offset to a suitable value. Around 50 is usually a good value.
3. Set the Ignition Lock Mode to Always On and set Firing Angle to a value that corresponds with a timing mark you have on your engine. This may be a simply TDC mark so the timing can be locked at 0.0 degrees.
4. Connect a timing light to the spark plug lead for Cylinder #1
5. Start the engine and check the ignition timing. In our example it should be firing the spark at TDC.
6. If the timing is not firing at TDC adjust the Distributor Base until it does fire at TDC.
7. Bolt down the distributor body and do not move this again
8. Set the Lock Mode back to Disabled.
9. Consider the real world ignition timing range we would like to tune the engine over. For example we may need 0deg (TDC) for engine starting, and as high as 40deg BTDC for light load conditions at high RPM. Find the mid point in this range, which for this example is 20BTDC
10. Physically rotate the crankshaft so that Cylinder #1 is at 20BTDC. It does not need to be degree-perfect, just position as well as you can to this amount.
11. Grab a paint pen of any type and mark on the distributor body directly under the middle of where spark plug lead for Cylinder #1 attaches.
12. Remove the distributor cap.
13. Loosen the two screws andr turn therotor button so that it points directly to the mark you placed on the distributor body.
14. Tighten the screws and put everything back together.
You are now set your TDC Offset and have Phased the Distributor correctly.