piston qsutin

[Grant Casey]

i got my 89mm piston 11:1 wisco piston kit today in the mail, isn't it weird that the top of the piston is flat and it is still 11:1.

[Grant Casey]

i thought i got the wrong pistin but then i match the numbers up and it was write

[Grant Casey]

bump

[troutman561]

I dont know the answer but im pretty sure all 4 stroke pistons stay flat so the valves dont hit. Perhaps its thicker, compare to the stock piston to see if there is a difference...

[†2005 400ex†]

no, they usually angle up the piston towards the middle to up the compression so the valves dont hit, you knnow inbetween the exhaust and intake valves

[Grant Casey]

yeah i think the bigger the dome shape on the piston the higger the copmresin, but this one is flat but it say its 11:1.
any 440 11:1 owners that can give sum feed back.

[troutman561]

Wont the rise be so small that it probably wont even be noticed, we are talking millimeters here...

[Grant Casey]

but the rise on the 88mm 11:1 piston was noticeable, maybe the 89mm piston is just taller.



we will have to get sum one on here that knows pistins

[dragex400]

I would say that the piston is longer. Measure it and compare it to your old piston. I would bet the high comp is longer. Kinda like changing the rod length without actually changing the rod length.

[blown331]

going from an "88mm" to "89mm" piston is going to be widening the piston.

You can run a "flat top" piston and achieve high compression. If i remember correctly the stock 400 pistons are like 9:1 or so?

it only takes a few additional cc's of material on the top of the piston to up compression considerably especially when you're talking about such a small combustion chamber

[krt400ex]

if u have ever looked at a high comp piston then u will see that they have recessed areas so the valves don't beng into the piston. if they don't(and i would think that it would because 11:1 is a big jump from 9:1) then i would chack it against other 11:1 pistons...also, i would do a comp check once it is installed and before u run the motor to see if it is the 11:1

[krt400ex]

u can see what i am talking about in the one above^^^^^ and the one below

[†2005 400ex†]

if the piston is taller then it could hit the valves, the only way i have ever seen a high compression piston made is with the piston doed and cut out for the valves to clear

[krt400ex]

Originally posted by †2005 400ex†
if the piston is taller then it could hit the valves, the only way i have ever seen a high compression piston made is with the piston doed and cut out for the valves to clear


exactly

[blown331]

even most dished pistons have valve reliefs cut into them :) that's common

nice illustrations :)

[jordan_m6]

what kind of piston did you get. My stock bore Wiseco 11:1 piston has a big rise in it. if it doesnt you got stock comp or 10.1:1.

[exrider008]

Originally posted by jordan_m6
what kind of piston did you get. My stock bore Wiseco 11:1 piston has a big rise in it. if it doesnt you got stock comp or 10.1:1.

same with me you could clearly see the rise in the piston.

[F-16Guy]

Originally posted by Grant Casey
i got my 89mm piston 11:1 wisco piston kit today in the mail, isn't it weird that the top of the piston is flat and it is still 11:1.
Here is a really old thread that contains some absolutely invaluable information concerning the Wiseco 89mm flat top piston. It will really help a lot with doing the build right, and give you a huge advantage over people that just buy the "440 kit" and throw it together. Another thing to mention is that it would really be in your best interest to install HD head studs while you have it apart now. With a high compression 440cc, it's not a question of if you will have head gasket problems, but a question of when. My head gasket blew due to a stripped stud on the starting line of my first race. It sucked. Contact GT Thunder. Here's that link:

http://www.exriders.com/vbb/showthread.php?s=&threadid=8316&highlight=have+you+decked+the+cylinder

[Edit] -- Read this article and you will see how much different EXRiders was back in the day. There were a lot of knowledgable people and a ton of information. I'm not saying that there isn't good info now, but it was a much more professional environment with hardly any bickering.

[Grant Casey]

hey 5-Guy

so my 89mm 11:1 piston is correct that it has a flat top and still is a 11:1.

[blown331]

whats the part # on the piston? should be on the underside

also, remember the thickness of your headgasket will also increase/decrease compression as well

[christophere251]

i have a 12.5-1 and it has a slight dome but all pistons even stock have valve reliefs cut in them. The wrist pin location is probally changed to compesate for the compression

[250X_project]

Wow there are few people on here these days who have that kind of knowledge. That is a great article. I never even thought about the deck height changing the timing....

Here's a great quote from there...."There's more power hiding in these motors, we just need to keep looking." I like that. Good find!

Also krt400ex, said take a compression test, but can you even convert this to static compression ratios?

My 12:1 in my 250X has a good sized raise between the valve pockets....

[F-16Guy]

Originally posted by 250X_project
Wow there are few people on here these days who have that kind of knowledge. That is a great article. I never even thought about the deck height changing the timing....

Here's a great quote from there...."There's more power hiding in these motors, we just need to keep looking." I like that. Good find!

Also krt400ex, said take a compression test, but can you even convert this to static compression ratios?

My 12:1 in my 250X has a good sized raise between the valve pockets....
Good reading, huh? A flat top piston is acually preferable to a domed piston because of the flat surface, but Wiseco didn't raise the deck height (lower the wrist pin hole), and it was actually made for the XR400, so they didn't account for the extra gasket thickness of the EX. If you set it up right by decking the cylinder, milling the head (depending on what CR you want), and degreeing the cam with an adjustable cam gear, you can really squeeze some power out of it. I think it's still a great mill for fast play riding and occasional local level racing.

[krt400ex]

Originally posted by christophere251
i have a 12.5-1 and it has a slight dome but all pistons even stock have valve reliefs cut in them. The wrist pin location is probally changed to compesate for the compression


actually i don't think that the stock 400ex piston has valve reliefs cut into them, but don't quote me on that

[250X_project]

Yes that is very good reading. No offense what were did people like that go!?

I'm sure the stock 400 piston has reliefs I know the 250X/300EX stocker does. Also the 250SX...Z50 :D ....I'm sure they all do. It would be crazy not to have them, That little space makes a huge difference.

[250X_project]

Also does anyone have any calculations on how much compression changes w/ different gaskets/decked cylinders or with a longer stroke? Thanks

[jordan_m6]

my stock piston is flat with no relief cuts in it.

[250X_project]

Really? Huh, that makes no sense.

[krt400ex]

Originally posted by 250X_project
Really? Huh, that makes no sense.


if u think about it, it doess make sense. the 400ex does not have any radical cam profiles stock, that valves don't open very far or for 2 long, and 9.1:1 is a low compression. the motor is in such a mellow state of tune that it doesn't need the valve reliefs.

[†2005 400ex†]

That makes sense now that you mention it

[GPracer2500]

The stock 400EX piston does have valve reliefs.

http://i24.ebayimg.com/06/i/000/7c/04/1efa_1.JPG

[krt400ex]

Originally posted by GPracer2500
The stock 400EX piston does have valve reliefs.

http://i24.ebayimg.com/06/i/000/7c/04/1efa_1.JPG
thats it then...i guess it does...thnx for clarifying(sp?)

[jordan_m6]

guess my piston was too dirty to see them. I sure didnt notice them.

[250X_project]

Now that makes sense. I have never seen a piston w/o reliefs.