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4 stroke dirt bike exhaust

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Ok I had a quick search but theres about a million threads to sift through.

I know 2 stroke exhausts work by pushing some of the mixture back in by back pressure (im pretty sure this is the basic concept anyway).

But a 4 stroke exhaust, well I thought it worked by basically making as much flow as possible, almost making a vacuum. So the idea is to make it not to large and not to small to make it flow fast and take with it all the exhaust gasses with it, but not try and make it huge volume which would decrease flow speed, Just get the perfect mix (for intented rpms, i dont wanna get to technical).

So, for an example lets use the standard can on my bike, a 07 TE 450. It comes with an arrow pipe from the factory. ( not as good as aftermarket but better than some stock exhausts).

Im going to assume that changing the header diameter would change flow/volume so lets leave that.

How would changing the muffler (the big end bit, i cant remember the name haha) effect my power? If the pipe is already flowing a certain speed/volume wouldnt the muffler decrease this? its there to reduce noise right?

Wouldnt it be easier to get the right header for flow/volume and remove the muffler completely if noise was no conern? Or does this help the exhaust? How do stock ones vary powerwise from aftermarket ones? Less restrictive im going to assume.

Now for the next part of my question. How does the FMF Powerbomb make more power? its just a random ( i know its calculated) wider spot in the exhaust? How does this make more power?

Long post I know, I would just like to now a bit more about bikes 👍

 

Read 4 Stroke Dirt Bike Exhaust Reviews

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You are pretty much right in your understanding of how a four stroke exhaust works. You want the pipe large enough to allow the exhaust gases to easily escape, but no so large that the gases escape with too low of velocity. The exhaust velocity is important so that the momentum from the moving gases will leave a negative pressure wave (basically a vacuum) behind each pulse. When an exhaust is tuned for a specific engine and RPM range, then this negative pressure wave will be at a peak at the exhaust valve when it opens the next time, creating a larger pressure difference between the combustion chamber and the exhaust port. This pulls the gases in the combustion chamber out, and helps pull the fresh charge of intake air/fuel in (this is where there is valve overlap). This is called scavenging. Beyond that, minimizing restrictions such as tight turns or decreases in pipe diameter are key to keeping flow up.

You are right that for the most part, a properly designed straight pipe will perform better than any muffler. This is why F1, NHRA, and many other high power applications with no noise limitations do not use any silencers. Since we have to deal with noise issues, the key becomes making a silencer that restricts as little as possible, but still decreases sound. Different approaches have been done on this such as using tapered cores, chambered cores, and baffle cones.

As for the powerbomb, it is technically called a Helmholtz resonator, and has been used on production cars to decrease noise for many years. It isn't really a wider spot in the header, it is a normal straight through header with a few holes drilled in it. Typically 4 holes are used, around 1/4" in diameter, 2 near the front of the "bomb", and 2 near the rear of the "bomb" turned 90 degrees on the pipe from the front set. A chamber is then welded over the pipe to allow gases (and more importantly pressure and sound waves) to expand slightly out into the chamber. The basic idea behind it is to use resonating sound waves in the pipe to help cancel eachother out to decrease sound. The byproduct in certain applications is a slight increase in power at certain RPM's, typically related to its effect on pressure waves in the pipe, though the power increase tends to be pretty small.

Another popular trick for high RPM applications is the use of a stepped header. This is where the header steps to a larger diameter a few inches from the exhaust port. Some pipes use multiple steps along the length of the header. Typically, there is a step where the exhaust port ends and the exhaust pipe begins. This is left as a step, rather than making the pipe diameter match the port diameter to help prevent reversion, that is prevent exhaust gases from flowing backwards into the combustion chamber after the piston reaches TDC. If the pipe is designed to be used at high RPM's, then the exhaust gases may not have enough velocity at low RPM's to prevent reversion, so this step helps prevent it, thus helping power at low RPM's. There have also bee anti-reversion cones used in the past, it is like a funnel placed inside the entrance to the header to further help prevent reversion. Steps placed further down the pipe create pressure drops in the flow of the gases, and can sometimes help increase power at higher RPM's depending on where they are placed along the pipe.

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First off your bike does not need and you do not want ANY backpressure what so ever. all that "you need backpressure" talk is just myth. As far as exhausts go an aftermaket exhaust will help because it will be less restrictive. the main goal of any 4 stroke exhaust is to get all of the exhaust out as fast as possible. Make the pipe diameter to large and the exhaust speed slows down. Make it to small it doesnt have enough volume. Its all a compromise. You need to have the biggest diameter exhaust possible without slowing down the exhaust speed. Or the smallest diameter exhaust possible without cutting the volume down to much and creating backpressure issues.

The theory behind the powerbomb is that the lil expansion chamber lets the exhaust expand for one quick moment which makes it cool and condense which according to fmf increases the flow speed of the exhaust gases. But I have personally never ridden a bike with one on it so I cant say if it actually does anything.

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Thanks for the explanation guys! Great work 👍

So I guess the reason why the new YZ450F runs the intake/exhaust switched around, so the exhaust pipe runs alot straighter than a conventional engine, which in turn make the exhaust work more effectively?

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the new 450 pipe has a loop so i do not think this was the reason, i think it was more to have the air box at the front of the bike and the tank lower to get the best mass centralization, also the head is more efficient on the intake because of the steep port angle witch would not be possible with the tb and filter in the usual position.

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Thanks for the explanation guys! Great work 👍

So I guess the reason why the new YZ450F runs the intake/exhaust switched around, so the exhaust pipe runs alot straighter than a conventional engine, which in turn make the exhaust work more effectively?

The bigger advantage is that you can keep the intake away from the exhaust heat. The conventional set up runs the exhaust right next to the intake and airbox, heating up the air and fuel as it goes into the engine. The other advantage is that Yamaha made the intake ports almost straight. Straightening out the intake will have more of a positive effect than straightening out the exhaust.

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Thanks for the explanation guys! Great work 👍

So I guess the reason why the new YZ450F runs the intake/exhaust switched around, so the exhaust pipe runs alot straighter than a conventional engine, which in turn make the exhaust work more effectively?

Actually no. The other guys explanations are right on but as stated but they had to put that loop into the pipe to make it longer. to short of an exhaust will cut the velocity down and the exhaust pulses wont have enough length to help create that pressure to keep the velocity up.

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rubbersdown, Just how does a shorter headpipe reduce velocity? Larger will reduce the flow velocity, but I fail to see how shorter will slow the flow or the tuning pulse in any way?

The loop in the pipe is to tune the length at the harmonic that they want to use. Shorter pipe would use a different harmonic which would typically tune for a higher rpm.

Swiss

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Okie dokie then 👍 So do you guys reckon we will see more engines like this in the future? I know the new Ossa trials bike uses it with fuel injection as well!

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rubbersdown, Just how does a shorter headpipe reduce velocity? Larger will reduce the flow velocity, but I fail to see how shorter will slow the flow or the tuning pulse in any way?

The loop in the pipe is to tune the length at the harmonic that they want to use. Shorter pipe would use a different harmonic which would typically tune for a higher rpm.

Swiss

I admit that what you stated is true, length of the exhaust will greatly reflect and change the torque curve. What I was theorizing and please correct me if you think im wrong (I probably shouldn't post my own theories anyways lol) but I figure (and have heard tuners say this before.) when the exhaust valve opens it sends a wave of pressure. when that pressure reaches an opening of the exhaust (the silencer) a rarefaction pressure wave is then sent back towards the motor. Now most people seem to believe that its best for this wave to reach the motor when the piston to crank angle is at 120 degrees to the piston. That is said to be the best position for the motor to scavenge the remaining exhaust gases from the chamber. This is actually how some tuners figure their exhaust length using a specific formula based off of that 120 degrees and exhaust speeds. So I figure, If the exhaust is a specific length to make sure these pressure waves reach the motor right at the perfect time for the rest of the gases to be expelled that will greatly reduce any unnecessary back pressure and increase velocity.

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Swiss, you are correct. RD has it partly right, partly wrong.

As with any gas or fluid conduit flowing at a given volume/rate, a smaller tube will increase the velocity, and a larger one reduces it. Length doesn't really affect it, except that as the length increases, the resistance to flow also increases due to the additional friction on the conduit walls. This last is not really much of a factor in exhaust design.

The harmonic tuning idea works like this: At the opening of the exhaust valve, a pressure wave is initiated at the exhaust port. Assuming a straight pipe, this wave will travel the length of the pipe until it reaches the end. As it exits the pipe, the pressure drop generates a negative pressure wave (alternately referred to as a depression, or less accurately, a vacuum wave), which travels back toward the exhaust valve at about the same speed. The plan is to have this low pressure wave reach the open exhaust valve during the overlap period so that it can aid in the final evacuation of any remaining spent gases, and also so that it can lower the pressure ahead of the incoming fuel/air charge to aid in the start of the induction cycle. This can be adjusted by varying the length of the pipe.

It should be noted that any sudden and significant increase in pipe section will have the effect of being an "end point" for the initial pressure wave and thus generate a negative pressure wave in response. Mufflers play havoc with this kind of tuning, and stepped pipes are one way around that.

Look at the links in this post, and you can see that there's a lot involved with the whole matter:

https://thumpertalk.com/forum/showthread.php?p=9333683#post9333683

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Actually Grayracer and Rubbersdown. When the exhaust valve opens there are 2 pulses that start traveling down the length of the exhaust pipe. One is the pressure pulse which is the exhaust gases moving out of the cylinder and the other is the Sonic (sound wave) pulse that moves MUCH FASTER and travels down the length of the pipe and is used for Tuning. You don't tune with the pressure pulse or the returning pressure pulse, you tune with the sound wave and how it works with and against the pressure wave.

Stepped pipes and megaphones are both effective in generating reflective pulses to tune with. The step pipe also generates some anti-reversion effect in the pipe which helps to keep pulses that are returning at the wrong time from passing up the exhaust port and into the cylinder and even into the intake port.

Port steps at the head/pipe junction and A-R cones also help to widen the powerbands by working against reversion pulses at all rpms. The Powerbombs and Tuning chambers like Yamaha and Husky and some other pipe makers are using are also helping to prevent the Reversion pulse from getting back to the port/cylinder by generating interference pulses in the headpipe that block the returning flow.

Regardless of what length the pipe is there is only a small rpm band where the exhaust is correctly tuned for the pulse tuning. The rest of the time the pulse is timed wrong and the exhaust valve is still open and the tuning pulse can force exhaust gasses back into the cylinder, reducing performance at those non-tuned rpms. There are many tricks to tuning and I have seen a Suzuki factory pipe off of a 750 use several of them in just one 4-1 system.

Swiss

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Sorry, Swiss and Mike, you have things a bit mixed up. Ironic in your case, Mike, since you provided the proof yourself. 🙂

You have confused "pressure wave" for the mass physical movement of the bulk exhaust gas volume. Sound IS a pressure wave, thus, the pressure wave travels at sonic speeds, and is used to, 1) accelerate the evacuation of the spent gases, and 2) to accelerate the filling of the cylinder with the fresh fuel air charge.

From the linked text above:

A pressure wave is generated as the valve continues to open. Gases can flow at an average speed of over 350 ft/sec, but the pressure wave travels at the speed of sound...

Continuing:

...two basic phenomenon are at work in the exhaust system: gas particle movement and pressure wave activity. The absolute pressure differential between the cylinder and the atmosphere determines gas particle speed. As the gases travel down the pipe and expand, the speed decreases. The pressure waves, on the other hand, base their speed on the speed of sound. While the wave speed also decreases as they travel down the pipe due to gas cooling, the speed will increase again as the wave is reflected back up the pipe towards the cylinder. At all times, the speed of the wave action is much greater than the speed of the gas particles

..and finally:

...Waves are also reflected back up the original pipe, but with a negative pressure. The strength of the wave reflection is based on the area change compared to the area of the originating pipe.

This reflecting, negative pulse energy is the basis of wave action tuning. The basic idea is to time the negative wave pulse reflection to coincide with the period of overlap - this low pressure helps to pull in a fresh intake charge as the intake valve is opening and helps to remove the residual exhaust gases before the exhaust valve closes.

Read.

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im going to explain basic muffler theory. Most mufflers have tes to direct the exhaust and use that to change tone. I'm assuming here (and we all know what that means) that the one your looking at is a high flow muffler. A high flow muffler has either no plates and use a glasspack design or uses very few plates. When you have an open muffler its easier for the gasses to excape therefore resulting in alot of power.

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im going to explain basic muffler theory. Most mufflers have tes to direct the exhaust and use that to change tone. I'm assuming here (and we all know what that means) that the one your looking at is a high flow muffler. A high flow muffler has either no plates and use a glasspack design or uses very few plates. When you have an open muffler its easier for the gasses to excape therefore resulting in alot of power.

Well there you have it folks.

Hope you were paying attention Grayracer.

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Well there you have it folks.

Hope you were paying attention Grayracer.

But you also lose tourqe that way, but gain in the higher RPM

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But you also lose tourqe that way, but gain in the higher RPM

That is a fallacy.

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