anyone know the reason for them to change the piston from having the two small holes on the intake skirt to having those big slots. did this correspond to the 350 to 400 change or the fuji to robin change (that had extra ports going stright to the crankcase bypassing the cylinder. don't be afraid to get technical. that's what i'm looking for. and what about the ports above the main intake port, are those to help push out exhaust gases. do hpd power maxx kits have these features?

 

powermax jugs have those and more.

the top two ones are to asist in throwing oil ontop of the piston.

the larger holes were just a piston design change to add more air/fuel.

 

did oem pistons ever have these slots. and if they did does any know if they added them to compensate for a different cylinder design. were the jugs with the extra intake ports designed specifically designed to be used with the big holes or the small holes. having both seems like overkill.

a thoery my mentor was throwing around was. are they there for actual intake or were they there to allow charge back into the intake flange and then out the holes above the intake. because if you think about it. at the time they are wide open isn't at the same time as the engine has vacumm to draw in charge.

and as the piston is on its down stroke it is forcing the charge up through the transfer ports as well as out that hole into the intake flange. then where is it going to go.

i'm actually just throwing ideas for puting a yz 490 piston in. of course it has no holes just a notch in the intake skirt like most piston reed motors. i have a couple of cylinders of each design. going to do some experimenting.

our previous experiment was using a LT 500 piston as it was the only other thing to use a 18mm wristpin and a bore that big. the piston we used was a 88.5mm piston. It had some problems because the distance from the dome was .270" taller from the wrist pin than a polaris piston. so i machined a spacer to lift the clyinder that much. then machined a bunch of skirt off and cut those holes in it. this motor was reliable enough but lacked bottom end. thinking that was because of the increase in crankcase volume. which lowered the intake velocity.

i'm thinking that if the clyinder had the extra intake ports does it really need those huge slots in the piston. or could i just put little ones in. because one thoery would be they only need to be as big as the small intake ports above.
and you say that they are there to put oil on top of the piston. i was thinking that they just some air to help push exhuast out . as they are in line with the exhaust.
anybody with some input would be cool and of course if do this mod let everybidy know how it turned out

 

In mid 1996 model year Polaris shrunk the holes in the skirt back to a size similar to old 350 size . The fragile piston material the stock pistons are made of failed to frequently with the large holes in them - I assume is the reason for the change .

You can mill some small slots in your YZ piston if you wish . That would work I would think .

Keep in mind those size pistons you want are already available all set and ready to go .

Personally , I would stay close to stock bores and have the head cut correctly and a good port job . The power gain vs hassle and reliability factor starts coming into play.

Rick

 

thanks rick you make some very valid points. but you seem to be the only one on this site who actually knows something. but won't come out and say to much detail. but i understand you are in this business to make money so I understand. will people actually sell big bore pistons for these if they don't do the clyinder work. hpd looks like they might and aeen. i've got a 85mm in mine that will work fine just finding an aternative to my brother who is all ready at 88.5 needs new piston. just ordered a 89 mm yz 490. We will give it a go

 

has nothing to do with knowledge but there no reason to reinvent the wheel, if your looking for general performance info for 2 stroke motors check your local library there are some good publications. If your expecting to find a polaris forum thats like say a banshee hq were everyone is shooting out different setups your not going to find it. In 4 years(est.) on this forum I have only seen one other person but myself port there own machine. Most people don;t tune 400 polaris's or would rather spend the coin and get a relaible port job and not have to mess with it.

Port jobs are so cheap for this machine its hard to even justify doing it yourself unless you already have the tools and bits. a good set of bits runs the same as a port job.

 

well truth is most people are missing out on learning to do it yourself. I am of course a not like everybody else as i am a machinists with a mentor who used to specilize in small engine boring, resleeving, head fixing(when things get embeded in them), crank repair and straightening. but his porting experience is limted because he spent most of his time turbocharging snowmobiles, cars, trucks. He gave up on making horsepower the old fashioned way. So i've got all the tools to use. I have ported my own now it is not that hard. the xpolerer will no beat my dad's 800 sp
even still with stock pipe and carb. bottom end was not sacrificed that much either. rick won't like me saying this but oh well. a polaris 400 is really easy to do a mild port job. because it is not like a motocross engine that is already ported to more less the max. It is very detuned, so basic things to wonders. but i'm just tring to learn some things about this engine. It has some strange intake designs not usaully seen on dirt bikes and snomobiles. so i guess i'll try the libary but I doubt that books will show two-stroke designs with huge intake windows in the piston.
my mentor has a land&sea dyno as well as a engine dyno. one day I'd love to get my motor on one and play. thanks for the reply

 

Here is a little porting advice for you on your 400 -

If you are trail riding a 400 explorer - raise the exhaust port as little as possible .

Don't be obsessed with *knife edging* the transfer dividers . The leading edge is better off being rounded - either way work in this area means little .

I spend most of my time in the roofs of the transfer ports . It is make it or break it in this area. Word of advise ---> "when in doubt - keep tool out"

Many poeple do a little home porting on their own - poeple get carried away with wall finish , it isn't shiney wall surfaces that make things go fast - it is contours .

Where are the best places to grind ???? The places you can't get to . *grin*

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Before you hook your engine to a dyno - watch someone elses bike get dynoed first . A land and sea dyno is the prefered dyno for these engines - but it is a little shocking to listen to a wide open motor get pulled down by the dyno .

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The window design in the pistons is quite common . I can think of many engines made in a similar fashion . Nothing unusual here .

Have fun in the shop !

Rick

 

thanks rick and yes the angle of how the transfers come out has a big effect on things right. if it comes out flat, it usaully is better for more top end power, if they come out at quite an angle like 45 degrees. it should give it a wide powerband.Yes the intake and transfers should have texture and the exhaut port can be polished. I do have access to a 45 degree pencil grinder but i stayed out of the transfers maybe next time i'll flaten out the roof of the transfer that has the bigger angle of the two. the one the intake side i beleive.

Does anyone ever tighten the crankcase in anyway to get better pumping action.

i'd let the dyno owner run the show he has dyno lots of motors and spent a better part of the 1990's building fuel maps for turbocharged snowmobiles with EFI

thanks for the advice rick.