[Isaac Zachary]

Originally Posted by Michaelflat1

I drive an electronic throttle car, and unfortunately below 1500rpm, if you come off throttle, there will be no fuelcut, if you come completely off throttle, TorquePro shows timing going quite retarded, (maybe -12 degrees) and there is still quite a substantial engine braking effect

Both retarded timing and closed throttle produce engine braking. A closed throttle produces more braking than an open throttle with no fuel.

Originally Posted by Michaelflat1

At the lowest possible throttle position (quite easy to do on an electronic throttle) i can have the same fuel use, and throttle position, but with +21 degrees advance, and a small amount of torque from the engine, an improvement from negative.

More advance (to a point, depending on RPM and other factors) will produce more torque or higher idle RPM or less engine braking. But retard it enough and the engine will brake the same as if fuel were cut. At idle you can get a bit of instant torque just by advancing the timing, which helps overall acceleration (and deceleration) response by controlling timing in addition to throttle.

Originally Posted by Michaelflat1

The fuel use parameter in Torque is not always accurate (it is during warmup), but is skewed with EGR operation, as Torque assumes all airflow is fresh air and does fuel use based on that (i believe)

I don't quite understand what you're saying. Fresh air needs fuel. The engine shouldn't be pouring in more fuel for recirculated exhaust gases. So if it assumes all airflow is fresh then that's a good way to measure fuel, as long as the AFR is the same since the EGR system isn't going to affect how much fuel is added to the fresh air.

Originally Posted by Michaelflat1

So, in these off throttle events, what is going on? Fuel being kept around stoch mixture (14.7:1 AFR) at/near the minimum throttle position? Is the timing purely to reduce torque to a minimum? Or is the mixture going lean and this is because of knock concerns? At roughly 1400rpm off throttle, Torque Pro shows fuel use to be roughly double that of idle, despite the engine producing negative/no torque.

I know that in fuelcut operation (openloop, -15 timing), the EGR and throttlebody are commanded to let some extra air through, and increase manifold air pressure (reducing effective vacuum)
and coming out of fuelcut, the throttle closes and pressure decreases.

Knock is never a problem at closed throttle. You can advance the timing all you want and you won't get knock.

One reason to retard the timing like that is to produce more engine braking while being ready for the engine to start producing torque again (because you hit the throttle again or because it slowed down to idle). That way you already have fuel in the intake and all you need is to bump up timing and you get instant "throttle" response even before the throttle moves.

Another possible reason may be help keep a stoich AFR (for emissions reasons) by opening the throttle a little while not losing engine braking at the same time. I know that this used to be necessary with many carburetors. Maybe a modern car has a hard time figuring how much air is going in at high RPMs and closed throttle.

Or it may do it to keep the engine and catalytic converter warmed up. I've been in older cars that cut fuel and on long downhill descents the engine can cool so bad you lose all heat for keeping the cab heater running not to mention how that might affect emissions.