DFCO for beginners - Chevy Impala SS Forum
 
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post #1 of 8 (permalink) Old 04-28-2011, 02:54 PM Thread Starter
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DFCO for beginners

OK, the label Decel Fuel Cut Off self-explains what it does, and when it does it, in theory.

I do NOT want DFCO to 'pull too hard', which [I heard] can cause cylinder bore and piston ring wear, so I doubt I want more than 15° DFCO Spark Retard. [Right?]

I do want DFCO to work more often, sooner, and longer than GM did - ideally, whenever the car is decelerating and the TPS is rapidly dropping below 5% because I'm about lift my foot off it?

For 'stock-ish' engines such as mine, what's involved?

(If I'm gonna change my driving style to improve MpG, I want my PCM to maximize the return on my efforts.)

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post #2 of 8 (permalink) Old 05-02-2011, 07:49 PM
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try going to http://www.monodax.com

ASE Master Certified L1 Tech's 9C1 that will become a snake eater. Best 13.62@ 97 with 270,xxx.T56. Who knew having a cop car could be so useful, everybody seem to part the seas for me. he he.
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post #3 of 8 (permalink) Old 05-03-2011, 12:27 PM
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man this is not an esay topic to write up

this involves a realtion of the PCM calibration and a the physical setup on the driveline

not just pcm numbers

NotoriouSS Tad
96' ImpalaSS - Johhny 5 - 7.9 @ 89.07 1.734 - sshhhhh......
96' 9C1 - POS / surf sedan / Beater - traps 103MPH / 30+MPG freeway~
67' El Camino - Rosie - 7.9 @ 86.5 1.77 60' on a street tire NA
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post #4 of 8 (permalink) Old 05-03-2011, 07:56 PM
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i agree there is one website for basic info on reprogramming. http://www.lt1pcmtuning.com/tips/

try that

ASE Master Certified L1 Tech's 9C1 that will become a snake eater. Best 13.62@ 97 with 270,xxx.T56. Who knew having a cop car could be so useful, everybody seem to part the seas for me. he he.
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post #5 of 8 (permalink) Old 05-04-2011, 10:19 PM Thread Starter
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As my sig suggests, I'm practically stock.

Do any Switches affect DFCO?

Constants …

1a) Is DFCO Enable RpM Threshold=1000RpM too low?
1b) Is DFCO Disable RpM Threshold=600RpM too low?

2a) Is the DFCO Enable MAP Threshold in reference to:
the MAP reading the moment when I lift (TPS < 10%)?
OR
the MAP reading the moment after I lift (TPS 0.00%)?
2b) Why can't I find this constant?

3) How high can I go with DFCO Disable MAP Threshold? 100KPa would always stall, right? How about 40Kpa?

4) I don't get the point of DFCO Disable RpM Decrease Threshold? How quick an RpM drop is too quick?

5) Doubt I should worry about the DFCO Enable Coolant Temp Threshold?

6) How low is too low for the DFCO Enable MpH Threshold?

7) Is it safe to set the DFCO Enable Delay Timer to 0.0 Secs?

In all cases, if power steering is lost, or the engine needs to be restarted, that would be using DFCO for too long, right? Are there other symptoms?

I fear piston/ring/bore wear; I won't ask for more than 15° of retard unless someone credibly convinces me it's safe.

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post #6 of 8 (permalink) Old 05-04-2011, 11:57 PM
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On mine, stock timer was 2.5 seconds, I found that for the most part, stock settings and a 1.2 second timer gave pretty good results. If you have timer set to 0, it ends up being frustrating, you back off and it suddenly starts to cut fuel, you end up with a normal operation and fuel cut off mode and no normal coasting.

Now, my car was a factory 3.42 car, so having those gears with the fuel off, you decel. You ended up frustrated. 1.2 seconds was about as low as I could tolerate it.

The problem with a deeper cut off, is overdrive. With in OD, it was hard to stay in DFCO for very long. If you went to 3rd, you can really run DFCO down pretty far, to around 800 rpm or so.

I tried keeping in lockup at closed throttle, it also helped a fair bit to let you run the DFCO down about as low as you could. I never had it stall.

I would keep close to GM settings, maybe tweak a little on the rpm limits and of course the timer.

The timing really isn't a big deal, there is no fuel anyway, why would it matter? If anything, it probably protects it when comes out of DFCO so it doesn't ping right away as fuel mixture is very lean.

Hope that helps some!

Quote:
Originally Posted by Marky Dissod View Post
As my sig suggests, I'm practically stock.

Do any Switches affect DFCO?

Constants …

1a) Is DFCO Enable RpM Threshold=1000RpM too low?
1b) Is DFCO Disable RpM Threshold=600RpM too low?

2a) Is the DFCO Enable MAP Threshold in reference to:
the MAP reading the moment when I lift (TPS < 10%)?
OR
the MAP reading the moment after I lift (TPS 0.00%)?
2b) Why can't I find this constant?

3) How high can I go with DFCO Disable MAP Threshold? 100KPa would always stall, right? How about 40Kpa?

4) I don't get the point of DFCO Disable RpM Decrease Threshold? How quick an RpM drop is too quick?

5) Doubt I should worry about the DFCO Enable Coolant Temp Threshold?

6) How low is too low for the DFCO Enable MpH Threshold?

7) Is it safe to set the DFCO Enable Delay Timer to 0.0 Secs?

In all cases, if power steering is lost, or the engine needs to be restarted, that would be using DFCO for too long, right? Are there other symptoms?

I fear piston/ring/bore wear; I won't ask for more than 15° of retard unless someone credibly convinces me it's safe.

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Last edited by RamAirRocket; 05-05-2011 at 12:00 AM.
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post #7 of 8 (permalink) Old 05-05-2011, 01:18 PM
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the trick is to stay in lockup and not have the PCM click off the deeper gear
that way the motor produces a large ammount of vacuum and keeps you in DFCO

the stronger the motor the less youll notice the delay IMHO
as cruise TP is so tiny anyway

with big stall the trick is mainly in the shift tables and TCC tables more than the DFCO settings

NotoriouSS Tad
96' ImpalaSS - Johhny 5 - 7.9 @ 89.07 1.734 - sshhhhh......
96' 9C1 - POS / surf sedan / Beater - traps 103MPH / 30+MPG freeway~
67' El Camino - Rosie - 7.9 @ 86.5 1.77 60' on a street tire NA
15' Cruze 2.0L TD <-- hate it <--
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post #8 of 8 (permalink) Old 05-07-2011, 06:53 PM Thread Starter
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Quote:
Originally Posted by RamAirRocket View Post
On mine, stock timer was 2.5 seconds, I found that for the most part, stock settings and a 1.2 second timer gave pretty good results. If you have timer set to 0, it ends up being frustrating, you back off and it suddenly starts to cut fuel, you end up with a normal operation and fuel cut off mode and no normal coasting.

Now, my car was a factory 3.42 car, so having those gears with the fuel off, you decel. You ended up frustrated. 1.2 seconds was about as low as I could tolerate it.
My educated guess is to determine how quickly my right foot can possibly go from 100% TPS to 100% brake (guess: under .3 Secs), then make sure that the DFCO Enable Delay Timer is longer than that.
Quote:
Originally Posted by RamAirRocket View Post
The problem with a deeper cut off, is overdrive. With in OD, it was hard to stay in DFCO for very long. If you went to 3rd, you can really run DFCO down pretty far, to around 800 rpm or so.
The way my shift strategy tables are set, I'm usually in either 2nd or 3rd (aka 3L60E); I expect most of my DFCO in 3rd.
Quote:
Originally Posted by RamAirRocket View Post
I tried keeping in lockup at closed throttle, it also helped a fair bit to let you run the DFCO down about as low as you could. I never had it stall.
So, Disable TCC UnLock @ Low TPS? Disable TCC UnLock on Rapid TPS Decrease? Or Disable both?
(And what's the ISSF filter's problem with the prefix 'D-I-S' anyway?)
Quote:
Originally Posted by RamAirRocket View Post
I would keep close to GM settings, maybe tweak a little on the rpm limits and of course the timer.
I plan to tweak all the DFCO constants a little bit at a time. Hope I don't stall much!
Quote:
Originally Posted by RamAirRocket View Post
The timing really isn't a big deal, there is no fuel anyway, why would it matter? If anything, it probably protects it when comes out of DFCO so it doesn't ping right away as fuel mixture is very lean.
I'll leave DFCO Spark Retard as is; I not only fear piston/ring/bore wear, but I don't want to suck motor oil into the combustion chamber.
Quote:
Originally Posted by RamAirRocket View Post
Hope that helps some!
If only the ISSF could get back what was lost … You've pointed me in the right direction, and helped to draw the lines within which I should remain. Thank you so much.
Quote:
Originally Posted by ProTools4 View Post
the trick is to stay in lockup and not have the PCM click off the deeper gear
that way the motor produces a large amount of vacuum and keeps you in DFCO
In principle, this makes no sense to me. What happens during a decel downshift that disables DFCO? (If this is the SINGLE redeeming quality of 2.56, it ain't enough.)
Quote:
Originally Posted by ProTools4 View Post
the stronger the motor the less youll notice the delay IMHO
as cruise TP is so tiny anyway
Just found a table whose existence puzzles me:
DFCO Enable %TPS Threshold vs RpM

I don't get it. This table seems to provide some sort of offset to the DFCO Enable Delay Timer?

A) None of the TPS values in DFCO Enable %TPS vs RpM ever exceed 9.4%
B) can't remember ever cruising with less than 10% TPS IN ANY CAR

1) How are 'A' & 'B' connected?
2) What is this table saying that I'm not getting?
3) Is this table related to why RamAirRocket needs 1.2 Secs of fueled coasting BEFORE DFCO kicks in?
4) If you set all DFCO Enable %TPS Threshold vs RpM values to zero, how would this affect DFCO Enable Delay?
Quote:
Originally Posted by ProTools4 View Post
with big stall the trick is mainly in the shift tables and TCC tables more than the DFCO settings
I'll never use a torque converter that won't let me haul at least seven passengers worth of weight. Don't know exactly how limits my converter's stall speed, but I think it pretty much limits me to a 12" TC.

Still, I don't know how much time I'd've wasted before fully realizing what you two have spelled out for me.
You guys are awesome! (Don't let that stop you from adding anything else that might help me or others, though )

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