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Discussion Starter #1
Just sent my cylinders out for porting and ordered the RK Tek head. I have the engine apart now, obviously, and am wondering about the issue I have been reading regarding welding on the 800 pistons for more power. Anyone else read of this or try it? I sent an e-mail out to the company and received a reply as to what happens.
"Bob, the pistons are welded on the exhaust side, and re-cut to my specs.
There is a hole in duration of the engine, which looses crankcase pressure
and this fixes it. I do believe it will improve your snowmobile! Any
questions call."
Feedback has been unbeleiveable regarding this work done. Any thoughts?
Also, what gives with the 90 degree fitting at the PTO side of the engine that is capped off? Cannot see it until you strip stuff off but appears as if it can feed oil to the PTO bearing.
 

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for the price and the gains to be made it sounds impressive,ill wait until the snow flies to hear how they are working out,quite a few on the western forum are doing this,and many of them are very knowledgeable people,so ill wait to hear their 2 cents :D
 

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Welding on pistons is kind of scarey. For short term like drag racing sure. But I wouldn't want to be in the back 40 and have something fail.
 

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I would not weld on the pistons of any engine.
 

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Welding would make the piston weaker in that area, cryogenic freezing makes metal stronger. Grinding also is not a great idea. For balancing. The pistons can come withing a few grams on a doo also. Buy a Mach Z, THEY'LL OUTLAST A bIG TWIN! oF COURSE WELDING ALSO VOIDS COMPLETE WARRANTY, unless money is coming out of you @@@! :p
 

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What this fixes is a "short-circuit" in the porting. It really tweeks my to find out this simple problem was in production for 3 years before Doo did anything about it.

What happens in when the cyl. fires is the combustions pushes down on the piston, and the exhaust starts to open. The header in the pipes starts to "suck" the spent charge out of the cyl. and during this critical moment, there is part of the intake port open on the side of the piston. What this means is that part of your nice clean fresh fuel goes right out the exhaust. Wasted.

Many of the guys are reporting that they are dropping down to 420 or 430 MJets with safe jetting. Just like the new REV that comes with 400MJ. I think Doo figured it out, and has kept it quiet. Supposedly they have new cyl and stuff on this years motors.
 

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if the new 800,s have 400mj that is a big change,and if 800mxz is right that would explain the flames out the exhaust and backfire when stopping the sled from a run down the trail,i always found it odd that i had 500mj and the cat 800 comes with 430 yet is a stronger motor and is very good on gas
 

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The reason why that the Rev may have leaner jetting is that it may have a more restrictive air box. Also Carb jetting has a lot of variables. The type of carb venting and where your air intake routes to makes a big differance on what your jetting is going to be.


Also 800mxz on your statment about fuel being wasted.

Without going into great detail of two stroke exhaust and ports. Most high performance engines uncover both the exhaust and intakes. THe vacuum created when the exhaust leaves pulls up the fresh charge. THe angle of the ports should not be aimed so that they are shooting the fresh charge right out into the exhaust pipe. A well designed exhaust system should push back the charge escaped back into the cylinder. The header length, size of the cones (ie fat part of the exhaust) and stinger diameter have an effect on the reversion on the waves that push the fuel back into the cylinder.
 

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Welding on a piston is most definately NOT accepted practice.
xotar, you are exactly correct with airbox & pipe explinations. A correctly designed pipe lets very little fresh charge out the pipe
 

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800mxz you said they were dropping mj down to 420-430 is that with the stock head?
 

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dont want to start a fight,i am no expert with the specifics of a proper 2 stroke,but several expert engine builders on the western forum have basically given this process the green light after calling the person who will be performing the welding job,and they have stated that it will not affect reliability,im not crazy either about getting my pistons welded,but i am anxious to see how it works for some of the riders this winter,becuase the mountain riders are the hardest on motors,running powder needs lots of wide open throttle running,and reliabilty is very important in the mountains.
 

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where are they welding on the piston?
 

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I am not 100% sure but it sounds like at the piston pin area. When the piston is up at tdc some of the exhaust gasses get trapped between the piston and exhaust port. So this area is welded up and machined to prevent this from happening.
 

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A piston is not round, if this is where they are welding & machining I would have a hard time believing in the reliability of this mod. Pistons are cam ground, narrow at pin hole, fat at intake & exhaust. I don't doubt the theory just the fix.
 

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if this is "the fix" the aftermarket will be quick to get the pistons out to fix this right.like I stated I would not weld on any piston that is going in one my engines.there are right ways to fix things and wrong and welding the piston is wrong.
 

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This is what I know about the problem. It lies at TDC.

I had the opertunity to look into this myself over the weekend.


At TDC, the piston uncovers the exhaust port. It is basically open while the engine cycles for several degrees of duration, loosing some of the charge. To me, this is not a good idea. Basically the some air is allowed into the crankcase and right back out.

As far as the pipe goes. Iam sure Doo did not do something to comprimise power in the pipe, especially going thru a totall re-design.

Shimming the jugs might help this scenario, but it would take some trial and error to fix it.

As far as welding on the pistons, there have been allot of welded pistons before this ever came allong.

I am looking into this.

The jetting specs just baffle me. Why 10 sizes different? The 800REv is strong, and runs even better than the standard ZX 800, so I would guess the power is still there.
 

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Discussion Starter #17
I think I will inquire tomorrow as to were the welding occurs on the piston and how much of it there is. You're right that if this does cure fuel consumption and the dreaded "blue flame" that it wouldn't be long before pistons come out with this update on them.
 

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I would wait for a company to come out with a modified piston (If not rotax) then to weld to the piston. For racing fine but welding weakens the metel around the weld and its not a question of will it holds its a question of how long will it hold. Then you have ballancing and all that stuff.

I would EMAIL the hell out of skidoo and fix the problem that way, but dont weld.
 

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e mail your brains out, ski doo will not buy anyone pistons for a performance increase. Only for durability issues. :doh:
 

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You have to be optimistic. :yee:

Maybe not replace pistons, but have them available for you to buy. If they dont already.
 
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