Was monitoring my intake temp today via my scan tool and noticed the timing advance was upwards of 35 degrees at casual acceleration. How far can it advance on a 99 c5? I thought I had read somewhere that the stock timing advance would not go that high, but then again I may have dreamed it. (boy i need a set of shop manuals.....I guess they contain that kinda info)

 

Is there such thing as vacuum advance on these engines, to add in extra advance at idle and other times.

Not a mechanical one, but using a MAP sensor or something?

 

All engine timing functions are controlled by the pcm. It's the MFWIC and uses the various engine sensors to call the shots.

 

I would say that's completely normal. I wouldn't be surprised to see 40 degrees advance at freeway speed cruising.

 

what got me interested was I was reading how high intake temps will retard the timing at a certain points. I am investigating CAI units and their effects on the motor. I discovered the timing advance situation while driving around watching the IAT temps and as questioned originally, how much "range" can the computer utilize in its timing management scheme?

 

On the same day i swapped over to my 2000 yukon and did a run monitoring the temps and timing. I swear I thought I saw 50 degrees of advance at one point. That to my un-educated automotive mind was the like seeing bigfoot or something....I didn't think that much advance existed...lol....so 40 is apparently possible. Now I also need to see if my scanner is confused and verify those numbers with another source. That is where the forum "tuner guys" can enlighten me I hope.

 

I will have to look at some of my log files, but low to mid 40 is not uncommon on my Escalade, however I never see them when I am driving my Corvette, but then maybe I am not just cruising in it.

 

Yup, even 50 is well within reason. I don't want to speak so absolutely because I haven't any experience tuning LS1/6 or GM V8s, but in most other applications I see something along the lines of:

Idle: 14-20 degrees, varies a lot by engine/head design.
Low speed (rpm) cruise: In the 30s
Freeway speed crusing: In the 40s
Moderate acceleration: Varies A LOT by RPM and specific load
WOT high RPM: Should be fairly steady in the mif 20s.

Timing will be the lowest at low rpm/high load. Like giving 50-70% throttle at 1500RPM or so might put you in single digit spark advance.

Again this is just general ballpark, and I have never seen GM spark advance tables so don't go taking any of this as absolute lol

 

This is interesting, I love learning about the LS engines, and I'm also still learning about the old small and big block engines, heck any engine for that matter.

It seems back in the day, before emissions stuff on a mechanical distributor, a vacuum advance could make a engine run smoother, with less throttle opening (set idle speed on carb) and also run cooler, say your in a long drive through Taco Bell.

However, when the emissions stuff came around, the vacuum advance was used as a "band aid" to help get the smog #'s down, such as no vacuum advance at idle, that along with low initial or base timing, engines ran sloppy, on brand new cars.

Supposedly the only time the vacuum advance would engage was at cruise.

The reason why was to get the NOX down at idle, if I understand, a less complete burn, lowered NOX, but then you had extra unburned gasoline, that then needed cats to burn off.

Anyways, I had been hooking up my vacuum advance wrong for years from what this old man that was a ignition guru said, for better idle and engine cooling, you need the vacuum source to be manifold, so it can get it any time the engine is in high vacuum, idle, cruise etc, vs port where you do not get the vacuum advance at idle.

Yeah yeah, why is this guy talking about old stuff in a LS discussion, well I'm thinking the basic idea applies to old and new, mechanical distributor ignition or modern LS ignition.

So about the high # of advance, in my reading on the old school stuff, say your initial timing is 14 degrees, your mechanical/centrifugal advance ads 21 degrees, and the vacuum advance ads 16 degrees, while cruising at light throttle on the highway, you could have 14+21+16=51 degrees of advance.

So maybe the modern stuff is similar, it just does it all by MAP manifold absolute sensors, and the ECM, no mechanical stuff like the old days, no weights springs to wear out, or distributor hold down clamp to get loose and timing be off.

 

The highest set value in the C5 base timing table (for a 2000 A4 car) is 41 degrees.
This assumes the engine is warmed up.
And is under crusing conditions, light throttle etc.


There are modifier functions that either add or subtract from the main table based on all sorts of sensor inputs.
The majority of the modifiers subtract timing, for instance the IAT sensor reading.

It's pretty interesting stuff.

If you want to learn, buy HPT or similar tuning software and have a look.

Ron

 

You're right. The same types of engine control functions are being performed, just now more reliably (less mechanical hardware, etc) and much more quickly. We continue to get closer and closer to the once unobtainable goal of: "Pick horsepower or fuel economy, you can't have both".
Btw, interesting you should mention distributors, the LS cam sensor is now located exactly where the distributor used to be located, and the oil pressure sensor remains in the same location as millions of old school SBCs. Architecturally speaking, after the LT1 fiasco, what was once old is now "new" again.

 

thanks for the input guys. I guess my readings are legit after all. After reading the posts and realizing I don't know anything when it comes to timing, I will take the advice of poster and do a little research on the web and software options. Thanks again for the knowledge.

 

Is it safe to assume that this software would be a source that would show all the points the PCM is monitoring and how those points effect the process (talking about the timing process here)......if so, that could be a very interesting learning tool to see all the points involved.

 

Yes. Shows them all.

Like Ron said, HP Tuners or EFI Live are a couple of the more well known, reputable, tuning suites out there today.

 

guess I need to expand my horizons and look into those items! I appreciate the direction.