0.250" compression height w/shim

[Brock]

Not a big fan of the "shim and larger compression height pistons "vs Polaris. Did this in mine and won't do it again. Seen to many kits with shims and raised compression heights in pistons that have failed and half the life of stock Polaris know. I tried it twice and no more.

Cat and wossner pistons with shim. Two different sleds. 1800 miles on cat pistons with shim and has a sharp lip on exhaust side of piston due to bigger compression height(0.250"+/-) allowing pistons to slap in bore and push piston to exhaust side even though it had 0.005" clearance new . They are now collapsed 0.009"total .

Compression height is from top of piston to center of pin and it's larger on cats vs Polaris. Polaris has the proper compression height.

Wossner durability kit pistons have 3300 miles and are collapsed 0.011-0.012" total

I don't like the compression height on cats or wossner 800 pistons and shim as this makes pistons rock in the bore even more then the stock Polaris pistons. The lower you put the pin on pistons, the more it will rock in bore

You guys can do want you want? Polaris has a better top of piston to pin then the cat or wossner(compression height)

Just sharing information from what we've seen

 

[po1ar1s]

Good information. We haven't had many piston issues on the 800 H.O. I feel like a coated OEM piston would last quite a while and would be what I would do on my personal snowmobile if I had to replace pistons.

 

[Dr.Notadoo]

cat pistons don't belong in the Polaris cylinder, wont last..i concur..

 

[Dr.Notadoo]

The thrust side is the intake side on any 2 stroke, the lip or edge you describe is a result of the piston trying to throw itself out the exhaust port, that's pretty common for that edge to form there.

 

[Brock]

Correct doc.

The lower pin placement on cat pistons vs Polaris forces the "piston to rock in bore" and puts "huge force on top exhaust side of piston trying to force it out exhaust port" resulting in a knife edge all the time. The wossner didn't knife edge as bad because of 0.020" offset of pin placement. Seen quite a few cat pistons with shim in the Polaris 800 motors now and the results are the same. It's basically a 1000 -1500 mile sled and change pistons. I did this and don't like the results seen and measurements after little miles on sleds. We all try stuff. Sometimes it works, sometimes it doesn't.

The proper compression height piston and 0.020" pin offset like Polaris doesn't do this . There is a reason Polaris has a shorter compression height and the proper pin placement.

I posted a general pic so everyone can see what is meant by "compression height"
The greater the compression height the more it will rock in the bore. The key is to have the smallest compression height(which Polaris has) but being 500 grams( which Polaris doesn't have) and tighter tolerances (which Polaris doesn't have). Polaris does have the correct pin placement but needs to tighten tolerances and taper and loose some weight to be the perfect piston in my mind.It's one of the better pistons Polaris has ever put out. But I would like to see what I mentioned above cause I'm very picky.
Wossner does make a direct replacement piston for the axys that is lighter and has tighter tolerances and less taper. CV-Tech sells them if anyone wants to know?


The only reason for shim is because air volume movement and restriction. The shim helps move more air volume which gains more hp plain and simple. But the downfall is larrger compression height.
There's ways around this without a shim

 

[Powaton]

What are the ways around the shim? Porting?

Polaris tolerance on the loose side to hedge aginst bonehead hard cold running?

 

[Brock]

What are the ways around the shim? Porting?

Polaris tolerance on the loose side to hedge aginst bonehead hard cold running?

Porting and machining to cylinder without shim can and will get the same results

My opinion on Polaris pistons having thick heavy pistons with a bigger taper then most has always been to prevent cold seize.

I've said this before. I wish Polaris would tighten their tolerances (piston-cylinder) and put a warm up program in ecu were you can't move sled for a certain period until it's happy at every start up. The bypass cooling works to prevent cold seize, just add a program in ecu so the boneheads can't cold seize as more protection.

 

[Powaton]

While on the topic of tolerances...What do you guys think about snowmobile squish clearance. Why do sleds still run so much squish clearance? U understand too much compression being an issue, but is there a problem with bringing squish below .030 and machining the dome to bring comp back? DTR has a good example of this on a cat 800.

I know 4 stroke guys have been doing it for years. I've done it on a couple bikes. Never knew why sleds don't do it.

 

[po1ar1s]

They are sort of starting to increase the cold engine temp protections. They control the EVES position based on engine temp, they progressively open as the engine warms up.

 

[po1ar1s]

The large squish probably has to do with wide variety of fuel quality. Some people run some really shitty fuel through their sleds so they need to run a larger clearance to try and help prevent detonation.

 

[Brock]

While on the topic of tolerances...What do you guys think about snowmobile squish clearance. Why do sleds still run so much squish clearance? U understand too much compression being an issue, but is there a problem with bringing squish below .030 and machining the dome to bring comp back? DTR has a good example of this on a cat 800.

I know 4 stroke guys have been doing it for years. I've done it on a couple bikes. Never knew why sleds don't do it.

Depends on shape of the dome and fuel for squish

The cats you're talking about are old efi systems, not an sdi for tight squish, totally different. I've ran 0.043" squish with the proper dome for 1000' asl and 12:1 afr . Depends on the how efficient the dome is cut . Most will be around 0.050"+ squish

 

[Dr.Notadoo]

The best way to understand squish is it has a direct relation to timing..think of less timing, is like having thicker squish and vise versa. So tighter squish has better throttle response as does more timing..the biggest issue with less squish is being able to resist det with pump fuel. There are other factors involved here such as proper coolant flow ..

 

[George A Twitchell]

If I have read most of Indy Dan's and understood those writings, what you guys are saying is basically just what Dan has said also, that being, to get away from longer compression height and back to stock pistons and maybe find better tolarences (thghter) and pay attention to warm-up procedures.

 

[Brock]

George.

The thing with these so called fix it kits or whatever you want to call them is just about more air for more hp . Restrictions are on intake and these raise to correct that . Some will say different, but it's just about that .

The compression height on the Polaris piston is correct.

Moving the pin placement lower in the piston like the cat/ cat wossner allows piston to rock in bore even more . It's best to get the pin closest to the top of piston as this won't rock in the bore or have tendency to push top on thrust side.

Look at the way nhra, top fuel , top drag cars have the compression height on pistons for instance? The pin placement is the closest you can get to top of piston. Put the pin lower By a 1/4"+ and see what happens???

Cv tech has the wossner lighter direct replacement pistons with same compression height as factory Polaris pistons but better tolerances. I'll look for the part # .

I've seen way to many shim kits over the past 6+ years now with larger compression height pistons vs Polaris and they all suffer the same fate.
Off setting the pin by 0.020" (wossner cat pistons)does help. But that's all it does. It just helps .

 

[Powaton]

The best way to understand squish is it has a direct relation to timing..think of less timing, is like having thicker squish and vise versa. So tighter squish has better throttle response as does more timing..the biggest issue with less squish is being able to resist det with pump fuel. There are other factors involved here such as proper coolant flow ..

With 4 strokes the game is to get squish as small as you dare. Independent of timing or any other variables. In some more extreme cases the piston will "kiss" the combustion chamber at max RPM. But the head gets a longer cool down period in a 4 stroke. The gain is from provoking maximum turbulence in the chamber, allowing higher compression before detonation and pushing as much combustIble material to the center as fast as possible.

 

[Powaton]

George.

The thing with these so called fix it kits or whatever you want to call them is just about more air for more hp . Restrictions are on intake and these raise to correct that . Some will say different, but it's just about that .

The compression height on the Polaris piston is correct.

Moving the pin placement lower in the piston like the cat/ cat wossner allows piston to rock in bore even more . It's best to get the pin closest to the top of piston as this won't rock in the bore or have tendency to push top on thrust side.

Look at the way nhra, top fuel , top drag cars have the compression height on pistons for instance? The pin placement is the closest you can get to top of piston. Put the pin lower By a 1/4"+ and see what happens???

Cv tech has the wossner lighter direct replacement pistons with same compression height as factory Polaris pistons but better tolerances. I'll look for the part # .

I've seen way to many shim kits over the past 6+ years now with larger compression height pistons vs Polaris and they all suffer the same fate.
Off setting the pin by 0.020" (wossner cat pistons)does help. But that's all it does. It just helps .

Cv tech...how much tighter and how much lighter?

 

[Dr.Notadoo]

These are just my thoughts and not saying that I know any more than the next guy. It was my understanding that the "fix Kit" came about with the longer "compression height" thus longer piston area to solve the preceived piston rock issues causing premature ring and piston skirt wear and ultimate failure with catastrophic cylinder failure. If I have read most of Indy Dan's and understood those writings, what you guys are saying is basically just what Dan has said also, that being, to get away from longer compression height and back to stock pistons and maybe find better tolarences (thghter) and pay attention to warm-up procedures. In my last five years of building and experimenting with different pistons and fittment, I have had very good luck with Arctic pistons and Sno-X (arctic replacements) with the fix kit spacer plate and have not tried Woosner because could not find anyone to buy from. What is the Woosner part number for direct Polaris replacement of the stock compression height piston and can I get that from CV-tech? Let's keep this thread going and let us know if we can find some better fitting and lighter pistons than the AXYS 2017 (which are good IMO).

gtwitch in wyoming

What I'm saying is Cat pistons don't work well in Poo cylinder due to the design and port location, nothing about pin height. Cats intake and exhaust are on the same side of the cylinder,the one side of a cat piston (ring pin side) is strait ,this is all for surport in a cat cylinder,not Poo..

 

[George A Twitchell]

Brock, The idea of getting the pin height closer to the top of the piston is where Indy Dan has gone with the long rod crank by using a longer rod and a shorter (stock Polaris) Compression Height and this accomplishes what you are talking about and I assume he is using a spacer plate to now get the deck height correct and by using Stock pistons, he has the squish according to the Polaris design unless the deck height is changed by the plate thickness and using the plate allows for more case volume and mass air flow! These long rod engines are getting a lot of attention and seem to be a long life engine if you hear what the users are saying!
gtwitch in wyoming

 

[Brock]

Brock, The idea of getting the pin height closer to the top of the piston is where Indy Dan has gone with the long rod crank by using a longer rod and a shorter (stock Polaris) Compression Height and this accomplishes what you are talking about and I assume he is using a spacer plate to now get the deck height correct and by using Stock pistons, he has the squish according to the Polaris design unless the deck height is changed by the plate thickness and using the plate allows for more case volume and mass air flow! These long rod engines are getting a lot of attention and seem to be a long life engine if you hear what the users are saying!
gtwitch in wyoming

Goerge, from an engineering stand point I think you can see why the shorter the compression height the better. Even common sense comes into play to know what's going to happen the more you move pin down on pistons.

Kits claim things but it comes down to the real reason ? Mass air flow and volume and nothing more.

I'd rather use the same compression height pistons as Polaris but get the mass air flow volume. Lots of gains here plus reliability.

Cv tech has a piston from wossner for 85 mm bore and 87.50 mm

 

[Dr.Notadoo]

Brock, The idea of getting the pin height closer to the top of the piston is where Indy Dan has gone with the long rod crank by using a longer rod and a shorter (stock Polaris) Compression Height and this accomplishes what you are talking about and I assume he is using a spacer plate to now get the deck height correct and by using Stock pistons, he has the squish according to the Polaris design unless the deck height is changed by the plate thickness and using the plate allows for more case volume and mass air flow! These long rod engines are getting a lot of attention and seem to be a long life engine if you hear what the users are saying!
gtwitch in wyoming

He is doing this because he believes the rod ratio is to short,but if you do the math the Polaris is the longest out of all three..