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Old May 25th, 2014, 11:06 AM   #26
InvisiBill
EX500 full of EX250 parts
 
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Name: Bill
Location: Grand Rapids-ish, MI
Join Date: Jul 2012

Motorcycle(s): '18 Ninja 400 • '09 Ninja 500R (selling) • '98 VFR800 (project) • '85 Vulcan VN700 (sold)

Posts: A lot.
Blog Entries: 1
MOTM - Aug '15
To simplify it, the Thermo-Bob is just a way to add a bypass to your cooling system, like most cars have.

In the stock setup, there's a single loop from the water pump, through the engine, to the thermostat, then the radiator, and back to the pump. If it's too cold for the thermostat to open, then basically no flow occurs in the system. This keeps coolant from flowing through the radiator and getting cooled even more, but it also keeps the warm coolant in the engine from getting out. When it finally does get enough warm coolant to the stat to open it, cold coolant from the radiator surges into the engine and the stat shuts again, leading to a cycle of surging temps.

With the Thermo-Bob, you have a loop from the pump, through the engine, then the Thermo-Bob housing, and back to the pump. When the thermostat heats up, flow is also allowed through the radiator to cool down and head back to the pump. When cold, this allows the coolant to keep circulating through the engine, keeping it a more constant temperature.

In one of the pages on his site, he talks about the effect of having the bypass always open when the bike is hot. It allows some coolant to bypass the radiator, but that means the coolant going through the radiator is slightly slower, so the increased dwell time means slightly more cooling. In the real world, this came out to about 3°F increase in temps when hot. With the stock setup, the engine input and output temps varied by at least 35°F, so 3°F in the worst conditions sounds pretty darn good.

I have no data, but it also seems to me that the water pump would be happier pushing the cold coolant around in a loop, rather than fighting against the blocked flow when the thermostat is closed.

The 500 has individual cylinder outputs for the coolant, which merge back at the thermostat housing. I'm working on modifying a stock housing to add a bypass port on the bottom under the stat. The 250 housing looks similar, so you could probably do that as well for a cheaper, DIY Thermo-Bob. This would leave the upper hose intact and get rid of the Thermo-Bob housing, but you'd still need to splice the bypass line back into the lower hose.
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