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356-930 356-930 is offline
Acceleration Junkie
Join Date: Aug 2007
Location: Laguna Beach, CA
Posts: 263
Originally Posted by fredmeister View Post
Great thread guys.....
I am having similar issues with pinging last nite since I changed from a 0.8bar boost spring to a 1.0bar spring.
My car is a 79 930 with the dual port can, the boost retard is open to atmosphere. Vacuum side is connected as normal.
Checked timing and here is what I have which confuses me:
at idle of 1000rpm..... timing = (neg) 7 degrees BTDC VACUUM PORT CONNECTED
at idle of 1000rpm..... timing = (neg) 29 degrees BTDC VACUUM PORT DISCONNECTED
Honestly I was shocked because I thought that the 29 was aweful high at idle because as I rev to 4000rpm (NOT DONE AS YET, SO CANNOT REPORT RESULTS) I expect the mechanical advance to go even higher.
Now the safe limit from other posts I have read under full boost and high revs indicates 26-28 degrees max timing BTDC......I wonder what it will be when I run it up to 4000rpm with vacuum disconnected.
1) In short, how much more advance does the mechanical advance add by itself?
2) Are my mechanical advance weights frozen at idle to explain the above results so far?
The results you posted are the result of vacuum and absence of vacuum. The mechanical advance, whether working or frozen, has no effect at low rpm. Itís a centrifugal mechanism and dependant on rotational speed and speed change to act on timing. Donít know where it begins to move but not likely under 2,500 rpm.
Remember, the term vacuum advance is misleading. When a vacuum is pulled on the vacuum port of the distributor, it retards the spark. As vacuum decreases with rpm increase and/or engine load, timing advanced due to the loss of vacuum.
Based on your posted data, you are 7 deg advanced at idle with vacuum. Loss of vacuum moved timing to 27 deg or added 20 deg of timing which is as it should be.
Mechanical advance may vary from distributor to distributor but expect 18 deg.
Timing with full centrifugal advance in will then be 45 deg. If you hook up the boost retard (uses same line as the vacuum line) 8 deg of timing will be pulled out above 5-psi and net timing will be 37 degrees.
Based on the factory setting (29 degrees is marked on the crank pulley) 37 is a bit much.
I find it reliable on my í79 930 engine to disconnect the vacuum to the distributor, plug the vacuum line, run the engine up to 3500-4000 and advance the timing until the engine starts to stumble, back it off till itís running smooth again and lock it down. The point the timing makes the engine stumble is a nominal maximum timing level. With the vacuum hooked up again, suspect you will be close to 0 degrees.
If youíre not running boost retard, Iíd be nervous with timing in excess of 29 degrees at 4000, especially under boost with a potential of getting to 1-bar.
You should also be nervous if you donít have a handle on the engineís AFR under boost.
Old 04-10-2008, 07:58 PM
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