Just a little FYI on the Polaris CFI Engine and surrounding rumors

[jjack]

all junk

 

[Kranked]

all junk

Great post. Thanks for your input jjack.

 

[Modman]

1) Rod Ratio is lacking: FACT: Polaris 800 CFI has the BEST rod ratio of any of the , modern, 800cc engines out there. The rod ratio is actually on the HIGHER end of the "acceptable" spectrum. Yes, there exists design guidelines for all engine builds.. and , again, the rod ratio of the CFI 800 is th ebest out there.

In short: There is absolutley no rod ratio issue. If there were, then Cat and Ski Doo would be suffering even worse.

I think people often assume there is a rod ratio issue because there are kits out there that install longer rods to address the issue. Just more of a mis-understanding thing I think.

2) There is a "Cold Shot" that causes engine failure: This is a good one and a huge gimmick.. Arctic Cat has had the SAME type as Polaris of coolant system for ever (no by pass loop) and has no issues .. the difference between Cat and Polaris is Polaris has a digital gauge so you can see the actual temps.. Cat has the same temps but no gauge.. Install a gauge on the Cat and ,SUDDENLY, it will have a Cold Shot problem as well... Only Ski Doo utilizes a by-pass loop and they only started doing this in 2000. For more details on why this is simply not an issue please check out this article:

Doo has been using the by-pass loop longer than that I think, the 670's and 583's came with the bypass loop that circulated to the surge tank and then back to the motor, at least as far back as 1995/96. Once the motor was warm, the t-stat opened to the coolers.

Cat also does have a digital temp gauge, you just have to hold down the up and down buttons on the top of the speedo for 10 secs and put into diagnostic. Never seen the cold spike on mine that you are referring to. This feature has been around since 2008/2009.

Not picking a fight or anything Kelsey - just clearing up some rumors.

 

[minnow10]

But the cold seize is not all about the fluid temp issue, the " cold seize" is majority the engine parts itself, does the loop become not important? is that why only 1 of the 3 use it? The gauge is to read that your engine is at optimal operating temp, this will allow you to add rpms as there will be no more expansion of metal, there is not enough " cold coolant" to cool the block enough to shrink it to " cold seize" . Just asking because this is what I had always been told.

 

[Vance Matheson]

But the cold seize is not all about the fluid temp issue, the " cold seize" is majority the engine parts itself, does the loop become not important? is that why only 1 of the 3 use it? The gauge is to read that your engine is at optimal operating temp, this will allow you to add rpms as there will be no more expansion of metal, there is not enough " cold coolant" to cool the block enough to shrink it to " cold seize" . Just asking because this is what I had always been told.

What you said sounds right. But would the coolant coming from the rear be cooled enough to give the cylinders a cold shot. What if I stopped for 5 or so minutes and the tunnel was full of snow. Also the outside temps were low. Seems like that could cause a cold shot. Maybe im wrong but the pro we have throttles back to around 6800 rpm after a long pull until it cools to a temp it likes. Even in deep powder.

Im no expert but it seems like a time bomb to me.

 

[Modman]

But the cold seize is not all about the fluid temp issue, the " cold seize" is majority the engine parts itself, does the loop become not important? is that why only 1 of the 3 use it? The gauge is to read that your engine is at optimal operating temp, this will allow you to add rpms as there will be no more expansion of metal, there is not enough " cold coolant" to cool the block enough to shrink it to " cold seize" . Just asking because this is what I had always been told.

I know what some guys say and that there is not enough coolant to cause the cylinders to "shrink" but the cylinders and cold coolant act like a heat sink and are only heating through conduction really, and this will take longer to heat than the piston itself, which is being heated directly by combustion. Think of it this way - when the fuel charge ignites, the piston at TDC or just before, and the combustion process is basically over rapidly. The majority of the cylinder is below the piston at this point, and therefore not exposed to direct combustion (besides whatever area above the ports are exposed to the end of the process as the piston moves down from TDC towards BDC).

The cylinder has coolant flowing around it constantly, the piston's only real mechanisms of cooling are the intake fuel charge itself, and conductive cooling (heat transfer) through the piston to the cylinder walls. The cylinder has actively flowing coolant for conductive transfer, convective transfer, its not getting much direct combustion, etc, and all that remains pretty linear as the RPM increases (coolant flow increases as RPM increases). The piston will heat up much faster than the cylinder and therefore expand much quicker. Whether its a "cold shot" or the fact the piston thermally expands much faster than the cylinder because of it, the piston expands, the cylinder does not and you have a "cold seize". Does this happen due to the coolant not being warmed up as fast as the piston before sustained high RPM or a cold shot of coolant from the coolers coming in? You choose. There is logic behind a hybrid theory IMO - if the piston is hot and the cylinder is warm, a "cold shot" of coolant could slow the expansion and as the piston continues to expand due to added heat (high RPM), the rates are much different now. Ultimately the rate of thermal expansion on the piston is much faster than the cylinder and this is why it ends up squeaking.

This is why factory applications of pistons use cast vs forged. Cast pistons are more dimensionally stable than forged and therefore the piston expands at a more uniform rate. They've assumed joe blow is going WFO from the parking lot with minimal warm up. The clearance requirements for cast vs forged pistons demonstrate this. Forged pistons are tougher and not as brittle but require more warm up time (also more expensive). This is why its also critical to warm up bigger bore motors, as the expansion stabilization rate issue is magnified on larger pistons.

The thermostat mitigates this, the bypass loop is there IMO, to help the motor warm up to operating temp faster and avoid the above scenario as much as possible, again, because manufacturers are assuming that we are riding cold sleds, full blown throttle as soon as the cord is pulled.

Goal of this whole long-winded post: let your sled warm up good and don't go WFO from the parking lot, give it a couple moments to warm up and cycle coolant after you stop for a smoke break, and you won't have a time bomb on your hands.

 

[jjack]

That's right modman different pistons different expantion rates,What causes all the problems is the piston expands faster than the cyilinder and it just wont fit any more.You can get a seize on a chain saw and there is no coolant but that's usually just on start up.what I do with al lmy sleds is put a 1/8 hole in the thermostat what that dose is allows a small amount of coolant to pass threw all the time so every thing warms up more even and no sudden opening of the thermostat dumping -30 fluid to the cyilinger.

Never had any problems do this on all my truck and equipment as well seems to work

 

[Vance Matheson]

Thanks for that info you guys. Im trying to figure out out to stop the pro from throttling back when hot

 

[jjack]

Thanks for that info you guys. Im trying to figure out out to stop the pro from throttling back when hot

you mean throttling back when its over heating

 

[rmscustom]

Thanks for that info you guys. Im trying to figure out out to stop the pro from throttling back when hot

I'm going to do the Doo t-stat on mine this year. I didn't believe the hype last year but paying close attention there is a loss of power beginning at 129 and by 133ish its turn out time.

 

[Vance Matheson]

you mean throttling back when its over heating

It dosent over heat, unless 130 to 140 is to hot. after some powder play and a few short climbs. If we sit and take a breather say 10 minutes then jump back on it won't rev to 8000 it will go to 6500 to 7000. Until it cools down. We only noticed this in slow moving deep powder on the trail it dosent happen.

I was thinking of the thermostat. But I checked it on the stove and it open before 100. Maybe I could just take it out and see or drill more holes in it.

 

[Vance Matheson]

I'm going to do the Doo t-stat on mine this year. I didn't believe the hype last year but paying close attention there is a loss of power beginning at 129 and by 133ish its turn out time.

Is that really to hot??..this one has hit 150 on the trails and it dosent throttle but if I go off trails and crack the throttle it will rev to 7900/8000. And cool down....im still thinking the slow moving in deep powder and no air flow maybe....

 

[jjack]

Is that really to hot??..this one has hit 150 on the trails and it dosent throttle but if I go off trails and crack the throttle it will rev to 7900/8000. And cool down....im still thinking the slow moving in deep powder and no air flow maybe....

I do belive but don't quote me that you have a exhast temp sensor,slow moving in powder might be bringing your temp up on the exhaust or you have a bad sensor,

 

[Vance Matheson]

Could be that sensor but its ok if we are on the trail. Im going to try the doo stat.

 

[jjack]

Could be that sensor but its ok if we are on the trail. Im going to try the doo stat.

If it was me id warm the sled up to 160 and if it doent kick out its not the engine temp doing it.then try restricting your exhaust and see if it that does itthen you know if you can fix the problem now then its ready for winter

 

[rmscustom]

Could be that sensor but its ok if we are on the trail. Im going to try the doo stat.

I'm not sure if we're talking about the same thing... I've never experienced what you have after shutting down for 10 and starting but I've ALWAYS have let my temps stabilize before getting on it. The 129 degree power fade I'm talking about would never be noticed on a trail. When I see it it's in deep snow WFO asking the most of everything. I'm not sure what the ecu is doing but it's for sure protecting the engine somehow be it adding fuel or a timing change. TRS Doo bypass is my first to do this fall to keep the temps below that (search SW if you don't know what I'm talking about).

 

[Vance Matheson]

I'm not sure if we're talking about the same thing... I've never experienced what you have after shutting down for 10 and starting but I've ALWAYS have let my temps stabilize before getting on it. The 129 degree power fade I'm talking about would never be noticed on a trail. When I see it it's in deep snow WFO asking the most of everything. I'm not sure what the ecu is doing but it's for sure protecting the engine somehow be it adding fuel or a timing change. TRS Doo bypass is my first to do this fall to keep the temps below that (search SW if you don't know what I'm talking about).

I believe we have the same issues it dosent throttle back on the trail. But if I go off trail and spin the track ...getting snow to the cooler and back on the trail it cools down. However in the deep on the hill it will throttle back. And the temps really are not that high. 140 max

 

[neilsleder]

Vance it could be not enough air getting to your clutches? Everything is getting hot not working properly.


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[Vance Matheson]

Vance it could be not enough air getting to your clutches? Everything is getting hot not working properly.


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it was happening that day at chappel and again at renshaw both deep pow days thats when its does it. i never thought of clutching tho.

 

[Vance Matheson]

BYPASS THERMOSTAT UPGRADE (PREVIOUSLY $28 upgrade) - SnoWest Snowmobile Forum

this is it rmscustoms?....