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Discussion Starter #1
Up front, let me say I've begun planning for my next winter projects. Let me also say I'm amazed by so much of the work of many on this forum, and I'm hoping those that have been there and done that can shed light on my apparent confusion..

Before we go further, I've looked at the cooling layout of the '18 XPT a fair bit, and I'm relatively impressed (with respect to the 2016 turbo's guys pain!). Still, guys smarter and more experience than I have performed mods I don't quite get, so I'm hoping ya'll can show me what the heck I don't get.

1) I don't appreciate the HCT "remove the thermostat and block the bypass" idea. In fact, for those that don't know, the XPT thermostat is dual acting, fully dynamic, and capable of completely shutting off the bypass on it's own. Why would I replace something that well thought out? Does it not work as advertised?

For those that don't get the coolant bypass thermostat, read this:

And:
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Now I know some pretty smart guys here are using this, so I'm awaiting their correction of what I've missed. (and assuming I'm about to learn something too).



2) I don't understand the split tank concept either. Sure, the coolant recovery tank is shared, but that doesn't "appear" to be mixing the two systems coolants in operation in any way. In fact, the stock tank doesn't seem to be in the normal operating flow path of either system. (unless I'm reading the coolant flow diagram of the service manual wrong, for I don't see supply side, only a return. Just like any overflow tank would have for refill during cool-down)

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Is the coolant somehow intermingling between system #1 and the aux cooling system #2 during operation (which would suck, and will have to be corrected) and I'm not seeing it?

If not, what then is the value of the split tank idea?

(Flame Suit "On")

Thanks in advance!

-d
 

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Discussion Starter #2
Perhaps I’m not alone in my doubt...

 

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Cooling system path in these cars seems complicated, but it isn't that bad.

When tstat is closed, coolant flows from tstat housing to water pump to engine to tstat housing, and just keeps circling. Some of the coolant in that system flows back and forth from the water pump to the oil cooler on the block, but it still stays within that looped system. The rest of the cooling system is just sitting there.

When tstat is open, it flows from tstat housing to radiator, to the filler neck, to water pump housing and then through the same path as when it is closed.

So, to answer #1,. the purpose of blocking off the bypass is to force coolant to always flow through the radiator, since the radiator is literally the only thing that helps reduce the coolant temperature. Everything else in the system is raising it.

To answer #2, the recovery tank is not directly in the path of flow. But it does see a ton of flow anyways. I realize the tank is connected on the "cold" side of the radiators, but when one system is getting hot and can't be adequately cooled by the radiator(as on a 2016), the "cold" side is still hot, and is intermixing freely with the reservoir, which then starts to bring up the temperature of the other system.


If you want to visibly see the flow in the tank, drain the tank. Refill it with just water while the rest of the system is green. See how long it takes for the tank to turn green.


My 17 XPT racecar had some cooling problems, but nowhere near what any of our 16's had. Bone stock cooling system, it would hover at 217-221 degrees most of the time, with an occasional spike to 232 degrees. Way too hot for me.

I immediately put in the split tank, blocked off the bypass line, and replaced the thermostat with a restrictor plate. Immediately dropped to 207-215 range, with occasional spikes to about 224. Still too hot.

So I put in an external oil cooler and remote mount filter. This bumped oil capacity to about 4.5 quarts, and dropped coolant temps a few more degrees across the board.

So finally I said to hell with it and put in a rear mount CBR radiator. I installed a single overflow tank, kept the tstat removed, and kept the bypass in place. Oil cooler stayed as well. Now it hovers around 173 -180 degrees. I think the hottest it has ever been since I put that radiator in was 192 or 193 degrees. While I can't see anything behind me, I have a ton of room for tools and a jack under the hood now, and removing front shocks is easy.
 

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Discussion Starter #4 (Edited)
ARR:

Thanks for sharing your experience and insights! I've a few follow-up's if you don't mind.

Re:
#1 (thermostat/bypass): Unless the thermostat is somehow blocking flow, it should be 100% diverted past the bypass circuit @203f. So it appears Poo designed the thing to have a normal operating range of 180f to 203f. Unless the T-stat is somehow restricting flow when fully open, I don't see how temps above 203f are any different vis a vis blocking off the bypass or not. Yet your experience shows you dropped to 207-215 from 217-221.

Heck, Newtons law tells us that thermal transfer is more efficient the higher the temperature delta, so the hotter a radiator gets the more effective it becomes. Yet here you saw a temp drop.

Since the split tank can only make the engine water temp higher (less radiator available to the engine coolant), that's not helping. So I'm forced to assume the Poo thermostat has a wide open flow below that of the restriction HCT suggests. Hard to believe, but I can't see any other conclusion.

Is that what you think is happening in that situation?


#2 (Split Tank): Very interesting. That shouldn't be possible, although if there is enough turbulence or restriction in the radiator perhaps the radiator overflow tubes that connect to the overflow tank are returning coolant even though they shouldn't. Or are you telling me you think both sides are somehow breathing/pumping coolant on the cool side in and out of the overflow tank?

I don't see anything in the charge air circuit that would create enough heat or imbalance to explain the latter, but perhaps if the engine side, what with thermostats and bypass circuits, might force some of this coolant pumping.

Even so, how is that making it's way into the charge air radiator? I'm going to accept at face value your test, but unless the charge air side is also breathing or somehow returning coolant it seems like a non-issue still. Did you happen to check charge air temps, or charge air coolant temps, to see if there was a change?
 

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Discussion Starter #5
While I have a fair degree of respect for the Poo cooling system, I do find one thing off.

Lets look at this first:
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Dear Polaris:

You install a thermostat that begins to open at 180 and remains partially in bypass until 203f. This at cylinder head water temperature. Then you turn on the radiator fans at 190f? While you're still in bypass? And you're doing this based on cylinder head temperature?

Huh?

It seems you want the thing in bypass as much as possible. It also seems you're trying to keep the engine close to 200f, keep water flow around the engine up, and limit the amount of cool water coming in unless we exceed 203f.

This is going to make our engine oil temps hang around 220f or so...

Hm.. This is probably best for power, efficiency and wear, so OK, I guess that's all reasonable enough. Sure seems hot though.

Now, about that themostat... There is an awful lot of stories out there suggesting your thermostat has a serious wide open flow limitation.
 

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Discussion Starter #6
Back on #2, I wonder if convective currents account for the flow between the two. And I wonder how an otherwise pretty well designed system got pooched over a 50 cent part
 

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Thermostats always function as a restrictor(even when open), which is why HCT(and other brands) include a restrictor plate to replace the thermostat. Unrestricted, the coolant flows way too fast and can't absorb enough heat from the engine. It's very easy to kill an engine this way. How restrictive is it? I don't know. If I find my old one I will gut it to measure.

My guess is that the bypass exists solely to keep coolant flowing through the oil cooler when the thermostat is closed.

As to #2, yes both sides are pumping in/out. The charge air circuit isn't bound by the thermostat. But, I honestly don't know offhand if the charge air coolant pump is operating 100% of the time, or only under certain conditions.
 

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The 2 system are definitely mixing the rad coolant and IC coolant although the diagram in the first post doesn't seem to show the full system routing
Just idling for 20 min or so the Rad tank is ..WAY .. hotter as compared to the IC tank
 

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On my 16 the electric pump runs all the time the motor is running. The 1/4” hose has a steady stream flowing into the coolant tank. In the stock configuration the engine coolant and intercooler coolant would mix constantly. I noticed the coolant flow after building my split tanks and topping them off while the engine was running.
 

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Discussion Starter #11
Thanks Max! Great info in both threads.

ARR:
Thanks for clarifying!

Yes, the IC electric pump is always on when the engine is running.
 

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Discussion Starter #12
Jammin, we must have posted about the same time, so I missed your comments. My apologies!

All:
OK, I call "No Mas". I'm pretty convinced the basic's of any pump states a pump must have equal flows in and out, so there should be NO WAY those two systems intermix. At the same time, real life proves exceptions to the rule all the time.

Issue #2: I hereby formally accept the two system are bleeding into each other. I remain skeptical about the actual degree and impact to the IC charge, but I'd already decided to build a test to learn for myself, so.. And I'm going to watch what Max tells us about his system!

#1: I continue to struggle with this too, but too many folks (here and in other posts) with real hard experience show "thermostat removal" has reduced water temps passed the temp the T-stat should be fully open. I'm not going to pursue it unless I develop a real heat problem, for I consider it an imperfect solution (another post another day), but I won't argue with folks that are doing it regularly.

Thanks again to all for the great information!
 

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They do not ... still the same as previous models.
 

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I run the HCT Split Tank, they block it to constantly cycle throught the radiator... pulling the t stat was covered above and the plate is a restriction plate so lower the pressure/flow back to where it should be.

I cant fully explain it as I did this years ago when my 16 was near new... So my memory of the conversation with HCT is pretty buried in my memory.

I can say that it does work well, I also swapped to engine ice and a little redline water wetter at the same time. I'm still debating on an oil cooler but still reading some. My temps now are not out of line. And my machine has not hit limp mode as of yet or been even close.
 

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I run the HCT Split Tank, they block it to constantly cycle throught the radiator... pulling the t stat was covered above and the plate is a restriction plate so lower the pressure/flow back to where it should be.

I cant fully explain it as I did this years ago when my 16 was near new... So my memory of the conversation with HCT is pretty buried in my memory.

I can say that it does work well, I also swapped to engine ice and a little redline water wetter at the same time. I'm still debating on an oil cooler but still reading some. My temps now are not out of line. And my machine has not hit limp mode as of yet or been even close.
I run the HCR Split Tank on my 2016 XPT. I also use their gauges and see about a 60 degree difference in the engine coolant and the intercooler temps. I didn't have and overheating issue with my RXR before I did the split tank. I run the Polaris coolant and ride in the desert of Southwest Arizona. I rode yesterday and it was 110 degrees here and I never got above 200 degrees on the motor . The tank temp showed 185 and the intercooler tank showed 120 .
 

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@CEP 341 good info.... thats the most solid information I have ever found on the effectiveness of the Split Tank set up
 

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I run the HCR Split Tank on my 2016 XPT. I also use their gauges and see about a 60 degree difference in the engine coolant and the intercooler temps. I didn't have and overheating issue with my RXR before I did the split tank. I run the Polaris coolant and ride in the desert of Southwest Arizona. I rode yesterday and it was 110 degrees here and I never got above 200 degrees on the motor . The tank temp showed 185 and the intercooler tank showed 120 .
Agreed... Next time i'm out i'll temp my tanks and stuff, i did many years ago when i installed it. But usually i just let 'er rip tatter chip.
 
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