The '90-'92 3.1l f-body's have a launch mode (LM) function in the ECM. This entails adding SA to quickly spin the motor up against the convertor. There are several qualifiers for this along with a time duration the timing is allowed to be added.

These vehicles run the $88 MaskID in a 1227730 ECM. It is a multi-port system. The torque convertors are rather loose and will stall up in the 2600-2800 RPM range.

Here are the launch mode qualifiers along with the modifiers:

These are the raw bin values and need to be converted. They are from AZTY which is the latest 3.1l SD Auto bin from GM.

Launch mode SA bias: used to bias the SA table. At zero the SA table is an adder. No values can subtract SA.

Launch mode knock retard multiplier: used to reduce the amount of knock retard. 208 / 256 = 81.25%, which is the percent any knock retard SA is reduced to.

Go to launch mode max mph * 3.2 (10 mph): vehicle needs to be below 10 MPH to enter LM. Will stay in LM above this speed.

delta tps required, 1.2%: delta TPS over 12.5 msec to enter LM.

else, delta map required: delta MAP required to enter LM

Note: either the delta TPS or the delta MAP will enable LM.

delta map allowed stay in launch mode: if delta MAP exceeds this value will stay in LM. Overrides timer value.

launch mode duration timer (2 secs): length of time to stay in LM. Can be overridden by delta MAP (secs = value / 80).

-delta tps% allowed to stay in launch mode: a slight lift on the throttle won't end LM, but a lift greater then this term will.


The LM SA table converts to SA by multiplying by 0.352. At 1200 RPM 100 KPa the added SA is 6° (17 * .352 = 5.984).

Notice the SA table and where the timing is added. Start low in the RPM, then the max at 1200 RPM, with a taper to 2,000 RPM. This causes the engine to literally leap to the stall speed with the result that the vehicle leaves the line quickly (these little 6-bangers need all the help they can get).

RBob.

{edit: corrected duration timer calculation}

 

Thanks for the info!! I understand most of it and once I get to messin around with it, hopefully I'll understand it all! I've got a bigger convertor in my 6 and I really need tuning to get my launches better.

 

Weird. Kind of cool and very practical for a low torque motor in a relatively heafty car with a large power robbing automatic. Got a datalog showing this in action?

 

JPrevost,

Something similar to this has been done for awhile with several of us here. The 100kPa column in my timing tables was always maxed (38d on my setup) from 0rpms to the stall of my converter (3600rpms). People would look at my table and say What the Freak? But, quick 60ft times resulted. I never tested to see how fast I could spin it up by going higher than my max timing. But, it worked really really well.

Check out a screenshot I posted from here...
https://www.thirdgen.org/techbb2/sho...hreadid=227912

Tim

 

Tim, I just can't believe you are able to run that much timing at such a low RPM without pre-ignition. I mean if you've got a large cam, and are running that much timing you're liable to backfire in the intake! You're effectively sparking the mixture while the intake valve is still open. Doesn't that sound a bit odd? Don't take this the wrong way but are you sure you've got your base timing set correctly?
Another thing, flame propigation speed. At low RPM's if you advance the timing too much (doesn't take much at all) you can effectively slow down the engine. The flame speed is pretty consistent so if you're making like absolutely no dynamic compression then I might be able to see how you're getting away with that high timing. Either that or you're actually never hitting those areas. Most drag racing you stage and roll into the throttle where the MAP is still low and you leave with a high RPM. Do you brake stand or do some form of standing with the throttle in the open position?
I'd really like to nit pick your setup in a seperate thread if that's okay with you. I'm very interested in how you're able to run nearly 40 degrees at those sub 1000rpm ranges... must be a HUGE cam, and like I said earlier, if that's the case you're getting really close to lighting the fire with the intake valve open.

 

The intake has been closed a long time at that point and he is accelerating through those points, not steady state...big difference. Also a rich AFR at those points too.

 

You're right , I was doing the math in my head and was using 90 degrees for a stroke, lol. Yeah, 180 degrees and it would take a lot of advance and duration. With ~300 degrees of intake duration and 45 degrees advance there's 50-60 degrees where the intake valve is closed before spark. Wow, talk about starting off on the wrong foot. Thanks for making me go back and check my work.
Yes, rich AFR's actually propigate faster so I'd think that at those low RPM's the overlap of the cam is the saving grace.

 

Now what would have been REALLY interesting was to feed it even more timing to spin it up even faster. I'm pretty sure it would have worked ... just would have probably needed some type of knock alert and/or stethescope during the tuning process. Would have been sweet though. I don't have the car anymore so I can't test it.

Tim

 

More timing, less timing, whatever, the goal is the same, whether you know it or not - get peak cylinder pressure somewhere between 15-17 degrees after top dead center, without knocking, pre-igniting, detonating, pinging, ponging, piddling, whatever. Most gasoline spark ignition engines will follow that general guideline.

Of course, nothing is 100% predictable, and sensors aren't 100%, and algorithms aren't 100%. But, if the RPMs are rising really fast, and you're using distributor reference pulses, you may need extra timing just to make up for the inaccuracy of the timing calculation.

On a knock limited engine, more timing is generally quite OK with richer mixtures, even though it's going to burn a little quicker. Cylinders and parts heat up quickly at sustained high loads, which may limit the amount of static timing allowed there (due to knock), but quick bursts may be beneficial, and knock free.

The best answer is probably a true seat of the pants comparison.

But, on the sceptical side, seems like it would break the tires loose easier (of course, maybe not with a 3.1). Not much launch with spinning tires.