Let's talk about exhaust, straight pipes, baffles, bogging, and back pressure

the_nutt

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The only problem I have with my '72 XS2 is that it bogs a bit in the mid-rpm range when accelerating anywhere about 1/4 throttle. I know that most bikes will bog if I try to WOT from low rpm's, but I don't think this bogging is normal.

I'm running Mikes cone filters and straight pipes. Mains have been bumped up to #140's and a plug chop shows correct mixture. The bike also starts first kick every time, and idles like a dream. Occasionally when it is really hot it will idle a bit high, but just steady around ~2,000 rpm, not runaway idle.

I bought baffles like these from Lowbrow (attached image). When I first put them in complete the bike would barely run. Too much baffling I assumed. I slowly shortened the baffles to where the red line is in the picture. It now runs with that much baffle, and the bogging is minimal now, but it sounds terrible. Kindof sickly.

What are you guys' experiences, what are you running? Can I have a nice sound AND proper back pressure without a muffler, or will I have to break down and rethink my exhaust system?

I'd like to hear how your bike performs, what exhaust you have, and what your jetting is if possible. Many thanks, gentlemen.


Cody

TL;DR Straight-piped XS2 runs great but bogs mid rpm range, baffles sound like shit. Have you found a happy medium?
 

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You might try experimenting with the length of the pipes. Very short pipes are the worst of the worst - and that's what I see on many of the builds here. They just hack the mufflers off and leave the pipes ending by the footpegs. Another option that I've seen NOBODY utilize might be the chrome muffler tips from VW Beetles. These are actually mini mufflers, straight through but with a louvered core backed by some sort of sound deadening material. To me, it seems they may be ideal, giving you all you're after - increased length, back pressure, and some muffling.
 
The only problem I have with my '72 XS2 is that it bogs a bit in the mid-rpm range when accelerating anywhere about 1/4 throttle. I know that most bikes will bog if I try to WOT from low rpm's, but I don't think this bogging is normal.

I'm running Mikes cone filters and straight pipes. Mains have been bumped up to #140's and a plug chop shows correct mixture. The bike also starts first kick every time, and idles like a dream. Occasionally when it is really hot it will idle a bit high, but just steady around ~2,000 rpm, not runaway idle.

I bought baffles like these from Lowbrow (attached image). When I first put them in complete the bike would barely run. Too much baffling I assumed. I slowly shortened the baffles to where the red line is in the picture. It now runs with that much baffle, and the bogging is minimal now, but it sounds terrible. Kindof sickly.

What are you guys' experiences, what are you running? Can I have a nice sound AND proper back pressure without a muffler, or will I have to break down and rethink my exhaust system?

I'd like to hear how your bike performs, what exhaust you have, and what your jetting is if possible. Many thanks, gentlemen.


Cody

TL;DR Straight-piped XS2 runs great but bogs mid rpm range, baffles sound like shit. Have you found a happy medium?

Have you done any searching on the forum for this topic? I know I have seen a few pretty extensive write ups all about exhaust and back pressure / back pressure myths...

I pretty sure Gordon Scott who makes some really nice systems for these bikes jumped in on one of the ones I read as well.

I don't have any experience with baffles myself, but thought I'd let you know I have seen some good write ups about this topic on here.

Ps I'm in SA...cool to see some locals on here
 
I just noticed your pods. They could be contributing to the problem. These CV carbs don't work that well with the pleated K&N type pods. Switch to UNI foam pods and your tuning headaches may disappear.
 
I'm no expert, but I have successfully made many exhaust systems, and I have successfully re-jetted many carb sets. You really haven't provided a lot of carb info, but I suspect your initial bogging could have been cured by leaning the slide needle. I also think that short pipes need a lot of baffle. I think that you need to do a total re-jet.
Come on by, lets do our trade and talk about your exhaust and carbs. Here is a link to an exhaust I made that works pretty much perfectly in conjunction with the carbs that I re-built:
http://www.xs650.com/forum/showthread.php?t=26822
 
You might try experimenting with the length of the pipes. Very short pipes are the worst of the worst -

I thought about this, but wanted to discuss it before I went in on a new system setup.

Switch to UNI foam pods and your tuning headaches may disappear.

Also a change I've had in mind. Might not have patience for mail order.. probably going to go to Cycle Gear tomorrow and pick a set up.

Come on by, lets do our trade and talk about your exhaust and carbs. Here is a link to an exhaust I made that works pretty much perfectly in conjunction with the carbs that I re-built:
http://www.xs650.com/forum/showthread.php?t=26822

Sounds good, I was going to call you tomorrow anyway.
 
To tune the length, find some pipe that will just slip over your current pipe and with some slits in it, clamp in place. Start with a pipe 1 or 2 feet long and cut 1" at a time off and test. If you can get it to slide on your pipe like 3", then you can cut 3" at a time off and just slide the pipe on more or less to test the lengths in between. Once you figure out what works best, then you can have new, pretty pipes made up to that length.
 
Drag pipe sag, yeah baby. I like their ".... easiest cure for drag pipe sag ..... " - they threw them in the dumpster, lol. Good place for them, the best really. This is a case of "You can't have your cake, and eat it too". You may run drag pipes for the "look", and you'll get it, but you may never get the bike to run really well with them. And even if you do manage to get it running smoothly throughout the entire RPM range, you'll be down on power at some points compared to a proper exhaust with good mufflers. All this seems to fall on deaf ears though, just like the warnings about using cheapo pods. Every couple days there's some new guy posting who sawed off his mufflers and slapped junk pods on, and now his bike runs like shit, imagine that, lol.
 
I enjoyed that video, and I learned from it, but I feel that in an effort to push his products he left some things out. For example, torque cones. Or a less restrictive baffle -- he admitted that the baffles he tried were the worst ones.


On my Sportster I have the same S&S Super E, drag pipes, torque cones and thumbscrews, and I'm happy.
 
I just watched the video again. He puts the drag pipes on, but does not put bigger jets in. Of course it's going to run terrible. He just put a free-flowing exhaust on without increasing the jetting, so it's lean. Then he does the (excellent) slinky/reversion analogy. He then explains that during the reversion incident, the engine is running rich. So he increases the size of the air bleed in the carb, further leaning things. Then he puts leaner jets in. Hey, he's an expert, and I'm just an ignorant schlub, but I think that is a deceptive 12 minute commercial.
 
We ran into this CONSTANTLY back in the `70s. Had to do the lecture about 'reversion', using pipe organ explainations, 'till we were blue in the face. These bikes, including the earlier XS1s, have cams that were considered 'hot' by hot-rod standards. Had one bike that had 'reversion' so bad, the pulse would travel back during cam 'overlap' and created a floating 'fog' of inlet/fuel in the carb bell. That bike had ZERO power at 4000rpm - 55mph/5th gear.

For some reason, the customers weren't buying it, kept insisting on running cut-off pipes. Like a rumor that won't go away. Still think it's an old racer trick, tell a flawed speed 'secret', throw off the competition.

There's enough stories about this to crash Travis's server, and I'm tired, so we'll stop here...
 
Okay, one last post, and then I'll shut up about this. The thumbscrew works pretty darn well in the video. Not as good as the mufflered exhaust, but close. Loud pipes are important to lots of people, and I think they would find the thumbscrew curve an acceptable trade-off of performance vs. loudness. If the guy had kept at it, and added torque cones and worked on the jetting, I'll bet he could have ended up with a much better curve.
 
When I built an xs with similar pipes, no filters, I had no bogging... I did however gain a noticeable amount of power through the entire rpm range by swapping to longer pipes with mufflers.
 
Extracted from the Minton Mods article at 650Central. Although experimentation is fun, it's wasted effort to go about re-inventing the wheel while hordes of them roll down the street past you. Folks with more equipment and more motivation to get it right have already been there. Just saying, looks sure as shit ain't everything, and if I'm putting my internal engine components at risk for early failure or some type of reduced durability, it's off the list!

http://www.650central.com/

EXHAUST: Although the 650’s exhaust system proved to make excellent power at higher rpm (more about that later), it also killed power in the 3,000 to 4,000-rpm range. Even before we took the bike to Jerry Branch’s dyno, it was obvious that the 650’s low-end torque and response could be improved, so we tried some aftermarket exhaust systems. We tried all the variations we could find: two-into-ones, replacement mufflers, and one set of replacement header pipes. They were all unsatisfactory in one way or another. The two-into-ones reduced ground clearance on the right so much that the bike was not fun to ride. Besides, they really didn’t improve the power enough to overcome the ground-clearance objection. Replacement mufflers had no chance to succeed; for, as we sawed the factory mufflers from the stock header pipes, we found the source of the stock exhaust’s anemic behavior: the inside diameter of the stock pipe is just 1.125 inches. Racing versions of the XS are regularly fitted with pipes that are a full 1.75 inches in diameter. The replacement headers were excellent: they were a desirable 1.5 inches in diameter and 33 inches long—almost perfect. The problems were that we could not find a satisfactory muffler to go with them and their exhaust flanges broke because they were too fragile. Through quirky circumstances we found a manufacturer who was willing to make exhaust pipes to fit our specifications: Bub Enterprises in San Jose, CA. We got our exhaust pipe specs from available literature regarding such things and from Jerry Branch, who is the most successful horsepower-getter in the motorcycle industry. Jerry agreed with our 1.5-inch-diameter-by-32-inch-length design, saying that it would probably give the XS650 the kind of performance we were looking for: excellent mid-range torque and the broadest possible powerband. The design later proved to be a complete success.



Bub Enterprises quickly made up a prototype pipe and rushed it through the chrome shop so that we could take the photos you see in this article. While he had the bike, he also made up a set of two-into-ones that were of the same diameter and length and would not drag. This is the pipe that is on the bike in the lead photo. Bub felt that many XS owners would prefer the slightly less expensive pipe even if the two-into-two proved to give better performance. Both Bub pipes were designed to give the greatest possible ground clearance and will clear the right engine case enough to allow the installation of the Lockhart oil cooler fittings, something the late stock pipes will not allow. The Bub two-into-two is fitted with Bub’s Norton-Commando replica, straight-through muffler. It is made of .065-inch-thick tubing and is well chromed. Although a little loud, the louvered construction of its silencer tube gives the exhaust a very pleasant sound that fairly shouts, "motorcycle!"
 
"Back Pressure" - One of my favorite buzz terms. Sorry, just have to put this in here...

Late 60s - early 70s, H*ndas starting putting that warning sticker on the frames near the mufflers, "altering mufflers will affect backpressure and void warranty", or something like that. Our h*nda manuals had some translation issues, somewhat amusing, like the modern day instructions: "For best happy fun, use not for the other use". I'm sure some of you are still head-scratching over those.

The way we understood it, from the way H*nda tried to explain it, was that they had come up with a muffler design that effectively mitigated reversion pulses, and marketing wanted to tout this feature, and manufacturing wanted to preserve it (with the warning sticker), but the term 'reversion' hadn't made to the translation books, and for lack of a better word, coined the term "backpressure".

Enter the fascinating world of the "grapevine rumor/misinterpretation/mutilation machine" and opportunistic entrepreneurs. Suddenly, products appeared touting increased backpressure, by actually restricting flow through various types of orifices, slits, slots, mazes, ...etc. You could even increase horsepower 25% by simply cramming a potato into the tailpipe! Order now!

The world would be a boring place without this stuff...
 
I had a suspicion that the bike wouldn't run well with these open pipes. After riding it around for a couple days with baffles I've found I like it being a bit quieter than when it was open headers. I think I'm going to sort out a couple of mufflers. Might use the stock head pipes, might not.. we'll see.

Anybody have experience with these stainless steel type universal mufflers?
 
A 12" muffler isn't going to muffle much, it's too small. They certainly look nice but I'm sure they'll be quite loud.
 
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