copied from sho forum
hit ctrl - f and search this post for the word “zero” … youll see that fuel DOES get cut off completely on the sho during decel
DECEL FUEL SHUT OFF STRATEGY
The Fuel Shutoff strategy is divided into two sub-strategies:
- Decel Fuel Lean Out.
- Manual Transmission Shift Fuel Lean Out.
The Decel Fuel Lean Out strategy reduces the fuel flow during specific load and RPM condition only during Closed Throttle Mode. The FUELPWs are multiplied by FN374 [Open Loop Fuel Multiplier vs. RPM] during a decel until the RPM is within the band created by [DSFRPM - DSFRPH] of the idle RPM range. Typically, FN374 is calibrated to provide a lean limit air/fuel ratio during the decel RPM range.
The Manual Transmission Shift Fuel Lean out strategy reduces (or eliminates) fuel flow at the beginning of the shift, until the RPM is within the band created by [SHFRPM - SHFHYS] of the idle speed (or until the transmission has been in Neutral for DSTM2 seconds). Typically, FN374 can be calibrated to zero during the range of shift RPMs.
CALIBRATION HINTS:
The use of Decel Fuel Shutoff can aggravate Clunk. Therefore, DFSO should occur only at low airflow (low MAP) in order to minimize the rate of change of Torque. CTDFSO [or the minimum time delay (in seconds) after entering Closed Throttle to enable DFSO] can be used to delay DFSO.
FN374 should be calibrated to avoid lean misfires. Therefore, it should be either zero (Decel Fuel Shutoff) or greater than the lean limit (Decel Fuel Lean Out).
CTEDSO [or the time delay (in seconds) during extended decels before entering DFSO, e.g. when travelling down a mountain or large hill] should be greater than CTDFSO to prevent DFSO after Tip-in, Tip-out.
As you can see from the excerpt above, the factory powertrain engineers recommend against excessive use of DFSO except in conditions where you can get away with it (e.g. in low airflow conditions after a specific amount of time while in a confirmed closed throttle state). Of particular note here is the encouragement for one to calibrate their Open Loop Fuel Multiplier function to work in tandem with DFSO so as to avoid lean misfires - a situation which is a very real possibility under the right conditions.
Overall, the “old” GU strategy utilizes 21 calibratable parameters and one calibratable function for Decel Fuel Shutoff operation (most of which were not listed or given in the excerpt above). However, since you are specifically asking your question from the standpoint of a newer, ATX-based EEC-V calibration, I would now direct your attention to chapter 9 of the CR strategy documentation (most of which applies directly to the '96-'99 V8 SHO calibrations), which provides an even better description of the DFSO strategy:
9.9.13 DECEL FUEL SHUT OFF LOGIC (CRAEA)
FUNCTIONAL DESCRIPTION
The Fuel Shutoff strategy is divided into four sub-strategies:
- Closed Throttle Decel Fuel Shut Off {DFS_CT_DECEL}
- Manual Transmission Shift Fuel Shut Off {DFS_MTX_SHFT}
- Auto Trans Override of C/T Decel Fuel Shut Off {TRN_DFS_DSBL}
- Auto Trans Request of Tipout Upshift Decel Fuel Shut Off {TRN_DFS_RQST}
Since the type of DFSO required for a given application depends highly on the selected Transmission, this file represents a common interface to all applications. The Transmission-specific logic was removed to a separate file, which is selectable upon Strategy Book build time (as long as the parametric interfaces are taken care of).
The following is a general description of all types of DFSO. Portions of the information may not apply in all transmission/strategy applications.
The base Decel Fuel Shut Off strategy turns off the fuel flow during specific load and RPM conditions only during Closed Throttle Mode. Fuel is turned off by the Torque Control module which ramps injectors off during Closed Throttle Mode. The FUELPWs continue to be updated for current LAMBSE values even though injectors are off or ramping off. The FUELPW values are required for ramping injectors back on during re-entry to “normal” driving.
Logic is available to ensure the vehicle does not cycle in and out of DFSO too frequently. A hysteresis flag has been added, which will avoid re-entry into DFSO during a closed throttle decel, so long as the driver has not tipped into the throttle. This is useful for manual transmission shifts for engine braking on long coast downs. It can be calibrated out with DFS_PRV_SW.
The Manual Transmission Shift Fuel Shut Off strategy ramps off fuel flow at the beginning of the shift, until the RPM is within the band created by [SHFRPM_CL] of the idle speed (or until the transmission has been in Neutral for DSTM2 seconds).
The Tip-out Upshift Fuel Shut Off strategy ramps off fuel flow when a rapid tip-out is coupled with a transmission upshift, in order to decelerate the engine fast enough to avoid a clunk feel when the upshift occurs. The determination of this condition is made in the transmission strategy which passes a flag (DFSFLG_TRANS) to Fuel strategy. This feature is able to be calibrated out within the transmission strategy. It mainly plagues the CD4E-based transmission/drivelines.
Calibration Guide:
The use of Decel Fuel Shutoff can aggravate Clunk. Therefore, DFSO should occur only at low airflow in order to minimize the rate of change of Torque. CTDFSO can be used to delay DFSO.
CTEDSO should be greater than CTDFSO to prevent DFSO after Tip-in, Tip-out.
For MANUAL TRANSMISSIONS: SHFRPM should be greater than DFSRPM.
Alternate DFSO calibration for Misfire Profile Correction:
The normal Decel Fuel Shut Off sub-strategy can be controlled by two sets of calibratable parameters, depending upon the state of the Profile Correction logic. Whenever the appropriate KAM correction factors have not matured, there exists the need to apply the DFSO strategy in a more aggressive manner in order to allow adequate unfueled engine rotation to determine the crankshaft profile signature. The RAM bit CF_KAM_MTR is utilized within this strategy to indicate the state of the Profile Correction KAM maturity. The more aggressive calibration constants will allow DFSO to occur in a wider operating region than those assigned in the normal powertrain development process. This alternate calibration is intended to occur for only a short period of vehicle operation whenever KAM has been reset. The strategy will automatically revert back to the normal DFSO calibration upon indication that the Profile Correction factors have matured. .
When compared to the older GU documentation, you can see how- even in description alone - the DFSO strategy itself has been significantly improved and expanded upon in just the 8 years that elapsed between the final GU strategy update (GUFB) and the issue date for the above CR document. One can also see how there are now several additional conditions and/or states whereby the calibrator may elect to use or modify the operation of DFSO on his or her particular car.
My tuning platform is rather limited so I always enjoy reading about other tuning applications & techniques.