1989-2004 KX500 Window specs

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10-23-20 UPDATE: Go to page two..

4-3-17... Latest info on Window mod..
After speaking to Stewart (Current KX500 Stock class Land Speed Record Holder) about the window here is the scoop.
Yes, he gained 3hp on the top end windowed vs non windowed.
Remember, this was a heavily modded engine generating a lot of heat.
Why the gain using a window?
Heat dissipation especially on the piston crown...
If your setting a land speed record, desert racing, Kart racing, Supermoto, or similar (Wide open for extended periods) then you need a window.
If your a trail rider or only see wide open throttle for short periods of time then you likely will not need it.
It will not hurt to have one as far as what Stewart could tell.
My piston is windowed and I run mine in the desert, dunes, and Street Tard.
My engine has a lot of hours on it over the last 10 years and no issues.
So, I will continue to run a window just cause it's cool

Here is the rest of the info..

For years the common belief of big bore KX riders was to window your piston.
According to all the information I could find it adds 2 hp and keeps your engine cooler.
So over at KXRiders several members are discussing the viability of it as well as modifications to the existing design.
What is a windowed piston?
Simply, a piston with a hole in the side.



Obviously the size and placement of the window will have a real impact on performance.
The post below are directly taken from KXriders. I didn't include all comments as there was some a little off track:
Motorrad:
Going to change your Piston window design.. I think the previous tuners design we are all using.. (no names needed)... may be too large, and LOW starving the boost ports to the transfers, as well as the floor ports...............
may go to a triangle shape.

Motorrad:
I think I have a good idea of changing it. would involve re-location. idea came from some of BDI's secret squirel porting... insted of porting the added ports into the jug.... put them in the piston.. more to come.
stewarts piston port area 1.003sqin
my idea .72sq in and it would free and clear the boost ports...
so roughly 75% size
if you dig HARD... I commented on this design to said builder back when he first came up with it.. wink but like many.. I drank the Koolaid, and listened to him, instead of myself.

Motorrad:
I guess my theory is... why would one add a piston port right in the middle of a Rear boost port... causing the intake charge to go to the crank case, instead of a direct path to where it needs to go....
I hope people jump in on this discussion. Im not a one man band here.

Motorrad:
Here is a VERY VERY crude drawing of what Im talking about... live with it.

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USMC 500:

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[Sandblaster]

Motorrad:
my theory on how air will travel. (look at arrows in middle section.. and imagine if there was a big hole cut in the piston... how that would starve the rear boost port (which is at the top of the middle section), and the floor ports... making the air travel MUCH further than it has to to get to the comb chamber)

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alward25:
Don't know, but it dyno proved 3 hp to have it. How does a posi track rear end work on a Plymouth, it just does );

Motorrad:
and my theory.. bring as much as possible flow to the engine for as much as possible performance, that is needed, but dont cut to much, to be safe that the cylinder dont crack up there... wink
I agree there is an improvement in running a window vs not. But I think we can improve on the current design.
to me , the current one is like slapping a U-turn in the transfer ports, making them go back to the crank case before they can then go to the comb chamber again..... kinda bass akwards

Motorrad:
the windows seem to just Skirt (yes pun intended) the re-inforcing ribs in the piston (wiseco) will have to test the wosssener. so that is a good thing.. that was one thing I was worried about.

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this design could also let ya cut the bottom of the intake skirt somthing like Yz250 skirts... for even more intake ............
(esentially a bottom window)

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[Sandblaster]

Good day folks. I think you are on to something here Motorrad. I too think that the large lower port starves the boost ports, as the boost port pressure differential is higher than the huge gaping hole in the middle of the skirt creating a short circuit. By moving the piston port to the sides like that, the air will be diverted to the sides much better due to the pressure differential in the intake being moved outwards to the side triangle shaped ports, like you have cut. I believe this will in turn direct much more air to the side boost ports while the piston is still covering the main intake port.

cbrfrenzie:
I have a dyno at my work that I have full access to, anytime I need / want. I am already working on a test using two different window porting options to see what the relative results are. Perhaps I can help you guys out with some distinct numbers on these mods and the result (torque curve is what we are really interested in here). I will be plumbing in a couple pressure sensors so I can see the resulting waveforms with respect to the dyno run, to give me a closer idea of what the air is doing in there. I will be logging exhaust pressure, intake pressure at 2 different points, EGT, RPM and torque.

Motorrad:
Awesome CB.. glad Im not out in left field here....

Another thing I was thinking about.. on my triangle ports. is make it so they DONT open (unshroud) till RIGHT after the transfer ports uncover. in my mind, making the piston act more as a PLUNGER and getting more pressure and air moving through the transfers before it has a chance to suck through the windows keeping the charge that is in the crank case from pushing back out of the window and ending up around the reeds, as the piston compresses the new charge,, (gets the flow directoin moving in the right way first)
this will also make the boosters start sucking before the piston windows pull anything... once again. getting the flow moving in the right direction first...
thoughts?

Motorrad:
Once again. this is all just REALLY REALLY ROUGHed out stuff. dont judge radius's, exact shape, size etc etc etc..
here is what I was thinking from the yz250 pistons.
this bottom cut CLOSES just as the transfers open...... which should keep it from starving the lower floor port and the boosters...
(last 2 pictures taken right as transfers open)

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SHLEPY:
This makes a lot of sense if the jug has the ports why not the piston...Looks good Mr M!!!

[Sandblaster]

cbrfrenzie:
Interesting idea there with the lower piston skirt shaping there in the last 2 pictures. I think we want to be careful with that area, as to not decrease the pumping effect of the piston moving down. If there is an opening there when the pressure under the piston is greater than the intake, air will flow in reverse into the reed chamber. Your pictures are very informative, I like how the angle you cut into the piston is different than than the angle of the boost port. This gives a nice transition into the piston boost port, which will reduce negative turbulence as the piston moves upward sucking air into the case. That angle and depth may be good parameters to experiment with. Keep it up, I'm liking your ideas!

Motorrad:
I 100% agree we need to be carefull in that area... I think as long as it closes the bottom before the transfers open up... we should be fine... Huh huh
heck.. would be better than that low window in the "S" pistons. that would be a problem just as you described.

Sandblaster:
How much closing is enough huh

Motorrad:
For refferance.... here is a "S" piston in exactly the same location as the last to pictures above. (exactly when the transfers open) you can imagine how that would starve the rear boost port....... and as far as lower skirt cutting.. the one I showed cut, that closes as the transfers open.. has to have better pumping action than the "S" piston, that is WIDE OPEN .... YES NO? Im just going off my thoughts here... What ya think CB?



cbrfrenzie:
Just for clarification, are you we talking on the down stroke or up stroke?
I like the split boost design better than the large central boost port in the piston. this will help split the air up and direct it towards the boost ports, with minimal short circuiting. How cool would it be to add extra oil holes on the piston wrist pin boss directly behind these new ports haha.

Motorrad:
I can tell you from now on I'll be adding oil holes to the exh bridges

81cr450:
Oh yeah "brilliant", done it on the cr5 for years , it never crossed my mind on the K5. Extra cooling duh...

Motorrad:
what does everyone think of..
1. not opening the piston window's till right AFTER the transfers open. (on down stroke)..
2. Closing the bottom of the piston cut right BEFORE the transfers open (on down stroke)
Playing some more.. I can get .839sqin of area (.419sqin per port) all without them opening untill the transfers are already open.. am I being a bonehead and missing somthing by having them open after the transfers?

81cr450:
Sounds like maximizing the pressure charge.
The sonic pulse from the pipe is supposed to be the end all but, I dont believe it. There's definite gain in lower crankcase volume & timing it to maximize it evil. I've also read that its not the size of the piston port... but thats not what she said.

Motorrad:
okay,. keep in mind this is on stock hight jug. and dont judge the CRUDE work. just playing and testing at this point, and the dremel and fast cuts is the easyest way to do that right now. once figured out, angles etc will all be cut on the mill...
bottom skirt cut closes 2mm before the transfers open..
windows start to open 1.5mm after transfers open..... they arnt fully open untill BDC..
the tape line is to cover where I cut further down the piston to start... decided we dont need to go that far. ??

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Here is BDC. note, They window doesnt go all the way to the bottom... in theory, to keep from starving the floor ports, and the booster ports.. (also need that hight, to keep them from opening before the transfers). hard to tell in the picture.. but when FULLY oepen at bdc. the bottom of the window is half way up the side boosters...

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here they are just starting to open... 1.5mm AFTER the transfers have opened.

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cbrfrenzie
I like this cut much better, with the tape covering the bottom of the piston window to delay them a bit more. It may even be beneficial to taper the opening of them a little more too, to create a smoother air flow transition from the air flowing though just the floor and side ports, to the floor ports, boost ports and the piston windows. That way there isn't a sudden pressure change just as the air starts flowing through the other ports. I will try my skillz at paint and see if I can draw what I am talking about. Basically the tape would be oriented at a slight angle, angling down towards the center of the piston, creating a "V" shaped piston port at the bottom vs a square-ish style shape. Other than this one thing (which may be tiddlywinks...), I think you have done a great job at maximizing the port area while maintaining the air differential at transfer port opening (on the down stroke).

Motorrad:
On the piston. Ive moved on to thinking that 3 SMALL holes are in order. not sure if the skirt is strong enough for what Im thinking though... undecided
2 like discussed earlyer, and on in the boost port, very high up compaired to the previous "port" renditions. to draw any charge that is trapped under the piston into the boost port.................
somthing like this.

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Motorrad:
Im thinking a top port like this, width of the rear boost.
and some skirt mods.
as I dont know if the skirts are tough enough for 3 holes.
will look into it more when there is a piston in my hand.

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cbrfrenzie;
Once you start carving up your new/used piston it would be nice to see where a top port like this is with relation to the transfers. Does it extend the intake duration beyond when the transfers cover back up?

Motorrad:
should be easy to see with my sweet half Jug that blaster sent me.
and the reason I want to add a port like that, is I believe the benefit of the S cut (big port in center) isnt more duration, but picking up trapped gass'es under the piston.
a higher up port liek the one I pictured would manage this, without messing with flow propertys of the flor and boost port.
as with those. we have same duration as a windowed piston... right............... dont think the skirt closes off those floor ports. Ill have to go look (memory isnt workign right now)
but Id like to add 2 more holes like the ones discussed earlyer, only smaller.
not sure if the pistons are stong enough for 3 holes...

cbrfrenzie:
Very nice, that will make this much easier to figure out. Are you thinking the skirt is too weak to run the two ports you were playing with a few months back?

Motorrad:
im still playing with those.
But im thinking the benefit that was found in the previous window isnt due to more duration. but to releasing ALL those spent gasses under the piston into the boost port..
as we all know the K5 shrouds itself alot in the transfers at BDC. even with all MY skirt mods I already do.
hence the reason Im lookin at the VERY high up port. perhapse combine with the BOTTOM of skirt trimming (like the YZ250 pistons) I put up before may be the strongest and best way to go.
the bottom skrit cut will deff help those of us that have the jug jacked up to the moon like me.

Motorrad:
kinda where im thinking of going.
not sure if pistons are strong enough.
(Hmm custom batch of piston time?? )

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Motorrad:
Hmmm.
thick wall skirt, 3 hole windows, exh cooling hole's , modded skirt at transfers, long skirt, long rod, 88mm (stroke). 90mm piston please.

Motorrad:
Okay. I wanted to run the top window port as wide as the widest part of the rear boost port.
mocked it up in my Half jug.
and. when the transfers JUST open. the top of that port will already be roughly halfway down the wide portion of the rear boost port.
sooooo.
I can either cut it as a upside down trapezoid, or make it square and the same width as the narrow portion of the rear boost port (same width as the previous "S" pistons)...
The pointed (stainless dental tool) marks where the top of the window port would be, when the ports open.

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here is the top of the boost port (dental tool)

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which is roughly here... special note (making my calculations based on WOSSENER piston machining, as there is a big step cut there in the crown... )

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81cr450:
I like the idea but
look at where mahle is puting their struts on modern designed pistons
I do like the middle high & tight like you have I think of the piston swinging around in there like swinging a bucket full of water in a circle, the most pressure is going to be in the very bottom. It also puts it opening above the reeds so they dont impede flow. I also vote rounded hole edges , nothing square or triangular as it focuses load. Just my 2-cent, from one who doesnt know

[Sandblaster]

Motorrad:
hey 81. that pic isnt showing up. can you re-post it ... Im curious what your showing me.. also what its off of would be nice

81cr450:
Theyre 4 stroke pistons for gas motors, but ... theyve moved the skirt support inward right to where your slicing the linear window.

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Having the support there makes the piston strong enough you can make a 4.350 bore bbc piston weigh 740 grams which is sbc stock piston weight, point being theres gotta be alot of load going on in that area of the piston.

Motorrad:
I like the skirt bracing. but the deletion of the side skirts on the K5 motor would cause PROBLEMS

81cr450:
Just structural integrity was all I was looking at, as far as where you had the linear ports marked, the exact location modern pistons carry their load.

Motorrad:
been thinking on this piston window thing.
I believe the side port idea may effect aux port flow. as when the aux ports are needed they are a more direct route for the charge hiding in the intake area infront of the reeds.
I believe that the center "S" design will and DOES effect the floor and rear boost flow.
I believe the HIGH center port idea, will effect the rear boost port, even though it may clear the hiding charge under the piston?Huh
Im thinking. that since the big K5 already has OVERSIZED transfer ports. we need all the squeeeze we can get.
Im now thinking that some Skirt modifications in the transfer port area like I already do is in order.
and some skirt modifications on the intake side. as like the transfer area, the radius's were made and designed for cast pistons to live. but since we are running FORGED pistons. we can get away with more... and thus, modify the bottom profile some.
thinking keeping the greatest ammount of area under (bottom profile) of the intake skirt open as long as we can till just before the transfers open up... is perhapse the way to go here...
so they will close JUST before the transfers open.. so we still have the nearly the same port duration as a window, with only a few degrees of closure, before the aux and boost ports pick it back up. (floor ports still cause it to be full duration open anyways) causing the piston to be a big plunger.... helping to shove the LAZY intake charge through those over sized ports....
thoughts??
warning. I didnt proof read this. just typed it............... rolleyes
keep in mind. I build power for DESERT PEOPLE. not drags, or hill climbs so windows may work better for those "other" types.

Motorrad:
Team green really had a problem of under lubed upper end bearings in the baja 1000.... rolleyes

USMC 500:
If this is figure is correct, I can say this.......its a K5, is 2 HP going to break my heart?.......If so I'm sure that 2 HP can be found somewhere else. It would be nice to have a dyno, KX500, multiple pistons with different cuts, and a Motorrad all in the same place for a few days to put an end to this debate but these conditions are hard to come by.

Motorrad:
And not saying "S" was wrong. or right... so lets assume its a good 2HP
many I have chatted with agreed that if there was to be a window.. to move it up, and make it smaller. myself included.
Im just saying. if the window was good for 2hp. (theory)... was it because of added intake duration (360deg)? venting the trapped gass's? or evil elf magic shocked ??
BUT.
what if another method was better?
Im thinking that with the window. you get a good initial "squeeze" on the charge. when the transfers first open, you get a rush, that is then short circiuted by the window. since it all trys to equilize due to the sort of short circiut the window and rear boost port cause. possibly... in theory remember. causing the second half (1/3 or whatever) portion of the charge to get lazy...
NOW..
with the skirt trimmed.
you still have Nearly the same 360 port duration, since the second the piston closes the intake, the transfers open up, causing the AUX ports and rear boost to take over the "360" effect the window had. then the piston has the chance to COMPRESS the crapola out of the charge, through the entire cycle, as it cant escape from the window into the boost port., causing it to keep the velocity up in the transfers (oversized remember), through the ntire cycle........ the only issue here is trapped gass's, and reverse flow, short circiut from the floor ports..? But the floor ports are fairly small, vs the transfers, so the charge should stick to the transfer route, and actually pull from the floor??
oh wouldnt a flow bench be nice. with colored air... and transparent Jugs.

sandblaster:
And a ultra high speed camera to watch it in really slow motion

81cr450:
ktm 500 way of doing it, I'd imagine they been to a dyno

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Motorrad:
Go check out late model KTM 300
And last of the cr250
Both of which run awesome

[Qsixteen]

There was a member over there that tested a piston skirt mod change if I remember right and had good luck with a improvement on power. This was back many years ago.

[Sandblaster]

Yeah I believe it was Stewart that tested and dyno'd it.
It was a bit before I joined there so I may be wrong.... Nah!
I'm not saying he didn't gain 2-3 hp, but judging from his work and business ethics I think there may be some skepticism.
Also, I think the window is to big and in the wrong spot.
But, the only way we will know for sure is to try some different configurations.
Hoping we will get some more ideas....
There is a private dyno in our area but I have not been able to get any time on it yet...

[Qsixteen]

No it was not stewart somebody else and it was not a window mod and I think it was a skirt mod. I know on the 125 motors you pick up about 1 hp from 4500-8000K rpm range and less up top end with a window verus a piston without window. The piston without window makes more power up topend. These pistons mods are a choice for somebody that what's to keep there cylinder stock.

[Sandblaster]

That's interesting...
I've got to get some dyno time with my SM to get a good base line while it is basically stock...
Then start adding mods one at a time with dyno runs in between..
It's too bad there are no other KX500 nuts locally with a dyno...