I've been seeing timing values reported in Datamaster that are higher than what is in my main and extended timing tables and I'm trying to trace down the source. I've learned some things from experimenting but am still fuzzy on how everything correlates.

Can anyone explain the function and net effects of changing the following ECM constants?
- Spark advance table bias
- Spark coolant temp correction bias
- Spark reference angle
- Maximum spark advance (sounds simple enough but I've seen timing go above this value on occasion - what would cause this?)

Appreciate any insight...

 

If you look up "mystery timing" you might start to find the answer. From my years of reading much more than posting on this boards (and most on this computer section), it seems no on has found the source of this additional timing. For me it starts at +2 in the lower RPM's and eventually gets to +4.

Please find it and post. I wanna know!

 

Probably the egr timing too if you havent zeroed that out. Pcm does its own deal as far as adding timing at high rpm it increases a little as you move on up the rev range. Adjust accordingly.

 

I have never had this issue.

 

Update - the additional timing I've been seeing appears to be related to IAT temps. I've zero'd out every table available in TC that could feasibly affect timing with no change in behavior. However, when IATs went up with some recent warm weather, I only saw 1 degree added vs. the 4 degrees I was seeing in colder weather. All other parameters have remained constant the only variable being IAT temps. For those interested, the IAT temps I'm referring to were ~59 degrees where it added 4 degrees timing and ~90 degrees where it only added 1 degree timing. Ambient temps were roughly 20 degrees cooler than reported by the IAT.

My working theory is that there IS a hidden IAT vs. timing table which adds timing in colder weather. For my application (roadracing) where I run it hard in all kinds of weather it forces a compromise in my overall tune but not a big deal in the grand scheme of things. Just thought I'd share my observations....hope this helps someone down the road.

 

Well, you are parallel with what I was thinking. Additionally, I also use my RX7 for roadracing. I really wish I had access to this IAT table. TC does not have it with OBD I.

FYI, My Firehawk had the perfect intake and placement of the IAT sensor and still always read 10 degrees above ambient. It is tough to completely isolate our intake charge from the engine's heat. Please someone suggest something here.

 

I put the IAT sensor in the cap of the K&N filter on my Callaway CAI. That eliminated the heat soak of the stock intake elbow location, and it made a perfect ambient air temp sensor. However, between the sensing point and the intake manifold, the air was still picking up heat. The ideal sensor would be located in the middle of the intake manifold plenum, and isolated from heat transfer from the adjacent aluminum manifold.

 

Some of the additions to TC in recent years have come from users figuring things out and then asking TC to find the table that does X.

So, do some experiments to figure out if the IAT-advance relationship really exists and what it looks like, and then contact TC with that info and maybe he can find the relevant code and table.

Or if you have a whole lot of time on your hands, jump into the code yourself.

 

I sure wish somebody could find the table to increase enrichment on rapid throttle changes. Kinda like the squirter on a carb.

About this iat timing changes I have never seen it. Have you guys looked in the "Spark Correction vs. map vs. coolant temp"? I know I use that on cars that are pushing it with compression on pump gas, when the coolant temp goes up I pull timing. I also add some at low coolant temps to help things along.

Fred I only agree with your statement about the iat sensor in a speed density application. In maf mode you would want it located at or in the maf sensor, kinda like the ls1 setup.

 

The stock MAF sensor has in internal IAT sensor that it uses for the calculations. The LS1 design is an "economy" design to eliminate the unneeded 2nd IAT sensor in the ductwork.

 

My comment on "squirter on a carb"
Probably not really needed info but I band-aided the need for the "Accelerator Pump" table WS6T3RROR mentions needing by lowering the injector flow rate and leaning out the affected tables

My comment on "Timing Creep"
I also have a chart showing the effect of MAP on RPM and Timing in one of my original posts... https://www.camaroz28.com/forums/sho...d.php?t=600649 or https://www.camaroz28.com/forums/sho...d.php?t=560089

Loaded a file with 10 degrees of timing in all tables beyond 50kpa at all RPM. Set all temp offsets to 0, set all min timing to 1 (pulled the Injector Fuse and spun a used OPTI with a cut off tool - this was also tested AFTER the BARO RESET mods - not sure if that had an affect on timing but it did on fuel)

Basically:
) Ran 50kpa MAP (0 boost) from 400-6375rpm, showed the following:
400-700rpm = 10 degrees timing
800-2100rpm = 11 degrees timing
2200-3700rpm = 12 degrees timing
3700-5300rpm = 13 degrees timing
5300-6300rpm = 14 degrees timing

2) Ran 75kpa MAP (7.5psi) at same RPM's as above with same timing results:
400-700rpm = 10 degrees timing
800-2100rpm = 11 degrees timing
2200-3700rpm = 12 degrees timing
3700-5300rpm = 13 degrees timing
5300-6300rpm = 14 degrees timing

3) Ran 100kpa MAP (14psi) at same rpms as above with same timing results:
400-700rpm = 10 degrees timing
800-2100rpm = 11 degrees timing
2200-3700rpm = 12 degrees timing
3700-5300rpm = 13 degrees timing
5300-6300rpm = 14 degrees timing

4) I then ran "stock" 100kpa BARO Key On with 100kpa load... same results (so I wont post them).


Sent all the above into to TC and got this response...
"Hi Dan - Hmm, strange. I can't think of anything that would add advance (timing) based on RPM like that but if you get a chance, send me the calibration (.bin) file you are working with so I can take a look at it.
Best regards, TC "


Thing is, this was done to prove safety of the the 2 bar mods and make sure this would not mess up timing, I could one day (or someone else) test this same scenerio using a cold IAT and one that's sitting in HOT water. This would be interesting.

 

Learn something new every day I guess. I always thought the iac on the lt1 air box was the only one. All i know is with the ram air box right over the radiator mine always heat soaked at low speed but would report normal at any speed above about 15mph. Do you have any further info on how the maf works exactly. My guess is they use some correlation with the constant of convective heat transfer to determine velocity through the maf and then use the cross section to get volumetric flow, then tweak the ideal gas law around a bit to get the mass flow. Iat would be one of the needed inputs to get density and determine mass air flow. I always wondered why not put the iat in the maf on the lt1's, silly me I guess they do.

dookie454; Your timing increase with rpm is what I have encountered many times, apears to be rpm dependant only. Thats why alot of n/a lt1's run better when the timing retards up top in the timing maps. People look at you cross eyed when you tell them that but theres a reason. It would be interesting to see you test it with a hair dryer on the iat or the maf to see if there was a change in timing.

I know the injector constant trick, its just kind of a big bunch of brain damage. Ie I just think there has got to be a better way. It becomes a whole bunch more work. Not in your case but in the usual case.

 

Yea, it does majorly suck when it comes to lowering injector flow rates on an otherwise good tune because it affects everything including crank fuel, initial start, etc... all those things a person never knows exists until you fix one thing and break another... ha, I only brought it up cause I couldnt tell if you were having problems and were kinda fed up and at the end of the road like I've been many times looking for that bump to get you back on track and thinking working on the LT1 PCM wasnt a complete failure... I am suprised that table hasnt been discovered yet though.. it's there, it has to be, PCM doesnt just know to throw in fuel during a TPS transition.

 

The LT1 MAF doesn't provide temperature information as an output, so it is not a temperature sensor.

In addition, a quick look at the FSM and google also suggests to me that the LT1 MAF does not measure IAT, because:

1) it doesn't see T, it sees delta T

2) the reference for the delta T is not intake air temp, it is ambient underhood temp.

Like I said, it was a brief look, so those may be wrong.

 

Delta t is fine, that is actually what you have in the convective heat transfer equations. I assume that they dont work that equation on the fly coming up with the coefficient is a real pita and complicated and actually not too accurate (if you get within 25% of actual you're doing good). I think they must use some kind of correlation from lab testing based on power ie frequency to maintain the temp of the wires to come up with the velocity. I think the iat whichever one is used is just there as an input for ideal gas law for speed density and maf mode.