In a perfect world, an internal combustion engine would burn 100% of the
fuel used. However even in a finely tuned engine, some unburned fuel exits
during the exhaust stroke because the intake valve begins to open before the
exhaust valve is fully closed. This is known as valve overlap. The more
radical a cam, usually the more valve overlap, and the greater amount of
unburned fuel dumped unused out the exhaust pipe. Fuel requires oxygen to
burn. So the air injection pump supplies oxygen into the exhaust stream to
promote the burning of the residual fuel (unburned hydrocarbons) in exhaust
manifold. The only thing is there is a very slight parasitic loss of hp
from driving the pump.
Assuming the air pump has done its job and the result thus far is that all
of the fuel is now burnt, you now have exhaust made up of Carbon Monoxide
(formed by the combustion of gasoline) and Nitrogen Oxide (created when the
heat in the engine forces nitrogen in the air to combine with oxygen) . A
catalytic converter is a device that uses a catalyst (in the form of
platinum and palladium coated onto a ceramic honeycomb or ceramic beads) to
convert Carbon monoxide and Nitrogen oxides into Carbon dioxide, water, and
nitrogen. Again, there are hp losses because the catalytic converter creates
back pressure in the exhaust system.
The two systems were complimentary because while the air pump reduces
unburned hydrocarbons in the exhaust, it increases nitrogen oxides due to
the higher exhaust temperatures. The catalytic converter then returns these
to Nitrogen and oxygen. However with out the air pump, if unburned
hydrocarbons make there way to the catalytic converter, they will be ignited
there because of the supply of oxygen created by the catalyzing process
itself. Igniting them in the catalytic converter is very bad, leading to a
cherry red hot catalytic converter and possible fire hazard.
MG 1275's from late 1968 have air injection systems.
48 state MG 1500's have air injection from 1975-1976
California MG 1500's have air injection and catalytic converters from
1975-1979
48 state MG 1500's have air injection and catalytic converters from
1977-1979
EGR (exhaust gas recirculation) is another method of reducing unburned
hydrocarbons. During low load situations (i.e. cruising, deceleration) high
manifold vacuum opens a port between the exhaust manifold and the intake
manifold allowing a small percentage of exhaust fumes to be reintroduced
into the combustion chamber for further burning. I am unsure but believe
that EGR systems were standard on all MG's with the 1500. If the EGR valve
opens to soon, too fast, or too much, a lean burn misfire will occur. This
feels like the car is "dragging" or fighting a headwind, and will sound like
the car is running rough.
David Riker
74 Midget
63 Falcon
70 Torino
http://home.pacbell.net/davriker
----- Original Message -----
From <Dbcooper292 at aol.com>
To: <spridgets@autox.team.net>
Sent: Wednesday, January 09, 2002 10:34 AM
Subject: try this
> since you knocked out that paint color question so fast here's another:
my
> car seems to have air injection equipment and no cat. it's a 75 and my
> understanding is the cat came in 77 except california. My further
> understanding ( very possibly wrong) is that the air pump is to aid the
cat
> in doing it's job. Without that it seems you're just diluting the
exhaust.
> Why does my car have one and not the other? was it originally a CA car
but
> lost its cat somewhere or is it a correct 49 state model?
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