back-pressure

[kasey200x]

hey i was wondering how exactly back pressure works on four strokes. i feel kinda dumb asking this but i really need some help.

[NOS_350X]

A four stroke donst have to have any back pressure. Look at drag bikes/cars. More hp less backpressure, More backpressure more torque. Its all about the ease of flow of the exhaust and intake through the engine.

[kasey200x]

but wont it bend the valves if it dosent have any back pressure?

[FlyingW]

It's the 2 strokes that run on back pressure.

[kasey200x]

but say you take the header of a 4stroke three wheeler. wouldnt that cause the valves to bend?

[dads]

They can do to accesive heat. most of the time you will scorch the exh. valve and seats and loose compression.

[random-strike]

depends on your application.

if you want all out high rpm horsepower, then yes backpressure is not the best.

if you want a useable power curve you need it.

reguardless you need some piping off the head, you won't bend the valves, but you can crack them because of the valves get hot and cold to much/fast and it causes them to crack. the valve can also suck in cold air through the open exhaust port.

[NOS_350X]

reguardless you need some piping off the head, you won't bend the valves, but you can crack them because of the valves get hot and cold to much/fast and it causes them to crack. the valve can also suck in cold air through the open exhaust port.

I keep hearing people say that. I have yet to see it, This is my OPPINION. With the rpm's of an engine. Your pushing hot air out so how can cold air go in. You stand behind a bike you feel the exhaust nothing can go back in. Ever put your had over a exhaust pipe? all it does is push out no pushing in.

Now i woudnt go around running my bikes with absoutly no pipe on them. I i have started engines without any pipeing coming off them at all. You have cold air comming in through the intake so why would it hurt if it came in through the exhaust port?

[kasey200x]

so if someone riged up like a fatty pipe and put it on a four stroke what would the affects be? more torque or just a stupid idea, assuming you had everything, such as pipe bender, welding machine, ect.

[Rex Karz]

It would breathe easier, so you could go bigger in the main jet and get more power. And it would look cool.

[random-strike]

so if someone riged up like a fatty pipe and put it on a four stroke what would the affects be? more torque or just a stupid idea, assuming you had everything, such as pipe bender, welding machine, ect.

stupid idea.

[random-strike]

I keep hearing people say that. I have yet to see it, This is my OPPINION. With the rpm's of an engine. Your pushing hot air out so how can cold air go in. You stand behind a bike you feel the exhaust nothing can go back in. Ever put your had over a exhaust pipe? all it does is push out no pushing in.

Now i woudnt go around running my bikes with absoutly no pipe on them. I i have started engines without any pipeing coming off them at all. You have cold air comming in through the intake so why would it hurt if it came in through the exhaust port?

this is common engine knowledge. if you run w/o any pipe at all the engine will run terrible anyways, you need some pipe because the vacuum in the exhaust pipe helps to pull out exhaust from the cylinder. and as i previously stated you will crack a valve running with open exhaust ports.

the reason you feel exhaust all the time out of the pipe is because there will always been exhaust in the pipe, the exhaust strokes come to fast for the exhaust to be completly out, plus there is a vacuum in the exhaust pipe.

coming in through the exhaust port will hurt because it covers the entire exhaust valve in cold air, much colder air than would be coming in the intake port and into the cylinder, and only the bottom of the exhaust valve will be exposed to that.

[cr480r]

I keep hearing people say that. I have yet to see it, This is my OPPINION. With the rpm's of an engine. Your pushing hot air out so how can cold air go in. You stand behind a bike you feel the exhaust nothing can go back in. Ever put your had over a exhaust pipe? all it does is push out no pushing in.

What happens when you shut the motor off? That is when damage could/would occur when running without an exhaust pipe...

[jenndnn3]

Pressure is the key. We are looking for equalibrium on a 4 stroke...

Heat creates pressure, Cold creates a different pressure. When the 2 are against each other you will break something because those pressures do not match or are not equal. More heat = more pressure......Heat will typically win every pressure war.

Everything has a break or pressure point, and the stress of heat and cold help those points along...whether negative or positive.

It is pressure not temperature that we are keying for an engine to run. How much pressure can we get into cylinder, to create the energy to move this item. In a 4 stroke this is controlled by the valves. Valves seal up the cylinder thus allowing the pressure to build. They break due to pressure outside is not equal.

So NOS is slighlty correct, it would indeed breathe easiest without no exhaust, Pressure is being released from a 4 stroke to allow for the cycle to start again.. This happens when the piston is moving back up after getting the big power stroke, and the piston pushes the burnt material out... IF here is the Key Something doesnt allow this to escape your going to build more pressure...Outside of the engine. (smaller pressured engines really do run well without an exhaust.) They dont build that different of a pressure. Except then we have cold which is a different in pressure. Since everything is escaping via valves, and only thing keeping them sealed is springs and the 2 pressure working against each other you get the cracked bad valve....

Then you key to this oh but Jen more pressure is a good thing! Yes more pressure creates more power, but not at the cost of breaking internal parts... Because, everything has a pressure point that it cannot maintian... 2 different pressure points against each other, one will lose... So you build pressure in one place, build it elsewhere (2 stroke) or allow pressure to escape so we can build it again (4 stroke) so we maintian balance and not break things..

The pressure has to be contained, built used and then released. If the pressure outside the the valve is less then the pressure inside what happens? the one with the most pressure wins... So it is extremely important to maintian that nifty balance. Therefore making everyone else correct when you have the heat versus the cold and a broken valve..Because now we are dealing with different pressures..And removing exhaust system of a super pressurized engine to an outside cold conditions creates this scenerio. Remember Heat is directed through the exhaust thus maintians that equalness in pressure..

And that is in the firing system alone... What about springs. They are under pressure as well, and other things...

Now I am keying on valves alone, I understand rings and other things play a major roll as well... IT all comes down to getting the best amount of pressure without breaking something...

In a 2stroke it uses other pressures against each other for forcing energy (backpressure) There is no valve system to contian the pressure we are trying to build, so we find it elsewhere.

Anyway thats what I think, or see things ...:thumbsup:

[random-strike]

it really won't run the best without any exhaust at all. it needs the vacuum in the exhaust to help pull out the exhaust during the exhaust stroke

[dads]

Maybe on a two stroke you need back pressure. But a four stroke runs best with straight pipes that give very little back pressure.

when the valve closes the exh. is moving out but stops and moves back tords the valve tite new exh. comes out to change the flow.

It happens with out a pipe too, only it pulls cold air back in. if you rode it hard and got the valve glowing, the temp change will make the stem crack and pop the head off the vale. or bend the valve stem.

[random-strike]

Maybe on a two stroke you need back pressure. But a four stroke runs best with straight pipes that give very little back pressure.

when the valve closes the exh. is moving out but stops and moves back tords the valve tite new exh. comes out to change the flow.

It happens with out a pipe too, only it pulls cold air back in. if you rode it hard and got the valve glowing, the temp change will make the stem crack and pop the head off the vale. or bend the valve stem.

they make the most peak power.

but not all machines want peak power, drag racing yes, woods riding and stuff not really :)

[jenndnn3]

Suck, squeeze, pop, blow = 4 strokes


Hey Kasey,
You can do a dogpile search on how 4strokes work and exhaust theory and other relavent info. Unfortantly you really have to stifle through the ads I sell this exhaust and general opinion based sights, but when reading this stuff you will get a basic general rule of thumb. (I mean most people agree this way.) There is a ton of info out there, and even in opinion some have valid arguments, well worth reading if you have the time. Knowledge is powerful and even if you dont agree with something its always good to understand why the opposit of your argument stands where they stand, or where they are coming from and why they they may or may not believe the way they do.

[kasey200x]

hey guys, just got the motor back together and on the frame. but it really sex cause it dosent have any compresion. so im ordering another set of gaskets, and rings. dose anyone have an idea on how muh it costs to get the cylinder honed?

[random-strike]

hey guys, just got the motor back together and on the frame. but it really sex cause it dosent have any compresion. so im ordering another set of gaskets, and rings. dose anyone have an idea on how muh it costs to get the cylinder honed?

you might have the cam timing very far off... didn't you say when you tried to turn it over it hit something and stopped? why don't you try and turn it the other way and see if it moves back and hit again???

honestly... you should either take the time to learn how to build that engine correctly, or pay someone to do it...

[ridetrikes250R]

fourstroke exhaust is run off of pulsation, thats why if you can build a fourstroke exhaust taking in mind exhaust pulse, valve timing and the general idea of when there will be pressure in the header you can build a superior pipe, there are all sorts of exhausts on the market that work awesome souly on the design of engine pulse.

also on the hot cold valve thing, its because the exhaust valve can go up anywhere from 100-300 degrees in temerature with a shoty pipe, allowing a valve seat to fall or the valve to crack, valve warp, is from the high heat of the valve dropping dramatically when turned off

[kasey200x]

you might have the cam timing very far off... didn't you say when you tried to turn it over it hit something and stopped? why don't you try and turn it the other way and see if it moves back and hit again???

honestly... you should either take the time to learn how to build that engine correctly, or pay someone to do it...
JUST SO U KNOW, I DO KNOW HOW TO WORK ON MOTORS, SO BEFORE U START TO CRITASIZE U SHOULD STOP AND ASK. ITS ABOUT THE TENTH MOTOR I HAVE BUILT.
I HAD TOOK THE HEAD BACK OFF AND DIDNT NOTICE ANY THING THAT WAS NOT SUPPOSED TO BE THERE, AFTER I REASSEMBLED THE TOP END AND RESET THE TIMING IT WORKED FINE. JUST NO COMPRESSION, SO I AM REPLACING THE RINGS.

[Mr. Richard]

a little history is helpful ,,,,, but i have to ask, while you were rebuilding this engine did you mic the bore and piston? or check the end gap on the rings? these are two very basic steps to a rebuild,,,,,, and when you say no compression did you do a dry and wet compression test to see if it was the rings or maybe a bad seal on a valve ??? just wondering.........

[random-strike]

JUST SO U KNOW, I DO KNOW HOW TO WORK ON MOTORS, SO BEFORE U START TO CRITASIZE U SHOULD STOP AND ASK. ITS ABOUT THE TENTH MOTOR I HAVE BUILT.
I HAD TOOK THE HEAD BACK OFF AND DIDNT NOTICE ANY THING THAT WAS NOT SUPPOSED TO BE THERE, AFTER I REASSEMBLED THE TOP END AND RESET THE TIMING IT WORKED FINE. JUST NO COMPRESSION, SO I AM REPLACING THE RINGS.

there will still be compression, the piston would have to be broken or melted through for there to be 0. is this the 10th ****ed up engine you've built??

no need for you to use caps and get all hissy.

[ridetrikes250R]

iam with random, there is something more then the rings, even a TOASTED motor wtih a hole in the piston will read like 25-60 pounds of compression (been there toasted that....) it defently sounds like either there is a HUGE mic difference between the piston and cylinder, or the more logical approach of saying one of the valves are bad.

[jenndnn3]

I too agree With everyone else Kasey200x, Dont get overly excited...

You have started several threads, all which we try very hard to answer your questions on. But it kinda seems you are little all over the place. Even I have the impression that you dont know what you are doing... Or what you want to do? That is not a bad thing so please dont be insulted. Sometimes I get there myself..

Take a moment to answer KASEY's questions

little history is helpful ,,,,, but i have to ask, while you were rebuilding this engine did you mic the bore and piston? or check the end gap on the rings? these are two very basic steps to a rebuild,,,,,, and when you say no compression did you do a dry and wet compression test to see if it was the rings or maybe a bad seal on a valve ??? just wondering.........


Then I am gonna add some questions myself,
What are you trying to accomplish? Basic rebuild or a we going up in the performance arena? Before any of this can happen, did you do the basics of what KASEY is asking? Then Random asked a very specific question as well,
you might have the cam timing very far off... didn't you say when you tried to turn it over it hit something and stopped? why don't you try and turn it the other way and see if it moves back and hit again???
I am seeing alot of questions about pop-up pistons and backpressure and so on...I can only assume with the info given these are all related to the same engine?

The funny thing is we dont even know which trike you are working on, if we go on this thread alone, we are assuming its a 4 stroke,,,,:doh:

[random-strike]

if the cam timing is off and one of the valves is open on the compression stroke it will have 0 compression...

[kasey200x]

the reason it didnt have any compression was the gaskets, i was given the gaskets which were the same as the stock metal gaskets(not shure if you can even still get those) by a friend who wasnt shure weather they had ever been used. i just decided to go ahead and get the new rings, couldn't heart any thing.

[random-strike]

the reason it didnt have any compression was the gaskets, i was given the gaskets which were the same as the stock metal gaskets(not shure if you can even still get those) by a friend who wasnt shure weather they had ever been used. i just decided to go ahead and get the new rings, couldn't heart any thing.

having a used gasket won't make it have 0 compression... even having no gasket, it'll still have compression.

[bansheeboy]

the reason it didnt have any compression was the gaskets, i was given the gaskets which were the same as the stock metal gaskets(not shure if you can even still get those) by a friend who wasnt shure weather they had ever been used. i just decided to go ahead and get the new rings, couldn't heart any thing.

i'll be honest and say that i'm going to bash you. it seems like your the type of person who already has it all figured out in your head and thats the way it is and if anyone says anything to the contrary then your going to be down on them from the get go. everyone here from what i've read have came up with alot of good ideas but you insist that you know your **** and it's either this or that. i've seen guys like you all over the forums i visit and i always have the same response... if your that ****ing smart then dont make 20 post's asking for advice just so you can argue with the people who respond trying to help you with your problem. it's very simple, either say thank you and ignore everyones advice, try what was suggested and find out it worked or didnt work then reply or just dont reply at all. it's realy not rocket science. you post a question and people are going to answer with there theories. if your convinced your ideas are automaticly better than theres than dont post questions you think you already know the answer to. if your just here to ask questions and then argue about peoples responses then you should go **** off somewhere and fix your bike yourself since your so damn smart. my opinion is that the bike would be fine with a little tuning but theres a douche bag working on it who would rather spend his time arguing on the internet instead of reading up on how to fix his problem.....

[Mr. Richard]

by a friend who wasnt shure weather they had ever been used.
dude your way off ,,,,, any competent engine builder would be able to spot USED HEAD GASKET IN 1/10 OF A SECOND,,,, your descriptions of all your problems are very novice to say the least,,,, without any real details ,,, so i am sure you will have many more problems to solve in the future,,,, the poeple who are trying to answer your questions have hundreds of years of knowledge ,,, i have 45 alone,,, so calm down ,,, we do know what we are talking about,,,,,,,,,,,,,,,,,,,,,,,, all of those who have responded did it for your benifiet not theirs!!!!

[dads]

The one thing you need to check is the valves. if the piston comes into contact with a valve they bend and you will not see it but it will never seal again.

[NightBiker07]

on a 4 stroke.....no backpressure= burnt exhaust valve. simple as that. the best 4 stroke exhaust your going to get for an atv are the aftermarket ones, they build them with optimized tuning........way better than anything your going to build.......

my cousin built an exhaust for his Trike. it ran GREAT, TONS of power, till the valves burned out a week later...........

[kasey200x]

hey guys, i just came back across this thread and realized how muck of a little doutch i was being. i think this was atleast a year ago, i honestly didnt know what i was talking about. i wanted too apologize, too anyone i may have ofended. i admit i didnt know wtf i was doing.. since then i have read hours and hours and hours worth of threads on here. i have learned whole lot of stuff about the atc's.

[TMERacingPhotography]

I keep hearing people say that. I have yet to see it, This is my OPPINION. With the rpm's of an engine. Your pushing hot air out so how can cold air go in. You stand behind a bike you feel the exhaust nothing can go back in. Ever put your had over a exhaust pipe? all it does is push out no pushing in.

Now i woudnt go around running my bikes with absoutly no pipe on them. I i have started engines without any pipeing coming off them at all. You have cold air comming in through the intake so why would it hurt if it came in through the exhaust port?

The only problem you would have while it's running would be the extreme heat from no exhaust and an improper carb setup. Engines are built to withstand a ton of abuse but they have their limits. Most of the problems occur when you shut the engine off. If it cools down to fast after it was way to hot you are looking for major problems.

[TMERacingPhotography]

Opps didn't see how old this thread was. Ignore my post.