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· Bald headed dork
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I guess I'm a little confused here. I'm planning on helping a couple of friends put new slip ons on their bikes this weekend. One of the guys is almost afraid to do it because he's afraid it'll cause his bike to run way too lean and cause engine damage. The other guy just laughed and said all the end can does is muffle the sound. We're not changing anything on the intake side, so it's not taking in more air that could cause a lean condition. I definitely agree with the second guy. I mean after all, isn't the end can just simply a muffler? I fail to see how putting a different muffler on a bike will cause it to run lean. I think it's one of those old myths that never seems to die. I've had slip ons on other bikes and haven't rejetted them and they were fine. Granted, a slip on AND a jet kit will work well together, but all we are changing are the mufflers at this time. The jet kits will have to come later as funding allows. In the meantime, we're just changing the mufflers.

So my questions to you engine experts are these:

1. Does adding a slip on muffler to a bike make it run leaner than it does in stock trim? If you think so, tell me why and give proof.

2. Isn't the end can just a muffler?

3. Simply changing mufflers shouldn't cause a bike to run lean should it? Why/why not?

4. Even if the new muffler is less restrictive than the stock muffler (which almost all aftermarket ones are) how and why will this cause a lean condition if nothing else is changed on the bike? Give proof for your answers.

Thanks for your time fellas, I'll make sure to take a printout of this thread to my buddy's house this weekend to settle our disagreement.

Scott
 

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You answered your own question...if it is less restrictive then yes it is flowing more air and that results in a lean condition. Proof is in the logic-just think about it. Ever put a pipe on a dirtbike? When you do you have to almost always richen the fuel/air mixture so it runs correctly. Besides most bikes are lean from the factory to meet EPA standards. That is why almost all ducatis pop and spit at idle in hot weather-they come here extremely lean. Most likely merely by adding slip ons to the R1 your friend will not notice and drivability issues except in abrupt throttle openings off idle where it may not pull as cleanly as it would if it were adjusted.
 

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446 Posts
You got to think about back pressure you’re loosening there using a less restrictive pipe. Also the size of the pipe & baffle counts. If it’s just a slip on you can probably get away not using a jet kit.
 

· Premium Member
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9,465 Posts
Just put the slip-on on. If it runs like crap after that then the bike had problems BEFORE the slipon install and needs to be "tuned" up anyways.
It's when you get to full exhaust everything changes
 

· DAMNED HELICOPTERS !!!
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411 Posts
This has rolled around my head in the past,i agree that a less restrictive exhaust [either a full system or just a can] will increase airflow,but, surely the "extra" air is still being measured either by the carbs venturi or the EFI mass airflow sensor and would thus provide the extra fuel ????Like the guy said PROVE or at least explain it to us.
 

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538 Posts
With an aftermarket slip-on or full exhaust system, you are basically increasing the flowrate of the exhaust. You are increasing the size of the piping plus you are reducing the restrictions in the piping to increase velocity.

With this increase in velocity, you increase a sort of "vacuum" during the overlap between the exhaust and intake cycle. That draws in more intake charge.

The other portion is that with most production combustion engines, the exhaust is the restricting portion of the system because of noise restrictions.

You need to think of a motor as an air pump. It can only ingest as much as it can expell. The intake can only suck in as much as the exhaust allows to escape. With the aftermarket slipon or full system, you are allowing the exhaust to expell more.
 
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