Ignition Timing - Lets get technical

[Schmieder]

i've had people tell me the PCM won't use torque calcs when the MAF is unplugged and burns the tranny... but i have no idea personally.

tuning the MAF to meet the VE SHOULD be a good method.

Burns the tranny with MAF unplugged? How does that happen? Or is that people who are relying on torque management being unable to perform w/ MAF unplugged?

btw, I just loaded a log file from VE Tuning where the MAF was disabled AND unplugged and the PCM still measured torque delivered to the trans. So there must be a method being utilized to measure torque with the MAF out of the loop. Possibly VE Table being used for torque measurements? Interesting indeed.

If it makes any difference I am using a 1997 Lumina PCM.

[Schmieder]

Here is the export file showing the torque measurements while MAF is disabled. You can see the MAF flow values remain constant and do not fluctuate.

For your curiosities

[SappySE107]

GM used the same VE table for a small port 3100 and a 2004 3400, minus 4 datapoints that were raised from 67.5% VE to 75%. The MAF table is slightly different however, though I have no idea if its the same MAF or not.

The trans uses the max line pressure when the MAF is unplugged, but you can set that to 45 or 55 psi and it won't be so hard on shifts compared to the 90 default. BANG!

I VE tuned the 5 speed DOHC in the 92 (97 ECM) and then plugged the MAF in. Its damn near the same fuel trims as without the MAF. I don't understand how tuning a MAF works without VE, because you can have the same flow but different load and require different fueling. This is exactly why I gave up on the 94-95 3.4 DOHC MAF setup.

[Schmieder]

GM used the same VE table for a small port 3100 and a 2004 3400, minus 4 datapoints that were raised from 67.5% VE to 75%. The MAF table is slightly different however, though I have no idea if its the same MAF or not.

The trans uses the max line pressure when the MAF is unplugged, but you can set that to 45 or 55 psi and it won't be so hard on shifts compared to the 90 default. BANG!

I VE tuned the 5 speed DOHC in the 92 (97 ECM) and then plugged the MAF in. Its damn near the same fuel trims as without the MAF. I don't understand how tuning a MAF works without VE, because you can have the same flow but different load and require different fueling. This is exactly why I gave up on the 94-95 3.4 DOHC MAF setup.

Update

The MAF tuning I read about involved gathering a lot of data and getting averages to make educated trials in shifting the MAF curve. It is a long, long process but worth it. All I do know in planning to tune the maf is logging everytime I drive. Soon I;ll compile the data and get average MAF hz per rpm/load and then figure where the fule trims are at the specific rpm/load either rich or lean and then adjust the maf curve at the Hz given for that specific rpm/load cell.

I also thought about exporting the log file and sort by MAF Hz from low to high and get and average fuel trim for the specific Hz ranges and adjust that way as well.

I read a few ways to tune the MAF but I think I'd be better off designing my own method as I have a few ideas that may work out well. If they do I'll surely post them up in How To form.

end update

GM is crazy. Same VE Table for a small port and large port?!?! Talk about lazy!

Yeah, shifts did seems hard whem MAF was unplugged.

Ultimately I am going to go SD for a while. I will keep my MAF in the garage just in case. But I'de rather run a 2-bar MAP so I can utilize the PCV system to pass emissions.

The way I had to set up vac lines with boost made it damn near impossible to feed the crankcase fresh filtered air AND have the MAF register the air being feed. So for now the crank case is ventilated to the atmosphere. Once my 2-BAR MAP is in I will place the fresh air feed just behind the filter, before the turbo. This way under boost the air being drawn in will create a slight vacuum at the fresh air feed which is good for the crankcase. Likewise under idle that same line will feed the PCV to draw air in like a stock n/a engine would, only difference is the check valve in place. SO the crankcase will have a more effective airwashing then a n/a engine would.

Maybe I'll use the MAF to measure my farts, lol

[Schmieder]

So I have one of the later ignition maps for the LA1 (3400). Now, I assume with confidence in that assumption the tables are still tuned for 87 octane pump gas.

In your guys experience, approximately how many degrees can be advanced from 87 oct to 93 oct? A friend said 1 degree advancement per 1 octane point, but that sounds to simple and seems like a rough guess at best.

With a turbo, I found the higher RPM's at WOT needed a strong retardation.

How about the over-run? When I hit KR, lets say I get 3 in cell 4400rpm/0.80g but that 3 is also present at 4400rpm-0.44g during the over-run stage. If I remove the knock in cell 4400-0.80g should the cell 4400-0.44g also correct itself? It seems cell 4400-0.44g is just retarded because the previous cell was.

btw, I put in the colder plugs and they got rid of the knock I was getting a 3+psi. Now I can get to 6 psi before any knock, it starts again at 7psi. I tried tuning it out but no effect so I believe it is the plug still. Two heat ranges colder should have corrected that though. So it seems it has something to do with compression or something I am over looking as the coolant temps are steady at 180-184*. Any suggestions?

Yesterday, I had a cop tag me and followed me around for about 10 minutes but didn't pull me over. I guess my paint job says 'speeder'. No turbo play then, lol

[IsaacHayes]

Some of the later 3400's had a ton of spark advance and on a stock tune people were getting KR. So that is something to consider.

What plugs did you end up with? What octane are you running?

Yup by having your car a custom two tone it screams "younger kid" and usually that goes along with "up to no good"... LOL. Having a GP is usually enough low-key, but now you've changed that.

My orange car is sure to draw attention. Bright paint, sporty looking, big ass cowl, and loud as hell. It will be "fun" trying to drive and tune that with as many cops in my city. You can't drive 3 blocks without seeing 2 city cruisers. Then you have the hwy patrol, capitol police, county/etc all in the same city... But mainly the city cops are the real a-holes and they are waaaay to many of them. I'll try to avoid all hills because it's a 5spd and my friends have gotten C&I's (careless and imprudent) for chirping their tires on hills taking off before. A C&I is devastating on car insurance. There is no common sense with the cops here. If they want to give you a ticket, you are getting one.

[Schmieder]

Some of the later 3400's had a ton of spark advance and on a stock tune people were getting KR. So that is something to consider.

What plugs did you end up with? What octane are you running?

Yup by having your car a custom two tone it screams "younger kid" and usually that goes along with "up to no good"... LOL. Having a GP is usually enough low-key, but now you've changed that.

My orange car is sure to draw attention. Bright paint, sporty looking, big ass cowl, and loud as hell. It will be "fun" trying to drive and tune that with as many cops in my city. You can't drive 3 blocks without seeing 2 city cruisers. Then you have the hwy patrol, capitol police, county/etc all in the same city... But mainly the city cops are the real a-holes and they are waaaay to many of them. I'll try to avoid all hills because it's a 5spd and my friends have gotten C&I's (careless and imprudent) for chirping their tires on hills taking off before. A C&I is devastating on car insurance. There is no common sense with the cops here. If they want to give you a ticket, you are getting one.

My plugs, I got double platinum Champion 7401's. It did the trick, no more knock. My old plugs were pinkish around the tips.

I am running 93 octane and had modified timing from a 2004 3400 file. My spark is down to 12* under boost but that is normal as far as I know. The rest of the less then boost sparl map are kept ecxept for a few knock instances that were tuned out. I believe I have a good, strong spark tune.


Cops, yeah thats a tricky issue sometimes. Hopefully the suit n tie in my drivers license photo and car seat helps out a bit. I happen to know many of the cops in my neighborhood and they are beginning to recognize my car so my local officers leave me be, thats another long story but we all get along well. As for city cops, I really don't see many. I'm in between rich and ghetto neighborhoods where they dwell the most. You know how it goes, they hang around the ghetto for violations (income) and hang around the rich areas for tax presence. Us middle class get the least attention. We don't commit enough crimes to make the city a good income and we don't pay enough taxes to have strong cruiser presence.

I'm working on a finer AFR tune. Using the AFR adder per RPM under PE. What I did was command finer afr's for different conditions.

idle 14.3
light load 14.3
power before boost 13.6 to 12.9
entering boost 12
well into boost 11.5 to 10.9

Instead of just 14.3 then 11.5 PE. The power n/a stage scales from 13.6 to 12.9 depending on the throttle position. Lower throttle in PE is closer to 13.6, and higher throttles in PE are at 12.9 all under vacuum. Same thing with entering boost, low throttle is 12 and high throttle is 11.7. When 5psi kicks in its at 11.5 and slides to 10.9 after 3.5 seconds of boost and stays at 10.9.

I did this by making the AFR per rpm at its richest (under PE) then scaled back with the throttle position afr adder. After some fine tweaking, it works great. I get 14.3 under idle and light load to save fuel, 13.6-12.9 AFR when I need power but not in boost yet to save gas instead of using 11.5 AFR, then having the full effect of 11.5 in boost and soaking to 10.9 to help cool things off after a few seconds of boost. It should give me maximum power while saving some fuel.

So if I was at idle (14.3) and floored it, the AFR goes right to 12.9 up to the upper 2k rpm then slides to 12 at 3200 just when boost is kicking in and hits 11.5 at around 3600 rpm and hits 10.9 around 5500 (or 3 seconds) what ever comes first.

But if I only used 35% throttle my afr would stay at 14.3 until about 2800 rpm and hit 13.6 and slides to 12 at 3200 rpm.

It's really cool how detailed a tune can get.

I thought about putting PE at 1% throttle this way I will always be in PE and can command a specific AFR at various conditions. Basically taking away control from the O2 but still have it to record the AFR for the PCM to know what AFR to command.

EDIT: Now back to topic, lol. I plan to add 1 degree across the spark map EXCEPT for areas I had already tuned out spark knock.

If the PCM can record distances travelled, then I could essentially do 1/4 mile runs recorded and test spark adjustments.

[Schmieder]

Still getting a misfire past 5psi. It was occuring at 3psi until I put in the colder plugs.

Timing when entering boost is around 14-8 degrees.

The power comes in great and then it shudders lightly. It registers as spark knock but the same readings occur even when I reduce timing, so I don't think it is KR but could be wrong.

My plugs are gapped at 0.060". I know these Grand Prix's have kick ass ignition systems but wondering if I could be blowing out my spark past 5psi. My ICM is older and my coil packs are new. No issues what so ever below 5psi, runs great.

Should I try gapping the plugs down to 0.050"? I don't want to start pre-ignition with too close a gap.

But as far as I know, it becomes harder for voltage to arc under greater pressures, so I really think this is the issue.

btw, running 93 octane and CR is 9.5:1 (pistons and heads are carbon free as of a few weeks ago, so no CR creep)

Advise please, thanks.

edit: btw, o2mV reading shot to 63mV in PE when they should stay steady 920-930mV during the misfire when beginning to hit 6psi. That tells me there is a brief misfire. Also, the timing jumped to +0.5* for a brief moment then returned to -8* even during the false knock, strange. I would believe the +0.5* is the timing being pulled by the knock sensor detecting the misfires and thinking its knock but the timing returns to normal during the false knock.

I say false knock because I really don't think it is genuine pre-ignition. Adjusting timing has no effect to the KR readings. The KR readings hit up to 5* but are usually 1-2.5 during the 6psi+.

I really think I need to gap to 0.050" but would like input before doing so.

Again thanks guys, you all are great, HAPPY EASTER!

[robertisaar]

But as far as I know, it becomes harder for voltage to arc under greater pressures, so I really think this is the issue.

correct.

and i doubt a closer gap will give you pre-ignition...

[Schmieder]

correct.

and i doubt a closer gap will give you pre-ignition...

Ok, I read a gap TOO small would spark hotter and the increased terminal temps may be hot enough to trigger combustion under boost.

But I believe the gap would have to be really tight for that to happen.

So in your professional opinion, what would be an optimal gap for my application?

9.5:1 CR
Cam Advanced 4* (mild increase in compression)
7-8psi max
Current gap 0.060"
Champion Plugs 7401 RS12PYP (2* colder then stock)
Estimated 233 HP, 330 lbs/ft

Thats probably info over-kill but it's there. I'm thinking 0.035" as I read a lot of turbo owners like that gap but I also think 0.050" since that is a modest 0.010" reduction and would probably stop blow out while keeping a substantial gap.

What do you think?

[robertisaar]

i would do it in steps to make it worthwhile...

i read an old addage somewhere that suggested for every atmosphere you add, to cut the original gap in half...

i'm not sure where this came from, but for ~7PSI, that would mean .045...

i'd say it's at least a good starting point. i know some TGP guys that run .038 with good results, but N/A 3.1s(and possibly stock TGPs as well?) run .045 plugs to begin with. and since they run ~7PSI from the factory, if that rule was followed, they would come up with .034...

worth a shot.

as far as causing pre-ignition, maybe if there was a direct connection from the electrode to the ground, in which case it would basically be arc welding itself.

[robertisaar]

i would have edited, but i think this post deserves it's own:

during this process of finding the best gap, do be sure to do plug cuts during this process to ensure it is FALSE knock. if you start seeing little specks of anything on your plugs, suspect that it is part of your pistons.

[SappySE107]

If you cut spark down to 8 degrees and get KR, its probably fueling or something physical (such as gap or heat range). The stock 97 3100 spark table looks like a better start to me than the 3400 code.

Without feedback on the small port 3100, I couldn't tell you if the VE tune is way off or if its close enough with the MAF to work.

Why aren't you looking at TGP code? That would make the most sense to me for spark advance.

[Schmieder]

If you cut spark down to 8 degrees and get KR, its probably fueling or something physical (such as gap or heat range). The stock 97 3100 spark table looks like a better start to me than the 3400 code.

Without feedback on the small port 3100, I couldn't tell you if the VE tune is way off or if its close enough with the MAF to work.

Why aren't you looking at TGP code? That would make the most sense to me for spark advance.

Ok, I'll look up a 97 3100 spark table. Not too many 3100 files in HP Tuners repository. Oh snap, I forgot we may have TGP code right here in this web page.

The VE is tuned very well. The LTFT's are all at 0. I then took a LONG drive with the MAF plugged in and then exported the results to excel workbook. I sorted by MAF Hz and did a LTFT average for each cell block in the MAF Calibration Table.

Example, for MAF cell blocks (1875Hz)(2000Hz)(2125Hz) I would find an average LTFT for frequencies 1938Hz-2063Hz (halfway points outside of 2000Hz) to determine the percentage off for cell 2000Hz. It was a tedious process that took a few hours but it worked beautifully. My MAF was originally off by 16% in the low end and 4-5% in the higher end.

Now my MAF is tuned in great, in fact I'll upload a copy in case anyone wants to try it out.

I have tried AFR's of 11.5 to 10.5 and didn't stop the knock, I even tried 10AFR once just to see. The spark plugs are 2* colder so they are well in the safe range. Since I have a gap of 0.060" I am becoming very confident it is a gap issue. Especially since posts here are stating turbos have been using 0.035". So I think I'll pull my plugs and investigate for piston material then gap to 0.045".

btw, what am I looking for specifically to determine if I am getting piston material on the plugs?

[robertisaar]

http://www.diy-efi.org/gmecm/papers/plug_cuts.html