Hi all,

I was adjusting my timing and noticed that I'm getting 34° at idle with the EST plugged in. Timing was set to 12° BTDC so I backed it off to 6°, but the timing was still holding at 34°... I was under the impression that the computer only added around 15° or so at idle. Spark plugs look a bit lean. Here's my sensor readings in park using my Alltest Brainmaster:
Temp: 95 c
Baro: 4.6
Map: 4.1
TPS: .5
Dwell: 25-30
O2: varies between .3-.9
RPM: 800-900

I'm using a prom I burned that's basically the ZZ4 tune but limits the added timing to 32 degrees (I'm using vortec heads). Swapping to a different ecm only increased the timing at idle to around 40°.

Is there a way to test any of the timing components? What inputs does the computer use to increase timing?

Thanks,

 

Set your base timing with the EST unplugged. When the EST is plugged in, the computer readjusts the timing to where it wants it depending on engine operating conditions. If you check the timing with the EST connected, the results are meaningless.

Since there are not a lot of sensors on these cars, the computer only has a limited amount to use for inputs.

Throttle position sensor TPS
Engine coolant temp sensor ECT
Engine oil temp sensor (If equipped) EOT
MAF or MAP sensor
Knock sensor KS
Intake manifold pressure sensor (If equipped) IMP
Barometer sensor (usually incorporated into the ECM)

 

I disagree - something seems wrong as the last time I checked the timing it only brought it up to 20-25° at idle with EST plugged in. Also, the timing does not back off hardly at all when I rev it up.

 

The HEI module affects the final timing too. It can actually generate an offset.

The table regarding spark latency vs rpm corrects for this. Non stock modules can sometimes violate that table. My Pertronix module had a 5 deg offset.

If you do a search on spark latency in the DIY Prom section there's a fairly recent thread on this.

 

I double checked my timing mark and it appears that my balancer shifted enough that 8° advanced on my timing tape is actually 0°... Should I replace the balancer? Also, can anyone tell based on my spark advance tables what the computer should be adding at idle (77 KPA, 95 c and 850 rpms in closed loop)?

 

How did you perform the check on the balancer?

 

EFFFFFF abuncha "mark", "light", etc. for a minute.

Does it run good?

Does it run better in some/any way AT ALL if you twiddle the dist one way or the other? IOW, is the timing actually "wrong", ACCORDING TO THE ENGINE? Does the engine whup out "book" and "light" and look at "mark" and decide accordingly how it's gonna run? What makes you think YOU know better than THE ENGINE what timing it needs?

"If it RUNS good, it IS good".
— paraphrased from one of the GOATs in a totally unrelated field

Set up your table to where the engine runs the best it can with the dist set to whatever static setting you can consistently replicate; and ENJOY it. Nitpicking over whether the "numbers" in the table that it appears to take for it to do so, is a waste of cognitive effort.

 

I used a piston stop. Rotated it both ways and marked the balancer using a fixed point on the timing tab. After that I divided the distance between the points and made a final mark.

 

The reason I started down this path was because of poor off idle acceleration. I decided to check my balancer when I saw so much timing at idle. Even with 12° of advance my marks were off 8° so I was really on running 4° of initial timing. I'm going to bump up the timing according to the new mark and see how it runs. I still think 34° of timing at idle seems a lot but I'll see how it runs.

BTW I don't think there's anything wrong with wanting to fine tune the spark curve for my specific combo - especially given vortec heads don't need as much timing.

 

I think the 4th gen LT1's were running about 23-24 deg at idle (as am I with my Miniram).

34 is a bit high for idle, yeah.

My experience is mainly with the 90-92, so I'm not as familiar with the 85-89 stuff.

Does your calibration have a closed throttle spark advance table? I think that one can also influence idle timing. Also, you should have a value for base timing in the calibration (i.e., your distributor setting). That should match what you set the distributor to.

 

OK fine.

First thing you have to remember is, the engine doesn't give a single rat fart HOW the timing gets determined; all it cares about is WHEN THE SPARK OCCURS (i.e. the OUTCOME of the determination process, not the INPUTS to it). When this is at its best, the engine will run the best. The engine will tell you that it likes or doesn't like any change in timing at any given set of conditions, by running better or worse. It's TOO EEEEEZZZZY: just ask the engine what it likes, and make sure you give it exactly THAT.

Sooooo:

Measure your static timing so you can put it back EXACTLY where it is now. If you don't trust your timing "mark" crap, then just mark the dist physically. Restore it to normal ECM control mode afterwards.

Twiddle the dist one way or the other after this measurement. Either way (I suggest starting with advancing it), any reasonable amount, doesn't too much matter, except that you want to move it enough to create a significant enough difference (hopefully) that it's likely to make a noticeable difference in how the engine runs, if there is any noticeable difference to be made.

Are we clear so far?

One of 3 things will happen to off idle acceleration:

Off idle acceleration will get better
Off idle acceleration will get worse
Off idle acceleration will not change at all

Ignore ANY AND ALL other effects of the change. Ignore idle itself, full-throttle acceleration, cruising, etc. etc. etc. Pay attention to off idle acceleration, and off idle acceleration ONLY.

If off idle acceleration gets better, do the same thing a little more. Better/worse? If off idle acceleration gets better, do the same thing a little more. Better/worse? If off idle acceleration gets better, do the same thing a little more. Better/worse? If off idle acceleration gets better, do the same thing a little more. Better/worse?

If off idle acceleration gets worse, adjust it back to where it was, and a little more. Better/worse? And so on.

Keep doing this until no further improvement in off idle acceleration can be made. Zero in on PERFECTION in off idle acceleration.

If it does nothing at all, put it back where it was and leave it alone, because there's nothing to be gained in off idle acceleration by changing it in the table either.

Capisce?

OK, now that off idle acceleration is THE BEST it can possibly be as far as what you can get by just changing the spark timing, measure your static timing again. Figure out how many ° you moved it, in which way. You just discovered that off idle acceleration is at its best when the total timing (static + table value) is best when it's xx° more/less now, than it was when you started.

Put the dist back where it was.

Reprogram the spark timing values related to off idle acceleration in your table accordingly. Doesn't matter in the least what values are in there now, or whether they "look" "too high" or "too low" or what, either before or after; just make them ± xx° different, by however far you found that twiddled the dist to optimize off idle acceleration. Let the engine enjoy having the spark where IT wants it instead of where YOU think it should be.

Or, for all you know, you might find during your test drive, that EVERY aspect of engine operation, or at least a significant majority, are better with the dist twiddled to this new location. In which case, leave it there, and maybe, MAYBE, use the tables to alter any aspects of operation that got worse. After all, if you get (for example) better off idle acceleration, AND higher highway gas mileage, cooler running, smoother idle, and better ET, but less trap speed, then you might want to simply accept the new static setting, and adjust the highest RPM values in the table to have total timing more near where they started out.

This is EXACTLY how we used to tune a carb and a mech dist back in the day. Only difference being, we used springs & weights & vacuum cans instead of a table to tune the dist, and jets/rods/PVs/etc. to tune fuel. We MADE A CHANGE AND ASKED THE ENGINE IF IT LIKED IT BETTER. The engine is always willing to tell us that.

If twiddling the timing made no difference to off idle acceleration, or if off idle acceleration got worse in either direction from wherever it originally was, then it's already where it needs to be for best off idle acceleration, and the problem to your off idle acceleration symptom lies elsewhere. Most likely in the fuel map.

And, now that you've got a feel for how to ASK THE ENGINE what it likes, you can repeat the same process for any other aspect of performance that seems less than ideal, or even, you can just ... experiment. After all, besides the cost of gas testing it, it's FREE to dink around. Max HP, best ET, good cruising gas mileage, crisp throttle response, or whatever else strikes you as something you'd like to improve. Obviously a chassis dyno and a WB O2 makes this easier and quicker to at least get close, but the process is the same.

 

Timing is the first tuning change you do to an engine. Once the best timing is determined, you don't change it unless you make a major change to the engine. Jetting, plug gap, valve lash, fuel map, etc, etc, will not change where the engine wants the timing. If you upgrade the carb, throttle body, cam, heads, etc, then you will need to find out where the engine wants it's timing again. If you change the timing, you will need to fine tune the rest of the tuning again. Changing the fuel type normally requires different timing as well as different fuels have different burn rates.

As for the timing pointer, I had a similar problem many many years ago with I had a SBC. Kept bumping up the timing and the car kept going faster. Finally stopped advancing the timing when it was around 46*. Decided to dial in the cam with a piston stop and timing wheel to find out the timing marks were off by 8*. What I thought was 46* was actually 38* and I currently run it at 37* on my BBC. To fix the issue at the time since I was still using the distributor to change the timing, I ground off the timing pointer on the cam cover and installed an adjustable timing pointer. The balancer was new and had timing marks engraved all around the balancer. Adjusted the timing pointer to exactly TDC then readjusted the ignition timing. Never had a problem after that.

 

The engine does not care what some "pointer" "points" to.

All it cares is, when the spark occurs.

Sure, having a pointer that points correctly is a good thing. I'm not saying that it's not. But it's TOTALLY unnecessary in almost all cases. The engine does not care. It can't see it, and wouldn't know what to do about what it saw, if it could.

The whole "mark" "tab" "pointer" "light" "Book" "spec" notion is built on the idea that you ALREADY KNOW what the timing should be, and you're simply trying to set it to THAT number. Thing here is though, you (OP) DON'T know what the timing "should" be; therefore knowing what it is, while ... interesting maybe ..., is USELESS. It's a little like having a GPS that tells you that you're at 43°17'31.24" -85°26'51.44" (interesting, maybe) but doesn't correlate that to a database of roads that can tell you what turns you need to make to get to the grocery store. USELESS in virtually ALL situations.

OP, don't get all hung up on "mark" and "tab" and all that. Get your TIMING right, so that the spark occurs when THE ENGINE wants it to. Piddling around with the "mark" won't do a damn thing toward making THE ENGINE happy. It's a shiny thing off to the side of the path that leads you into a distraction from your purpose. Your purpose is to make your engine run right, not to generate pretty but useless numbers.