valve overlap

[Jim Yeagley]

    Hi.  I'd like to know more about these "negative overlap cams".
Seriously!  What are they, and how do they differ from the norm?  Ditto on
your comments about finding a cam grinder who will tailor YOUR cam to YOUR
application.

Walt,

I'm not sure about 'negative' overlap, but I'll assume they are referring to
less overlap.  With a normally aspirated (unblown) engine, more valve
overlap, or a larger lobe seperation angle, aids mid to upper rpm hp by
allowing the outward flowing exhaust gasses to create a sort of vacuum in
the combustion chamber when the piston is around tdc and both valves are
open, where this vacuum starts to 'pull' the intake charge in before the
downward moving piston begins to draw.  At lower rpms, it creates that lopey
idle we all know and love, but robs low end torque.

With an engine under pressure, this larger overlap actually hurts, since the
intake charge is under pressure and waiting for the valve to open so it can
rush in.  With more overlap, this intake charge rushes right out the exhaust
valve initially, instead of creating more pressure in the combustion
chamber.

This is what I think they mean by using negative overlap cams for turbo'd or
blown engines.

Forgive me if I got too basic, it's something I actually understand!

Jim Yeagley
1996 Dodge Indy Ram
See it and many others at: www.indyram.org
1975 Pontiac Grandville Brougham Conv. (in baskets)
webmaster at indyram.org
jimyeagley at stratos.net