DFCO help and other ?s [Archive]

twcblackhand6970

07-07-2009, 02:40 AM

I'm trying to tune my car for some better mpgs because I've been averaging about 10mpg lately with mixed hwy and city driving. I've turned the TC to lock earlier and advanced the crap out of my timing in the lower rpms at all MAP values but mainly the 45-60 range where my car is usually at while I'm cruising. I was wondering if putting the most spark advance without knock nets the best mpgs because that is what I've been doing and I'm just trying to make sure that it's not wrong. Also I've been tinkering with the DFCO. I thought DFCO would either null the blms or make them lean but while I'm in my DFCO range I never see that happen. I never see my timing get retarded either, it'll just float around 34* on a decel. Is my DFCO wrong or something?

My values are:

DFCO Enable RPM Threshold: 1700rpm
" " Disable " ": 950
DFCO Enable MAP Threshold: 28kpa
" " Disable " ": 70
DFCO Disable RPM Decrease Threshold: 3500rpm/sec
DFCO Enable Coolant Temp. Threshold: 20*C
DFCO Enable MPH Threshold: 14mph
DFCO Enable Delay Timer: 0.0sec
DFCO Spark Retard: 30*

I'm shooting for a aggressive DFCO system that'll basically cut fuel when I let go of the throttle at cruising RPMs but not while I'm going too fast so I don't have a enormous amount of vacuum. My car idles at 76kpa with 12gps airflow according the the MAF sensor. The enable MAP value of 28kpa is basically when my car is about 75mph at highway speeds. Are my values backwards always disabling or is it just activating and I'm not looking for it correctly?

MEAN LT1

07-07-2009, 06:54 AM

This is what I used. Its long but it helps.....

Increasing MPG

These days, getting better MPG (miles per gallon) out of your vehicle is even more important, with prices at the pump increasing so quickly.
Thankfully, there are several ways to substantially increase your LT1's MPG (as well as with other vehicles with similar PCMs).
There are at least two main ways to affect MPG: fueling and timing. The first is probably obvious, but many people don't realize that reduced timing can reduce combustion effeciency, which in return uses more fuel to get less torque (which moves the car).
Fueling

We'll look at fueling first, since it's the easiest way to get better MPG. There are several areas and tables to be addressed, as there are several types of driving that affect MPG.
Fuel Pressure

This has to do with general tuning too, but can severely impact MPG.
If your fuel pressure is set higher than stock (43.5psi on the LT1), and you have not accommodated for that in the PCM tune (by adjusting the fuel injector constant), then your car will run rich. The O2 sensors will sense this and try to correct, but the stock tune can only correct so far. Until it corrects, you'll be running rich, which decreases MPG.
Running rich, if it happens long enough, can also foul the O2 sensors, which will put you in a never-ending cycle, as the fouled sensors read less oxygen and think more fuel is needed for efficient combustion.
Fuel Injector Constant

This can be another obvious culprit, but can sometimes be partially overlooked depending on the brand of injector.
Some aftermarket injectors are rated at a pressure other than GM's stock of 43.5psi. Even within the same brand (such as Accel, see here (http://ltxtech.com/files/AccelFuelInjectors.pdf)), some injectors flow differntly than their rating at 43.5psi.
For example, Accel's 24# injector flows 24.3# at 43.5psi (more than rated), but their 26# injector flows 25.6# at 43.5psi (less than rated). If you did not know this, and simply used their advertised flow rating in your tune with the stock fuel pressure of 43.5psi, your BLMs would be off. Obviously, the ones that flow more than advertised will make you run richer if set to the advertised number.
Do your best to find out what the injector flows at 43.5psi, and use that number. If, for some reason, you are using a different fuel pressure than the stock 43.5psi, adjust appropriately.
Fuel Injector Offsets

Many people are familiar with this table from using Bosch/Ford SVO injectors, which have a longer offset than the stock AC/Delco injectors.
In my experience, raising these values (to a point) will richen up the mixture. However, they should be set correctly for your injectors, as having them set incorrectly can cause driveability issues like bogging and backfiring. Do your best to find the correct offsets for your injectors, and your MPG should increase.
Open Loop AFR

This is a good table to use to increase MPG. Open loop is the area of driving when the car is first started, and its length will depend on coolant temperature and elapsed time after startup.
By increasing the values in this table, less fuel will be used before closed loop (O2 sensor feedback) is entered. You can run up to 14.9-15.0 at idle and part-throttle MAP values when the car is fully warmed up (80*C+), without much if any impact on driveability (back-firing, etc.).
You can also increase the length of time that the PCM stays in open loop, to give you a leaner AFR for more time, before closed loop is entered and an AFR of 14.7:1 is attempted.
Initial Startup AFR Enrichment

This was a fairly late table addition to TunerCat. Its use is not completely understood, but it must have to do with a richening of the AFR right after startup, depending on coolant temperature. It must be used at least during open loop, and perhaps also during closed loop if it is entered quickly.
The obvious way to increase MPG is to lower these values, and even zero them at fully warmed up coolant temps (80*C+). Less fuel will be added by the PCM during startup, which could be substantial if you use your vehicle for lots of short trips.
O2 Rich/Lean Swing Threshold

Here is the great secret to increasing MPG! A fairly late table addition to TunerCat, this one tells the PCM which voltage from the O2 sensors is the halfway point between rich and lean. Most injectors have their stoichiometric (14.7:1) point output at about 0.450V. With that value entered in this table, the PCM will think that any voltage above .450 is rich, and any voltage below .450 is lean.
The way to increase MPG is to lower this value. What you will be doing is telling the PCM that a lower voltage is the halfway point, which it will do its best to reach via the normal BLM setup. The result is that what used to be considered lean (say, .400V) will now be considered ideal, and the PCM will correct for that leaner output of the O2 sensors, thus giving an AFR of leaner than 14.7:1, which will use less fuel and increase MPG.
Because stock-type injectors are "narrow-band" (they are only very accurate along a short range of voltage), you cannot go too far or you'll end up in the less accurate range of the sensor, and may get inconsistent results (a wildly swinging AFR, perhaps even dangerously lean). Exhaust temperature will also affect voltage, meaning at a different exhaust temp you may be at the same AFR but the sensor will read a different voltage.
I can't recommend any particular value because of this limitation of the sensor; but anything lower than stock should increase your MPG. It is up to your accepted level of risk of running overly lean that will determine how far you should go.
Spark Timing

Timing makes a large difference in combustion efficiency. A more efficient combustion means less fuel is needed for the same amount of torque output, which also results in better MPG.
As mentioned above in the Idle tuning section, Christian Milliard discovered that substantially increasing idle timing reduced both airflow and pulsewidth requirements for the same RPM, and also decreased coolant temperature (http://www.carprogrammer.com/Z28/PCM/Ignition/idle%20spark%20tuning.htm) (probably due to less unignited fuel burning in the exhaust).
Especially with aftermarket cams and displacement, more timing advance will generally equate to more MPG. The only reliable way to tune accordingly is trial-and-error, as mentioned above for spark timing tuning. Most often, the top-end of timing that you can without any knock will give you the best combustion efficiency and, therefore, the highest possible MPG.