> My guess about the non-firing-at-idle cylinders, was that the weakly-closed
>valves were letting exhaust gases into the intake tract in precise phase to
>prevent the "bad cylinder" from getting any intake gas. It wouldn't take much
>exhaust gas to pretty well destroy the manifold vacuum. What do you think?
This is in fact precisely the problem with high-overlap (high-performance)
camshafts, that the intake and exhaust charges get mixed up in the ports
because there's a period during which both valves are open. At high RPM
the velocity in the intake port is great enough (because the piston is
dropping quickly in the bore) that you get very good filling of the
combustion chamber, so good in fact that a little of the air-fuel mixture
gets pushed out the exhaust valve to make sure there's no exhaust gas in
the cylinder. Likewise, from the exhaust stroke's viewpoint there's a
high-pressure front of incoming air-fuel that helps push the exhaust gas
out and means it takes less engine work for the piston to push the burned
gases out. (Remember of course that this also has to work in concert with
the header, intake tract, valves, and port sizes. System, system, system.)
The problem is that at low engine speeds, there isn't such a high velocity
in the intake ports -- particularly if they're large, high-performance
ports -- and you get this tendency to burble and stumble at idle. This is
why most high-performance street cams warn you about rough idle, and why
most race motors idle at 2500+ RPM. Factor into this the consideration that
a big port (remembering Venturi and Bernoulli) requires a high flow velocity
to fill it before you start using its full capacity, and you've got a setup
for poor running till you "come on the cam." Once the velocities reach the
point at which they all start to harmonize -- quite literally, as the
fluid dynamics of this are said to be much like those in a wind instrument
or a pipe organ -- things begin to work in a manner that can be dramatic.
The sudden, almost turbo-like push in the back that my race car gets at
4000 RPM is a dramatic example of this.
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