On Fri, 14 Mar 2003, Mark Keller wrote:

> The Digifant Fuel system works largley by monitoring  input from the O2
> sensor.  By monitoring the O2 sensor with an averaging multimeter you
> can see the average fuel mixture theengine is actually getting. Because
> the engine is constantly getting a little less and a little more than
> this average, it probably isn't even producing the same power it could
> if it got a constant mixture.

Another issue that you only lightly touched on is emissions.  Best
emissions in a catalyst-equipped engine are obtained with a mixture that
alternates between slightly rich and slightly lean, hovering around the
mixture where the available fuel exactly matches the available oxygen.

Best *power*, on the other hand, occurs with a mixture that is richer
than that value.  Unfortunately, that results in a lot of unburned fuel
(hydrocarbons) going out the exhaust, which is a no-no from a pollution
standpoint.  You can see this pretty graphically if someone driving a 60s
muscle car floors the throttle.  The exhaust will turn grey from the
partially-burned fuel being pushed out.  These cars were generally tuned
to run quite rich at full throttle, both because it generated more
horsepower and because it tended to prevent detonation and keep combustion
chamber temperatures down.  Gas was cheap and wasting a little to protect
the engine was considered acceptable.

What's no doubt happening when you go to full throttle is the ECU is going
into a "full throttle enrichment" mode, where it adds a bit of extra fuel
to the mix so the engine can run richer and develop more horsepower.

David Brodbeck, N8SRE
'82 Diesel Westfalia
'94 Honda Civic Si