"Negative Overlap" or more commonly known as Pressure Balance cams

[Robert Harris]

The concept is quite simple and Dave Vizard in his book about Chevy Cams
covers it extensively.  Like all his books - more fact and theory than brand
specific.

The assumption is that since their is both positive manifold pressure and much
higher exhaust manifold pressure thru much of the power region, use no
overlap.

Overlap in a NA engine used the exiting exhaust gasses to draw fresh charge
into the cylinder and if the negative pulse from the exhaust is coupled to the
intake, create even more flow.  With a turbo, because of back pressure, for
much of the throttle, the exhaust back pressure is higher than the intake
pressure - so overlap actually forces exhaust up the intake manifold.

So you time the closing of the exhaust valve to get maximum exhaust extraction
and then delay the opening of the intake valve until well past TDC to about
the point where the downward moving piston has reduced the residual exhaust
pressure to the intake pressure.  At this point - about 30 after TDC - you
open the valve and the intake pressure - being marginally above the residual
exhaust - forces charge into the engine.  Absolute minimum exhaust charge
dilution.

Closing of the intake is about normal depending on boost pressure - but long
duration late closing is not optimal - pressure fills cylinder and holding
open late reduces the trapped charge and works the intake pressure against the
rising piston.

What results is a pure otto cycle engine - short cam on intake and normal
exhaust.  Very torquey on the low end - Vizard reports 1000+ hp with enough
boost on a 350 chevy so there is no lack of top end power.

1963 Ford C-600 Prison Bus Conversion "Home"
1971 Lincoln Continental 460 "Christine"
1972 "Whale" Mustang awaiting transplant
1978 Dodge Long Bed Peeek Up "Bundymobile"

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