SPICA performance

["Scott Johnson" <scott@domain.elided>]

The short answer is "send it to Wes Ingram". Because the SPICA system is an
open-loop analog computer, if you push the engine performance beyond what the
guys in Arese thought about, the SPICA system can't compensate. Mostly this
limits your cam selection. Apparently the SPICA system will respond to a cam
"one model hotter", whatever that means, and then nothing else. Most other
volumetric efficiency gains will also be ignored beyond a certain narrow range.

However, if you send it off to Ingram, and tell him what you're doing, he'll
custom-grind a new 3 dimensional cam for you that will re-map your fuel delivery
to match all the other gizmos.

The way to picture how this cam thing works is to find one of those cheap
plastic ash trays that are in IHOPs and Dennys' everywhere. Pick up one (empty
hopefully) and hold it perpendicular to the table. Imagine the ashtray is inside
the pump, with the ashy end pointing toward the big SPICA pulley. Put the tip of
your finger on the outside wall of the ashtray on other side, with your finger
in an L shape like you're trying to scratch something. 

The ashtray is the cam, and your finger is the "follower". This is how the 3d
cam is set up inside the pump.

As you push down with the throttle, the cam will rotate (twist the ashtray),
moving the follower up or down. There are four large steel balls on the side
with the pump pulley (attached to an inner part of the cam that acts like a lazy
susan), and as you accelerate they push the cam forward, again moving the
follower up or down. A spring pushes it back when the engine speed slows. (You
would think these balls would cause the cam to rotate a bit as well, but the
throttle linkage holds it steady.)

The follower, which looks like the business end of a ballpoint pen, controls a
rack that controls how much gas is fed to the injectors (which have pistons and
are actuated by a crankshaft, for all the world looking like a tiny little
engine), which controls how fast you go.

There's a couple of other things that control the positioning of the cam, but at
its heart this is how it works.

What Wes does is re-shape the cam, which re-maps the delivery to match your
mods.

Anyway, hope this helps!

Scott Johnson
Alfa Spider FAQ Author

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