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timing under boost Posted by Notnoel [Email]  (#23) [Profile/Gallery]
(more from Notnoel) on Fri, 14 Apr 2017 14:00:08
In Reply to: Re: Plug gap, timing, and boost, Oh my! (long), Simon S, Fri, 14 Apr 2017 11:09:54 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Messing around with the spark plug gap is really not the way to go at this. You are seeing effects in your experimentation, but you'll never get there because the timing is not static, but dynamic across the boost range. Also, I'm not sure what Simon is getting at by "the timing control of this car consists of a vacuum bottle that retards timing a little at slight boost."
The timing retard is controlled by the small capsule that sits on the distributor. For an engine at factory settings (16 degrees BTDC), it basically retards the timing to 10-11 degrees BTDC under full boost at about 10-12 psi for stock setting (you can go higher with APC tweaks - I run at about 18-20 psi, but you still want peak retard of about 10-11 psi at full boost). As the boost pressure increases, it puffs out a diaphragm inside the capsule and that pushes a small arm inside the distributor an retards the timing (its the mirror immage of a "vacuum advance").
So, if you advance the timing to 18 degrees BTDC, the capsule is only going to retard the timing to 12-14 and you are not going to get an efficient boost. You can however, bend the stop on the little arm that moves within the diaphragm so that it will then travel all the way to 10-11 and get an efficient spark at that boost. Someone else came up with the idea of removing the stop, trimming one side into a cam, and creating an adjustable stop on the capsule.
The next problem that then arises is that Saab put an anemic spring into the capsule and it does not deliver the boost smoothly across the entire range. It tends to dump all of that retard at about 3-5 psi, moving the retard too quickly to 10-11 degrees and creating a "flat spot" in your acceleration curve. That can be addressed by replacing or supplementing the diaphragm spring. A company called Group 9 used to remove the spring and replace it with a stiffer one and that radically improved the performance of the device. Brad at KC Saab did something similar.
That modified mechanical device (which does something similar to the MSD, which is all electrical)is called an AID (for "advanced ignition diaphragm or device), but these were made a decade ago by Group 9 and are nearly impossible to find anymore. I did some tinkering and came up with a way to supplement the stock spring so that it wasn't necessary to cut open the capsule and get the same results; I also incorporated the cam idea. You can make one yourself in an hour or so if you're good at that sort of thing - see the link below. I also incorporated the cam adjustment idea that I found while researching the subject.
There's a lot of really good research and discussion on this topic on the performance board. Search "AID" and capsule, and diaphragm and you'll find most of the research that led me to my little invention. Ultimately, a new old stock Group 9 AID came up on FleaBay for $100 and I just had to have it to satisfy the purist in me (and it is prettier than mine). Both did the trick.
http://www.saabnet.com/tsn/bb/performance/index.html?bID=112282" target="_blank">Link to my homemade AID (read the whole thread)
->Posting last edited on Fri, 14 Apr 2017 14:13:53.
1
, Fri, 14 Apr 2017 13:55:06
, Fri, 14 Apr 2017 14:42:431, Fri, 14 Apr 2017 14:00:08 <-- Viewing This Message
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