| Author | Message | ||
     Midknyte |
W/o degenerating into just a post of this muffler vs. that muffler (please) - what are the benefits of reducing backpressure by using an aftermarket exhaust? Ok, to some extent, the answer is obvious. But I thought I'd pose this as a thread hoping that we could get into some detail about what happens when there is too much or to little muffler action. Feel free to wander off topic and interject nuggets about ceramic coated or wrapped headers. | ||
| Glitch |
I can't find it now, it's in the knowledge vault somewhere. Aaron Wilson wrote a very nice article on how exhaust works. I'm still looking. Header wrap keeps the heat in the pipe, making the gases exit faster, therefore helping cylinder fill at overlap. I haven't noticed any difference, but it sure looks cool! | ||
| Kds1 |
Most will argue my input because it goes against what we've all been taught.....too much backpressure=loss of power.....too little backpressure=loss of power......knowing how much and when is the key, the rest is just paperwork and formulas that you can control on paper but not in the real world......I apologize if I'm starting the wrong way, but this is how my exhaust works....wrapped headers for me anyday, heat moves faster than cold, ceramic is more vulnerable to heat loss through passing air than the wrap, put a layer of ceramic coating in your attic and let me know how much heat it holds.... I have no formulas , papers, or scientific data that i can understand to back these comments up, but I know my muffler works..... Kevin www.kdfab.com | ||
| M1combat |
I know his muffler works too  . WRT scavenging, there are two sets of pressure waves. One is generated by sound, the other by changing exhaust gas velocities. The trick lies somewhere in getting the negative pressure waves to arrive back at the exhaust port as it opens. This will help to pull the spent mix out of the combustion chamber and pull combustible mix into the combustion chamber. Well, that's the EXTREMELY simple answer anyway... I seem to remember an excellent post in the KV on the subject as well... | ||
| Midknyte |
The trick lies somewhere in getting the negative pressure waves to arrive back at the exhaust port as it opens. This will help to pull the spent mix out of the combustion chamber and pull combustible mix into the combustion chamber. I can understand that concept. Reading into that, it's not a matter of simple backpresssure. I had thought of the muffler being primarily a noise suppresser. Being a trade off of between it's silencing ability and therefore the backpressure a tradeoff for noise reduction. This puts it in a new light. So actually, a well designed muffler is actually helping pull exhausted air out of the system. With this in mind, is that why too little back pressure (or even straight / open pipes) would be power robbing as well? | ||
| M1combat |
They aren't power robbing "exactly" . They work very well in a narrow RPM range. That's pretty much the job of the exhaust system is to "scavenge" the burnt mix out of the combustion chamber. You can't really make an exhaust that will work "perfectly" for the entire RPM range though (although Buell did one heck of a job with the stock 12 exhaust). You need to decide what range you will be using and tune your exhaust for that range. My preference would be tuning more so for the lower end of that range because it's better to gain speed early than late. I have two pipes... A Drummer and a Force. The Drummer makes about 12 more HP under 3000 RPM. It's WAY better for the street. The Force makes about 7 more at about 4000. The Drummer then passes the Force again as you cross over about 4800 RPM. I would use the Force on the track because on the track you woudn't really ever be dropping below 3000 anyway. I don't like the Force for the street because when I'm leaving a street light I typically have the clutch fully released by about 2300RPM. The lack of power until I get to 3000 is VERY annoying on the street. Of course, I can feather the clutch for a proper "launch" but I don't want to have to do that on the street. That said... If the track had multiple longer straight where I wouldn't be dropping below 4800 much then it's back to using the Drummer for the track as well because it does make more power than the Force on top too... I understand you weren't interested in a this vs. that debate, but I was just trying to illustrate that the design of a pipe needs to be heavily dependent upon what you will use it for. | ||
| Lucas70374 |
I can relate to what M1combat is saying about the Force to drummer, it is very annoying with that power loss to 3k. I never had a drummer but I did have the D&D and I am noticing a huge difference between those to and the Drummer is suppose to be that much better in low RPMs then the D&D. Very good M1combat | ||
| Tq_freak |
the muffler does regulate and promote exhaust exit speed, a great example of this is the new corvette, next time you see one in a parking lot look inside the exhaust tips, the engineers went as far to dimple the inside of the tips like a golf ball, this in turn takes a lot of the turbulance out of the exiting gas's hence making the spent stuff leave faster. (thank you thermodynamics class ) | ||
| Blake |
If by "back pressure" we are describing the resistance to flow inherent in the exhaust system then there is only one correct answer: The less back pressure the better. But that is not the only exhaust system parameter affecting engine performance; in fact in some cases it may not even be the primary factor affecting engine performance. An engine exhausts hot combustion gasses not in a steady state continuous flow, but in discreet violent pulses. Each exhaust valve opening and closing event defines the timing and duration of each exhaust pulse. At the front of each of those pulses is a high pressure wave front where extreme combustion chamber pressure first encounters the much lower pressure inside the header pipe. At the tail end of each exhaust pulse is a low pressure wavefront, a "trough" if you will. Whenever these pulses both high and low encounter a discontinuity (any change in surrounding geometry) a portion of their energy will be reflected back up the exhaust track towards the exhaust port. This is similar to the behavior of waves in a body of water that encounter obtacles or discontinuities in boundaries. The reflected portions of exhaust pulse energy can either assist cylinder evacuation (low pressur epulse) during the exhaust event or they can hinder it (high pressure pulse). But most importantly there is a tiny window of opportunity as the exhaust valve is about to close where the intake valve is opening. This is the "overlap" of the exhaust and intake events. If the exhaust tract is designed such that a reflected low pressure (suction) pulse arrives back at the cylinder head's exhaust port during the overlap event, it will actually assist in evacuating spent combustion gasses and even help to fill the combustion chamber with new incoming air/fuel charge. It acts like a vaccum cleaner sucking up the spent combustion gassses. If however a high pressure pulse arrives at the exhaust port during the intake/exhaust overlap event, it will hinder evacuation of spent combustion gasses and even push new incoming air/fuel charge back out of the combustion chamber. A high presse pulse then act like a blower trying to push spent combustion gasses back into the combustion chamber. Not good.  So why all the hoopla and competition in aftermarket exhaust designs? They are all free flowing. They all seek to put the reflected low pressure pulse at the exhaust port during the overlap event. So why are the results different? The difference is in the intent of the muffler designer and where he/she seeks to optimize this low pressure pulse performance wrt engine speed. See, the exhaust pulses travel at virtually the same speed, no matter the speed of the engine. Regardless of engine speed (RPM) the pressure pulses of the exhaust take a set constant time to travel to discontinuities in the exhaust tract, and then be reflected traveling back to the exhaust port. It is only when the duration of the reflected exhaust pulse round trip coincides with engine speed such that the negative pressure pulse arrives during the overlap event that we obtain optimum performance. Muffler designers are basically configuring the geometry of their products to achieve that optimum performance for engine speeds which they feel are most important to their customers. They also may utilize varying geometry (megaphone, stepped pipes, muffler valves) to achieve a wider more consistent range of optimum performance across the operational range of engine speeds. That is the challenge. And no one system will provide optimum performance across the entire rev range of the engine. Not yet anyway. The optimum design would be a perfectly free flowing straight pipe that could change its length with engine speed, starting out longer, then getting shorter as engine speed increases, thus always causing the low pressure trough/pulse that is reflected back from the tailpipe opening to arrive at the exhaust port at the most opportune optimum time. Like a sliding trombone. Freer flow is a start. Optimized geometry is key. You can actually significantly improve performance of an engine at a certain engine speed by strategically placing a small obstacle (geometric discontinuity) within the exhaust tract. It is not the added backpressure (increased resistance to flow) that aids engine performance. It is the low pressure wave that reflects off that obstacle that is aiding performance. A stepped or megaphone style straight pipe is the absolute optimum configuration for an exhaust tract as they each provide a number of discontinuities optimally configured for the desired powerband while avoiding serious impediments to flow. They are also horrendously loud. Pretty sure there are animated gifs on the web that illustrate this whole situation. Maybe someone can find one and post a link. | ||
| Glitch |
Pretty sure there are animated gifs on the web that illustrate this whole situation. Maybe someone can find one and post a link. You knew I'd take that bait, didn't ya!?  Any way I was mistaken, it wasn't Aaron Wilson, it was Blake. Thanks for that Blake, I love the way you explain things to where even I can understand. On to the animation. I've had this one for a while, just needed a brain-jog. While this is a two-stroke, it's a very good illustration to what Brother Blake has explained.  The main difference you see are that a two-stroke, needs the pulse to push unspent gases back into the combustion chamber, because of the lack of valves. Our four-strokes would need this pulse timed to have it at a "pull" to help with scavenging, and cylinder fill. | ||
| Kds1 |
Our four-strokes would need this pulse timed to have it at a "pull" to help with scavenging, and cylinder fill. I'm sure glad my cams have that ground in them, lobe centers, separation, and overlap...don't forget their job...you can run one all day without a pipe but no cams, we got problems... | ||
| Drift |
HeHe!!  you said strokes.... | ||
| Blake |
Glitch, Pretty sure Aaron has posted on the topic here before. Couldn't find a four-stroke one eh? heheheh Didn't have you in particular in mind, but I was hoping someone would google up an example. Good point Kevin. One muffler might work great with a particular set of cams but not so great with another. It's all one big happy system, or it can be unhappy if there is just one uncooperative link in the chain. Not sure I'd say that the cams have the exhaust tract's pulse/reflection characteristics ground into them though. Gotta have both for optimum performance. You just want to keep all this secret don't ya? | ||
| Glitch |
It's all one big happy system, or it can be unhappy if there is just one uncooperative link in the chain. Yep, one can't have a top performer, and only work on one end of the engine. | ||
| Midknyte |
Thanks! I had thought it was all just a simple trade-off of power loss for noise restriction. Very good and understandable answers. If I may - could you extend my understandings here to include how catalytic converters work and effect this system? I know what they do, but not really how. | ||
| Glitch |
Well since catalytic converters rob power, weigh a ton, and my bike ain't got one, I'll point you here. | ||
| Jerseyguy |
When folks extol the virtues of opening up their exhaust as much as possible I usually suggest they remove the muffler and header completely & run their bike that way. That generally causes them to reflect on how there may be something just a bit more complex going on here.  | ||
| Blake |
http://www.rbracing-rsr.com/exhausttech.htm Good explanation of things exhausting. | ||
| Mikej |
rbracing used to have some info on their site about Buells and some history, but that info appears to be gone now so I won't repeat it. They do good stuff though. | ||
| Steve_larson |
Have to admit, this is some of the most intelligent discussion we have ever seen on a web forum about exhaust. Blake, yours was the most lucid and concise description we have seen in a short response. We are impressed! We particularly like the part in the RB Racing article about Hydroforming, "....an increasing volume situation for people who are not going to build a $100,000.00 hydroforming die to turn a bent primary tube..." Maybe you Buell owners now have something available to you that others in the tuning world only dream about! I would like to add my thoughts on the top question in this thread, as it seems to have evolved from a question about mufflers to one of header/muffler design. As has been pointed out, it is not about "back pressure", but wave action, both in the header AND in the muffler, which is part of the header for the Buell. So to answer the original question, the choice any Buell rider should make about muffler changes should be viewed in light of the overall header design and desired use, and how the change effects the wave action, and thus the ending power output. Too many people want to know "how much power does it make", when the real question is "what kind of power does it make?" A "number" means almost nothing except to coffee shop racers who want to think they are going fast. So if "x" dollars gets you more peak and less low end, or "y" dollars gets you more low end and less mid-range, these are all choices you have to make about how much you are spending, how you ride and what you are getting for a power curve. Good racing systems (or slip-ons if that is what one is looking to buy), should not lose power ANYWHERE. Why would somebody buy a product that takes away something they already have, unless you are headed to the Bonneville Salt Flats? Most performance riders are willing to give up a couple way down low, you don't spend a whole bunch of time at idle when performance riding. But from there on up you should want to keep at least as much (if not more) power then stock as you head to midrange, and then get a big hit in the middle and top end. That is what performance riding is about, and the exhaust should work as such. Rolling into the throttle should get you clean acceleration, followed by big, solid, flat and linear power delivery so all the rest of the parts can work with you. (Imagine nothing and then 120 hp instantly, you would not be going faster, you would be heading for a crash!) Generally speaking for the real world, we feel that the bigger and flatter the curve, the more interest one should have in that design. Conversely, the more the "flat" torque curve is being "altered" the more one should investigate further to see if the losses are worth the gains as they relate to your riding style and purpose with the bike. When everything is made to work with each other, the graph should reflect a solid, strong and flat torque curve, which means you have a good combination. And that combination should be what every rider is looking for no matter what part of the exhaust system they are trying to upgrade or replace. Steve Larson Customer Support Micron Exhaust | ||
| Kds1 |
Steve, That was so well laid out, way to go!!!! Some use fancy equipment, some use what comes on the bike.... Kevin www.kdfab.com | ||
| Blake |
Steve, Thank you for such a very kind review.  Kevin, So true. It is the results that matter. | ||
| Phantom5oh |
You weren't kidding Blake, that site gives great info with clear examples! Thanks for sharing! |
