Back Pressure-Yeh or Neh?

7 replies to this topic
  • BRP-rider

Posted October 17, 2005 - 03:50 PM


Do 4stroke dirtbikes need back pressure in the muffler for
it to run properly? :banghead:

For instance, if baffling is removed from an exhaust pipe on a four stroke bike, and it then has no back pressure, will it gain power or lose power? :banghead:

Is it possible to remove just the baffling from an XR600 exhaust pipe without destroying the muffler and/or the spark arrester? :lol:

  • XRsteve

Posted October 17, 2005 - 08:38 PM


With no backpressure you lose bottom end. Top end would probably gain a little, but I don't think it would be worth the loss at low rpm. And it would be very loud!
If the XR600 has a bolt in baffle its easy to pop out.

  • BRP-rider

Posted October 17, 2005 - 08:50 PM


Yes, mine has a bolt on baffle on the end of the pipe and it has already been removed. :lol:

Is there more baffling actually inside the pipe or is it just a resinator with a spark arrestor? :banghead:

Should I remove the reamaining baffling, (if there is any more), or should I just leave it, because I prefer good/ strong bottom end power. :banghead: :lol:

  • BRP-rider

Posted October 17, 2005 - 08:56 PM


:banghead: How does that work that you would lose bottom end power with no back pressure? :banghead:

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  • Thumpmeister

Posted October 17, 2005 - 11:26 PM


I believe its a resonance thing; it has to do w/ the pulses from the exhuast strokes, and weather the exhuast lets them flow free or not.
With a wide open (or no) muffler, the bike will want to run wide open because theres nothing to restrict it, and it will run better like this. As for the exact physics of it, I don't know of many of us (myself included) who would completely understand the concept of backpressure and low-end. My only guess is that since its slightly more restrictive, that it would run a bit better at less then WOT since theres something to hold it back.
Sorry, I doubt that really answers your question, but this is kind of an in-depth concept.

  • Shawn_Mc

Posted October 18, 2005 - 04:47 PM


The back pressure and the physics of how it helps or hurts a fourstoke engine are based on the overlap of the intake and exhaust valves built into the camshaft. At one point the intake and the exhaust valve are open at the same time. The scavenging effect the exhaust pipe has will enhance or degredate the intake charge coming from the carburetor.

There are a couple basic and major things going on in the exhaust. Heat and sound waves. The stepped headers try to take advantage of the heat. As the pipe becomes larger, the heat in the exhaust wants to expand and the larger steps allow it to do that. But, the exhaust charge is of finite size. So when it expands, its drawing a vaccum on its point of origination. Now for the sound...the sound also travels up the pipe. Pipe makers mess with lengths, sizes and such to dial in the RPM where they want the sound to reflect back down the pipe. What they seek to accomplish is, the heat scavenges out of the combustion chamber all the exhaust and even some of the incoming intake charge. With the sound wave, they then hope to pack the portion of the intake charge back into the combustion chamber, so the pipe actually allows the engine to pull in more than it noramlly could on its own. Timing the valve events with the exhaust is critical. But thats why they call it, "coming on the pipe"

Ive got more for if you want...I live for this shit.... :banghead:

  • wr450fyamaha

Posted October 18, 2005 - 05:25 PM


well.....ive run my bike with no packing (yoshimura system) and it had a noticable gain in low end torque and a loss in peak power.....but it was very very very loud.

  • hill5150

Posted October 18, 2005 - 05:48 PM


well.....ive run my bike with no packing (yoshimura system) and it had a noticable gain in low end torque and a loss in peak power.....but it was very very very loud.

It was really loud, how did that happen??????????? :banghead: :busted: :busted: :lol: :lol: :banghead: I almost peed my pants........... :lol:

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