I want to adjust the timing on my engine, maybe advance it just a bit. I don't have a timing light. A friend told me something about advancing it 'til it knocks, then retard a little. Is it that simple, or do I need to use the light?

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Take 'er easy, and if
she's easy, take 'er twice!!!

 

Hum,

You can do it "by ear", and I've done it that way more than once. However, even assuming the timing mark on the damper outer ring isn't correct, it's always nice to be able to see just how far you have moved the timing so you have a reference if you have to adjust it further.

One problem arising from setting the timing without a light and scanner is that the ECM/EST and detonation sensor will retard the spark in steps to prevent engine damage from detonation (knock). This can turn your timing advance into a lower-performance modification, since advancing by just one degree too much will cause the EST and ECM to retard the spark in about four degree steps. Once detonation is detected, the ECM will continue to retard spark in steps instead of a degree at a time. You could potentially lose up to eight degrees total advance if you advance two degrees too much. The only way you would know that would be through reduced fuel efficiency and lower ETs (unless you have a dynno handy).

Of course, the ECM can only retard so much, so there is a point where the ECM can no longer protect the engine from detonation, but that would require a base timing way beyond 15° on most systems.

Set it by ear if you must, but at least try to make some sort of reference mark so that you can adjust it from that point if necessary.

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Later,
Vader
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"I'm'a do Things My Way - It's My way or the Highway."
Adobe Acrobat Reader

 

You might wanna look around at a few garage sales in your local area. I found a used timing light for $5 and it worked perfect. You can also buy a new Craftsman for like $50, thats what I did as soon as I got the money.

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1987 Chevy Camaro IROC-Z
L98 TPI 350 (5.7L)
TH 700R-4 Transmission
Borg Warner 7.75" 9 Bolt Rear End

Current Mods: LT4 Hot Cam, Comp Cams 1.52:1 Roller Rocker Arms, Edelbrock TES 1 5/8" Headers, Hooker 3" Aerochamber Cat-Back System, Performance Resource Chip, Accel Ignition Coil, Cap, Rotor, 8.8mm Wires, K&N Filters, JET TPI Air Foil, All Free Mods, Falken ZIEX Z-Rated Tires.

Best ET (w/o LT4 cam): 14.32 @ 97.7mph
(corrected for elevation)

7.5" 10 Bolt with 3.42s soon to come!

 

It's been my experience that playing with the ignition advance is only the first step down a long, long road of other engine modifications.

Might as well bite the bullet and buy a timing light because it'll pay for itself in no time.

You can rest assured, it won't be the last thing you buy for that 'sucka' either.

Jake

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1986 Corvette Coupe, 415 CID, Edelbrock 6073s, ZZ9

 

Jeff (Vader) hit the nail on the head. While the GM eprom is FAR TOO CONSERVATIVE on its spark advance; advancing it just a little can cause you to loose a lot of advance. Worst, once knock retard is enabled, it "lingers" quite awhile PLUS I just discovered it "pulls" you max timing out until you have either driven for over a couple of hours or turned off the engine.

I just discovered this problem this weekend when I booted my car (had a little knock retard) and saw it pull 3* out of my "highway mode spark adavance (GM's hidden code that needs to be modified to get it to work)".

I suspect this is a part of GM's "Octane" code that is used to detect lower octane gas. So there is in fact two retards in the GM eprom: 1) Short term ... that lingers for about 5-10 seconds...just enough to screw up a 1/4 mile run and 2) Long term that is designed for the open highway. I am looking for a way to modify this "long term".

The short term is easily handled by adjusting the "attack/decay" rates within the eprom...but I can tell you that you must be careful. Or you will turn that "inaudible knock" that is over compensated in the eprom into an "audible knock" that is NOT compensated enough.

But, just to let you know how far you can go when you get it tuned properly in the eprom. I now run 34* WOT and only loose a couple degrees when knock is detected and I get far more power (32* effective). And with Highway mode I have ran an AF ratio of 17.4:1 with 46* spark advance on an 8* grade with NO KNOCK.

I am still experimenting with that and I will be trying 18:1 with 47.5* spark advance. I suspect this is the maximum point of leaning/advance. It is amazing how much advance (and leaning) you can do in part throttle cruise. Every time I think "this is it, this MUST be the maximum"; I find I can go further. The net result is a massive increase in fuel economy with no effects on WOT...different part of the eprom programming with no effects on part throttle. You CAN have your cake and eat it when you fiddle with the eprom.

 

PS: When I have looked at most TPI engines with Diacom, I found that they get only 29* spark advance at most and generally trigger 5-8* of spark retard. Your effective spark is only 21-24* and then the ecm will pull a further 2-4* for "octane". A typical L98 TPI is lucky if they are getting much over 20* effective spark advance. No wonder those Mustangs keep up so well.

Compound this with GM's overly rich eprom and you have a much slower car than it really should be.

 

I read recently that the LS1 won't go back to normal timing once it's detected knock until gas is added to the tank.

I know it sounds crazy, but the tech article said that the timing will remain in the retarded state until the gas tank is refilled.

I've watched Diacom at work on my L98 and found that the ECM will return to normal timing after knock retard has taken place, but the timing is put back in a lot slower rate than it was taken out.

And, yes, the ECM always pulls out a lot more than is actually needed to kill the knock. That coupled with the slower rate of timing being put back in will kill power on that particular run.

For example, if the ECM pulls out 8 degrees of timing because of knock, once knock ceases, it will take several seconds for all that timing to be put back in by the ECM.

Jake

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1986 Corvette Coupe, 415 CID, Edelbrock 6073s, ZZ9

 

Jake, I cannot say how the "low octane" code works on MAF, but on SD after you reach a "threshold" kpa and encounter knock, you do loose upto 9 degrees long term. This is completely different than the "short term" knock sensor that relies on the "attack/decay" rates.

With SD cars, the stock eprom pulls timing out "long term" if the knock occurred with any load over 70 kpas. There are also multiplier tables involved both on kpa and rpm. But it manipulates the "base" timing to give you less than full timing after a knock incident with more load than 70 kpas.

 

The only results I've seen are for MAF systems using Diacom. My traces and those sent to me by others (who were trying to diagnose problems), all show about the same thing: timing is taken out quickly but returned very slowly.

The system seems to remove timing in about 4 degrees increments and one trace I have has the system pulling 20 degrees of timing out. I believe the PROM had been modified to allow this much removal, since I think the stock PROM has a lower Max removal setting.

Jake

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1986 Corvette Coupe, 415 CID, Edelbrock 6073s, ZZ9

 

The short term knock sensor "attack/decay" should recover in 10-20 seconds. You can monitor the effects by adding your base timing to your relative. This usually is smaller than the "Total" by the amount of Knock Retard.

The long term "octane" affects the maximum allowable in the "Total". If you monitor it long enough and "reconcile" the displayed "Total" to all the various "spark tables", you will notice a few degrees mysteriously missing. I know my "spark tables" quite well now, and that is how I noticed it when I KNEW that I should have been running 45.9* in HMSA mode yet I was only getting 42.0*. That is when I tracked it down to the "octane spark retard code".

I've just "defined" the various "constants" with my TDF Editor to make a "special" TDF to play with these values and hope to figure out how to "modify" them to alter the effects of this code.

I can appreciate what GM intended with this code (allow you to safely run "bad gas"); but I would like to alter the way the knock sensor is behaving to more the way I want it too - a safety feature but with speedy and full recovery.

I am POSITIVE that similar codes exists in the MAF system. Just buried somewhere waiting to be found.