i have had this question for some time now and no one has been able to give me a straight answer... How important is it to have back pressure on the motor? the reason im asking this is because i made a set of fully functional 3" mitter cut chrome exhaust stacks for my vinson 500... because i removed the muffler for the most part my exhaust is just like a straight pipe and other then a T in the cross over pipe, my exhaust no longer creats any back pressure on the motor... (well i dont does anyways) i have discussed with other atv ppl and as far up as some guys that i work with the build high end drag car engines... no one can give me a straight answer on what break pressure really does and why its needed... or if its really needed at all... as for my machine it seems to run just fine... i went on a 20 mile ride to day with it and other then needing ear plugs it was great... well if any one has some insight on this topic please fill me in... im just curious... oh, plus if i could figure out how to load some photos on here i would put some pics of it to show my stacks... thanks again for any info on this topic

 

back pressure is more of a critical thing with 2 strokes its needed to prevent unburned fuel/air mixture to pass right through the cylinders into the exhaust, 4 strokes actually dont need backpressure but straight pipes on a 4 stroke thats designed to operate with backpressure also isnt good either....if you dont change things around a little, the air/fuel mixture, sometimes ignition timing, intake design also sometimes, also picking the right size pipe for good velocity is key, to big a pipe will actually create backpressure this is due to the exhaust gas expanding and therefore cooling, the cooler exhaust gas is denser, this slows the exhaust flow, large exhausts will be better off at high rpms but fail at low rpms and smaller pipes will create more low down torque but wont flow as good at high rpm thats why you need a good balance, backpressure is needed to remove gases from the engine,back pressure also creates something of a suction to help remove spent gases when it isnt too restrictive, too much back pressure and it cant all escape, too little and the engine will pull it back in through the exhaust valves when open, in order for straight pipes to work well on an atv the engine has to be modded right, or at the least changed air/fuel ratio and timing...its really a pain in the *** to explain hopefully that helped u out a little.

 

As long as the air/fuel ratio is correct you won't do any motor damage. So either rejet or get a fuel programmer that you can create your own map, Such as MSD. Then you need a air/fuel ratio gauge to create a map.(only applies on a fuel injected machine) When i put Duals on my Rhino I had to add 23% more fuel to certain areas of the RPM range.

I'm thinking with your exhaust being that big that you are going to be running REALLY lean and losing power.

 

Backpressure causes a "pulse" in the pipe. The above statement of "it's more important on a 2 stroke" is correct.
Altho, you will burn pistons sooner w/o backpressure on a 4 stroke. I personally feel that backpressure is important for performance.

 

back pressure is what generates torque. you will have tons of horsepower with free flowing exhaust. but on this particular machine i would think you would want torque instead of horse power

 

The problem with a completely open exhaust is the possibilty of a warped exhaust valve. The valves get extremely hot when the engine is running, and are still that way when you shut it down. With a completely open exhaust, the colder outside air is allowed to rush into the pipe and to the head, hitting the hot valve. With rapid cooling, the valve stem itself can actually warp. That is why when dragsters shut down, they plug the stacks. It allows the hot air to stay trapped and cool down gradually. You could run it that way forever without any problems, but its something else to consider.

 

thanks for everyones input... its a big help... i know that i have some pack pressure on the motor... i know i said it was a straight pipe but its not techincally straight through... the exhaust is still original all the way back to where the muffler used to be... from there i put a 45 degree bend and continued the 1.5" stock size pipe back to my crossover pipe... at the crossover pipe it "T"s... its not curved of bent its a direct "T"... from there it goes to each side of the wheeler and the "T"s again... this time into 2.25" up pipes that my 3" stacks slide over and run up through the plastic and back rack... so its not a true straight pipe other then the fact that there is no baffles in it any more... i road for 20 miles in all conditions... mud, trails, and some road riding where i was able to open it up... i havnt done anything to the carb but i have put and free flowing air filter on it... it seems to have alot more torque on the low end and the high end feels about the same... i know that this is the opposite of what everyone has told me but thats how it is... it seems to run great... idk....... im not an expert i just thought that i would be different and look cool... i just dont want to screw up my motor thats all... thanks again

 

Any pictures? Sounds like quite the setup!

 

If you head pipe is still stock, you are creating enough back pressure..... The majority of required back pressure is in the curve of the head pipe, not the muffler.

But, the muffler does create the back pressure that keeps torque numbers in line with the HP numbers to keep the power curve where it needs to be. So while you won't hurt your engine, you might change the power curve of that engine, and cause it to make more power on top end and less on bottom.....Which on a Ute might not be the best thing.

You may also cause a slight lean condition, in which case you should also re-jet as required.

 

thanks again for the info.... any little bit i get helps... i would put some pics up but i dont know how too... i think i need to resize them but im not sure how to do that....