Timing is everything

6 August 2014  

We get a lot of work from the motorsport community which often means working with unconventional systems and a lack of information. One such example this month was a non-starting 205 GTi, suffering from a lack of compression. However, the engine had been fitted with a flywheel from another vehicle so that an aftermarket ignition system could be fitted. Before the rebuild, the owner wanted to set up the ignition ECU, so a replacement distributor was fitted – this had been locked and the vacuum capsule removed to prevent any centrifugal advance.

Gathering data

An ECU requires certain information in order to calculate the ignition timing, i.e engine speed, engine position, engine temperature and engine load.

We needed to work out which flywheel type had been fitted so the ECU can calculate engine speed. Using the oscilloscope and counting the number of sine waves answered that question, 60-2. Using live data we could now confirm the correct cranking speed. Engine position is an altogether different proposition. The flywheel could be fitted in any position as the dowels had been removed when it was lightened and the owner wanted them replaced when the engine was rebuilt. We needed to program the exact reference point into the ECU but the problem was how, without any timing marks and without knowing the position of the flywheel. This is where the pressure transducer proves its worth.

We knew it would identify TDC for cylinder 1 as this would be the highest point on the waveform. The missing tooth signature should be before TDC to allow the ECU time to calculate the timing. Using the cursors, we were able to measure the number of degrees before TDC and program the details into the ECU.

Using 0? advance as a starting point we could check the ECU was providing the spark at the correct time. This of course may require a small adjustment of the distributor, in this case, 1.7?. Figure 1 shows how this was done.

With the distributor in the correct place, the base map was reinstalled and the engine burst into life ready for mapping on the rolling road.

Find out more

If you are familiar with using a pressure transducer you should have noticed the engine has low compression. If you would like to know more about ignition system analysis or using pressure transducers to aid diagnosis, contact Andy at Got Boost on 01656 670741.

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