Cams

["Rich Wagner" <drsavage@xxxxxxx>]

There has been a fair amount of discussion about cams in the past few
Digests.

Bob Brady wrote:

>So far, after installing 10548 cams (the euro 2l) in my '74 gtv, the
>improvements have been much less dramatic...
>Finally, I haven't dialed in the SPICA to work best with these cams.  
>The mixture is basically where it was with the stock units.

Dana Looms wrote:

> My experience has been that the "48" cams do provide a subjectively
> substantial increase in performance, with the greatest gains occurring
> between 3000 and 5000 RPM.  To get the maximum benefit from the cams,
> however, it is not only necessary to time them correctly, but to 
increase
> fuel flow at higher RPM to take advantage of the improvements in
breathing.
>  Changing the idle mixture won't necessarily do the job.  On early SPICA
> cars, moving the ambient temperature-adjusting lever on the FI pump from
N
> to F helps some and doesn't cost a penny.  The best solution, however, is
a
> high-performance SPICA pump calibrated to your cams.

Dana reprises a very important point which I talked about many Digests ago.
 Before you go out and buy a new set of hot cams to put into your car, you
need to consider the trade-offs.  A simple cam change alone does not
necessarilly offer increased performance.  In carbureted cars, an increase
in lift, duration, or overlap will often provide a noticable increase in
power.  Increasing the intake lift or duration provides for less restricted
breathing at high RPM, pushing the flat section of the torque curve higher
into the engine's RPM range.  However, it also results in an increase in
emissions at low revs.  If your car has Webers, and you know how to adjust
them, you can account for this problem.  If not, you may have just
converted your car into a "gross polluter".  An increase in overlap
provides for better scavaging of exhaust at high RPM.  Too much overlap,
however, increases emissions at high RPM, and causes exhaust regurgitation
at lower RPM (reducing low RPM power).  If you live in a state with tight
emissions standards, you want to be very careful about putting hotter cams
into your carbureted engine, or buying a car which has been so modified.

A SPICA-injected engine will react differently to a cam change.  The SPICA
pump is calibrated to deliver a given amount of fuel after calculating the
mass flow rate of air going into the engine.  That's a very important
distinction between the SPICA system and the Bosch systems.  The Bosch
systems actually measure the mass flow rate.  The SPICA system does NOT
measure it.  It calculates it.  The mass flow rate is a function of RPM and
throttle opening and, in the SPICA system, the fuel delivery calculation is
actually carried out by the 3-D cam inside the pump.  This is the reason
it's so important to adjust the linkage between the pump and the throttles
carefully.  If this adjustment gets out of spec, the mixture is no longer
correct.  Now, if you make a change to a hotter cam, intake breathing will
be improved at high RPM.  However, the SPICA pump can't measure this
difference.  It's still calibrated to deliver the proper amount of fuel for
the stock cam, which means the mixture will go lean at high RPM.  This
doesn't mean the engine will miss or backfire.  It does mean that power
delivery will probably not change -- the engine will feel like it did with
the stock cam.  At low RPM, even with injection, the hotter cam causes the
same rebreathing problem it causes on a carbureted engine, making for a
rich mixture and higher emissions.  To really derive the benefits of a
hotter cam in a SPICA injected engine, you need to recalibrate the pump by
changing the 3-D cam profile.  This, I believe, is what Ingram offers in
his high performance pumps.

I'm not telling people that they shouldn't experiment with cams.  They're
easy to change on an Alfa, so why not play?  I do, however, want people to
be educated enough to understand what they may be buying, and what effect
it will have on their cars.  There's no point in making your machine
perform better if it can't be driven.  There's also no point in paying for
increased performance that you don't get.


Rich
Manitou Springs, CO
'82 GTV6 Balocco

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