Another cammed LT1 with negative timing

So another cammed LT1 in for a tune and it starts up and sounds glorious. Once I start dialing in the maf (adding since it was very lean) the timing just drops. Car drives great but the negative timing at idle sounds terrible (no cam chop) and the headers will glow and burn everything close if you let them.

On a side note when I check the VE the car is asking to pull fuel. How is this possible? By the maf it's lean and by the map it's rich? By significant amounts none the less. Anyone run into this? The car has been running great, no start up issues, no driving issues.

What other fields control the idle timing? I've tried the VTT tables but I've gone in both directions with no difference.

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Originally Posted by JASON11WS6

So another cammed LT1 in for a tune and it starts up and sounds glorious. Once I start dialing in the maf (adding since it was very lean) the timing just drops. Car drives great but the negative timing at idle sounds terrible (no cam chop) and the headers will glow and burn everything close if you let them.

On a side note when I check the VE the car is asking to pull fuel. How is this possible? By the maf it's lean and by the map it's rich? By significant amounts none the less. Anyone run into this? The car has been running great, no start up issues, no driving issues.

What other fields control the idle timing? I've tried the VTT tables but I've gone in both directions with no difference.

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Can you post your tune and log? When you say the car is asking to pull fuel, are you saying the fuel trims are negative? how much? and which fuel trim? short term? long term? I don't see how you know its rich or lean by the ve, map, or maf reading? We never know for sure how "correct" our ve and maf calibrations are in terms of actual accuracy, but we do know from wideband feedback if we are lean or rich.

Did you make the tune changes for the cam?

VT CONTROLS idle.... period

Any cam setup will always need a reduction in VVE around idle cells, aftermarket cams are less efficient at this speed so a reduction is needed

yup you need lower VVE and lower VT in the idle areas. Make sure you do Airmass and MAP and the E85 tables if you have enabled E85 in the tune. The MAP values where the new cam idles (assuming 60kpa) need to be lowered to at least the values that where it used to idle stock (30kpa). Also, when you change the MAF you are changing the final Airmass calculations and this can move the virtual Torque reference higher or lower depending on which direction you changed the MAF. A higher MAF value would cause a higher VT reference which would lower timing vs stock in the idle areas.

During the maf tuning the car needed more fuel (it was very lean). So I added to the maf curve and that dropped the timing. I find it weird that I need to lower the vve and raise the maf.

I should also mention these cars are supercharged.

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Originally Posted by JASON11WS6

During the maf tuning the car needed more fuel (it was very lean). So I added to the maf curve and that dropped the timing. I find it weird that I need to lower the vve and raise the maf.

I should also mention these cars are supercharged.

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ITs always been this way with cam cars.. blower cars will effect VVE but it will always need to be lower with a larger then stock cam period.

When you added fuel you increased cylinder load and therefore tq

Quote:

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Originally Posted by JASON11WS6

During the maf tuning the car needed more fuel (it was very lean). So I added to the maf curve and that dropped the timing. I find it weird that I need to lower the vve and raise the maf.

I should also mention these cars are supercharged.

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It is weird, but remember the days of LT1_Edit before LS1's were even out? You put a big cam in and then you would have to drop your VE tables but increase the MAF. The general operational theories(or maybe facts) of all of this stays the same like it did back when 1990's LT1s were everywhere and LS1's were touched by the hand of god. It's just the computers and software got a lot more complex.

Fair enough guys. Thanks for the help. I'll keep working on the VVE tables until I get some timing and hopefully a stable idle. Is there anything you look for in the scanner to see if you're going in the right direction...or to make sure you didn't overshoot your goal? I'm thinking like a specific torque field? Like max torque or delivered eng torque? Is there a good procedure or set of pids to watch out for?

There is a thread somewhere on here where myself and a couple others laid out the basics.

You want to log these:

Peak Engine Torque
Max Engine Torque
Immediate Engine Torque Commanded
Engine Torque

Driver Demand Torque Request
Axle Torque Commanded
Actual Axle Torque


In the first group you want Max Engine Torque to never exceed Peak Engine Torque or it will limit with spark or throttle, technically you want at least a 100 lb-ft buffer where Max is below Peak. Immediate Engine Torque Commanded and Engine Torque should almost mirror each other or you have some airflow or torque values off that is throwing this off. You do not want Immediate Engine Torque or Engine Torque to exceed Max Engine Torque.

In the second group you want Driver Demand Torque Request to always be higher than Axle Torque Commanded & Actual Axle Torque when accelerating. If they are not you can have timing pulled or the throttle closed by the ECU because you're getting more torque than the driver is asking for with the gas pedal.


MAF, VVE, VT & MBT tables all affect the final torque calculations in different ways. Screw too many of them up and you have a jacked up torque model on your hands and the car drives terribly. The REAL Torque values don't really matter that much, just that they all stay in line so that none are limiting and that Spark Advance and Torque Management Advance all behave properly.

I always scan those for full throttle. I get how that all works. I was just hoping there was one that would indicate why timing is being pulled. I'll keep working at it

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Originally Posted by JASON11WS6

I always scan those for full throttle. I get how that all works. I was just hoping there was one that would indicate why timing is being pulled. I'll keep working at it

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Depends how timing is being pulled. Torque Management Advance, particularly at idle, is influenced by Virtual Torque. Lower your VT Idle areas in MAP, Airmass, MAP85 and Airmass E85 and the ECU will think its making less torque and it will throw in some timing to maintain idle. Immediate anything refers to timing and Predicted anything usually means throttle when it comes to the tables in Editor. Idle External Load plays a role as well but I wouldn't mess with this until after VT and VVE.

Look at the cars normal Idle values pre cam. It is probably around 35KPA and .15-.18 Cyl Airmass. Look at the values in the VT tables for these areas. Copy these values to the new idle areas of the cammed car which is probably a similar or slightly higher airmass, but a much higher KPA(60-70). At minimum you would need the 30-40kpa values moved to the 60-70kpa area, but will likely need 20% less than that. Then Airmass you probably won't need to lower as much as MAP in the VT tables. You will then likely need to apply the same MAP changes to the VVE table for idle. If you're idling at a higher KPA then the pressure ratio will also be a higher value at idle and you likely have too high of values in the stock VVE table to idle properly. You need to tell the ECU that it is using less of the incoming airmass at idle for combustion and you do this with the VVE tables.

In the end it was a damn vacuum leak. Sorry for wasting your time guys. Someone took a plug out of the back of the supercharger for no reason really and it never got put back in. Either that or the car came in that way (the minimum spark tables were set above zero when the car came in).