At 02:40 PM 8/30/2000 -0700, Max Heim wrote:
>Isn't a 7 lb. cap a little low? A sealed system would have a 12 or 14 lb.
cap, I would expect. It would operate more efficiently at a higher
pressure, I would think. ....
Nope, nope, and nope. With a 50/50 mix of permanent antifreeze
(etheylene-glycol) and water, and a 7 psi pressure cap, it can peg the
temperature gauge at 230dF without boiling. The only purpose for increased
pressure is to prevent boiling at higher temperature. If the MGA ever goes
over 230dF you're in deep stuff for several reasons anyway, such as boiling
the fuel in the carburetors (vapor lock). And I wouldn't venture to
increase the pressure without installing a gauge to read the higher
temperatures that might be encountered. The reason that newer cars run
higher pressure is because they are designed to run higher temperatures for
purposes of economy and emission control.
So long as you maintain liquid coolant in the engine, no air, vapor or
steam, the engine will be cooled adequately. As such, so long as the
coolant is topped up and not boiling you're okay. Witness this for example:
http://www.ntsource.com/~barneymg/pic89/trmont.htm
With increasing temperature the first problem you will notice will be the
carburetors acting up from the start of vapor lock condition. This you
will generally not notice as long as the car is moving at an appreciable
speed on the highway, even if you do peg the temperature gauge. But it is
a problem with stop and go traffic in hot weather, where the under hood
temperatures soar dramatically.
After keeping the coolant in the cooling system, the next objective is to
keep the coolant moving. If the water pump should fail, the pulley lock up
or the belt get loose for instance, and the coolant flow should either
cease or subside to a minimum, then you get local hot spots in the head
around the exhaust valves that cause localized boiling. The steam and
vapor which occures locally in this case is a lousy thermal conductor, so
you then run a high risk of cooking the head and/or valves. The bad part
here is that you won't necessarily notice either a rise of temperature on
the gauge of a loss of coolant, and a highere pressure relief cap won't
make any difference, because the fluid won't be escaping out past the cap
anyway. The boiling is very localized at first, and the steam or vapor
only travels a short distance before hitting cooler fluids and condensing
again. The only telltale signs of this condition may be a percolating
sound in the engine that you will probably not hear over other engine
noises, and most likely a significant loss of power and possible pinging
in the engine as the cumbustion chamber get too hot.
Also, the stock MGA radiator wasn't designed to take that much pressure.
The original factory spec for the pressure cap was 4 psi, with the 7 psi
cap being an option (dealer installed I presume). For a long time I had a
reoccuring problem of solder seam leaks where the top and bottom tanks are
soldered to the core flanges. Every year or two I was hauling the radiator
back to the shop to R&R one or the other of the tanks to resolder the
seams. The fact that the radiator is bolted directly to the body and sees
all that direct vibration doesn't help the situation. I did actually have
the top tank split the seam once while cruising through the Arizona desert
with a 7 psi pressure cap. The original design of the seam has the solder
joint in tension so it tends to pull apart under pressure and vibration.
The ultimate solution was to have the radiator shop reform the shape of the
tank flanges. Original design had the flanges sticking outwards with one
on top of the other. The new shape has the (top) core flange bent upwards
and the (top) tank flange bent downewards (and mirror image for the bottom
tank) such that they get soldered together in a vertical plane. Then when
under pressure the load on the joint is then in shear rather than in
tension. They pressure tested this arrangement at 25 PSI, so I could run a
12-13 psi presure cap (if I thought it was prudent). So far I still use
the 7 psi cap with no problem. The nice advantage here is that I have had
no service call for the radiator now in 6 years and 110,000 miles.
Tech session over. Have a noice day.
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com.~barneymg
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