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I'll give it a shot:
If you are looking for exact values that work for someone else, the chart at Jouko's site has that information. X Timing @ Y Boost (bar & psi), initial timing at 19.5 BTDC.
You don't mention it, but I am assuming that you have disabled the pressure retard portion of your advance/retard unit. If not, you need to.
You will not need to keep changing the regulator setting, because it's purpose is to prevent any additional retard from occuring at and above a preset boost threshold, 15 psi seems to be the recommended setting and I also agree with it.
Every engine is going to behave a little differently. What you really should do, is map your retard/advance unit, especially if you haven't disabled the retard part it. You'll need an adjustable timing light, vacuum/pressure pump and an accurate gauge. I'd map both pressure and vacuum. This way you will know the characteristics of the mechanical part of your timing curve; this can't be changed, except for the initial timing setting. The unit will retard/advance from this base setting. Knowing this information will help you decide where to set your initial timing and give you a good idea of where to set the BTM. Mapping is a good idea, I once posted the results of my unit and it was significantly different from the specs that SAAB shows, it's always nice to know what you are working with.
Depending on your initial timing setting and the fuel grade used, you may not need any pressure retard until 3-9 psi of boost. The goals are to:
Run as much initial advance as possible, but probably not exceeding 22 degrees BTDC, 18-20 is probably more realistic.
Ensure that your timing is not less than 9-10 degrees BTDC at 15 psi of boost and that no more retard will occur beyond that point. Retarding timing beyond this point, to run higher boost, will not increase power.
Delay pressure retard until it is needed to offset detonation and retard only the amount necessary. The point retard is necessary will depend mostly on your initial timing setting and fuel quality. Although this is a goal, it's not possible using the BTM (unless, see below),because timing will be retarded as soon as boost is sensed, at a rate based on the BTM setting. Increasing initial timing allows the retard to start happening in a more advanced place than it would have happened if set to stock, 16 degrees BTDC.
I used to use a slight bleed, T'ed into the the distributor vacuum hose with a check valve. This allowed the 1st few psi of pressure to bleed off and not move the retard actuator until boost rose to higher point and still maintained vacuum advance. The unit was also modified to allow more retard than stock. You would still need the check valve if something similar was put into the hose to the BTM, otherwise you'd have a small vacuum leak. This would be especially useful if you found that you could safely delay pressure retard until 3-6 psi of boost.
Experimentation will be the only way that you will find what works best for your engine. I know that I have probably just made things more complicated for you, but I hope this helps some. Good luck getting it right.
Jouko's Site, BTM timing chart
, Wed, 4 Jul 2001 13:05:13
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