Had Some "Fun" Today

[studigggs]

1990 K1500 5.7. I have never felt good about setting base timing. Always felt like I was just getting it close and going with it. This is probably fine, but it never sat right with me and I always questioned if my base timing was really at 0deg. Well, today I believe I got it right. I connected a pressure transducer and ignition probe to cylinder 1 and used a scope to set base timing. First picture is what I started with (basically where my timing light skills are) and you can see it was a couple degrees off even though it looked (to me) dead-on with a timing light. I guess this could also be a symptom of a stretched timing chain, which is likely after 30 years and 160k miles.

Second picture is after retarding a hair and you can see the ignition probe lines right up with TDC. With the scope I liked being able to hold the engine at 1,000 RPM, record data, stop the engine, review it, adjust, and test again. So much more relaxed. I wanted to calculate exactly how many degrees off TDC I was at the beginning, but lost the data file. Of course, after all that, it runs and drives the same, so probably a waste of time, but thought some would find it interesting.

Pic #1: Starting point Timing light. You can see I'm a little advanced. https://imgur.com/RdB7Y4w
Pic #2: After adjustment with transducer and ignition probe. Spot on. https://imgur.com/tu4737B

 

[Schurkey]

I connected a pressure transducer and ignition probe to cylinder 1 and used a scope to set base timing.

Interesting concept.

I've used the "magic wand" magnetic probe and an inductive pickup to set timing, (Snap-On "Lumy-Mag II" MT 1480) but never a pressure transducer.
Photo 1. http://hbassociates.us/Snap-On_MT1480_01.jpg

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First picture is what I started with (basically where my timing light skills are) and you can see it was a couple degrees off even though it looked (to me) dead-on with a timing light. I guess this could also be a symptom of a stretched timing chain, which is likely after 30 years and 160k miles.

Why would a stretched timing chain affect ignition timing when using a timing light? You'd just twist the distributor to compensate. Therefore, why would ignition timing set with a light, be different when set with a pressure transducer?

Second picture is after retarding a hair and you can see the ignition probe lines right up with TDC. With the scope I liked being able to hold the engine at 1,000 RPM, record data, stop the engine, review it, adjust, and test again.

1000 RPM??? IS THE ENGINE RUNNING?

If the engine is running, peak pressure is NOT going to be at TDC. It's going to be several degrees after TDC, when the air/fuel mixture has had some time to combust. Even if #1 isn't running, you could have some offset between peak pressure and TDC, depending on the response time of your equipment. I wouldn't guarantee that the ignition module doesn't change timing somewhat, even with the ESC wire disconnected. It's actually fairly common for a magnetic pickup-based ignition to retard about one degree for every thousand RPM, but that gets "cancelled" by the spark advance mechanism, so folks don't usually notice it. The slower the engine is turning, the more-accurate your system is likely to be--at least down to "cranking" speed.

You were better off with the timing light, IF (big IF) the timing mark on the damper aligns with "0" on the timing pointer when the #1 piston is at true TDC.

 

[studigggs]

Why would a stretched timing chain affect ignition timing when using a timing light?

Good point. It wouldn't. Rather, I would be compensating for user error (which is prevalent in my garage). Timing with a light can vary depending on what angle you are looking through the 'wedge" to the harmonic damper. With a pressure transducer, TDC is peak pressure, every time. Response time?...a pressure transducer is just a resistance measurement on a strain gauge, nothing digital about it. Channel 1 (ignition) and channel 2 (pressure) were measured with the same scope so the sync should be there or the scope would be useless for any diag. Yes, the engine was running. Interesting info regarding setting timing at idle wrt spark advance and RPM. Initially, I was taking recordings at idle and the timing would jump around relative to pressure TDC. When I went to 1,000 RPM, the timing was at a consistent location in the pressure curve. Maybe I'll mess around some more tomorrow.