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June 14, 2003

Ken Charles!
You can run heated or unheated. As
David Jacobs states, the MG Metro
runs heated. And as David says, you
can run either way and see which way
is best but, before you do.....
you need to understand the effects of
water manifold heating and why it exists.
It is basically there for economy of operation in light load
driving...like around
town sort of stuff. As David says, heating
the mixture allows for better vaporization
of the fuel. True. This allows for a "leaner"
mixture for more economical operation (better gas mileage) in part
throttle, light
load cirumstances.
Now the makers know that you don't
need "heat" to help vaporize the fuel
at large throttle openings...flow velocity
will do this for you. And of course, excess
heating of the mixture causes a loss of
power in the combustion process when
you put your foot in it to truly "go"!
Heat is fine for economy, not fine for
power. Guess what? The manifold makers
know this!!
So, they calibrate the amount of water
heating the manifold gets, knowing in
advance the temperature drop expected
within the intake manifold as mixture
velocity increases when you put your
foot in it.
End result, a nice economical mixture
for economy of operation puttering around
town at light loads, and basically no heating when you need power, the
temperature drop within the manifold
easily overpowering what heating is being
supplied to the manifold....all carefully
planned for by the makers!
Remember, when velocity increases,
temperature decreases (as does pressure
but, this is not important in this discussion!:)
The trick is, if you use a water heated
manifold, you need a needle calibrated
for a water heated manifold in this
If you use an unheated manifold, you
need a needle calibrated for an unheated
manifold in this application.
The needles will be calibrated the same
around mid-range and higher rpms.
Below this range, the needle for the
heated manifold will be thicker (leaner)
than the needle for the unheated manifold
(which will be thinner for richer 'cause
unheated mixtures have a more difficult
time fully vaporising at low velocities).
Obviously, it can be seen that the water
heated manifold is the way to go, if given
the choice.
Next, I do not know what your HIF6 came
off of so I don't know the needle and air
valve spring it is presently calibrated with.
Assuming your parts man carries the
1.75 conversion for a 1275 A Series, he
should be able to guide you in these
respects. If he carries this conversion
kit for the STOCK engine, you will need
a needle a little more rich than what this
kit comes with, considering your choice
of cam is "up" from stock. You are not
going to bigger valves or having flow
work done on the head so the air valve
spring supplied with a 1.75 conversion
kit for a STOCK 1275 Series A should be about right. If anything, you'll
need one a step more "stiff" I would imagine, so the air valve doesn't
get to fully open 'til the motor reaches
about "redline" wherever that will be
with the cam you select.
BTW, this new redline may be a little higher than with the stock cam?
reason to get the motor "balanced" to
at least street spec., eh?! :):)
Cap'n. Bob
'60 :{)

PilotRob@... 22663

Posted on June 14, 2003 09:37 PM
Posted to category(s) Carburetor | Cooling System | Cylinder Head | Engine (external) | Engine (internal) | Exhaust System | Exterior | Fuel System | Gauges | Intake System | Restoration | Specifications | Suspension & Steering | Technical Info | Tuning

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