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Will amend the diagram, it was rather rushed.
Driving 'The Fat One'
Here's the Cosworth drawing for suggested Duratec cooling . The 620 appears to follow this bar the extra circuit cooling an oil cooler on this diagram .
Which is why your all experiencing the problems you are.....
The Cosworth version runs a Modine coolant/ oil heat exchange which improves the cooling circuit over yours as there is an increase in flow to the thermostat so this will improve response times - though still no where near as effectively as using the bypass circuit, since the flow is considerably less and the water is fed to the rear of the stat and not onto the wax element.
Expecting your cooling systems to warm up quickly, maintain a constant correct baseline temp, and respond in a rapid manner to sudden thermal changes when the ponies are unleashed is madness considering the only communication to the stat is via the bleed pipe, which is by design restricted to a 3-4mm bore !
Is there a " standard " " back plate " that one could fit on the rear of the block and take a larger diameter bypass connection from ?
If so it would be an easy T in to the current pipe that runs from the expansion bottle .
Right mister, a lot of stuff you write makes sense. What is the cure?
Possibly too small but what about removing the temp sensor on the left and using that tapping ?
Here you go.... there's a stock Ford hose (CC carry this in stock circa £16) which goes from the pressed in elbow to the other pressed in elbow in the block by the stat housing.
Although listed as no longer available I bet a bit of googling will throw one up otherwise you could modify the stock Cosworth version.
Havent seen the exact outlet on the 620 but this offer location for temp senders etc too but I'm guessing its this one...?
Arh ! thats the photo I was looking for !
Modify yours one to accept a bypass outlet underneath - same as the Burton one.