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Mechanical or Tech

Baffles and back-pressure

  • 06-14-2013 | 03:02 AM
  • Porkchop
  • So some of you may remember seeing the pics of my bike, and if you didn't the bike was purchased used through a dealer. The bike was already fitted with Leo Vince SBK pipes, of which were completely unbaffled. For such a small motor, the thing is absurd sounding and often get complements from the 1098/1198/1199 riding brethren at my local D.O.C. for sounding louder/better than their bikes.

    Fast forward a bit, now that the weather is really nice almost all day long I commute on the thing everyday. I've only put 14 miles on my car since June 1. The semi-problem I am having is that I'm a night owl. I come home really late from hanging with my brother/friends, like between 2 and 4am, and I am worried about bothering the neighbors. Most often I cut the ignition as I hit my street and coast into the driveway. I recently ran across the Leo Vince box and found the original baffles... or what I thought was the original baffles. I think from what I see in the illustrations the baffle was an endcap with about 3-4 inches of tube sticking back. All I have are the baffle caps. Upon further inspection, I think the original owner pulled out the original baffles and cut the tube off the end of the caps, most likely looking for a sound increase. When he didn't get it, I think he said fuck it and pulled the caps off completely.

    I was intrigued to hear what it would sound like with even just the caps installed. Thank god the Leos have an easy screw in baffle caps. So I did and tested it in the garage, and even at idle, me and my roommate concluded it was a noticeable difference in not only overall volume, but the booming bass. I left to meet some friends for a movie this evening and there was some differences. Accelerating you still get great sound, but on the highway it was most noticeable the noise difference. Even to the point that I noticed wind blast and front tire noise much more now. Gone is the booming sound of going through underpasses too...

    But past the noise... something happened. The bike's mid-range torque has jumped a surprising amount. The bike really had a dead spot in the mid-range between where the chuggy v-twin torque ends and the thing starts to really rev out and make peak horsepower. It's a window of about 2k rpm and it bugs the shit out of me. Even to the point that I have been chomping at the bit recently about upgrading and moving to the M1100EVO or Streetfighter 848 for a little more oomph. But this is just so confusing. I even stopped for a minute to make sure I wasn't making things up in my head and did a few pulls through all the gears... no dead spot. And throttle response is vastly improved too. Deceleration on/off throttle moves used to be like a light switch and could be herky jerky if you weren't super precise. Smooth...

    So I started thinking, how much does a little change in back pressure really effect a motor? I mean this isn't even the WHOLE baffle, just the endcap. How many people are riding aftermarket pipes for the sound aspect that are actually hurting performance? Does anybody know the science or mechanics behind this? It's just really astounding.
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  • 06-14-2013 | 07:30 AM
  • Turbo Ghost
  • Anytime you modify an exhaust system you are affecting the performance. The thing to remember is the manufacturers have to design an engine with noise in mind and so, the engine tune is matched to be the best it can be with that particular exhaust. The engine can be modified to perform better IF you change a combination of items so that they match (jetting, intake, fuel maps, etc.) the new exhaust design once baffles are removed.
    Most people just want the noise and think if it's louder it must be faster.
    Now, on 2-strokes, the exhaust design makes a HUGE difference! That is an art/science unto itself!
    On the other hand, with a turbocharged engine, everything after the exhaust turbine is irrelevant and unnecessary. A short open pipe is best for performance and usually is not much louder than a standard engine with a muffler due to the turbine breaking-up the exhaust pulses.
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  • 06-14-2013 | 08:13 AM
  • Particle Man
  • Funny you mention 2-strokes. It's amazing how many snowmobile riders think they can just mod the exhaust and nothing else. Those are usually the ones you see sitting on the side of the trail scratching their heads wondering how their sled started to run lean and ultimately toasted their power plant.
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  • 06-14-2013 | 09:17 PM
  • Porkchop
  • Just rode home... still can't believe it. It's a completely different bike. I'm starting to wonder what the bike would sound/perform like with the whole baffle.

    What I do miss is the crackling and popping during deceleration. It's almost completely gone. I did have one really loud "pop" last night which was odd.
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  • 06-15-2013 | 10:30 AM
  • njchopper87
  • Quote Porkchop
    Just rode home... still can't believe it. It's a completely different bike. I'm starting to wonder what the bike would sound/perform like with the whole baffle.

    What I do miss is the crackling and popping during deceleration. It's almost completely gone. I did have one really loud "pop" last night which was odd.
    Does that pop sound like a car backfiring? If so, my bike used to do that as I was coming to a stop at lights every once in a while. I believe I was relying on engine breaking a bit too much, because once I started to pull in the clutch and hit the break earlier it hasn't happened.
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  • 06-16-2013 | 01:25 AM
  • Porkchop
  • Quote njchopper87
    Does that pop sound like a car backfiring? If so, my bike used to do that as I was coming to a stop at lights every once in a while. I believe I was relying on engine breaking a bit too much, because once I started to pull in the clutch and hit the break earlier it hasn't happened.
    Yep. I use a TON of engine braking. This thing has always had it in boat loads. Oddly enough, it has diminished a bit since I made the changes, actually more smooth.
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  • 06-16-2013 | 01:51 AM
  • azoomm
  • I would suggest contacting Leo directly and see if what you have can go to it's intended install. Let me know if you need a contact...
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  • 10-06-2015 | 09:58 PM
  • harley_cruiser
  • It sounds like without the baffle you were getting reversion. Reversion is when the shock wave travels down the pipe, then back up and hits the valve when open at certain rpms. If it hits when the valve is open it keeps exhaust form exiting, and keeps the intake gasses from going onto the cylinder.
    On real bad cases it can even fart out the carb.
    The baffles break up the shock wave and smooths it out so that there is not a reversion back up the pipe.
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