Could someone please explain to me

1. ## Could someone please explain to me

how back pressure works in detail. and why does it burn your valves to ride without a pipe or a different pipe? thanks.

2.

3. As I understand things, what is called backpressure is better thought of as resistance in the pipe. Stock pipes are essentially more restrictive than most any aftermarket, or no pipe. The higher the resistance of the pipe, the higher the pressure immediately as gas exits the cylinder through the valve. You have to add some math and chemistry here, but suffice to say that an increase in pipe resistance will increase backpressure and force the pistons to work a little harder to push the exhaust out the pipe. Now this does not necessarily mean a drop in horsepower, but someone else will have to explain that chemistry. I do know that having no pipe will result in valves burning out because of essentially backfiring. Any unburnt hydrocarbons normally flow down the pipe and then ignite instantly and violently when they meet fresh 02 (normal backfiring). If you have no pipe any such backfires will happen right at the valve, resulting in heat/vibration related damage.

-Sandworm

4. it's all about exhaust velocity. gasses swirl through the exhaust and basically create a siphoning effect. there is a formula for figuring out exactly what the best diameter is for an engine but if what your wanting to know about is why no pipe would burn valves we won't get into the size stuff. having the pipe drawing the hot gases away from the valve in effect keeps them cool. having no pipe to do such would cause the heat to not be puled away from the valve and valve area thus heating them up to extreme temps and burning them. the face of a valve is thin and thus easilly heated. a valve itself isn't very big either so it wouldn't be able to dissapate heat fast enough to keep the face from burning.

5. Stuff like this belongs in Tech and Mods. Please keep that in mind for next time......

With too little backpressure the gases are still burning very hot when they come out past the exhaust valve. That and due to the overlap in the intake to exhaust timing some of the intake charge can leak out along with the exhaust at lower speeds and that also burns in the exhuast port. All of which leads to a far hotter exhaust valve than you should have. The valve doesn't "burn" per se but the extra heat erodes the valve and valve seat and then it can start leaking. And when it leaks the burning mixture in the cylinder vastly speeds up the erosion and you end up with a "burned" exhaust valve and seat.

The lack of back pressure from an open exhaust doesn't cause it by itself. Many engines over the years were designed to run with very short exhaust stacks just fine. Take the old WW2 aero engines that used little 6 inch stubs. The key is to have the valve timing to deal with the mixture flow.

Our bike engines are cammed to work with some back pressure and if you greatly shorten or open up the pipes you mess up the overall balance. So don't do that. However swapping to some other brand of slipon or full system will be fine. You're reducing the back pressure but not to a huge extent like short or oversized pipes will do.

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