valve overlap

[Roger Heflin]

On Tue, 23 Mar 1999, Jim Yeagley wrote:

> 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!
> 

I believe you also want to use the less overlap cam's on a N2O engine,
for similar reasons.    For my car there is a 224/236-112 cam, and a
224/236-114 cam, the 114 cam is for blown/N2O applications, and the
112 cam is for NA engines.  I am running the 114 cam because I want to
run N20 someday.   I believe the stock cam in my car was somewhere
around 202/205-117 so it has even more separation.

				Roger
				93 LT1 Z28 (350)