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Discussion Starter #1
Please jump in with any and all ideas, I've got to try something, and don't know where to start. I've ridden my TX500 to work several times now. Replaced stock exhaust with some shortie megs from Mike's XS that look awesome, are way too loud, and have raped my mid range torque. The bummer is I never rode this bike in stock condition to have a baseline to compare to.

This weekend I packed the mufflers with fiberglass, and cut another piece of fiberglass that I placed upstream from the spark arrester, hoping that would kill some sound. I'm sure it must have added some back pressure as well.

Well on the ride this morning, the pipes were not noticeably any quieter, but the bike ran and responded MUCH better. A big enough difference to rule out "mind tricks". I found some low-mid range grunt somewhere. (I had a hard time finding any power before 5500-6000rpm, and cruise to work at about 4500. Now, there's all kinds of grunt from about 2500 to 5000 - I didn't spend any time higher than that today.)

Now, these shorties are about 12" shorter than the stock exhaust was and completely open. So I'm assuming my engine likes to have at least SOME back pressure to run right. Carbs have NOT been rejetted, as I have no idea which way to go.

Anyone have or know of a good resource regarding exhaust tuning, the process of rejetting, etc? I promise I combed the forum before asking, but that doesn't mean I didn't miss something.
 

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As I understand it, you have a a couple of trade offs. First, at the end of the power stroke and beginning of the intake stroke there is allot of exhaust gas pressure. Because of the cam overlap both exhaust and intake valves are open simultaneously. The shape of the pipe needs to give the pressure pulse a way out. Too much back pressure and the pulse attempts to return to the cylinder thereby compromising the fresh intake charge with hot spent gasses. The flip side is a system that flows too much. In that scenario typically, pipe diameter is too large and results in reduced exhaust gas velocity which in turn makes a quality intake charge more difficult due to a reduced "scavenge effect" during the overlap.
So basically you need to split the difference between high flow and enough back pressure to ensure decent scavenge on the intake charge. Megs attempt to do that. Sounds like your set up wants a little more back pressure, I'd add some length to the pipe before the meg.

This might be BS as I have not read:
http://www.bentleypublishers.com/gallery.htm?code=G309&galleryId=1483

cheers, bcr
 

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73tex, Just got back from the kiddos concert. My earlier post made no mention of carburation and that is a mistake. I high flow exhaust system will move allot more air than stock. You will absolutely need to richen your mixture. What do the plugs look like? I'm betting you're lean.

have fun with it, cheers. bcr
 

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I cannot understand your verb-age and the things that confuse me are the two terms, "spark arrester" (which you packed with fiberglass) and "12 inch shortie"
 

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and also I do not understand where you packed this fiberglass "upstream from the spark arrester"
 

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Megs have a specific function which works with a specific engine, cam, rpm range. If you filled some of the volume of the megaphone section with packing...then you altered the volume of the meg, and altered the way the meg handles the exhaust pulse wave and reversion wave. Which could totally alter the rpm range at which the exhaust works efficiently, or create better efficiency, or destroy all efficiency.

Forget "back pressure" that's a laymans term...no engine ever runs better with a positive reverse pressure in the exhaust system. What makes it appear that way is that often an "open" pipe will have very strong pulse characteristics, either good, or bad, or very narrow in range. Often stuffing something in/on the pipe disturbes the way the pipe handles the exhaust pulses, which could make the engine appear to be responding with increased power to some restriction in exhaust flow...actually all it's doing is changing the dynamics of the exhaust to an rpm range that you might or might not find more usefull.

In other words an short, open meg might make great power at 14,000 rpm, but unfortunately your engine will blow up before that, so you run it at 10,000 rpm, where it appears that the open meg runs like shit. So you stuff something in there, which changes the dynamics of the way the system handles the exhaust pulses....now it makes power at 8,000 rpm. A layman says...wow, the engine needed back pressure....no....the engine just needed a system that wasn't narrowly tuned to 14,000 rpm. Now with something stuffed in there, instead of having a pipe that make tremendous power at 14,000 and crappy power at 8,000, you have a pipe that makes acceptable power at 8,000 and freakin zero power at 14,000. For the racer....a net loss in power.... for a regular bike....it's more "usable" power on the street. And this phenomena makes the average person think "back pressure" helped the engine make more power. When actually it makes less power...just makes more power where you happen to want it....this time. Might be different next engine, next pipe etc.
JohnnyB
 

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uhh.....what he said. now, if you'll excuse me I'll be in the personal lubricants isle. cheers,bcr
 

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I was thought that exhaust pulse was more effective with two strokes than 4 strokes. I'd love to read that book - I've been having trouble sleeping lately.
 

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Johnny is right as far as he went. Headpipe length is also a factor. Generally speaking the longer the headpipe the lower the RPM where HP becomes useful. You may want to try an extension to your headpipe that will move your muffler farther back. You may gain the advantage of lower RPM power and moving the exit of the exhaust farther back so you don't hear as much noise. The neighbors will still hear it all just fine.

Ken
 

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Discussion Starter #11
Hmmm. Well jbranson has just effectively labeled me as an average layman. I've been discovered.
Ok, to clarify in pictures what I lack in descriptive ability:

Stock exhaust -


New exhaust -


Here's what I'm trying to explain (one of the bonuses of being stuck in front of CAD all day, I can fake like I'm actually working)
Mock cutaway of muffler -


Explanation of "flash supressor" -



Where I wrapped the fiberglass -


And where I may have done something stupid, but seemed to help -


From your discourses, it makes sense that Steve's TX500 probably runs quite well relative to the stock setup. Same or close to same length and diameter. (After cutting the stock exhaust up, you see that it's really a small diameter tube inside the larger outer tube, so Steve's looks very close to what's actually inside the stock exhaust.)
 
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