Swapping the timing sensor leads (from old site)

<TABLE cellSpacing=0 cellPadding=0 width="100%" border=0><TBODY><TR vAlign=top><TD>Swapping the timing sensor leads

contributed by Rich Russell
A useful gain in low-end torque for free? Read how....


The timing of the spark in each cycle is critical to a petrol engine - it needs to be adjusted to be slightly earlier or slightly later depending on engine speed and load. This is necessary to make sure the fuel is ignited at such a point to produce maximum power, whilst preventing an uncontrolled burn which could damage the engine, or other undesirable effects.

The ECU performs this function, by looking up in a table when (in degrees of rotation after the engine passes a certain point) to discharge the coil to generate a spark. The spark voltage is directed to the appropriate spark plug by the distributor.

The ECU measures the manifold vacuum by means of a diaphragm and magnet inside it's casing. This tells it how much load the engine is under. Less vacuum implies heavier load, and so the spark must be slightly later to prevent detonation. This delaying of the spark is called retarding the timing.

It determines the engine speed by means of an inductive sensor above the flywheel. The flywheel itself has a series of teeth around it's edge - 40 in all. As these pass under the sensor, a stream of pulses is generated, the number of which occur in a given time gives the engine speed (this signal is also used to drive the rev counter, and operate the fuel pump). The faster the engine, the earlier the spark is needed - ie advanced. The sensor for this can be seen in the second picture, removed from the engine, and it's location in the car above the flywheel in the third (not very well though!)

The GT Turbo also has a 'pinking detector' (basically a microphone on the cylinder head), which if the ECU detects this problem, will cause it to automatically retard the ignition 4 degrees for a short while to prevent it, at the expense of power. You can see how these are all connected to the ECU in the first picture.

A known point in the cycle is needed to reference the delay worked out by the ECU (from the vacuum and speed factors) so the sparks occur at the right point. This reference is known as TDC - or Top Dead Centre. It is when the piston in cylinder 1 is at the top of it's compression stroke. So the baseline for timing is that spark 1 fires at 0 degrees after TDC, then the next spark (3) fires at 180 degrees, then the next (4) at 360 degrees past TDC and finally the last one (2) at 540 degrees. The advance or retard is applied to these basic positions, to get the exact point.

In the Renix system fitted to the R5, it determines when TDC happens using the same sensor as is used for engine speed. The flywheel has two teeth that are longer than the normal ones. These represent 90 degrees before TDC and 90 degrees after. The ECU will expect a speed pulse to come, but there will be a gap at these two points on the cycle, so it knows from these 'missing pulses' that the engine is either at 90 degrees before or after TDC. In fact it doesn't actually matter which of these points it is at - because the timing advance or retard will be the same for all four cylinders, so as long as it generates a spark when it gets the 'missing pulse' (plus or minus the delay for engine conditions), and another 180 degrees later, all will be correct. The fact that there are two rather than just one marker for TDC is due to the fact that it takes two whole rotations of the engine to fire all four cylinders on a four-stroke engine, with a spark occurring every 180 degrees of rotation.

So each time a 'missing pulse' is detected, a spark is produced. You can see these teeth on the flywheel in the fourth and fifth pictures. Note there is a second toothed ring with smaller teeth behind the timing one - this is for the starter motor to engage.

If you were to move the baseline back a bit - ie advance the timing for all conditions, you would gain a bit of power, assuming that the fuel is able to handle that advance - this resistance to uncontrolled burn is the octane rating of the petrol. The GT Turbo was designed for 97 RON leaded petrol. So if you run 95 RON premium unleaded, you should retard the ignition slightly to prevent pinking. If you run a higher octane fuel, such as Shell Optimax (around 99 RON) or add an octane boosting substance to the petrol (such as Castrol Valvemaster), you can safely advance the ignition slightly. Ever wondered why dragsters with 5 litre engines can generate several thousand bhp? They run on fuels with octane ratings much higher than unleaded petrol, with severely advanced ignition (and massive superchargers)!

One way to move this baseline is to physically move the TDC sensor forward a little. Unfortunately the standard one is fixed. You can buy an adjustable version from the main tuning companies though. However, on a high-boosted turbocharged engine, there's another factor to take into account- heat. At high revs (ie high boost pressure), the inlet air is very hot, which can lead to detonation. So ideally you don't want too much timing advance at high revs.

By an amazing stroke of luck - you can achieve about a 4 degree advance simply by reversing the wires on the TDC sensor!

The easiest way to do this is by cutting the wire (shown in the first picture) to reveal the red and white wires within. Crimp/solder bullet or spade connectors onto the ends (and insulate well) - then you can swap the wires when you want to, but still have the option to swap them back if you have to run on lower octane fuel. See the sixth picture for this.

You should find that the car is more responsive at lower engine speeds - I have yet to do a before and after comparison on the rolling road, but the result is confirmed by many people. You will find that in the region before the turbo spins up (around 2500rpm as standard), the car doesn't feel as weak, so you should have more confidence in pulling away from junctions and traffic lights.

How does it work? My explanation may be inaccurate in places, but I'm certain that the general idea is correct!

The signal from the timing sensor is a sine wave type shape whenever a tooth passes under it. By inverting the wires, you invert the waveform, so what was a peak is now a trough etc. The ECU it would seem looks for peaks above a certain threshold to count as a pulse. So when the longer 'missing pulse' teeth come round, the previous trough (now a peak) is counted instead of the actual start of the longer tooth. So the TDC time is detected as being half a tooth/gap section earlier - which happens to be around 4 degrees of rotation.

Extra information

THE MASTER
"The advance is 4.4 degrees on the initial timing. It's something we used to do to improve driveability out of corners (makes it more responsive) on the cup cars. But that was at a boost setting of 1.0bar and as the AEI retards the timing the more boost you have I'm not sure if its a good thing if your running around 2.0bar if the fuelling is not set up correctly (just makes the fuelling more critical with a high boost setting).."


Make sure you use high octane petrol with this modification, or you risk damaging your engine. Super unleaded, Optimax or additional octane boost are recommended. (Article revised Mar 2007)
</TD><!-- Spacer --><TD width=15></TD><!-- Images --><TD width=200>https://www.rtoc.org/articles/i/timing/diag_2.jpg

diagram

https://www.rtoc.org/articles/i/timing/sens1_2.jpg

sensor1

https://www.rtoc.org/articles/i/timing/sens2_2.jpg

sensor2

https://www.rtoc.org/articles/i/timing/fly1_2.jpg

flywheel1

https://www.rtoc.org/articles/i/timing/fly2_2.jpg

flywheel2

https://www.rtoc.org/articles/i/timing/conn_2.jpg

connector


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