[DeTomaso] Thermostats and bypasses

[MikeLDrew at aol.com]

In a message dated 3/22/11 15 55 56, fresnofinches at aol.com writes:


> 
> We often discuss the problem created when a stock 351C uses a Windsor 
> thermostat.
> The Windsor stat will allow some coolant to always bypass its proper flow 
> through the radiator,
> instead it is allowed to flow back to the engine block. This happens 
> because the Windsor stat does
> not have a 'hat' that expands when warm to block the engine bypass 
> passage. Allowing some coolant to
> bypass the radiator is what the 351C stat allows, but only to assist in 
> engine warm-up.
> At that point the 351C stat closes the "hat", blocking the bypass passage 
> and forcing
> all coolant to travel through the radiator.
> 

For years I believed this to be true, but in fact it's not.   At least, not 
exactly.

If you look at the Windsor series of engines (289/302/351), they all have 
an external bypass hose that affixes to the thermostat housing, and run from 
the front of the engine, right next to the upper radiator hose, down to the 
water pump.   This hose is unaffected by thermostat operation; water flows 
through it continuously, meaning that a percentage of the coolant is always 
bypassing the radiator.

John Christian (who has forgotten more than all of us put together will 
ever know) educated me on the 351C system last year, and pointed out that the 
diameter of the 351C thermostat 'hat' is considerably smaller than the 
orifice that it purportedly plugs up.   When you calculate the surface area of the 
orifice, and subtract the area of the hat, you're left with a fairly large 
number--in fact, almost exactly the same area as that on a 289/302/351W 
bypass hose!

So, all things being equal, when a Windsor or Cleveland thermostat is open, 
about the same amount of water bypasses the radiator and circulates through 
the engine.   The difference is that when the thermostat is closed, a 
substantially greater amount of water circulates through a Cleveland (or rather, 
the total restriction is much less).   Too, if a 351W thermostat is used in 
an unmodified 351C, excessive amounts of water will bypass the radiator when 
the thermostat is open, contributing to overheating problems.

And by the same token, modifying the 351C to block off the bypass entirely 
places a strain on the system, because now the water pump is attempting to 
circulate coolant and there is zero movement available.   There is simply no 
place for the water to go until the thermostat opens.   The water that is 
getting hot, close to the hottest parts of the engine, is unable to actually 
touch the thermostat, and localized hot spots can occur until the heat is 
transmitted through the (stagnent) water enough to open the thermostat.

Mike (learning something new every day from genuine smart guys like JC!)