Congratulations mgs list. Finally you're getting very close to the truth
on this topic.
On Wed, 19 Mar 97 18:56:16 PST "Richard H. Cady"
<cadyxrh@ctxix.sod.eds.com> writes:
>Running the MGB engine without a thermostat or substitute flow
restrictor effectively reduces the water pressure inside the engine
block. The fluid flow restriction provided by the thermostat increases
the coolant pressure internally resulting in a more uniform flow of
coolant throughout the entire engine. Without a thermostat, the coolant
flows very fast through the front of the engine. This causes the upper
rear of the engine to be starved for fresh coolant where it eventually
overheats. .....
Changes the flow is correct, but not much to do with pressure. If you
remove the thermostat and look below it inside the water cavity of the
head, you will see a very open area in the water jacket at the front of
the head. Without a thermostat, water flows very freely through this
area, with some of the water approaching the outlet from the sides.
Circulation comes largely from the water pump up through the front of the
block into the front of the head and out through the water neck. When
you install either a thermostat or a blanking sleeve, you are putting in
a barrier like a small circular fence around the opening. This barrier
restricts the lateral flow at the front of the head, forcing more flow at
the rear of the head, which of course comes up from the rear of the
block. The result is more uniform flow through the whole engine and
better cooling at the back.
On Wed, 19 Mar 1997 21:13:39 -0600 (CST) jddamon@ix.netcom.com (JD Damon)
writes:
> ..... Sometimes when I'm pushing the Sprite pretty hard (which is not
unusual on Car Club runs) it will reach 230 degrees on the gauge.
Problem? Nope! Cast iron engines will not suffer any undue wear or stress
at temperatures even higher than this IF the cooling system is working
properly! .....
Yes sir !! Absolutely true. As long as the coolant is contained (no
leaks), and does not boil (keep the pressure), it will carry away the
heat very efficiently.
J Damon also asks:
> If moving the coolant through the radiator slower causes it to lose
more heat, would the coolant not gain more heat as it moved through the
engine slower? Or does your coolant travel at different speeds?
The optimum flow rate for cooling is: Enough flow to carry away the heat,
but not so much as to use gobs of power to run the water pump. For
maximum cooling you need maximum flow, but not so much as to cause
cavitation (those vacuum bubbles don't carry much heat). But most
engines do not need maximum cooling except in rare circumstances, so you
can slow down the pumping (somewhat) and save a lot of power. But, if
the flow gets very slow or stops, the coolant will boil inside the head,
those little steam bubbles will stifle the heat flow from the head, and
the engine will be damaged, so you can't slow it down that much. In
practice, water pumps are generally designed to provide just a bit more
than the minimum required flow at idle speed.
An engine generates a certain amount of excess heat which must be removed
by the cooling system. The final exit for that heat is from the radiator
to the air. Air flow through the radiator is treated similar to the
water flow in that the fan is generally designed to blow a little more
air than the minimum required at idle.
Heat transfer is a function of the temperature difference. Internal heat
of the engine raises the coolant temperature inside the engine. Cool air
outside absorbs heat, lowering the coolant temperature in the radiator.
And the excess internal engine heat will always get dissipated to the
air, as long as the coolant stays in and does not boil or cavitate. It
stays cool.
The point of the whole thing is how to keep it from boiling. If you have
a marginal cooling system, and you want maximum heat dissipation from the
radiator to the air, you want the radiator to be as hot as possible. To
this end, you would like a very high coolant flow rate. When the coolant
flows faster it spends less time in the radiator on each pass, looses
less heat on each pass, and runs at a higher temperature in the radiator,
resulting in more heat dissipation to the air. At the same time, the
coolant spends less time inside the engine on each pass, picks up less
heat on each pass, and runs cooler inside the engine, resulting in more
heat flow from the engine to the coolant. Now the coolant is indeed
carrying away less heat on each pass, but is making many more passes
through the system..
There is a point of diminishing returns to this flow thing. When the
flow rate gets very high, the coolant temperature in the radiator
approaches the coolant temperature in the engine. Then more pumping
consumes more power but makes little change in temperature or heat flow,
so you don't need to pump that much. To save power, the pump is designed
to move just a little more than the minimum requirement. The whole
system is a design compromise, with the goals usually being the lowest
cost to manufacture and the lowest power consumption, in that order
Incidentally, a larger radiator does not mean you can pump less water. A
certain minimum flow rate is required to keep the coolant from boiling
inside the engine. Also, as long as your radiator can dissipate the heat
the engine is generating, a larger radiator will not dissipate more heat.
The thermostat will slow down the flow to maintain the correct operating
temperature in the engine, which is exactly what a thermostat lives for.
J Damon further comments:
> I find it hard to believe there is a vapor lock problem on the pressure
side of a fuel pump. Vapor lock was a problem back in the old days when
the pump was mounted on the side of the engine and the vacuum line was
quite long. Vacuum lowers boiling point, pressure raises boiling point.
Box body Sprite pumps are in the back of the car so if your car exhibits
vapor lock symptoms you'd better check pump volume and pressure. Surely
you have a delivery problem. Trash in tank or pump I suspect.
True enough, but not the only kind of vapor lock. When the fuel boils in
the feed line before the fuel pump, the fuel pump cannot pump the vapor
efficiently, and the engine dies or will not start. The other type of
vapor lock (more common today) is when the fuel boils in the carburetor.
In this case you have a vapor (or a mixture of vapor and liquid fuel)
being metered through the jets in place of liquid fuel. The vapor
contains a lot less fuel than the liquid, so the intake mixture goes very
lean, and the engine coughs and sputters or may even die. This situation
is aggravated by alcohol and gasohol fuels because alcohol boils at a
lower temperature than gasoline.
This is where a heat shield for the carbs can make a difference. And if
that doesn't cure it, try to avoid the alcohol bearing gasoline.
Meanwhile, as a daily fix for this type of vapor lock, pan the throttle
until you can get the engine up to about 3000 RPM. Hold the engine at
this speed until you can draw fresh fuel into the carbs. The fresh fuel
should cool the carbs enough to banish the vapor lock. If you have a
manual choke, pulling the choke full on will speed the cure. With my
MGA, 3000 RPM and full choke will clear up a very sever case of
carburetor vapor lock in about one minute.
Thanks for the patience. I have the need to do this occasionally.
Barney (man I hate alcohol) Gaylord
1958 MGA
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