Originally Posted By: 440Magnum
I like the feature. Obviously my SRT doesn't have it, but most vehicles with automatics I've owned since the early 90s have that feature. It only kicks in if your speed exceeds about 5 mph over the setpoint. Perfect for avoiding a ticket on a gentle downhill. I remember that a particular software re-flash added that trick to my wife's 93 Vision TSi, and I was blown away at how effective it was. My limited experience with cruise control prior to that car was 80s GM junk that would lag 15 mph below the setpoint on an uphill and then downshift 2 gears and scream... followed by floating 15+ mph over the setpoint on a downhill and making me brake, then have to re-engage the cruise. Naturally the 93 Vision was the first car I had where I actually started USING cruise control.
A diesel is another matter- diesels have virtually no engine braking by reducing throttle to idle- that's the whole reason "Jake Brakes" and other retarders exist- to give a diesel better engine braking comparable to (or in the case of jake brakes, better than) a spark-ignition engine. Downshifting at idle without a retarder on a diesle will only spin the dang thing faster, not actually increase drag like a gasoline engine will when the throttle butterfly is shut.
Diesels engine brake. They engine brake much more than gas-engines. Engine braking is a factor of compression not manifold vacuum.
Jake brakes just engine brake better:
Quote:
A compression release brake, or jake brake, this is the type of brake most commonly confused with real engine braking; it is used mainly in large diesel trucks and works by opening the exhaust valves at the top of the compression stroke, resulting in adiabatic expansion of the compressed air, so the large amount of energy stored in that compressed air is not returned to the crankshaft, but is released into the atmosphere.
Normally during the compression stroke, energy is used as the upward-traveling piston compresses air in the cylinder; the compressed air then acts as a compressed spring and pushes the piston back down. However, with the jake brake in operation, the compressed air is suddenly released just before the piston begins its downward travel. (This sudden release of compressed air creates audible sound waves similar to the expanding gases escaping from the muzzle of a firearm.) Having lost the energy stored within the compressed air, there is no 'spring back' from it so the engine must expend yet more energy pulling the piston back down again.