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Timing (was: I ordered my 155 SR 15 Vredestein Sprint+ today)
Posted by Jim Williams (yes, again!) (more from Jim Williams (yes, again!)) on Fri, 05 May 2000 01:11:27
In Reply to: I ordered my 155 SR 15 Vredestein Sprint+ today, rolf, Thu, 04 May 2000 15:14:13
First, Rolf, about that link to the Saabregistret page with the comparison chart on the various Sonetts: I pasted it in the link box but it didn't appear. I think the board automatically filters out URLs in the text, but perhaps I can slip it past if I note that you need to go to Aitch Tee Tee Pee colon slash slash www.saabregistret.se/data/utvsone.htm
Now, about that ignition timing stuff: Here maybe one of the experts will chime in and enlighten us, because I've just been through the searh for the ideal timing spec, too. I'm pretty happy with where I've got it now, but it certainly would be nice to avoid the voodoo ceremonies I had to go through to get there! There are SO many different variables including distributor type, emissions plumbing, etc., that it's hard to come up with a simple formula. What little I have learned so far:
First, when this discussion came up on the mailing list a while back, I think the consensus from the real experts was that the best procedure is to set it at 6 degrees BTDC at *cranking* RPM (with vacuum hose off and plugged, as with all numbers I sling around here.) Jack A., among others, pointed out that the centrifugal advance on some models of distributor kicks in at such a low engine speed that the spark will be advanced a bit even at idle. (I wonder if this is why the specs call for 6 BTDC @ 800 rpm for 95/96, and 3 BTDC @ 900 rpm for the Sonett...?)
However, I wasn't able to use this procedure because I do NOT have anyone to help me with my car! (I know, there are *lots* of jobs that are easier with a helper, but I just don't have any such people available. Someone here once suggested I bribe a neighborhood kid, but let's face it: in this day and age, that just isn't practical. I can just hear some kid yelling, "Mom, that guy with the weird car tried to get me into his garage again! He said he wanted me to crank it up for him because he couldn't do it himself, or something...")
Anyway, my solution was to set it to 6 BTDC at the slowest speed I could get the engine to run, and this spec seems to be working well. If I used much more advance, the engine would rev like crazy but wouldn't idle below 1500 rpm; very tiring for in-town driving. If I set it to the 3 BTDC @ 900 rpm called for in the book, it idled beautifully and ran smoothly, but wouldn't pull the hat off your head. The 6-at-slow-as-it-will-go number produces good power, doesn't knock, but will still idle (with just a bit of "farting," but that may be something other than the timing. Does anyone know the explanation of this phenomenon?)
I don't think this is a universal formula, though, because there are so many variables. For example, there are distributors with a single-chamber vacuum control and others with dual chambers that can either advance or retard; also, some engines still have their PVS (Positive Vacuum System) switch, which switches the distributor between low and high vacuum depending on engine temperature, while others don't have a PVS. As best I can figure out, the net effect is that you could set two Sonett engines to the exact same timing spec at cranking speed, and wind up with completely different advance curves at higher speeds!
Has anyone succeeded in fully sorting out this situation?
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