Injector timing and ignition timing, Please look.
11-23-2010, 11:12 PM
#1
Injector timing and ignition timing, Please look. Can someone explain how the PCM knows the crank angle. I have been dealing with other types of fuel injecgtion systems and I understand how those work, but the factory PCM I am not sure.
Most other fuel injection or ignition systems use a toothed wheel on the crank with one or more missing teeth that tells the PCM were Crankshaft TDC #1 is (or at least you program the PCM telling it how many degrees after or before the missing tooth is). Then ofcourse there is the cam sensor that tells the PCM the cycle.
Here is my confusion on the factory PCM, best I can remember there is no missing tooth on the reluctor, so how does the PCM know were TDC #1 is?
Is it the cam sensor that does this task as well?
If so then when I installed my cam and I advanced it another 6 degrees(or retarded it, I cannot remember which, I set the ICL at 105) did I just throw the PCM timing off? Would this explain why I can run about 12 degrees more timing (in my tune, not verified with a timing light) then most before I see knock? If this is the case then did I also throw my injector timing off too?
Playing with other aftermarket PCM's that give you the ability to define and adjust the injector timing I know injector timing makes a huge difference in engine performance.
I hope someone can shed some light on this subject.
Most other fuel injection or ignition systems use a toothed wheel on the crank with one or more missing teeth that tells the PCM were Crankshaft TDC #1 is (or at least you program the PCM telling it how many degrees after or before the missing tooth is). Then ofcourse there is the cam sensor that tells the PCM the cycle.
Here is my confusion on the factory PCM, best I can remember there is no missing tooth on the reluctor, so how does the PCM know were TDC #1 is?
Is it the cam sensor that does this task as well?
If so then when I installed my cam and I advanced it another 6 degrees(or retarded it, I cannot remember which, I set the ICL at 105) did I just throw the PCM timing off? Would this explain why I can run about 12 degrees more timing (in my tune, not verified with a timing light) then most before I see knock? If this is the case then did I also throw my injector timing off too?
Playing with other aftermarket PCM's that give you the ability to define and adjust the injector timing I know injector timing makes a huge difference in engine performance.
I hope someone can shed some light on this subject.
11-28-2010, 02:41 PM
#3
Most other fuel injection or ignition systems use a toothed wheel on the crank with one or more missing teeth that tells the PCM were Crankshaft TDC #1 is (or at least you program the PCM telling it how many degrees after or before the missing tooth is). Then ofcourse there is the cam sensor that tells the PCM the cycle.
If you want, pm me your email and I can send you some more info on the lsx ignition system.
11-29-2010, 04:46 PM
#4
TECH Senior Member
Hi Texas,
The 24x reluctor wheel is encoded with some sequence of narrow and wide teeth, this is what the attached oscilloscope waveform image shows.
CKP = crank position
CMP = cam position
The 24x reluctor wheel is encoded with some sequence of narrow and wide teeth, this is what the attached oscilloscope waveform image shows.
CKP = crank position
CMP = cam position
Last edited by joecar; 11-29-2010 at 04:53 PM.
11-29-2010, 05:49 PM
#5
After looking at a pic of the reluctor, I can see that. From what i read, the cam sensor can crap out and the engine will run just fine. So my theory that the cam timing could be throwing my ignition timeing off is proven wrong. Maybe I got the wrong pistons, I asked for -12cc to make 10.25:1 compression, maybe they are 9:1 pistons instead. Thats what the engine builder thought he ordered. It would make sense.
11-29-2010, 08:09 PM
#6
TECH Senior Member
The cam sensor identifies the two possible #1 TDC's to the PCM (TDC on compression/ignition, and TDC on valve overlap), this allows the PCM to fire the injectors and spark plugs individually in firing order sequence (1-8-7-2-6-5-4-3).
When the cam sensor fails, the PCM batch fires sets of injectors together (odd bank 1+3+5+7, even bank 2+4+6+8) and fires the spark plugs in TDC pairs (1+6, 8+5, 7+4, 2+3), the engine still runs fine, you won't even notice that this is happening.
There is some amount of leeway with cam signal position relative to crank signal, I have seen some of those waveforms where the cam step did not exactly align with the crank pulse edge, but I do not know how much leeway the PCM allows; maybe the cam company knows how much.
[ as a reference, the narrow CKP pulses are 3° duration, the wide CKP pulses are 12° duration;
I have reason to believe that TDC may actually be at the end of the 5th narrow CKP pulse,
this means that advancing the cam (CMP waveform moves left) has no effect on PCM's perception of which TDC it sees ]
There may be another possibility to your problem (which I sincerely hope is not the case): the crank reluctor wheel has been known to spin/walk if not tack welded to the crank.
Either way, if you have the wrong pistons and will be tearing down, take a close look at the crank reluctor wheel and check it's relationship to the crank.
.
When the cam sensor fails, the PCM batch fires sets of injectors together (odd bank 1+3+5+7, even bank 2+4+6+8) and fires the spark plugs in TDC pairs (1+6, 8+5, 7+4, 2+3), the engine still runs fine, you won't even notice that this is happening.
There is some amount of leeway with cam signal position relative to crank signal, I have seen some of those waveforms where the cam step did not exactly align with the crank pulse edge, but I do not know how much leeway the PCM allows; maybe the cam company knows how much.
[ as a reference, the narrow CKP pulses are 3° duration, the wide CKP pulses are 12° duration;
I have reason to believe that TDC may actually be at the end of the 5th narrow CKP pulse,
this means that advancing the cam (CMP waveform moves left) has no effect on PCM's perception of which TDC it sees ]
There may be another possibility to your problem (which I sincerely hope is not the case): the crank reluctor wheel has been known to spin/walk if not tack welded to the crank.
Either way, if you have the wrong pistons and will be tearing down, take a close look at the crank reluctor wheel and check it's relationship to the crank.
.
Last edited by joecar; 11-30-2010 at 01:05 AM. Reason: added more info
11-30-2010, 12:59 AM
#7
TECH Senior Member
Hi Texas,
The LS1 PCM is even smarter than I thought... ignore the red part of my post above (altho there are various engine controllers that work exactly like that) and see attached doc (gleaned from the GM Service Manual)... the LS1 PCM in the absence of a cam signal tries its best to synchronize the crank signal by seeing if each ignition event causes the crank speed to increase... this is a work of art
The LS1 PCM is even smarter than I thought... ignore the red part of my post above (altho there are various engine controllers that work exactly like that) and see attached doc (gleaned from the GM Service Manual)... the LS1 PCM in the absence of a cam signal tries its best to synchronize the crank signal by seeing if each ignition event causes the crank speed to increase... this is a work of art
Last edited by joecar; 11-30-2010 at 01:07 AM.
