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fredmeister fredmeister is offline
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Join Date: Oct 2002
Location: Baltimore, MD, USA
Posts: 2,232
Originally Posted by 356-930 View Post
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.
This is exactly what I stated in my post, except I see 22 degrees of advance without vacuum connected because it runs right up to the 29 degree timing mark, I guess this is still within tolerance of 20 degrees which you said was normal.
But what bothers me is the fact that with 18 degrees allowable in the mechanical advance, this puts me at 47 degrees BTDC at 4000rpm with my current set up.
Now if I hook up the boost retard line and get back 8 degrees that leaves 39 degrees total under boost conditions. This still is about 12 degrees too far if you target 27 degrees BTDC at 0.8-1.0 bar boost which from previous posts on the forum I see as safe limit. Is 27 degrees too conservative or can I run more safely? What is a real safe timing limit above 27 degrees I hope.
In order to get here I need to run 5 degree ATDC timing at idle.....and from my experience the driveability of the car at low rpm is absolutely horrible at this setting. I have even run around 0-1 degree BTDC before and this is the reason I am now at 7 degrees BTDC.......
I cannot see how the factory set the car to run at 5 degrees ATDC and got the car to move out of its own way at low speeds.
Last season when I was running the 0.8 bar spring at these same timing settings it ran well and I never heard pinging. I just wonder if it was still happening just not audible enuff for me to pick up on it, or if something has changed in my distributor over the past few months.
Need to try teeing the vacuum line into the boost retard fitting on the dizzy can and at least getting that amount of retard back....thanks for the tips.
1979 930 Turbo....3.4L, 7.5to1 comp, SC cams, B&B intercooler, Snow Perf water/meth injection, Rarlyl8 headers, Garret GTX turbo, 36mm ported intakes, Innovate Auxbox/LM-1, custom Manually Adjustable wastegate housing (0.8-1.1bar),--running 0.7bar max
---"When you're racing it's life! Anything else either before or after, is just waiting"
Old 04-11-2008, 09:08 AM
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