Or is it just me? There's a particular downhill road I drive down pretty much every day. When it's dry, I have to apply the brakes to stop myself speeding up. When the weather is damp, I don't. Why does that happen? 
Why does engine braking work better in the wet?
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Soldato
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Or is it just me? There's a particular downhill road I drive down pretty much every day. When it's dry, I have to apply the brakes to stop myself speeding up. When the weather is damp, I don't. Why does that happen?
When it's wet the water on the road adds extra friction.
ORLY?
It reduces friction between the tyre and road surface, though a significant depth of water would increase rolling resistance. On an averagely "wet" road at lowish speeds I wouldn't have thought it would make much difference.
More likely that the cool, damp air is actually increasing engine braking slightly. Moist, dense air would increase the pressure drop across a closed throttle, increasing pumping losses.
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How does that work then?
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Indeed. The coefficient of friction is reduced when the road is wet.
Is it because you're driving slower as its wet?
I'm with this guy, you're must be driving slower but when it's wet there's extra noise so probably makes if feels/sound like you're going as fast.
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Is that to say you have never noticed a change in engine charateristics between a scorching hot day and a cool one?
Sure, certainly recently noticed it run nicer in the cool evening than the scorching midday sun. That doesn't really have anything to do with engine braking though, in which I can't say I've noticed any difference. Frankly I can't see why it would make a difference anyway given that engine braking is caused by the lack of air in the manifold.
I'm not driving slower. I always check my speed at the top of the hill, and it's always 29-31 (indicated). In dry conditions this creeps up to about 35 by the time I reach the bottom unless I apply the brakes. In wet conditions, it goes up to maybe 32 if I don't apply brakes.
It's a fairly straight bit of wide road so there's no reason to drive slowly on account of rain.
It's a fairly straight bit of wide road so there's no reason to drive slowly on account of rain.
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I actually do thing it's this. Extra water needing clearing will increase effort required to push it out the way, increasing the rolling resistance of the wheels on the engine.
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