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Old 01-01-2021, 11:30 PM   #10 (permalink)
Tahoe_Hybrid
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Join Date: May 2019
Location: California
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2020 - '08 Chevy Tahoe H
Last 3: 18.4 mpg (US)

2021 - '08 Chevy Tahoe H
90 day: 17.08 mpg (US)

2022 - '08 chevy Tahoe LT
Last 3: 14.38 mpg (US)

2023 - '08 Chevy Tahoe
90 day: 25.57 mpg (US)
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Quote:
Originally Posted by Isaac Zachary View Post
Both retarded timing and closed throttle produce engine braking. A closed throttle produces more braking than an open throttle with no fuel.


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.



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.



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.

engine braking is transmission is related..

If i'm in drive no engine braking and the "regenration is not even noticeable " the engine speed is not married


Now on the other hand if I have in in mountain mode or M4 as it's known... Engine braking is noticeable I can come to a complete stop almost .... to about 8mph.. from say 60mph if i go off the throttle.. but I lose CVT 5th equivalent gear, though

as you can see in these videos the v8 engine braking is quite effective..



Last edited by Tahoe_Hybrid; 01-01-2021 at 11:38 PM..
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