Re: Spica air box question

["Jim Neill" <papajam1@domain.elided>]

   In AD V8 #35, Ed L. asks some questions about a Spica airbox;

This box has a butterfly valve in the air horn which
looks to be controlled by vacuum, plus it also has what appears to be
an air temperature sensor in the box itself, mounted on the lower side
of the box. I'm wondering about the purpose of these two items. I don't
think they are meant to be used on my '74 but am still curious about
what they were for.

   The device is called (at least in GM jargon) a TAC or thermostatic air
cleaner. It's purpose is to provide warm intake air to the engine during
engine warmup. When the air temp at the TAC temp sensor is cold enough, the
butterfly valve in the airhorn (snorkel, snout, air cleaner intake tube,
whatever) is closed to ambient air. Intake air to the engine is provided from
the engine compartment thru a flexible tube that usually originates from an
air box of sorts that wraps around the exhaust manifold. Warm intake air aids
in engine warmup but hurts performance (leans the mixture). When the air temp
in the snorkel is warm (or hot) enough, the temp sensor directs engine
manifold vacuum  (which is different than engine ported vacuum) to the vacuum
actuator on the snorkel. The butterfly valve then closes off the hot air
source and opens to ambient (and hopefully cooler) air increasing performance.
Cool dry air is better that hot moist air because it is more dense and
therefore packs more "stuff" into the intake charge. An easy performance
enhancing check is make sure that the butterfly valve is open to ambient air
when the engine is at operating temperature. Also make sure that the plastic
hose from front of the car to the air cleaner is not missing although not all
cars had these anyway. Most TACs default to the warm air position so if the
temp sensor is on vacation, simply install the manifold vacuum hose directly
to the vacuum actuator to have cooler intake air all the time.

Regards,

Jim
68 GTV - with velocity stacks and no TAC
papajam1@domain.elided

--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided