ox sensor on sequential efi,high overlap cam....

[Greg Hermann]

>Wow,  if this isn't an opportunity to make a fool of myself I don't know
>what is.  Overlap is introduced by the cam designer to improve scavenging of
>the cylinder and maximize fresh charge (new fuel / air) into the cylinder.
>To do this, the concept of overlap sacrifices fuel economy and an orderly
>intake/exhaust processing - rather muddling both processes together.  The
>idea works generally in a narrow rpm band (the more scavenging, the narrower
>the band - thought this is also influenced by valve lift and assymetrical
>cam lobes, etc).  BTW, this can be fine tuned with intake and exhaust
>lengths (that's another story).
>
>The point of all this (and I'm very new to efi so take this for whatever you
>feel it's worth); but from a newbie's point of view, O2 sensing specifically
>and air/fuel measurement in general are pretty much worthless in this
>situation just because of the lack of process (intake/exhaust cycle)
>control.  Everything is sort of happening at once, so trying to "tune" the
>mixture based on a test sample of burnt fuel along with some fresh charge
>seems pretty worthless.  This is why those ultra simple constant flow
>injection setups worked so good!  FWIW, carbs also have some problems with
>cam profiles like this.
>
>Actually, might be worth the effort to forget the sequential efi (unless you
>want to impress everyone in the staging area with your flawless idle
>quality) and just batch it (I just found out about this and it has changed
>my whole outlook on efi!).   You have that much cam, you're not idling
>anyway!
>
>rick
>
Amen, And this is why efi setups for REALLY hot cams use a tps input
(rather than MAF or MAP) along with rpm as the second coordinate of the
air/fuel map in the ecu--TPS is simply a far more accurate indicator of
what is going on in the engine at low throttle openings and low engine
speeds than either of the other sensors under these kind of cam conditions.
Some which are set up this way even switch from TPS input back to MAP at
higher engine speeds and loads--the exact area where those funny looking
cams really DO work!!

Drag racers have always been pretty damn good at figuring out what
WORKS--and not always so good at explaining why!!

Even a fairly tame engine can be made to run very well indeed with a
TPS/RPM based fuel map--witness the SPICA (mechanical) injected 1750 Alfas
of the late sixties/early seventies. It is just not as easy to do as it is
with MAP or MAF, and it is WAY more sensitive to changes in mechanical
engine conditions. For instance: setting the valve lash on one of the above
mentioned Alfas .002" tighter than factory spec would create a flat spot
every time!!

Regards, Greg