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

[Vogt Family]

On Tue, 23 Mar 1999, bob at bobthecomputerguy.com (Robert Harris) wrote:
> 
> 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.

Bob, are you on the Ford Trucks list also?  Very good explanation,
thanks a lot.  This is what I have been waiting to hear for some time
now.  I am building a Ford 429 for my projectmobile 4-wheel drive
truck.  Basically the major concern is maximum torque right off idle. 
My question is, would this type of cam be a good idea without a turbo,
since I am of the limited funds, or should I buy a normal "torque" cam
for the first stage of the build, and get a turbo grind later?  Also, I
have been wondering about the suitability of an automatic transmission
with this type of power delivery.  My major intended use is super slow
crawling.  The turbo idea is to also make it "mud-bog-able".  Any
thoughts, anyone?

Birken