Little bit of counterpoint. :-)
The heating of the intake manifold is to help promote vaporization of the
liquid gasoline droplets. This results in better running, more power, and
lower emissions. 'Tis true that this also heats the air, reducing the density
of the air going into the cylinders. That results in a power loss. So it's a
balancing act as to whether or not heating the intake manifold gains more power
through better gasoline vaporization then it loses through reduced air density.
I've never seen a definitive answer, and I doubt there can be a single
definitive answer. It would depend on the engine design and weather
conditions.
In the case of the Spitfire though, I have wondered if the water flow through
the intake doesn't actually cool it down. The exhaust manifold is in extremely
close proximity to the intake manifold, touching it in several places. This
could cause a lot of heat to be transferred to the intake. If that is the
case, blocking off the coolant flow through the intake could result in the
intake manifold getting even hotter. I don't know the answer to this one.
>>> Richard B Gosling <Gosling_Richard_B@perkins.com> 02/01 9:54 AM >>>
Kevin,
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, when the
manifold heater won't do any heating; once the manifold has heated up from
this coolant conduit, the engine is hot and you don't need the choke anyway.
Moreover, hot air is less dense, so if you suck a given amount of hot air into
your engine you are sucking in fewer molecules, so you have less power - this
is why turbo- and super-charged engines use intercoolers, to cool the air
before it enters the engine. The whole thing seems daft to me, but I have
never got around to seeing what happens if I bypass the manifold coolant pipe.
It sounds like your PO has tried what I wanted to try (or maybe just broke the
pipe connectors by accident and decided it was easier to bypass than fit a new
manifold). If your engine runs OK, I would leave well alone - unless you are
concerned with originality, or making it look nice and tidy.
Richard & Daffy
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