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Hey all,

I know one of the members did something to the stock muffler so that it's louder and lets more air through. I was wondering if someone could direct me to where it's posted, or post here how to do it. Thanks,

Pete
 

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THE JUDGE
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711 Posts
I did the mod to my pipe before I got my D&D.

My husband simply took a LONG screwdriver and punched holes in the baffle. He just punched a bunch of holes randomly. Honestly I can't say that it made a big difference. My husband said he could hear a difference, but I couldn't. (of course he knows what to listen for more than I do) I do know that the vibration at 4800 seemed to decrease when we did this though....so, who knows. Try it....you can always ask for a slip on for Christmas if you don't like it.

That is the one problem with doing this....once you've done it, there's no going back without buying a new stock pipe and I think they are almost as much as a slip on....go figure!

There are always many opinions on this. Some say it hurts your bike, and others say it doesn't do anything harmful to the bike....so, take each, and decide what you'd like to do!

Hope this helps!
 

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Silver Member
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Yep I did the same thing, belted the centre circle out of the baffle with a cold chisel, then folded the edges back to give it a clean finish.

I would say there is no increase in noise whilst revving it, but a nice burrble on the overrun.

What I did get was a distinct improvement in revability from about 5000rpm. The thing really spins now. I'm no engineer but having the baffle ( effectively a plate with a zillion tiny holes in it ) removed has to improve flow and eliminate a lot of backpressure.

I would love to throw on a Yoshi or similar but leaving my building at 6.30am most days this compromise keeps the neighbours happy.
 

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THE JUDGE
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Luckily, our neighbors have bikes themselves, so they don't care. That's REALLY nice when I want to ride the Harley to work...talk about LOUD!! I told my husband, if I ever had someone complain about my pipe mod on my ZR7s and say that it was illegal and they'd report it, I'd simply tell them I will just start riding the Harley when I leave my house at 5:45 in the morning. It's TOTALLY legal and WAAAAAAYYYYYY louder! haha
 

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I asked the service mgr at my local dealership about this mod and he recommended against it. He said that the bike needs a certain amount of backpressure and this mod takes away from that.

Question - Doesn't a slip on increase the air flow and take away from backpressure.
 

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Dirty Harry.... Moderator
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I think so, Sandeep. When I took the Yoshimura slip on out of the box before installing on my ZR, I noticed that you see right through it unlike your stock pipe. I've been informed on this great forum that the next step is to remove the EGR assembly to stop the "popping" sound you get when downshifting or throttling down. I haven't done that yet nor have I re-jetted the carbs, but my 7 runs like, as my Dad used to say, "a scalded-*** Ape." I still have the stock pipe in my garage, "untouched" in case I need to re-install it for "warranty" or sale reasons. In my humble opinion, I wouldn't go "stabbing" the stock pipe. But then again, I paid $280 for the Yoshimura round race slip on. Hope this helps some, Tom :)
 

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Yep I agree you need a certain backpressure, but I believe the baffle is only placed there as a quick/cheap way of getting past EPA regs. Like here in aussie I think the noise level can't exceed 100dba at 50% redline, so they baffle the thing to keep the noise down.

The stock muffler has a zig-zag internal plumbing anyway so a certain backpressure is retained. I guarantee though that by removing it you will see a marked improvement in revability from about 5000 revs, and externally no one can tell you have been fiddling with it.

:)
 

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i did the same thing as titomike. i bought netlore's old stock muffler and chopped the first baffle screen out with a large screw driver and hammer. folded the leftover edges back.

the exhaust sounds similar to stock but with a much lower note and just a little louder, more growl, less wisper. i'm sure i could start it up at 6:00am and not wake the neighbors. no rasp like a high performance slip on. exactly what i wanted :)

5k vibes are stilll there, feels like a few more vibes than before below 5k but nothing to get in a tizzy over, idle is still silk smooth. i also noticed it seems to rev better in the upper half. 4th and 5th gear freeway roll ons seem to have better acceleration.

the remaining baffle is a zig-zag tube so i'm sure there is still plenty of back pressure, more than any aftermarket pipe. since some of the aftermarket pipes say you can install them without re-jetting i'm not worried about it running too lean.

and since i used a second muffler i can go back :~

scott :)
 

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Ok I run a carbon slip on now, but the thing I did like about knocking the baffle out is externally no one can pick it. EPA, COPS, Insurer, Dealer etc. No one :D

:)
 

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Hey, Andy, your photo makes it look a little like a Velocette ...
 

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titomike, i like that too. not only llooks but sound is stealthy since it's not that loud. same thing with bigger jets which i may also do :) the idle mixture screws, well, at least they have to dip their head down and take a look ;)

scott :)
 

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Wheelie for Safety
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Good question Robert.

In reality you don't. You can get the most horsepower if you didn't have any exaust system at all. However that would be impractical because of the noise and pollution spewing out under your feet.

For the reason above we need to have some sort of pipe that directs the pollution away from us and to lessens the noise. The introduction of a pipe ends up producing sound waves and pulses that can hinder the amount of gas flow. At lower RPM having some back pressure can aid the gas flow by squeezing the pulses tighter together. This will allows the gas to move along faster giving more power.

Most of the modern sportbikes have a movable baffle that closes some at low RPM to create more back pressure and open at higher RPM.

I found this information at:
http://www.howstuffworks.com/framed.htm?parent=muffler.htm&url=http://www.nsxprime.com/FAQ/Miscellaneous/exhausttheory.htm

Andy
 

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in order for an engine to operate properly it needs a certain amount of pressure pushing back on the exhaust. even straight pipes on dragsters provide a certain amount of back pressure. since these engines only run in a very narrow RPM range the pipes ar tuned for that range in particular. performance at other engine speeds is sub optimal but that's not important for that specific application. most street engines run at a great range of speeds and need acceptable performance at all levels. that means a stock muffler is a compromise between acceptable EPA noise levels and trying to get rid of all the flat spots in the power curve caused by the muffler.

scott :)
 

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Wheelie for Safety
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Hi Scott,

I'm not sure your example of a dragster engine using short pipes is to add horsepower. There could be other reasons such as; point the exhaust away from people, prevent the intake from sucking in exaust, and to lower noise.

In theory, any amount of back pressure would decrease power.

Andy
 

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sorry, andy, i guess i was a little vague. yes, the main purpose is obviously to direct the exhaust. but the length and bend of the pipes are tuned for optimal back pressure in a limited RPM range, the range where the engine spends most of its time. for dragsters this is minmal backpressure at high rpm's.

while it might be correct that backpressure at a given RPM on a given engine is harmful to power output, an exhaust system tuned for an engine and how it will be used may provide more power at some speeds and (hopefully) better, more usable power and torque curves across the RPM range.

i used to play with VW engines. at times i ran them with no exhaust system whatsoever, exhaust coming right out of the heads. by manipulating the throttle you could get the engine to bog, run unstably even though the throttle was held constant or even sputter and die.

the entire intake and exhaust systems have to work in concert to provide the right dynamics for everything going in AND out of the engine. since our engines are used over a wide variety of RPMs the total system is a compromise to provide the best performance at many different points.

the stock muffler is very restrictive because one of its primary goals is to satisfy EPA noise requirements. it's obvious from everyone's posts here that louder aftermarket mufflers can provide more power and still keep the power usable. but even before noise regulations many racing engines still used mufflers. the engines that don't are things like dragsters and i think some flat track bikes. these engines both operate in a limited RPM range and have exhaust systems tuned to deliver power in that range. noise and other ranges of the RPM are secondary.

just as a wide open air box can cause bogging and other problems so can a wide open exhaust. it's all one sustyem and all has to play nicely together.

scott :)
 

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I thought that you needed 'some' pipe so that the valves wouldn't cool too fast causing them to warp.

Is the Yoshi street pipe very loud?
 
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