Can anyone find me an 86 200X 11:1 piston kit?? 12:1?? GOT ONE??

[Billy Golightly]

OK, here we go.



CR = (CV+CCV) / CCV


Formula key:

CR= Compression Ratio
CV= Cylinder Volume
CCV= combustion chamber volume.


Cylinder volume must be calculated, luckily its pretty easy.


CV= (3.1416xD²xS) / 4000


Formula Key:

CV=Cylinder Volume
D² (Which is to the second power)=bore in mm
S=stroke in mm


Combustion chamber volume usually must be measured manually via oil in a syringe or a burette of some sort.



So now that we have the mechanics of the forumlas and they've been roughly explained by a mathematical genius like myself We will continue on to actually executing and USING them.

First lets find your cylinder volume. Obviously the 86-87 200xs are advertised as 199cc. You could use that for a quick and dirty measurement but thats exactly what is if you do not account for cylinder over bores. For ease of calculating, you can figure every .040 thousandths of overbore is equal to 1mm. Its not exact but its close enough for what we're doing. I don't know the stock bore of the x off hand, but if its 64mm and you've got a .080 over cylinder the bore is actually 66mm.

I'm gonna whip out a calculator (the one that comes with windowsXP works fine for me). First I'm going to multiply pi (3.1416) by 66² (which came out to 4356) multiply again by 60mm stroke and I came up with 821086. Now divide that by 4000 and I've got a 205cc cylinder volumed engine.


So now that we for sure know our cylinder volume, I'm going to start working out the compression ratio formula.

CV(205) + CCV(16) (I don't know this #, but I'm gonna take a guess and use 16cc as a number) Those two numbers make 221. Now divide 221 by your same # you used for the CCV (16) and you arrive at a 13.8125 compression ratio. This is pretty high, so lets try it again with an 24cc volume instead.

205+24=229

229/24=9.541

So CR=9.5

This gives us a 9.5:1 compression ratio from the parameters we've used which is very close to the factory advertised compression ratio.




If you want to bump up Compression ratio (CR) play with the combustion chamber volume (CCV) numbers. There is a way I'm sure of calculating and figuring exactly how much volume would be removed from the top of a piston by converting the area and depth of a cut int a cc volume, but I don't know that off hand either and it would take some more research to find it. It should be a pretty straight forward deal (lengthXwidthXheight for cubes, you'd have to use pi in there somewhere since we're doing round cuts) but like I said I don't know it off hand. Some of the other infinitely more informed math gurus here can answer that one for us

[Dirtcrasher]

Ooooooooooooooooooh K......................

So, I guess I'll just throw it in the lathe and wing it like you did

J/K, gee, what a shame geek isn't here to help

Anyhow, thats some crazy math but easy nonetheless. Thanks for taking the time to post that

The dome of the piston is curved so each lathe pass removes even more material. There must be a way to measure it with liquid?

Friggin lack of parts for the 86!!

Oh well, we gotta figure it out sooner or later

[Billy Golightly]

Yeah the high compression 4 stroke pistons normally have the domes with the valve cut outs. Hell put some water in a small plastic container with some scaling on it like a 1/8in deep and see how much it rises with that much of the piston sticking down in it? More then one way to skin a cat

[Dirtcrasher]

More then one way to skin a cat

LMFAO.......

[honda250sx]

Steve-o how about a nice plastic syringe. They can be bought in most cooking aisle's and pet stores and measure in CC's. Come in thru the spark plug maybe? I was thinking oil instead of water?

[OldSchoolin86]

didn't like the 2001 400ex come stock at 11:1...maybe i'm an idiot.

More like 9:1 compression.

11:1 is the most you want to run in an air cooled bike on pump gas but even that can ping depending on your total set-up. If you have E85 available you can run 12:1 all day without pinging but then you have to find it when riding out of your area. As far as getting 11:1 SWIGIN has the right idea. Just get the 12:1 and have a shop turn it down. Any competent motor shop should be able to do it fairly cheap. Even if they're a little off you'll never notice a difference between plus or minus .1

[Dirtcrasher]

Steve-o how about a nice plastic syringe. They can be bought in most cooking aisle's and pet stores and measure in CC's. Come in thru the spark plug maybe? I was thinking oil instead of water?

Hey bud, thats an idea, I already have a few of those for bleeding brakes and injecting grease etc etc. I have a huge one I use to put SLIME in tires with .

Brings about more thoughts.......

I was thinking about another way that may be a fair way of getting about 11:1.

What if I bought both an 10.25:1 and 12:1 from Wiseco. Then I measure out say 1000cc in a container, insert the piston dome side down and measure the amount it rises. Then, do the same with the other piston. Each piston would be inserted to exactly the same position using the wrist pin hole as a guide.

Then I can toss the piston in my SouthBend Lathe and turn it down a bit, toss it back in the liquid and continue this until I am between the initial 2 numbers. That would bring me about 11:1.

Any thoughts on this??

[SWIGIN]

or just measure the highth of both and go in between

just dont make the top any thiner then a stock piston

[Billy Golightly]

Thats similar to what I had suggested DirtCrasher. I don't see any reason at all why it wouldn't work.

[Dirtcrasher]

OK, so I'm gonna go 12:1 and maybe turn it down based on all the above info.

Then I'll break it in and run it. I have numerous motors so I can expirement a bit anyhow, I just can't ruin this head I have.....

Anybody got an overbore 12:1 WISECO piston kit for this??