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Discussion Starter · #1 ·
Is this normal? I've been playing with my recently acquired LS1M almost non-stop for the last few days, and it's brought up a few questions.

It's telling me that my timing during normal, 70mph driving is between 30-40°...then I floor it, and it drops all the way to 8???

I am getting a few degrees of KR (just put in TR-6s with the 3.4", Wizair, and ubend/res delete). I'll test out plug wires when I get a chance. I just bought the car a month ago and they've probably been on there for the 69k previous miles. Normal driving stays at zero, but WOT gets up to 5 or 6° sometimes on 93 octane. Any other thoughts?

Also, during warm-up, after the car sits overnight, as it's idling, I get almost rythmic misfires. Idle idle idle idle grumble idle idle idle grumble.

Hmmm, I'm sure there will be more questions soon. Any help is appreciated.
 

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All vehicles will increase timing if cruising. If you remember the old vacuum controlled distributers, increasing vacuum would increase timing, as soon as you stepped on the throttle, vacuum decreased and so did timing. The computer mimics this behavior.
 

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mine cruises with about 40* of timing too. I think they do that for emissions. I you light the mixture early, more of it will burn, and less raw fuel will come out the exhuast.

I don't know if that is why they cruise with so much timing, but it makes sense to me.
 

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1998 GTP coupe. All stock and rust free. That will change soon.
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kubiache said:
Is this normal? I've been playing with my recently acquired LS1M almost non-stop for the last few days, and it's brought up a few questions.

It's telling me that my timing during normal, 70mph driving is between 30-40?...then I floor it, and it drops all the way to 8???
Perfectly normal. During closed throttle / part throttle cruise, timing is advanced to improve emissions and throttle response. When you floor it, you have increased engine load and the PCM uses different parameters to set timing.

I am getting a few degrees of KR (just put in TR-6s with the 3.4", Wizair, and ubend/res delete). I'll test out plug wires when I get a chance. I just bought the car a month ago and they've probably been on there for the 69k previous miles. Normal driving stays at zero, but WOT gets up to 5 or 6? sometimes on 93 octane. Any other thoughts?
Sounds normal. With your mods you may need a 180 tstat, a 3" DP, and maybe rockers to eliminate KR.

Also, during warm-up, after the car sits overnight, as it's idling, I get almost rythmic misfires. Idle idle idle idle grumble idle idle idle grumble.
Your TR-6 are 2 steps colder so they are more likely to have minor fouling until the engine reaches operating conditions. It will help if before you shut off the car, give a couple revs to "burn off" any fouling or drive a little aggressively before parking to clean the plugs.
 

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1998 GTP coupe. All stock and rust free. That will change soon.
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no, it gets the plugs hot to burn off any deposits that may have formed.
 

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Discussion Starter · #7 ·
No N20 said:
Your TR-6 are 2 steps colder so they are more likely to have minor fouling until the engine reaches operating conditions. It will help if before you shut off the car, give a couple revs to "burn off" any fouling or drive a little aggressively before parking to clean the plugs.
Will this always be the case then? Or with more mods, making the plugs more appropriate, does it taper off? Not a big deal either way.

I had a 180° t-stat at the time too, forgot to mention. 3" dp will be soon, since there's a set of PEMs from Pector on the way.

I'm just still getting used to boost and the many problems that come with it. :D
 

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Don't rev the engine just as you are shutting the key off. It won't "heat" the plugs any more than idling and will allow unburnt fuel into the cylinder.
 

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1Hot97 said:
Don't rev the engine just as you are shutting the key off. It won't "heat" the plugs any more than idling and will allow unburnt fuel into the cylinder.
Is this the cause of the so called misfire then, because my car does this also, I am also running the TR6 plugs??? If a rev or two will not help, what can be done to correct this, or does it really matter???
 

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If you are close to stock, you probably shouldn't run colder plugs.
 

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Ive got about the same mods as you, U-bend/res delete, 3.4" pulley, 160* t-stat, tr6 plugs, FWI, boost bypass mod, TB spacer, removed the TB screen and my PCM should be here in about a month. I'm also planning on much more this winter, I just got the car in april so I think I will leave the TR6's in
 

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1Hot97 said:
All vehicles will increase timing if cruising. If you remember the old vacuum controlled distributers, increasing vacuum would increase timing, as soon as you stepped on the throttle, vacuum decreased and so did timing. The computer mimics this behavior.

The vacuum port for distributor advance was above the throttle plates so as air flow increased so did the vacuum. It would suck on the distributor diaphragm increasing advance as the motor sucked more air.
 

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This was copied from a hot rod site: (sorry for the long post, but he said it better than I could have)

"Check out this clipping:

<<It was written by a GM engineer.

As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more."
 

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WOW! That was good info.Thanks
I cut this out of your post.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more."

This is all I ever had before 88 when I got my first fuel injected car.
 

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Glad you liked it, hope you understood that the vacuum advance actually never advanced anything.
 
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