Substitute Thermostat

[Jim Van]

I know a Stant 13868 thermostat will fit the ST1100 and seems to work OK. I've been using this replacement Stant thermostat for the last 6 years.

But I have been looking at different thermostats on the Rock Auto website and found a MotoRad 2040180 thermostat. It is suppose to be a "high flow" thermostat that opens wider than a standard thermostat such as the Stant 13868. I'm going to try the MotoRad to see if it will lower the engine temp at lower speeds in hot weather. I do most of my riding on the slab and the temp stays down with the Stant, but at low speeds in stop & go traffic the temp gauge is usually a little above the 12 o'clock position.

See : http://www.rockauto.com/en/moreinfo.php?pk=4483842&cc=1166587&jsn=491&jsn=491&jsn=491

 

[dwalby]

I think you're probably wasting your time with that Jim. I suspect your main enemy at low speeds is airflow across the radiator, not coolant flow rate.

 

[Jim Van]

I will find out for sure, one way or the other, Doug. Let you know in the spring how it shakes out. More flow has to definitely help to some degree, even after the fan kicks in. That Stant thermostat has been in there for about three years already, time for a new stat, might as well try the MotoRad.

Mr. Bush : My old stat will be on sale for $2 ( plus shipping - special deal, just for you ).

 

[dwalby]

I think its possible to pump more coolant through the system per unit time, but actually transfer less heat. If the coolant flows too fast, it spends less time in the radiator, so less time for heat transfer to take effect. Then it returns to the engine too warm, and the cycle is repeated. The system is designed with flow rate as one of the parameters, so arbitrarily increasing the flow rate isn't going to necessarily make things better. I'm curious to see what happens, if anything, and somewhat skeptical regarding how much additional flow the 'high flow' stat will actually provide.

 

[Slydynbye]

I think I notice that the Stant runs a bit lower than STock and wonder if that lower temp actually decreases Gas mileage.

 

[Jim Van]

Doug :

That seems to make sense, but that's not how it works. Have you ever seen a curve for a heat exchanger ( radiator ) ? The higher the flow, the higher the heat transfer.

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[Jim Van]

I think I notice that the Stant runs a bit lower than STock and wonder if that lower temp actually decreases Gas mileage.

If it runs cooler, that should reduce efficiency ( gas mileage ). The 13868 Stat is a 180 degree stat, I believe same as the OEM, tho. I tried a 190 degree stat and it didn't seem to affect my mileage, so I went back to the 180 degree Stant, BTW.

 

[dwalby]

Doug :

That seems to make sense, but that's not how it works. Have you ever seen a curve for a heat exchanger ( radiator ) ? The higher the flow, the higher the heat transfer.

Darn, back in the '90s when I was homebrewing it seemed like the chiller worked better when I slowed down the flow rate, but it may have been some other variable I changed, its been too many years (and beers) to remember. But a quick google search found the same info you show.

edit: a quick memory jolt and now I think I remember. I went from running the warm beer through the inside of the copper tubing, with the tubing in a bucket of cold water, to the reverse. I put the cooling coils in the bucket of warm beer, and ran tap water through the coils out to the drain. That way I had an endless supply of constant temp water, whereas the bucket water would heat up eventually and lose efficiency.

 

[dwalby]

OK Jim, now I can see through your clever little plan.

You say you do mostly freeway riding, where the temp is low anyway. So now you put this high flow stat in and what happens then? Higher flow, with good airflow across the rad, the coolant will get too cold too fast and the stat will start to close. Then the coolant will warm up and open it fully again, and so on. You've created an environment where you'll have a lot more open/close cycles on your stat, which means.................. yep, more preventative maintenance to make sure the stat isn't going to fail on you after all those cycles. You'll be able to justify changing it every year now instead of every three.

but seriously though: The coolant flow is regulated by the stat so that the operating temp stays fairly constant while moving, its designed to work that way, as opposed to the open/closed scenario in the previous paragraph. I suspect the high flow stat is going to open at 180F, and then regulate flow until it reaches the 180F equilibrium point, and stay there, but it probably won't be fully open. You'll have about the same flow as stock. But when stopped, it will theoretically keep the coolant a bit cooler because of the potentially higher flow rate when its full open, so it might take longer to get to vertical on the temp gauge. Is that kinda how you figure it will work?

 

[SMSW]

But when stopped, it will theoretically keep the coolant a bit cooler because of the potentially higher flow rate when its full open, so it might take longer to get to vertical on the temp gauge. Is that kinda how you figure it will work?

When he is stopped, won't there be less air flow through the radiators? I expect we have more air flowing thru the rad at highway speeds than is drawn through the rad by the fans alone in stop and go traffic. Is this thinking correct?

 

[Jim Van]

That's what I'm hoping for, Doug. Like I said before, time will tell. I'll report back in April after the bike is back on the road.

And yes, SMSW these ST1100's run nice & cool on the slab even on a hot day because of the great flow through the radiator at speed. But in town, at low speeds, all we have is the fan running to help us out. But more flow should help, even with the fan running according to the curves above. Quite a difference even at 5 mph with more flow.

So, I guess the take away is that I learned that there are different thermostat designs that I didn't know about. There is also a "reverse poppet" style that claims when it fails it stays open, not closed, to prevent overheating.

After doing more searching, it also looks like a Stant 45868 is the high flow version of the Stant 13868, but I'll try the MotoRad stat.

BTW, I used to think the worst condition for an automatic trans would be running on the slab at high speed, on a really hot day. I was wrong on that idea. I installed a trans temp gauge on my 1990 Voyager years ago and found out the coolest trans temp I saw was during the previous described conditions. The hottest temp I saw was after going up a fairly gently hill at 25 mph on the Michigan Tech campus in Houghton on a fairly cool day. Not much happened on the gauge going up the hill - there was a time delay and then the temp rose quickly about 2 minutes later.

Those Rock Auto prices are so good, even I will b able to afford to replace the stat every year. The local auto parts store wants over $13 for a Stant 45868. I can get one from Rock Auto for about half that, including shipping. But, I'll start budgeting now for the next replacement ...........

 

[Jim Van]

I received the MotoRad 2040180 and a Stant 45868 thermostat today. First of all, of course both have a flange diameter of 52 mm, but the MotoRad has a height closer to the OEM thermostat. The Stant 45868 is quite a bit taller.

The MotRad has a larger diameter opening than the Stant. I placed both in a pot of water and heated the water on the cook top. Both are rated at 180 degrees and opened almost at the same time and close to 180 degrees, according to my cooking thermometer. Of course, I don't know how accurate it is , but both stats opened at almost the exact same temperature.

Like I mentioned previously, the MotoRad has a significantly larger diameter opening and it opens about 50% more than the Stant.

My recommendation is that if you aren't going to use an OEM stat, try the MotoRad 2040180 instead of the Stant 13868 or 45868. It's under $4 at Rock Auto. Look up a thermostat for a 1989 Honda Accord.