Injector Math

[Tom Sharpe]

Walter Sherwin wrote:

> >Negative overlap cams

> This design greatly reduces the mid range
> >torque of an engine in exchange for fewer concerns with EFI tuning.
>
> Thank-you Gary!  One question though.  I kinda figured that the "negative
> overlap" cam title referred to reduced overlap, during the period which both
> the intake and exhaust valves would be simultaneously off their seats, near
> TDC.   Did the "n.o." cams go to the extreme of actual "zero", or
> "negative",  seat-seat overlap?  Or,  just significantly reduced overlap
> versus their naturally aspirated counterparts?

IMHO you really need to look at degrees open (duration and location) at .020 and
.050 for the intake and exhaust. roller hydraulic lifters changed all of the
rules of thumb I'm used too as they only have very small clearance ramps.

Traditional performance cams have wide centerlines (to close exhaust later and
help idle) and extra duration on the exhaust to make up for a poor exhaust
port/system. They run better advanced as the above changes make the intake lobe
retarded relative to a std cam profile. The intake closing point has the most
effect on torque (peak). Spreading the CL and adding exhaust duration really
just opens the exhaust earlier.

Roundy round SBCs usually use lots of lift/duration and narrow centerlines (lots
of overlap). Just make them rich enough and they will idle (fail em. tests) but
boy will they make torque - and RPM.

Pick the intake lobe that gives you the required RPM range and HP/torque, then
pick the exhaust lobe & CL (exhaust close) that will get you past the
sniffers.

Just my .02  Tom S