>
> John, would you please expand on that process a bit.  As I have always
> understood it, in gasoline engines,  "compression braking"  relied on
> the compression stroke to provided resistance to slow down the vehicle.
> Without fuel and ignition the engine just basically becomes a glorified
> air compressor, and the compression resistence will act to slow down the
> vehicle. Please enlighten us with more detail   as to how "vacuum" plays
> a part on "braking" with a gasoline engine.
>

Well, there are four distinct strokes the piston makes: compression, power,
exhaust, and intake. The compression stroke is kinda lik compressing a
spring. It does take energy to compress that air, but most of that energy
gets put right back into the system when the piston comes down, on what
would normally be the power stroke, because that compressed air can't go
anywhere and is still exerting pressure on the piston. Diesel trucks have a
"Jake Brake", an electric solenoid that pops open the exhaust valve at the
top of the compression stroke to release the compressed air before it can
push the piston back down. The exhaust stroke is just pushing the air into
the open exhaust manifold, so there's negligible resistance there. On the
intake stroke, though, the piston is trying to suck in air from the intake
manifold. With your foot off the gas the throttle plate, that big butterfly
valve in the middle, blocks off the source of intake air except for the
small amount through the bypass. This means the intake stroke is pulling a
pretty hard vacuum, which gives you your braking effect.

--
John Bange
'90 Vanagon - "Geldsauger"