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I've known fro some time about the limits of the 8V stock turbo vacuum advance unit. I noticed how the car runs up against the limits of it's performance depending on the set ignition timing. If set at the spec location of 20 btdc the performance off the line is great, while the high end boost suffers. If you set the timing to 16 btdc, the off the line performance suffers a bit while the high end boost is much better - a powerful surge up to it's designated APC limit rather than a slow climb. With the timing set at 20 btdc, I also noticed the tendency of the engine to ping more under warm to hot driving conditions, thus making the boost experience all the more glum. Because consistent reliable boost was preferable (why even have a turbo car?), my choice was to keep the timing set at 16 and let the low end performance suffer. Poor gas milage too with this setting.
My reading of this site informed me of the few ways to improve the performance range of my car within the limits of it's stock configuration. I could install a water injection system, an electronic ignition control system, and the SPG9 performance A.I.D. vacuum advance unit. Any one or all three. The simplest initial option from what I could understand was the purchase of the SPG9 A.I.D. unit, which addresses the limits of the stock distributor vac advance unit. In other words, having the best of both the 20 and 16 btdc settings - good low and high end performance.
Then there is the cheap and mysterious DIY vacuum advance unit modification where you drill out the riveted stop bolt (a round bolt-like plug)on the turbo vac advance unit, thus allowing the timing to retard more under boost. A poor man's SPG9 A.I.D.. I had to try it. Removing the stop bolt was easy enough. After setting the base timing to 20 btdc, on my first test run I experienced the results of too much retard. Lots of psi, very little actual boost. Even if I advanced the timing more than 20 to lessen the retard, the boost was too unpredictable. Clearly there had to be a limit on the retard, thus the inclusion of some sort of adjustable stop bolt.
So I modified a 10mm bolt head to fit where the stock stop bolt had been. I filed down* one of the six flat surfaces so that when in place and rotated, you had a variable range (or degrees of retard) of where the stop came into effect - from none to unlimited. I set it for what I could estimate was 4 degrees of additional retard under boost. My aim was to have the best of both 20 and 16 btdc ignition settings. The first test drive was dramatic. I had achieved exactly what I intended! The best of both settings. Even in very hot conditions I achieve the boost I want, with no ping. (the IC obviously doing it's job)
I've yet to experiment to see just how far I can extend the range beyond this initial setting. Perhaps if I advance the timing at 22 and extend the vac unit retard range to 6, I may have an even better overall response. There's got to be a limit though. Anyone know the timing curves at play here?
* in order for the bolt head to fit within the space provided and so the unit will sit flush against the distributor body, you must also grind down the bolt head at an angle opposite the side serving as the stop surface. Two washers were also needed as spacers to raise the level of the bolt up to where it could be effective as a stop.
posted by 64.12.10...
, Fri, 5 Jul 2002 14:10:22
, Sat, 6 Jul 2002 00:23:27, Sat, 6 Jul 2002 12:01:53
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