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Discussion Starter · #1 ·
I am looking for a new exhaust to swap out the stock one on my 04 636. I am looking for something smaller than stock but louder.

After searching through the forums, I found the "exhaust option" thread. I saw that one member posted his 04 with a 09-12 M4 exhaust...

Will using that newer exhaust cause any mechanical issues on my 04? A lot of the options in that thread are outdated so if you have any recommendations on an exhaust about the size of the M4 that would be great!
 

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Besides fitment, which I can't speak too, there shouldn't be any mechanical issues with running the newer can on your 04. And pretty much any aftermarket option is going to be louder than stock, and most should be smaller at least by a little.
 

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The M4 is certainly going to be MUCH louder than stock. Most people say it's one of the loudest out of the common exhausts people usually get.

Usually with an aftermarket exhaust, you may take away some backpressure in the exhaust which may usually result in some low end torque loss but its minimal and won't even be noticed by the average rider who isn't riding their bike to its max potential anyway.
 

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Discussion Starter · #6 ·
Awesome guys Thanks!!

I think half the fun is fabricating and making things fit anyway, I just wanted to make sure there were not going to be any mechanical issues. I will post some pics when this happens!
 

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Are people still talking about backpressure? Ideally you want zero backpressure and it's scavenging you want so it pulls the exhaust out of the cylinder.
Small diameter increases scavenging and gives you low end torque but creates back pressure in higher RPMs. Larger diameter loses scavenging however lets your exhaust flow better at highwer RPMs giving you more power up top.
 

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Are people still talking about backpressure? Ideally you want zero backpressure and it's scavenging you want so it pulls the exhaust out of the cylinder.
That statement is so extreme it's false.

Small diameter increases scavenging and gives you low end torque but creates back pressure in higher RPMs. Larger diameter loses scavenging however lets your exhaust flow better at highwer RPMs giving you more power up top.
It's all about balancing your torque and hp, and this of course depends on the type of riding you're doing.
 

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Backpressure doesn't do anything. It is the resistance to the exhaust flowing through the exhaust. You want negative pressure in the correct pulse with your valve overlap. Too many guys who don't build engines talk about things that they don't know and it becomes common knowledge
 

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I know, from dyno testing the same bike with multiple exhausts, that the more free flowing the exhaust the more the torque values and low end hp numbers dropped.
back pressure modifies scavenging making it more efficient in a different RPM range than with less or no back pressure. If you run your bike with exhaust or headers (no back pressure whatsoever) your bike will run like absolute shit.
 

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That's not from backpressure, it's lack of negative pressure to create the suction to pull the exhaust from the port. Backpressure is a restriction, negative pressure is the lack of pressure behind the exhaust pulse the pulls your next pulse out with it.

Think of a can of cream of mushroom soup and when you turn it upside down and the suction that stops it from coming out. Now if that was a longer tube and you had 2 gelatinous blobs of cream of mushroom (yeah, I have weird analogies lol) and both ends of the tubes were open. If that first blob moves, it pulls the second along with it from the suction (negative pressure) and that's the same with exhaust pulses. If you have a narrower tube then it pulls more effectively where as a larger diameter the pressure is so dispersed it isn't as effective. Removing the header is just having the exhaust "fall out" as opposed to being pulled out.

Backpressure itself is the resistance to all of this happening. No backpressure would be 100% efficiency.
 

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You want negative pressure
The term is VACUUM

But everything is a huge compromise with engines for at least a few more years until we get crazy complicated valve trains and we can modify timing to whatever the hell we want. Even scavenging is a compromise because we could so much more easily just push fuel/air through the cylinder and into the exhaust unburnt (read: two stroke).

But scavenging isn't needed up top with a high rpm, ram air engine. It's only needed at low rpm and without a ram air effect to give a somewhat decent volumetric efficiency, at higher rpm and load our engines surpass 100%. To what extent I don't remember, although I know my car hits 110% around 6-8k rpm
 

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Think of a can of cream of mushroom soup and when you turn it upside down and the suction that stops it from coming out. Now if that was a longer tube and you had 2 gelatinous blobs of cream of mushroom (yeah, I have weird analogies lol) and both ends of the tubes were open. If that first blob moves, it pulls the second along with it from the suction (negative pressure) and that's the same with exhaust pulses. If you have a narrower tube then it pulls more effectively where as a larger diameter the pressure is so dispersed it isn't as effective. Removing the header is just having the exhaust "fall out" as opposed to being pulled out.
Exhaust velocity is what you're going for.
 

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Yes, and scavenging is what does it at lower RPMs.
Cause and effect is fun.

And vacuums are the lack of pressure. It's only a negative pressure depending on which unit you're using, as well as the whole absolute versus relative thing.
 

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So this is what it's like talking to someone with a deep understanding of motors...I like it lol
 

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Exhaust efficiency as it relates to hp and torque is not a simple concept with lots of factors playing in, more than just how fast you can move the exhaust gases. As has been stated we are also dealing with vacuums/negative pressure caused by the exhaust pulses. This concept is most noticeable with 2-stroke machines, where the engine's torque and hp curves can be drastically altered by the shape and length of the exhaust and it's expansion chamber. I'm fairly certain that both of you, Inked and Steel, understand that concept.

What I'm trying to say is that back pressure (the restriction of exhaust gasses) can help your motor achieve better low end power because if effects the scavenging in such a way as to make it more efficient in the lower RPM range. Because as we all know if your exhaust gasses are moving too fast then the scavenging effect can suck out too much of the new air/fuel mix when the piston and valve train are opened for their longer, low RPM, durations.

Am I making any sense?
 

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So this is what it's like talking to someone with a deep understanding of motors...I like it lol
Steel understands too well and can confuse himself by over-thinking absolutely everything ;)
 

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You make sense and what you are saying is true to an extent but the whole idea of R&D is to elminate as much of the backpressure and use your scavenging. If you don't optimize the scavenging then back pressure can be a help get you back to a better place
 
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