Just trying to understand how & why this works and what impact it has on performance and economy.

I have read a number of threads on the subject but how it works and what it is doing is escaping me!!

 

Basically no fuel injected = no fuel consumed. On hard decel the engine doesn't need fuel, it serves no purpose. The engine is either fluid or mechanically locked to the torque converter or clutch and continues rotating due to the driveline. There is no need for combustion, so there isn't any. You save a little bit of fuel and the catalytic converter stays happy.

On hard decel the ecu doesn't turn on the injectors. The engine pumps dry air in and out.

 

Decel Fuel Cut Off: No effect on performance,,, slight increase in fuel economy.

The above explaination is correct, under decel, the fuel injectors are not pulsed. Since the engine can turn because it is connected to the drive wheels which are turning, the engine will not stall out. When the engine RPM drops to a certain speed, the injectors are turned back on, to prevent the engine from stalling out.

 

ya know.... I logged on the ask the same question.....

Thanks Chalky

.... and thanks to DOC99SS and Andereck for the answer.

R/

Frat

 

Good answers! How come under my DFCO, I still have an A/F of ~11.8:1?

 

Because yours is apparently turned off.

 

Thanks for the explanation. Why would anyone want to or need to alter these?

 

I think the biggest reason it is turned off is during tuning. You want as many variables taken out as possible. Once the tune is dialed in most turn it back on.

I have read somewhere people gaining some gas mileage by adjusting the tables. May try searching here and on the HPTuners forum.

 

Some times people will use gasoline to cool the motor down turing it off will allow you to cool the motor with extra fuel that isnt burnt. Ths dosent help the average person any but if you are road racing it can be helpful.

 

I see no good reason to mess with the DFCO tables, except maybe for high stall torque converters. But since I have an M6 car, I cant really say.

 

Cars with cams may need to tweak the tables that enable DFCO. I rarely see much below 23kPa on my H/C SS on decel. The stock DFCO settings wouldn't allow it to activate until I adjusted the MAP enabler.

 

Quote, "I think the biggest reason it is turned off is during tuning. You want as many variables taken out as possible. Once the tune is dialed in most turn it back on."

Minor point here but I always leave them turned on during tuning because they have no affect during anything but deccelleration and if they do, I want to see it. Plus, I think that the potential residual fuel left in the exhaust by turning DFCO off distorts tuning info more then leaving it on. For those who live in mountainous roads, DFCO really saves fuel on those long downhill runs.

 

The problem with leaving it on during tuning is it can skew the low-MAP side of your VE table because the WB will read it as a lean mixture. Then, you get AFR%Error OR BEN's upwards of "1.30" saying you need way more fuel here because the commanded AFR listed by the PCM is still 14.63 (or whatever you have it set at). I usually get it close with the WBO2, re-enable DFCO and tweak the final low-MAP areas using the STFT's/LTFT's.

 

How about DFCO on LS2s?? the settings to tweak it are very different.

 

Made me wonder about my settings. If DFCO was not being activated, would that cause some of the popping I am getting with my Borlas?

SSpdDmon, how did you go about determining enable/disable settings?

 

I find that I get popping only when DFCO is enabling or disabling.

 

My vet has always had some exhaust noise even stock but with the Borla's it is even worse. How are you determining when it is DFCO is enabling/disabling?

 

Few ways

1) You can feel the engine breaking effect (in an M6 anyway)
2) You can see the timing reduce dramatically
3) You will see your AFR hit 19:1+ on your wideband
4) If you were to view a graph log you would see that you hit the DFCO enablers/disablers

 

I think the popping on decel sounds awesome. My dad's C5 does it and it reminds me of a race car. I think it has to do with the design of the exhaust...x-pipe and what not.

Set you an email about the how...

 

If you are dumping excess fuel during decel, then likely that is the cause of the raz you are hearing. The raz is the excess fuel lighting off in the exhuast pipes. Personally, I like a little raz, but at $2.30+/gallon, I can live with out it.

A little tweaking on the DFCO spark ramp in/out rates should help with the raz during DFCO transistions.

I also had to raise the DFCO speed enablers to avoid mucking up my idle transitions. Lightened rotating assemblies are a bit more finicky.