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

[Bruce Plecan]

-----Original Message-----
From: Robert Harris <bob at bobthecomputerguy.com>
To: diy_efi at efi332.eng.ohio-state.edu <diy_efi at efi332.eng.ohio-state.edu>
Date: Tuesday, March 23, 1999 9:59 AM
Subject: "Negative Overlap" or more commonly known as Pressure Balance cams

Trouble is, appling theory, has it's limits.
To get good cylinder filling with the shorter Intake event (degrees
duration), you have to go for the gusto on valve accleration on the intake.
Too high accleration rate bad on lifters.
Then on the exhaust side, closing the valve too fast allows it to slam, and
then bounce a couple times.  Not good on valves, or seats.
Bruce


>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"
>
>Habaneros - not just for breakfast anymore
>