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Old 02-04-2014, 12:26 PM   #170 (permalink)
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Originally Posted by oldtamiyaphile View Post
I've tried this principle (in my Jeep), with the engine off (at the key), openning the throttle to WOT, and all I get is massive engine braking (along with a roaring sound!). I think more air being sucked into the engine means more pumping losses. I know my Fiat with no throttle plate has zero engine braking though.

What's going on?
I think the roaring sound is tricking you into thinking engine braking has increased. Engine off coasting at WOT reduces engine braking. Always has, always will. The roaring sound is your exhaust - now that the engine has something to pump, something comes out of the tail pipe again.

Remember when the engine is off, compressing a full cylinder of air is like compressing a giant spring. So all the effort required on the compression stroke is returned on the [ex] power stroke when this giant spring pushes back down, resulting in a net power consumption of zero. Minus friction, heat, etc.
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