>The original design had coolant running (from the top of the thermostat
> housing) through a conduit in the inlet manifold. The idea was to heat the
> incoming air, to reduce the need for the choke. This never really made much
> sense to me, as you only need the choke when the engine is cold, <snip>
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While this sounds good, it's not *entirely* true - Simply put, (as simply
as I can put it anyway) The PRIMARY purpose for this type of heating is for
better atomization of the fuel at cruise and part throttle/idle conditions.
It also allows the choke to be removed sooner, but it's reason for being
there is entirely independent of the reason one uses a choke, although it
is related to it.
When the engine is cold, the intake manifold is also cold. As we all know,
when a liquid/air emulsion (your fuel mixture), contacts the cold walls of
the intake manifold (and to some extent the cylinder walls and pistons) it
condenses into large globs of fuel (which don't burn efficiently or
completely) on these cold surfaces. This is further compounded by the
relatively low pressure intake charge velocity in these areas during idle
and low (intake charge) speeds. By heating the manifold during these
conditions (read AFTER the fuel and air has been mixed) then this condition
is greatly reduced or eliminated and LESS fuel can be used because most of
it is NOT falling out of suspension, resulting in a much more atomized
mixture. This leads to increased combustion efficiency, less WASTED fuel
and less harmful emissions. Remember, fuel NOT burned, is NOT producing
power. Fuel by itself, that is fuel (gasoline) not atomized with oxygen
(air) in some way WILL NOT BURN. A glob of liquid fuel, by itself, will
distinguish a lighted match (kids don't try this at home) So by heating
the manifold this results in increased drivability during part
throttle/cruise/idle conditions and a much and more efficient engine at
those speeds. I myself really question the benefit of not heating the
manifold especially for the reasons stated above AND ESPECIALLY ON ANY
STREET DRIVEN VEHICLE. A PROPERLY designed, heated manifold, when under
full throttle, WILL flow as good as a non heated one and the intake charge
will be only less dense for a small instance in any case. The large
pressure drop and greatly increased flow during these conditions will
result in the intake charge absorbing most of the heat from the manifold
(remember I said properly designed) resulting in a cold manifold at this
stage. Any decrease in performance at this point will be small if any, and
certainly not worth the drivability and fuel cost's associated with not
heating on ANY street driven vehicle - Think about it for a minute. If
it's realistically only a 1 or 2 percent gain that's less than a 2 hp
difference on the highest stock hp Spitfire and any gain will be at the
highest rpm/power curve. That's at about 5500 rpm on that vehicle. Heck,
removing your fan will give you more than that :-) and it won't cost you
fuel or drivability problems!
Barry Schwartz (San Diego) bschwart@pacbell.net
72 PI, V6 Spitfire (daily driver)
70 GT6+ (when I don't drive the Spit)
70 Spitfire (long term project)
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