Coast down timng and RATE

[The Alchemist]

Under "Spark"
theres a coast down spark table with a description detailing how this is trimmed up or down depending on how far out the car is from a "desired" coast down rate.
This naturally infers that there is a coast down rate table that could be adjusted to suit cammed engines etc etc, much like throttle follower does in the LS1 but with timing control rather that IAC control.
Being an important drivibility setting I would like to see this in new calibration updates.
Is there such a table GMPX???
Cheers,
Mike

[swingtan]

The E38 has a "throttle follower" table, at least in some calibrations. Check B1844 and B1845 for the "coast down" parameters. these control how the ETC adjusts the throttle opening in coast down conditions.

Further control is available via the spark tables B5123 and B5124. Lower values in here will result in the engine taking longer to slow down to idle speed.

What are you hoping to achieve by adjusting the ramp down rates ?

Simon.

[The Alchemist]

still looking for a calibration with the B1844.....Found it now....step rate/size...
The B5123 and B5124 are a timing offset vs the deviation from the "desired ramp down rate"
"If the actual coast down RPM is above the desired coast down RPM, then this table is used to make adjustments to spark timing usually by retarding timing away from the base value to slow the engine down."

To change the actual rate you need access to the " ramp down rate table " so to speak...this is what I was referring to in the earlier post.

Is there such a table yet to be added to the calibration files?

Just trying to achieve knowledge really I guess.....coasting throttle is an important drivibility setting especially when at cruise down hill when at 100km/hr when throttling off...
Mike

[swingtan]

The descriptions I have for B5123 and B5124 are slightly different. They don't actually reference a "ramp rate" but work on the RPM error between the desired RPM and the actual RPM. So if the throttle was at it's idle opening, then the desired RPM would be whatever the idle speed was meant to be. Any value over this would result in timing being pulled to slow the motor down. The speed at which the motor slows, would be proportional to the amount of timing pulled, so the ramp down rate "is" this table.

To bring the RPM down quicker, you would pull more timing, faster. That is to say, you would put lower values in the cells with less RPM error. To slow the "ramp down rate" you would have higher values in the table. I've tried playing with B5123 and found that rather than changing the ramp rate, it's more like a ramp down delay. Running too high a setting causes the revs to "hang" rather than actually drop slower.

What are you actually trying to achieve? There may be other ways that give better results.

[The Alchemist]

Sorry Simon, I meant rpm not rate....I see what you mean now....
"So if the throttle was at it's idle opening, then the desired RPM would be whatever the idle speed was meant to be. Any value over this would result in timing being pulled to slow the motor down. The speed at which the motor slows, would be proportional to the amount of timing pulled, so the ramp down rate "is" this table."
A good explanation.... thank you.
But what about when you are going at say 100km/hr and you throttle off when going down a hill the revs don't just drop to 1000rpm or whatever they slowly coast down as the engine slows don't they. what controls this "rate of decline"?
Where you said:
"So if the throttle was at it's idle opening, then the desired RPM would be whatever the idle speed was meant to be" wouldn't this only apply when the vehicle was stationery and you just gave it a free rev???

[swingtan]

Np, glad to help.

So are you trying to hold the RPM's up a little more between gear changes? Are you by any chance running a manual?

[The Alchemist]

more trying to manipulate the coast down effect on off throttle on down hills etc... Auto trans
edited above post, please re read.

[swingtan]

But what about when you are going at say 100km/hr and you throttle off when going down a hill the revs don't just drop to 1000rpm or whatever they slowly coast down as the engine slows don't they. what controls this "rate of decline"?
Where you said:
"So if the throttle was at it's idle opening, then the desired RPM would be whatever the idle speed was meant to be" wouldn't this only apply when the vehicle was stationery and you just gave it a free rev???

When going down a hill and you back off, the ECM will attempt to pull the revs back to the commanded idle setting. However the fact that the car is in gear means that the engine RPM will not drop until the cars momentum is slowed as well. In this case there can never be a set "rate of decline" as there are too many external factors involved, car speed, gear, steepness of the hill, head winds, towing a load, any applied braking.......

So there is no actual "rate of decline" table or setting. It's more like a "rate of correction" and this "can effect the rate of decline". So if you want the car to not slow down as much, run "less correction" in B5123. So the timing should remain higher on average. If you want the car to slow down quicker, then have "more correction" so the average timing should be lower.

[The Alchemist]

awesome thanks mate !

[gmtech16450yz]

Don't give up yet, there are several tables that you can try to tweak to do what you're trying to do. Yes, you can make the coast down speed drop faster, try these tables...

B1829, obviously lowering these numbers lowers airflow.
B1604, lower numbers will close throttle faster on decel, try 10 or below and a free rev in park will pretty much close the throttle as soon as you let off the pedal.
B1651 and 1652, try playing around with lowering these, another one that will definitely make the rpms drop faster. Too low and you won't have enough rpm control when cold or under load. (Basically the max tps that will be used during idle control, in all situations.) 2.2% in these tables equals about 21% tps(TAC), another table that you have to figure out what the numbers actually relate to!

There is a ton of idle and coast down control in E38's, it's just a little tricky figuring out what does what and what tables interact with each other.

[hymey]

Coast down spark, is basically idle spark with the car moving ie above 3km/h depending on the threshold.

The coast down spark mimics the idle spark table and u can raise it higher or lower pending on how fast u want rpms to decrease. The coast down correction will correct spark as it tries to approach idle rpm whilst moving.

This was very important to me as tuning the supercharger is harder then a NA car especially manuals. The e38 has a maximum hardlimit of 18.8% SAE.TP. Anything lower then this from correction or so forth the rpms will drop very quickly. So I increase coast down spark to settle it down. If you command 12g/s in min idle air it will be at its 18.8% limit so I add 12s to the min idle air tables until i approach idle speed and ramp it down.

I have also learn't that min idle airflow is actually "desired airflow" there is also Min idle area. This is a lower hard limit and we cannot command higher then 18.8% for the upper limit. The e38 sees "desired airflow" as a goal idle percentage ie 10g/s is around 17.6%. So if we set to 10g/s(17.6%TP%) and then uses correction to alter from the base of 17.6% to obtain idle speed,If the TP% reduces to correct idle it is necessary to reduce commanded(desired) idle airflow "min idle airflow" to ensure as little correction as possible.

Which is why its very important to sometimes disable correction for coast down and idle. ie idle is the base to start at and once its right everything else falls into place.