Timing 101

[HITN94]

I want someone to thorougly explain all the aspects of timing to me on an LT platform. base, advance, total, what all types is there and how do you figure out what you've got?

[gregrob]

You really, really need to do some research and reading my man.

An LT1 is a small block chevy, period. Yes it has a different ignition and induction system but it is still a SBC.

And an SBC is an internal combustion engine for that matter, and they are all very similar in how they operate.

As for the terms you threw out there, I'll give you a little definition but you really need to read up on it.

Base This term probably comes from older distributor type systems where you set a static ignition timing, then you have a vacum, centrifugal, or electronic advance that adds spark at certain rpms... The LT1 does not operate this way. Since it is a OBD-I (Actually OBD 1.5 but don't get caught up on that term) or OBD-II the spark curve is infinitely adjustable and there's no need for a fixed "advance" to be built into the distributor. You want more spark at 2500 rpms, you type it in the spark table..
**Caveat here** there IS a small advance built into the LT1's PCM firmware that adds a little on top of what you command. This is 3-4deg at most and only noticeable at higher rpms in my experience.

Advance I've slightly touched on this in the paragraph above but to further elaborate, "advanced" timing usually refers to {ignition} timing where the spark happens BTDC (BEFORE Top Dead Center). The stock LT1 93-95 years commands 33 degrees of "advance" at WOT and high rpms, for example. This ends up being a little higher when actually viewed on a scanner or better yet an oscilloscope because of the reason mentioned above (hard-coded firmware advance ). This means the spark plug is firing (or being commanded to fire) 33deg of crank rotation before the piston comes to top dead center. The reason for this is simply if the PCM waited until the piston was AT TDC to spark, there would not be enough time to complete the burn and get the most out of the combustion process. Hence the reason for starting the combustion process early (advanced). Conversely, retarded would be commanding the spark AFTER the piston has reached TDC and begun to go back down.

Total boy that's a loaded term, but in the simplest terms Total timing would usually be Base + Advance in an old distributor type system. For example you set Base timing at 24deg BTDC and the advance adds 10, you have 34 Total. Again, not really applicable to the LT1 in the way the terms are normally used.

Some other things you need to be aware of when people talk about "timing" regardless of the engine are when they say the word timing, they are almost always referring to ignition timing. But "timing" simply means the space in time that something happens, and is usually relative to something else when talking about engines.

For some fun googling and more reading on this subject by people a hell of a lot smarter than me (and so I dont have to type anymore lol). Do a search for:
Ignition Timing
Cam Timing
Valve Timing


quickly:

Ignition Timing = When the spark happens
Cam timing = What ICL your cam is set on relative to the cam's LSA which indicates how much retard or advance the cam has ground into it, or how much retard or advance the cam is set with using an adjustable timing set.
Valve Timing = Valve events, I.E. when the valves open and close...

[Wooshie]

Good *STICKY* info right there.:)

[HITN94]

ok, yeah thats a good write up, thanks.

[HITN94]

yes, as stated that was a good write up. for the record i am very aware of most of what you mentioned. i think the way i should have phrased that is i was more looking for info on the amount of advance that you cannot change like you stated. and if you were looking on the tuner (tunercat is what i use) where all you would need to go to see what your total timing is and where all you would need to change it. the reason for this thread is when running nitrous timing is one of the biggest factors and just wanting to see if i can learn something i dont know about how the timing is controlled in the pcm on this platform.

[gregrob]

yes, as stated that was a good write up. for the record i am very aware of most of what you mentioned. i think the way i should have phrased that is i was more looking for info on the amount of advance that you cannot change like you stated. and if you were looking on the tuner (tunercat is what i use) where all you would need to go to see what your total timing is and where all you would need to change it. the reason for this thread is when running nitrous timing is one of the biggest factors and just wanting to see if i can learn something i dont know about how the timing is controlled in the pcm on this platform.


In Tunercats you would use the two main timing tables to dial back the timing for nitrous. I currently see about 4 degrees higher on the scanner than commanded in the tune. Just have to tweak it til you get where you want to be then spray it.