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

[EFISYSTEMS at aol.com]

Hi All,
       There are lots of great theory's out there regarding camshafts....and
one of them is to have negative overlap and open the intake valve after tdc,
while this sounds good in theory it doesn't generally work dynamically or
mechanically.....I don't want to go on forever with this so I will just
address a couple key points.....In theory it would be great to open the intake
valve after tdc.......if we could SLAM it open with no delay AND have the
air/fuel immediatly start filling the cylinder,,,,but such is not the case in
any engine....the valve gradually opens and doesn't reach peak lift till over
100 degrees atdc...now yes with turbos and superchargers the air is
pressurized into the cylinder, but if you open the intake at the right time it
will force a scavenge cleaning and cooling the chamber with much less
overlap.......the more boost the less time the overlap cycle has to
be......this number is both boost and rpm dependant....it is time that matters
not the cam numbers,,,they are just what the result is......one other
thing,,,,,if your trying to depend on the piston to get the air moving in the
intake manifold, prepare to be spanked at any event you enter......No offense
intended to anyone and I do like David Vizard,,,,,,but any engine can make
1000hp with enough boost.........frustrated, and a book about camshafts with
10,000 examples and dynos sheets would be a good start,
-Carl Summers


In a message dated 99-03-26 19:01:40 EST, you write:

<< Subj:	 Re: "Negative Overlap" or more commonly known as Pressure Balance
cams
 Date:	99-03-26 19:01:40 EST
 From:	vogt at oro.net (Vogt Family)
 Sender:	owner-diy_efi at esl.eng.ohio-state.edu
 Reply-to:	diy_efi at esl.eng.ohio-state.edu
 To:	diy_efi at esl2.eng.ohio-state.edu
 
 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
 
 
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