high compression

[tyman2395]

Hey guys if i go with a 11:1 compression piston can i run regular gas?

[drew416ex]

Are you running a stock cam? Ive never tried it on a stock cam. The reason I bring this up is because if you use a stock cam, you will have more cylinder pressure than with a aftermarket cam because bigger cams have longer durations which loses cylinder pressure. For example with that piston and a stock cam you may have 200psi in the cylinder, but with a bigger cam and that piston you may have 185psi. So if you are running a aftermarket cam, you should have no problem with 93 octane, but if its stock im not sure.

[Brauap]

You SHOULD be fine. They say with the stock 9:1, you should run like 105 oct. but with the 11:1, a whole lot of people run pump gas, and havn't had any problems.. just run 93..

[tyman2395]

ya i will be running a stage 2 hotcam. what is the power difference between the normal compression and high compression?

[Brauap]

I heard it is more low(er) end torque..

[Pipeless416]

Originally posted by Brauap
They say with the stock 9:1, you should run like 105 oct.

:huh :huh

[Brauap]

Originally posted by Pipeless416
:huh :huh

I am pretty sure the manual says 105.. (Clymers)

Check it, its in the begining.

[BlaineKaiser450]

yeah you should be fine. But 11:1 is right on the line

My manual said the stock 400ex should have it was eithor 91 or 93. It was w/e the highest octain gas they sell at the pump. Most people run the cheep 87 stuff (or w/e the lowest is) in the stock engines fine. Yz426's are 12.5:1 compression from factory and run fine on pump gas. Normaly liquid cooled engines can go higher compression on pump gas than air cooled engines can.

11:1 on a 400ex some times needs high oct. If your bumping the compression up from stock the 400ex needs heavy duity head studs ($90) or they will pull and your top end gaskets will leak.

[Brauap]

Originally posted by ltr450fly
My manual said the stock 400ex should have it was eithor 91 or 93. It was w/e the highest octain gas they sell at the pump. Most people run the cheep 87 stuff (or w/e the lowest is) in the stock engines fine. Yz426's are 12.5:1 compression from factory and run fine on pump gas. Normaly liquid cooled engines can go higher compression on pump gas than air cooled engines can.

11:1 on a 400ex some times needs high oct. If your bumping the compression up from stock the 400ex needs heavy duity head studs ($90) or they will pull and your top end gaskets will leak.

I thought you only needed Hd studs for 12:1+ .. but you need them for 11:1 too?

[400exrider707]

Holy smokes....

Why do people post information when they have no clue what they're talking about?


11:1 SHOULD be OK with pump... if you have a cam, even better. Now not ALL 400ex's are built the same. I know some guys who used 11:1 and did have to mix 50/50 race/pump. Myself I had an 11:1, a cometic thin head gasket and a stage 1 hotcam and I got away with 87 oct.


For those that do not know. All aftermarket pistons for the 400ex are actually XR400 pistons, and the advertised compression ratio is NOT what you will get. You will be lower. An 11:1 is closer to 10:1 in reality. In fact I dont think it was even that high. You'll notice (if you ever do a rebuild) that your piston will sit .040" down in the cylinder at TDC. I dont know why Honda did this, but they did.

I suggest using the cometic thin gasket as well it was only $20.

Also any time you have the motor this far apart, replace the cam chain and use the CRF450... any year. Its the same as a $110 heavy duty one, but its cheaper than a stock 400ex one. Its something like $35 with shipping from service honda.

[BlaineKaiser450]

finnally someone who knows what they're talking about

[400exrider707]

Originally posted by BlaineKaiser450
finnally someone who knows what they're talking about

I can not believe the amount of misinformation in the 400ex section anymore... It's truly astonishing.

[Brauap]

Originally posted by 400exrider707
You'll notice (if you ever do a rebuild) that your piston will sit .040" down in the cylinder at TDC.


Yea, I noticed that too.. werid isn't it?

So you'd be fine with a 11:1 piston
stg 1 hot cam
6* timing key
thin gaskets
on 93 oct?

I'd think with all that you'd get overheated.. but since 11:1 is actually around 10:1.. :confused:

[400exrider707]

Originally posted by Brauap
Yea, I noticed that too.. werid isn't it?

So you'd be fine with a 11:1 piston
stg 1 hot cam
6* timing key
thin gaskets
on 93 oct?

I'd think with all that you'd get overheated.. but since 11:1 is actually around 10:1.. :confused:

Ditch the timing key and you'll be all set. Timing keys (in my honest opinion) are a bad idea on anything other than a 100% bone stock (motor wise) machine. All its going to do now is create more heat. Get rid of it if you're doing a piston.

[tyman2395]

thanks man thats some good information. helped out a lot

Originally posted by 400exrider707
Ditch the timing key and you'll be all set. Timing keys (in my honest opinion) are a bad idea on anything other than a 100% bone stock (motor wise) machine. All its going to do now is create more heat. Get rid of it if you're doing a piston.

The heat increase isnt even noticable. Maybe if you were riding like 1st gear for a few miles. 11:1 thin gasket and the key on 93 I would not recomend. I have heard of 10.8:1 and the key and it would knock on pump gas. I wouldnt tempt it. 10:1 and the key your fine.

[400exrider707]

Originally posted by FoxHondaRider
The heat increase isnt even noticable. Maybe if you were riding like 1st gear for a few miles. 11:1 thin gasket and the key on 93 I would not recomend. I have heard of 10.8:1 and the key and it would knock on pump gas. I wouldnt tempt it. 10:1 and the key your fine.

Which is why I told him to ditch the key. They're good for stock motors and thats about it. If you read Curtis Sparks recommendations, he even says on the advertisements for the timing key for stock motors.

You say the heat increase isn't noticeable, but then go on to say it knocks on a 10.8:1 piston? This is misleading!?

What data do you have that it "isn't noticeable?"

Thanks for your input.

[Pipeless416]

Originally posted by 400exrider707
I can not believe the amount of misinformation in the 400ex section anymore... It's truly astonishing.

and its usually spread by a select few.. :rolleyes:

[2muchquad]

and its usually spread by a select few..


LMAO...:D

Originally posted by 400exrider707
Which is why I told him to ditch the key. They're good for stock motors and thats about it. If you read Curtis Sparks recommendations, he even says on the advertisements for the timing key for stock motors.

You say the heat increase isn't noticeable, but then go on to say it knocks on a 10.8:1 piston? This is misleading!?

What data do you have that it "isn't noticeable?"

Thanks for your input.

dont think you realize engine physics or w/e you want to call it. Its not the heat its pre detonation. Your making the spark occur sooner and the higher compression combust the fuel sooner since the pressure builds quicker which is what advanced timing does. The key to building a race engine is the perfect mix between timing and compression. Myself I havent noticed the engine any hotter than it was before with the key. I know what a 440 feels like they will get pretty darn hot this isnt even noticable. Now I just thought of a way to retard the timing 6 degrees and run a higher compression on pump gas, just flip the key

[400exrider707]

Originally posted by FoxHondaRider
dont think you realize engine physics or w/e you want to call it. Its not the heat its pre detonation. Your making the spark occur sooner and the higher compression combust the fuel sooner since the pressure builds quicker which is what advanced timing does. The key to building a race engine is the perfect mix between timing and compression. Myself I havent noticed the engine any hotter than it was before with the key. I know what a 440 feels like they will get pretty darn hot this isnt even noticable. Now I just thought of a way to retard the timing 6 degrees and run a higher compression on pump gas, just flip the key

What do you think causes detonation? There is no pre detonation... its pre-ignition, OR detonation. They are two different actions. There is no spark occuring in either of those situations, they are HOT SPOTS. The key to building a race engine is knowing what you're doing. There are many things going on inside a motor while it's running, and timing and compression are just two of them. Compression as we're talking about it is actually not very important as we are talking only static compression. Dynamic Compression is what is important, and the timing key will effect this.

You haven't noticed the engine any hotter? What temps was it at before and what is it at now? Doesn't sound like you're going about it very scientifically...

Try retarding it 6 degrees and run a real high compression piston... let me know what happens. I want to see pics after.

I'm not trying to knock you... no pun intended (engine knock..hehe), but it is a general rule of thumb that the timing key is a bad idea with higher compression motors on the 400ex.

[400exrider707]

Originally posted by Pipeless416
and its usually spread by a select few.. :rolleyes:

I haven't been around enough to pick out the select few, but my god they seem to be in every single thread...

I wish there was a smiley for pulling your hair out...

I dont mind helping out those who just dont understand... honestly I learned everything I know from this very site myself, my main problem is with people posting INCORRECT information... and it seems to happen more and more. If you dont know or dont understand, just ask, there are a lot of knowledgeable people here...

[2muchquad]

Agreed.


Me personally i find it rather hilarious how uninformed people can be on here.Me i dont consider myself a "mr.knowitall" by any means,but sometimes people dont want to heed advice if it isnt from 'sparks or gt' or they didnt read it in atv action.Like every parent will tell ya'sometimes you have to let your kids make their own mistakes.Well in reference to the question,im running high compression and i plan on doing the key as well,actual compression will vary with valve overlap so detonation isnt even a issue,nothing but 93 oct pump for me.;)

Originally posted by 400exrider707
I'm not trying to knock you... no pun intended (engine knock..hehe), but it is a general rule of thumb that the timing key is a bad idea with higher compression motors on the 400ex.

High compression is 12.5:1 and up.

[400exrider707]

Originally posted by FoxHondaRider
High compression is 12.5:1 and up.

I'd put the limit at 12:1 myself on a 400ex... but thats just me, there is no specific cut off on high compression.

Anything over 11:1 on a 400ex and I'd be running heavy duty head studs and a heavy duty rod. Known weak points on the 400ex. Sure some have run on stock rods at 13:1, but for how long?

The main enemy of the 400ex is HEAT. HEAT is what causes the aluminum to break down and the head studs to pull out of their threads.... timing keys cause HEAT.

[2muchquad]

.... timing keys cause HEAT

So does starting it up.Heat is a byproduct of the process.Any time you increase performance your gonna make more heat,thats the trade off,theres no free lunch.

[400exrider707]

Originally posted by 2muchquad
So does starting it up.Heat is a byproduct of the process.Any time you increase performance your gonna make more heat,thats the trade off,theres no free lunch.

A cam will reduce heat...:eek2:

A water/meth setup on my truck will reduce heat and increase performance!!!

There are quite a few things I can think of that will reduce heat and increase performance.

Though you are right, there is no free lunch.

Originally posted by 400exrider707
Holy smokes....

Why do people post information when they have no clue what they're talking about?


11:1 SHOULD be OK with pump... if you have a cam, even better. Now not ALL 400ex's are built the same. I know some guys who used 11:1 and did have to mix 50/50 race/pump. Myself I had an 11:1, a cometic thin head gasket and a stage 1 hotcam and I got away with 87 oct.


For those that do not know. All aftermarket pistons for the 400ex are actually XR400 pistons, and the advertised compression ratio is NOT what you will get. You will be lower. An 11:1 is closer to 10:1 in reality. In fact I dont think it was even that high. You'll notice (if you ever do a rebuild) that your piston will sit .040" down in the cylinder at TDC. I dont know why Honda did this, but they did.

I suggest using the cometic thin gasket as well it was only $20.

Also any time you have the motor this far apart, replace the cam chain and use the CRF450... any year. Its the same as a $110 heavy duty one, but its cheaper than a stock 400ex one. Its something like $35 with shipping from service honda.

There's a lot of people that don't know what they're talking about on here. I think it's gotten too big for it's own good.


As far as the piston, the piston itself is not different. For some ungodly unknown reason honda decided to put a thicker head gasket and base gasket on the 400ex. Who knows why.

With stock width gaskets, and 11.1:1 is actually more equivalent to 10.8:1 compression, not 10.1:1.

With an xr400 gasket kit you will run a true 11.1:1

But the cam helps dramatically lower the compression too like you mentioned. On the stage 2 hotcam the duration the valve stays open is so much longer your compression would go from 9.1:1 to somewhere give or take around 7.9:1

That assuming your using a stock piston.

[400exrider707]

Originally posted by DMC-4OOEX

As far as the piston, the piston itself is not different. For some ungodly unknown reason honda decided to put a thicker head gasket and base gasket on the 400ex. Who knows why.

With stock width gaskets, and 11.1:1 is actually more equivalent to 10.8:1 compression, not 10.1:1.

With an xr400 gasket kit you will run a true 11.1:1

But the cam helps dramatically lower the compression too like you mentioned. On the stage 2 hotcam the duration the valve stays open is so much longer your compression would go from 9.1:1 to somewhere give or take around 7.9:1

That assuming your using a stock piston.


I still think its more dramatic than that. I dont think the XR has the issue of the piston sitting so far down in the cylinder. I didn't think the base gaskets were any different.

Some folks have run No base gasket, and a thin head gasket to get it closer to the XR. Last I knew it still didn't match the XR's compression ratios as advertised, but maybe I'm not remembering this so clearly. It's been a while for me.

You are right about the cam, but lets not confuse those who do not know. You are mixing static and dynamic compression up now and I dont want to confuse the situation any further, though you are correct.

[hornetgod13]

This thread has been good reading! Thanks for the entertainment.
Here recently, I've been watching and reading all the half truths and mis-information and haven't been posting much but, this thread is just too priceless not to chime in on.

I removed my advance timing key when I went with 11:1 Wiseco Piston, XR400 Thin Gasket, and Stage 1 Hot Cam. Quad runs GREAT with 91 Octane. Rock Solid! Also has stock studs. No stripping or loosening.

Originally posted by 400exrider707
I can not believe the amount of misinformation in the 400ex section anymore... It's truly astonishing.

Show where I said wrong! I was going to say the piston compression is normaly lower than what it says but I didn't. Theirs not one thing I said wrong! Dont think you need heavy duity studs? 3 years ago I bumped up the compression and was still on pump gas using a 400ex engine and didn't change the studs. They pulled and the top end gaskets leaked so bad that the engine was always smoking from the oil all over till I sold the race engine for $280 with a fresh rebuild.

See. Look in front of the engine. Look at the front and you will see black oil all over :rolleyes:

And XR piston? The 400ex is a xr400 engine slightly changed of corse it uses a xr piston and thats why smart people buy xr400 stock cam cause its like a stage 1 hot came but cheep cause $40 vs $150!!!
http://i108.photobucket.com/albums/n26/getdrunk1990/sacaxaS.jpg

http://i108.photobucket.com/albums/n26/getdrunk1990/4wheeler089.jpg

Originally posted by 2muchquad
So does starting it up.Heat is a byproduct of the process.Any time you increase performance your gonna make more heat,thats the trade off,theres no free lunch.

Timing key is only for stok engines. They are a totaly no no on modded engines!!! And one of the only mods you can do to gain hp and run cooler is by changing the exhaust system. That gains hp and makes the engine run cooler.

[Brauap]

Originally posted by 400exrider707
...just ask, there are a lot of knowledgeable people here...

I try. When I do, I get called names and I get bashed. :rolleyes:

[Brauap]

Originally posted by ltr450fly
Show where I said wrong! I was going to say the piston compression is normaly lower than what it says but I didn't. Theirs not one thing I said wrong! Dont think you need heavy duity studs? 3 years ago I bumped up the compression and was still on pump gas using a 400ex engine and didn't change the studs. They pulled and the top end gaskets leaked so bad that the engine was always smoking from the oil all over till I sold the race engine for $280 with a fresh rebuild.

See. Look in front of the engine. Look at the front and you will see black oil all over :rolleyes:

And XR piston? The 400ex is a xr400 engine slightly changed of corse it uses a xr piston and thats why smart people buy xr400 stock cam cause its like a stage 1 hot came but cheep cause $40 vs $150!!!
http://i108.photobucket.com/albums/n26/getdrunk1990/sacaxaS.jpg

http://i108.photobucket.com/albums/n26/getdrunk1990/4wheeler089.jpg

You can use the XR400 piston & cam? Whats the advantages of the piston?

[Brauap]

I was just looking on Service Honda @ the XR400R cams, and they, too, are $150. Click on this to see for yourself! I'd much rather have a HotCams Stage 1 Cam that is made for the 400EX for the same price, wait, cheaper, than have something in my valuable engine that wasn't ment to be in there! I don't know about you, but that is just me.. :ermm: (http://xtremeusa.com/fiche_section_detail.asp)

[2muchquad]

Timing key is only for stok engines. They are a totaly no no on modded engines!!!

Wrong,they can be beneficial to a modded engine as well.You think drag races that run MSD ignitions are using stock engines?Surely a drag engine is an internal combustion engine just like the 400ex,same principles apply.Of course sparks is going to be conservative with their key because too many morons would take things the wrong way.Bottom line is i plan on running the key at 11.5 to 1 and wont think twice about it,but i also am famaliar with how the internal combustion engine works and have been doing my thing since the mid 80s,this was way before the internet came out.So for the uninformed,just leave the performance aspect of things to "us people" that know what we are doing and talking about.Rest of you guys just do as sparks,duncan,4st tech,etc says

[400exrider707]

ltr450fly.... at no point did I say YOU were wrong, but you were sure quick to defend yourself huh? haha.

The XR cam is no where near the stage 1 hotcam, though it is better than the stock 400ex cam.

They use the same piston yes, but the 400ex one sits lower in the cylinder. Proven fact.

Also like I said the studs pull out of the cylinders due to HEAT. Its generally not a problem with the compression itself, but compression raises heat, the heat breaks down the material in the cylinder, then the studs pull. Usually caused from running too low of an octane fuel and detonation... seems you were running high compression and low octane? What compression were you running?

Hornetgod - the cometic thin gasket is thinner than the XR gasket just FYI.

Brauap - keep asking, who cares what the goons think. If you want good answer my advice is think your question through well before you post it. Try to make it clear and legible and use punctuation. A good clean question usually gets good clean answers. Questions are the only way to learn. I have more respect for those who come on here and ask 100's of nonsense questions than the guy who comes on and bashes people for asking stupid questions. You have to learn somewhere and chances are if you have a question on it, someone else does too. We're all here to help each other out, lets remember that... we are all here for a common reason... ATV's. We should be helping each other not bashing each other.

2muchquad - again you're getting into stuff that is out of the realm of this... I mean yes you are correct for sure. We are not talking drag engines here, we're talking mid 80's technology air cooled motors. Air cooled motors dont do well with heat. Simple but true. If you are the engine guru you say you are then why are you here? You should be helping us with your knowledge! Not everyone knows everything you know and vice versa. I know it's frustrating sometimes, but some people really just need a good explanation of why or why not to do something. The timing key is generally a bad idea because of the extra heat it produces on a motor that already has heat issues with mods. Also a drag motor would have a variable timing, not just a set advanced like this key does. Something else to think about... Thanks for your input on this thread though, its appreciated.


Happy ATV'ing everyone, and Merry Christmas!

[2muchquad]

I mean yes you are correct for sure. We are not talking drag engines here, we're talking mid 80's technology air cooled motors. Air cooled motors dont do well with heat. Simple but true. If you are the engine guru you say you are then why are you here? You should be helping us with your knowledge!

im always right..lol some people dont want to hear it if they didnt get it from sparks themselves...thats just how it is.i come here just to see what some people are doing with their quads as far as mods and stuff,im surely not here wondering if drilling holes in the crown of my piston will increase compression:confused: theres a lot of bogus info on the web,i wish i could sort this kinda stuff out:rolleyes:


Merry Xmas everyone:)

[400exrider707]

Originally posted by 2muchquad
im surely not here wondering if drilling holes in the crown of my piston will increase compression:confused: theres a lot of bogus info on the web,i wish i could sort this kinda stuff out:rolleyes:


Merry Xmas everyone:) \


Gas ports? Usually used on two strokes, but yes it could work... hehe.

[Brauap]

Originally posted by 400exrider707
\


Gas ports? Usually used on two strokes, but yes it could work... hehe.

Yea, thats just used for lubercation.. but raising compression? wtf. If someone honestly said that..

[400exrider707]

Originally posted by Brauap
Yea, thats just used for lubercation.. but raising compression? wtf. If someone honestly said that..

Gas ports seal the rings against the cylinder harder... I have no clue how that would be used for lube...

Its commonly used with low tension rings, which can also be used on a four stroke, but its more commonly found on two strokes. Low tension rings...

Rings account for something like 40% of a motors drag, so low tension rings will reduce this. Low tension rings offer a smooth way for the piston to go down in the cylinder, then as its raising up the compression will go through the gas port and force the rings out causing them to seal hard against the cylinder wall.... how is this BS?

[Brauap]

On my YZ125, it had 2 holes on the scurt, called lubercation ports. Our machanic opened them up even more with up with a tap for better lubercation. It holds the mixture, and lubes it as it goes up and down the piston.

You know how when the piston goes up, on the intake side it is open? Well as the gas squarts in there and some goes to the lubercation ports on the other side of the piston. The the piston goes down and completes its normal stroke.

Remember, I am talking 2-stroke here. Not a thumper.

[Brauap]

Originally posted by 400exrider707
Gas ports seal the rings against the cylinder harder... I have no clue how that would be used for lube...

Its commonly used with low tension rings, which can also be used on a four stroke, but its more commonly found on two strokes. Low tension rings...

Rings account for something like 40% of a motors drag, so low tension rings will reduce this. Low tension rings offer a smooth way for the piston to go down in the cylinder, then as its raising up the compression will go through the gas port and force the rings out causing them to seal hard against the cylinder wall.... how is this BS?

I thought you where talking about the lubercation ports, but where is the gas port? Wait, the gas port is on the intake side, correct? I am talking about the lubercation ports on the exhaust side..

Originally posted by Brauap
On my YZ125, it had 2 holes on the scurt, called lubercation ports. Our machanic opened them up even more with up with a tap for better lubercation. It holds the mixture, and lubes it as it goes up and down the piston.

You know how when the piston goes up, on the intake side it is open? Well as the gas squarts in there and some goes to the lubercation ports on the other side of the piston. The the piston goes down and completes its normal stroke.

Remember, I am talking 2-stroke here. Not a thumper.

It's not called a lubrication port. I forget the term but it's somethign along the lines of a transfer port.

And that's not just for lubrication. There's WAY more to it then that.

When the piston is on it's upstroke, air/fuel through those ports on the piston. Then the piston goes down and forces the fuel/air up to the top of the cylinder through the transfer ports (In the cylinder, not the piston transfer ports)

It then burns and the sonic wave theroy comes into effect. Which is a completely different thing.

SO yes, those ports in the piston are for lubricating the crank (the gas has oil in it of course, and that's the only lubrication the crank gets. Not like a four-stroke) but there's way more to it then just lubrication.

[Brauap]

Originally posted by DMC-4OOEX
It's not called a lubrication port. I forget the term but it's somethign along the lines of a transfer port.

And that's not just for lubrication. There's WAY more to it then that.

When the piston is on it's upstroke, air/fuel through those ports on the piston. Then the piston goes down and forces the fuel/air up to the top of the cylinder through the transfer ports (In the cylinder, not the piston transfer ports)

It then burns and the sonic wave theroy comes into effect. Which is a completely different thing.

SO yes, those ports in the piston are for lubricating the crank (the gas has oil in it of course, and that's the only lubrication the crank gets. Not like a four-stroke) but there's way more to it then just lubrication.

Oh.. well thats what my mechanic told me.. :ermm: are you talking about the ports on the intake or exhaust side of the piston? I know about the intake side, and thats what you where talking about...

[400exrider707]

A gas port is a hole drilled on the TOP of the piston not on the skirts. This hole on the top of the piston is a port that leads to the ring valleys. So as the piston is coming up into the cylinder, the force goes through the gas port and forces the rings outward to seal against the cylinder walls. This allows you to run a real low tension ring so that on downstroke there is less drag on the cylinder and more power to be had.

Originally posted by Brauap
Oh.. well thats what my mechanic told me.. :ermm: are you talking about the ports on the intake or exhaust side of the piston? I know about the intake side, and thats what you where talking about...

I don't know about the yz125, but the banshee doesn't have ports on the exhaust side of the piston and shouldn't have.

[drillteamleader]

You are fighting a loosing battle my friend. I know what you are talking about, and unless you show them a very detailed pic, I don't think they will get it. JMO By the way, do you know anyone who makes the low tension oil ring for these motors?

[400exrider707]

Originally posted by drillteamleader
You are fighting a loosing battle my friend. I know what you are talking about, and unless you show them a very detailed pic, I don't think they will get it. JMO By the way, do you know anyone who makes the low tension oil ring for these motors?

Not for the 400ex motors no... I know they can be had for the 450R's, but I guarantee you can have one built by CP or any of the other top companies. You can custom order pistons anyway you want them.

[Brauap]

This is the closest that I could find.. its not the normal YZ125 piston, but they said it was for a YZ125.. (but I think this was just a random picture, notice the rings, the YZ125 only has 1 ring)

See the two holes? Thats exactly what I am talking about..

[drillteamleader]

Those two holes are where the oil goes after the oil ring scrapes the oil off of the cylinder. That is how the rist pin gets oiled and the piston and rod get some oil to keep them cool. The gas ports he is talking about are drilled from the top strait down around the edge of the piston to intesect with the top ring. So if you were looking down your cylinder at the top of you're piston you would see about 12 tiny holes drilled down into the top of the piston around the outside edge. When oyu start to compress the air fuel mix, the preasure builds up and goes through those holes and forces the back side of the ring outward towards the cylinder, thus creating a tight seal. It doesn't raise your compresson per say, it allows you to use low tension rings to reduce frictional losses, But the gas ports allow this without loosing compression passed the rings from not having a tight seal.

[2muchquad]

you guys still talking bout'dis stuff...:D

[400exrider707]

Originally posted by 2muchquad
you guys still talking bout'dis stuff...:D

its about time we got some good tech talk going in the 400ex section

[Brauap]

Originally posted by drillteamleader
Those two holes are where the oil goes after the oil ring scrapes the oil off of the cylinder. That is how the rist pin gets oiled and the piston and rod get some oil to keep them cool. The gas ports he is talking about are drilled from the top strait down around the edge of the piston to intesect with the top ring. So if you were looking down your cylinder at the top of you're piston you would see about 12 tiny holes drilled down into the top of the piston around the outside edge. When oyu start to compress the air fuel mix, the preasure builds up and goes through those holes and forces the back side of the ring outward towards the cylinder, thus creating a tight seal. It doesn't raise your compresson per say, it allows you to use low tension rings to reduce frictional losses, But the gas ports allow this without loosing compression passed the rings from not having a tight seal.

Where could you find piston and rings like that? Would that make it less reliable? Also is it made esspecially for like a drag racer.. or a MX'r or anyone?

[tyman2395]

all i can say is guys nice job thumbs up you guys know your stuff

[drew416ex]

Originally posted by Brauap
This is the closest that I could find.. its not the normal YZ125 piston, but they said it was for a YZ125.. (but I think this was just a random picture, notice the rings, the YZ125 only has 1 ring)

See the two holes? Thats exactly what I am talking about..

Yeah thats not a 125 piston. Thats for a yamaha 4 stroke. Notice the 5 slots for valves on the top.

[400exrider707]

Originally posted by Brauap
Where could you find piston and rings like that? Would that make it less reliable? Also is it made esspecially for like a drag racer.. or a MX'r or anyone?

For a 400ex applicaton you'd have to have the piston custom made. I'm sure JE or CP or any of them will do it. I know CP does for a fact.