Timing VS. TDCOffset

[Rodd]

I have been giving this some thought and have decided to post it here for opinions. At the other place GMCTD posted the facts/mysteries about TDCOffset. Although it was helpful I don't know if I fully comprehend it. I actually started thinking about this about 6 months or so ago when I posted my data from my scanner and GMCTD said the timing was a little off. I'm thinking I may have been wrong in the process.

After replacing my IP I set the TDCO at -1.93 and then set my timing. Could that make the timing be off? What if I had set the TDCO at the stock -.50 to -.85 range, then set the timing and then drop the TDCO to -1.5 to -1.93 and not reset the timing. Would this change the timing or keep it where it was set.

Thanks!

 

[Turbine Doc]

Yes, way I work it on a unknown truck is to do time set command and see where I am, 0 desired with 3.5 avg when in time set should be where you are initially if at factory TDC offset.

If necessary adjust IP if reqd, so that truck time 0 desired and 3.5 avg actual when in time set, actually I shoot for an avg. of 3.8 in time set, which nets -1.3/-1.95 TDCO after commanding the learn using ko/ko in a OBD-II truck.

If you have a T2 tool, this 3.8 deg avg actual in time set mode gives you a "window" of TDCO if you rev engine slightly and command TDCO learn, then PCM can "capture" an advanced TDCO setting.

I've not played with OBD-I myself so this info is what I do with a OBD-II

 

[Rodd]

Thanks Tim. I knew you would chime in. I haven't check my timing since I installed my new IP back at Thanksgiving. I need to check it and change it if necessary. So basically I should check my timing and if the avg is 3.5 - 3.8 I'm good. If it is higher I need to move pump to passenger side if lower toward driver side. Once it is set where I want it I need to do the TDCO. Is this right?

 

[Turbine Doc]

I'd check to see where I'm at, if at 3.5-3.8 check your TDCO, may only need to do ko/ko with fully warmed up engine and all stored codes cleared, if TDCO is below where you want it, then bump it to driver sdie to advance it, pass side to retard it, and don't worry where time set is, you are in "ballpark", in gasser terms I'd call 3.5 avg in time set as the "base timing", once there, you'll ignore checking it when tweaking for ideal TDCO, small bumps when moving IP for that.

Loosen bottom nuts on IP, make adjust with top one only, until you get IP/TDCO where you want it,

What kind of scan tool you using to set/read TDCO with

 

[gmctd]

Simply put, actual injection timing is adjusted by rotating the IP, and the setting in degrees BTDC is stored when TIME SET is commanded - TDCO is compensation used by PCM to run at that setting, and is self-adjusted by PCM when TDCO LEARN is commanded in OBD1 systems, or KoKo is performed in OBD2 systems - each timing setting has a range of TDCO values selectable by revving the engine within a band above idle but below 1500rpm - the more negative TDCO value within each range usually gives the best performance at that timing setting

NOTE: If PCM DTC's at any time, timing procedures will not work - all DTC's must be cleared in order to procede - that includes "TDCO out of range" when rpm goes above ~1500rpm

The KoKo routine was included in the OBD2 systems - it does TIMESET and TDCO LEARN automatically, therefore is very important that you first do a preliminary scan to determine current timing setting to use as the starting point - because it is automatic, the resultant timing setting cannot be seen with a scantool, in accordance with the fsm statement that timing changes will not improve performance (think EPA and deviation\violation taxes) - TDCO can be observed in OBD2, and is indicative of current IP timing

So, first, run a preliminary scan to determine where timing is currently set - oem is +3.5*BTDC, TDCO -0.5~ -0.75* - performance is +8.5* BTDC, TDCO -1.94*
- if lo, rotate IP towards drivers-side
- if hi, rotate toward turbo-side
- 1mm movement = ~2.5* change

Do TIMESET, then TDCO LEARN with OBD1

Do KoKo with OBD2

Do a rescan to verify results - TDCO LEARN can be used to tweak up desired numbers -
-1.94* is the de rigor number for hi-perf, but I detuned to -1.5* one brisk autumn morn, as the intense cold-rattle was more than I could stand, with visions of the pre-cups insanely rattling around in their nests

 

[Rodd]

Thanks guy's. I guess this is the next project after fixing the baffle in the tank.

 

[buddy]

The timing that you set by turning the IP is the injection timing, but that is also going to be influenced by the programming in the PCM.

Time set disables the stepper motor, so desired 0* as stated, and we have the IP's set around 3.5* with stepper motor disabled, that is timing them. Then the PCM programming could bump it to 4.5, 8.0, 8.5 at idle, whatever it is programmed to and stepper motor makes it so.

 

[gmctd]

Not even true, Buddy, which erroneous posting makes it difficult for some to grasp the truth - see the new TDCO sticky for full disclosure

 

[Rodd]

Thanks! Now I know I need to read this 10 times before it sinks in.

 

[buddy]

The procedure worked different than you explained, like 1000 rpms, mine certainly doesnt do that.

The Stanadyne IP rebuilder explained that to me, perhaps too basic.

Many people would want to know why they move their IP but timing never changes, because the PCM will set the timing the same regardless of base timing. My based timing can be 3.5*, but my desired and actual will show 8.5* in the time set, until PCM desires 0, then the actual goes to base timing.

I can get 3 different sets of desired and actual timings from 3 different PCM programming chips without loosening a single bolt.

 

[gmctd]

A - Stanadyne flowbench procedures are not comparably compatible with those required by in-vehicle systems

B - If the EFI values are already correct when doing TIMESET and TDCO LEARN, no change in operation will be observed - if the IP is moved, several things change noticably, beginning with the 1000rpm idle which, hopefully, prevents engine dying during incorrectly performed procedures

I compared procedures using a new TECH2 and GMTDScan Tech to find no difference, except that I prefer GMTDScan Tech as more user-friendly - same with Autoenginuity

 

[mitchedo]

Not even true, Buddy, which erroneous posting makes it difficult for some to grasp the truth - to wit:

BASE timing is the most retarded position of the camring\Optical Sensor, limited to and set by the physical position of the IP

ACTUAL timing is the measured position of the camring\Optical Sensor while the engine is running, and is variable to match rpm and loading requirements

DESIRED timing is the amount of advance PCM has determined as required to match rpm and loading requirements

DESIRED and ACTUAL timing should always match within a degree or so

+3.5* oem or +8.5* performance Base timing is resulted from the locked-down-by-three-12point-nuts physical position of the IP on the timing cover - PCM cannot change or alter BASE timing, but can DESIRE timing that is advanced wrt Base timing

When TIME SET is commanded PCM first sets idle rpm to 1000rpm to prevent the engine dying during timing procedures, disables DESIRED timing advance, using 0 advance to move the camring to it's most retarded position, then wratchets the Timing Stepper Motor to rotate the camring\Optical Sensor between the physical limits set by the current location of the IP, comparing that range of movement to crank TDC - PCM is measuring the available range of advance in degrees at that position - camring\Optical Sensor rotation is limited to 22* total by the design of the advance piston, which gives 11* total advance at crankshaft - minimum timing position with 0* advance is actual BASE timing, or +3.5*BTDC crank, with total advance at that position of 14.5*BTDC - rotate the IP ~2mm in the advance direction adds +5* for +8.5* actual BASE timing, with max advance of +19.5*BTDC - PCM never commands, and the IP cannot respond to, negative advance, or retard, below actual BASE timing - 'nuther words, it cannot go below actual BASE timing value .......unless the IP was mistakenly retarded to some degree less than oem minimum of +3.5*

Once PCM determines IP position in *BTDC during TIMESET, it wratchets the stepper to move the camring into BASE timing position, again, determined by the physical position of the IP - that position is actual BASE timing with 0* desired advance - OBD1 can read this number, OBD2 fudges the +3.5* number to fake out any enquiring minds, in accordance with the FSM blurb on timing alteration and performance gains - when TIMESET is de-commisioned, PCM resumes normal timing operation - now, you're setup do TDCO LEARN

Next, when TDCO LEARN is commanded, and here's where OBD2 KOKO comes from, PCM once again dials in 1000rpm, sets DESIRED (advance) to 0 to fully retard the camring\OS for starting position, wratchets the TSM thru min to max timing, prolly verifying what was measured in TIMESET, then moves the camring\optic sensor back to actual BASE timing position with 0 advance - PCM then computes a Top Dead Center Offset based on that measured range - now, altho PCM has disabled DESIRED timing function display in TDCO LEARN, as engine rpm is increased PCM calls for, or DESIRES, timing advance as required to match the increased rpm - this hidden advance is where the range of TDCO values for each actual base timing setting comes from: PCM continually recalculates TDCO based on the varying advance required for the varying rpm as if that were the actual BASE timing - so, you get a range of values for each actual BASE timing position: at +8.5*BTDC you can get -1.5* to -1.94* TDCO while in TDCO LEARN merely by increasing engine rpm until you see the desired value (pun intended, snicker!), then exit TDCO LEARN to lock that value into PCM

OBD2 KOKO procedure takes advantage of the positional advance remeasuring (TIMESET) that TDCO LEARN goes thru prior to calculating the correct TDCO value - thus, you don't need TIMESET in OBD2

And, that's how it really works, folks, word up.................

:bow: Sticky this please!

 

[Turbine Doc]

GMCTDs results are same as I see when I run with a MT2500 tool, on one visit to Houston we compared my MT2500 to his GM Tech 2 to know the differences.

As far as Stanadyne test bench comparisons, the test bench runs IPs at a test profile as the test bench's computer dictates, which does not duplicate what a PCM does with the IP, as I found out when visiting Pensacola Diesel shop and helping build/test a couple of IPs there myself.

This is not wholly correct; but when I explain how timing/TDC offest works, I compare it to a "spark advance positing" in a gasser, yes actual spark is controlled by the computer, but you can bias the actual timing that the computer biases it's timing adjustments by physically moving the distributor in advance or retarded from TDC so computer controlled spark is influenced by distributor positon in relation to the cam, this is assuming the audience has ever played with a distributor controlled spark system, fewer of us folks around, even rarer still is guys that know what ignition points are.

So sort of similar for the 6.5, this is what you are doing by moving the IP in time set to be slightly advanced from 3.5 base time if going away from factory TDCO spec., you mechanically change where the timing ramp begins on the cam ring inside the IP, PCM is blind to this and why the TDCO has to be "taught" to the PCM so it knows electronically where to begin adjusting timing.

Also why I prefer to go with TDCO vs. optic bump, as with optic bump you can be mechanically too far out of window and PCM will not be able to learn position & know physical position and timing advances for cooler weather will be all out of whack/hard starts happen/errant timing happens mpg can suffer.

 

[buddy]

Could you eplain though the difference between base timing on distributor and how a gasser uses that to influence its timing curve and idle timing, compared to our electronic computer controlled pumps.

What is base timing doing for us, other than setting a bottom. Does that actually vary any timing when PCM controls inection timing to whatever it wants per programming? If I set it to 3.5* base timing, and get my TDCO to -1.5 somehow, is that any different than setting my base timing to 7.5* and TDCO of -1.5? When my PCM is going to advance it to 8.5* at idle.

 

[gmctd]

Closest parallel I can think of is the old vacuum-modulator on the side where the distributor was base-timed with the vac disconnected - when reconnected the vac assist made the engine run better at both ends of the mechanical advance curve - TDCO does way more than that

Base timing is just that - a starting point and bottom limit, where timing can never be more negative than that Base - it also limits the upper end, only 11* away, the most important limit when saddling up to ride

The +3.5* factory setting was GM's compromise for emissions, producing the most power within those federal emissions standards - however, we know that increasing Base timing +5* raises and broadens the torque curve by several hundred rpm, and increases hp, also moving it up in the higher rpm bands, suitable for racing'n'stuff, as demonstrated on any number of dynomometers with different operators, different brands, different conditions

Again, each BASE timing has a range of TDCO values - you cannot, fe, get -1.5*TDCO at +3.5* BASE timing - that can happen after replacing the IP, where the IP is at +3.5*, but the original IP was set at +8.5* - it will DTC mightily, and the engine may not even run until the cause is corrected

Whatever your Base setting, the fuel\rpm\advance curve will call for ~+8.5* advance at ~695-750rpm idle at 195*F ECT operating temperature - doesn't matter if Base is +3.5* or +8.5* - what does matter is if you're looking for increased power when max advance is only +11* away from Base: (+3.5* + 11* = +14.5*BTDC) or (+8.5* + 11* = +19.5*BTDC)

Remember: this is timing to get the fuel into the combustion chamber at the top of the compression stroke in time to make a good showing - gassers (patooie!) truck the fuel in with the incoming air charge on the intake stroke, the compressed gasoline\air mix has been in the cylinder since the start of the compression stroke, and is just looking for a light at that point
- gasoline is extremely volatile and unstable in the presence of a spark
- Diesel fuel is slow burning, requiring way more energy for ignition than gasoline, so time is of the essence when the injection window becomes narrower and narrower as rpm increases - higher rpm requires more advance to get the fuel in earlier, since it is difficult to get it in quicker - we only got 11* total available, so yeah, Base timing matters when you're playing

TDCO allows function across the total injection timing range, allowing for normal throttle sensitivity across the range with advanced timing, helping to get the fuel in quicker in higher rpm ranges by allowing variance of the start of pumping, difficult to do in a mechanical injection system

 

[buddy]

Its important to note what PCM's normal idle injection timing is and not advnace the base timing over that.

If the observed idle timing is 8.5* that is not base timing, so when moving IP can't work off of idle timing because it will stay the same. You will only see the base timing when in time set mode with 0* desired.

So the screen you want to see is
desired/actual/TDCO
0*/8.5*/-1.94

because seeing 8.5*/8.5*/-1.94 is not necessarily the performance timing for top end high RPM advance

 

[gmctd]

You completely ignored my statement that DESIRED and ACTUAL must match within a degree or so - then you post more confusingly erroneous mis-information - what is it with you, Buddy? Are you dyslexic?

 

[Turbine Doc]

Its important to note what PCM's normal idle injection timing is and not advnace the base timing over that.

If the observed idle timing is 8.5* that is not base timing, so when moving IP can't work off of idle timing because it will stay the same. You will only see the base timing when in time set mode with 0* desired.

So the screen you want to see is
desired/actual/TDCO
0*/8.5*/-1.94

because seeing 8.5*/8.5*/-1.94 is not necessarily the performance timing for top end high RPM advance

???? Now i'm lost and haven't a clue what you are getting at

 

[gmctd]

Folks, if Base timing is set to factory spec of +3.5*BTDC, PCM will DESIRE +8.5*BTDC ACTUAL timing at idle, thus stepping the TSM thru +5* of advance to get there: +3.5* + +5* = +8.5* ACTUAL idle timing, thereby losing almost half of the advance range in the process

If Base timing is set to +8.5*BTDC (for increased performance), PCM will still DESIRE +8.5*BTDC ACTUAL timing at idle, thus no need to step the TSM, with the entire 11* of advance now available for mo' power

 

[buddy]

Folks, if Base timing is set to factory spec of +3.5*BTDC, PCM will DESIRE +8.5*BTDC ACTUAL timing at idle, thus stepping the TSM thru +5* of advance to get there: +3.5* + +5* = +8.5* ACTUAL idle timing, thereby losing almost half of the advance range in the process

If Base timing is set to +8.5*BTDC (for increased performance), PCM will still DESIRE +8.5*BTDC ACTUAL timing at idle, thus no need to step the TSM, with the entire 11* of advance now available for mo' power

Thats just what I said, whats the difference?

You did not understand. I said base timing can only be seen in time set mode, where desired will be 0*, and you want to see actual of 8.5* for the performance. So what you want to see is 0* desired, 8.5* actual, and TDCO of -1.94 when in time set mode, which will be your base timing.

If you see 8.5*/8.5*/-1.94 on the scanner at idle (not in time set mode) then that is not base timing, that is just what the PCM is doing, and base timing might be 5*, or 3.5* or 6.2* or whatever. The stepper motor will make it 8.5* in normal operation no matter where the base timing is. That is unless your PCM is programmed for something other than 8.5* idle, which I have seen 4.5*, so you cant put base timing over the PCM's desired idle timing.