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Discussion Starter #1 (Edited)
As far as I know, the Exhaust Valve embedded in the stock muffler (aka Yamaha called it EXUP) is used to close the exhaust a little so as to engage the Helmholtz Resonance; thereby, improving the low-end torque.

I just looked at the Yoshimura documentation and it said to disengage the 2 cables that drives the servo since the Yoshi slip-on does not have a valve. I'm assuming most others do not have it either.

So, what's the technical merit for removing the stock OEM muffler? Just for "improving" the sound but at the same time losing low-RPM torque? Doesn't make sense to me.
 

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Traditionally its looks, sound or weight and sometimes saving the cost of factory replacement in the event of an off.

In the case of the SC77 its really just looks and sound as the muffler is Titanium and quite light - but it also makes it cheap to replace in the event of an accident - believe it not the SC77 Ti muffler is $2000 (before discount) from Honda.
 

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Discussion Starter #3
What about EXUP for low RPM torque? No one is interested in that compared to sound? They are willing to forego that?
 

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What about EXUP for low RPM torque? No one is interested in that compared to sound? They are willing to forego that?
For looks and sound on the street, probably. In the case of the SC77 the US bike is so restricted stock that the gains of a full exhaust and ECU flash will outweigh any midrange lost from losing the EXUP.

Which is probably as much an easy low speed noise abatement device as it is a low rev torque enhancer. The loss of torque is not huge, measurable but not huge.
 

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Besides once the bike is tuned correctly for the exhaust (Or other mods) the power curve and delivery is so much more smooth that you wouldn't notice any loss in power down low anyway.
Just a slip on and loss of the exhaust valves isn't gonna be a change in low/mid torque you can't live with.
Now add that to an aftermarket race filter, headers, and velocity stacks and that's a different story
 

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Discussion Starter #6
[...]
Now add that to an aftermarket race filter, headers, and velocity stacks and that's a different story
What possible changes could the 3rd party vendors do to improve performance from the headers to the muffler? widen the single pipe after the 4 pipes transition into it? Just curious ...
 

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Discussion Starter #7 (Edited)
I think this explains a little clearer how the EXUP improves low RPM torque.
The key idea is the velocity of the exiting gases.
At high RPMs, the velocity of the exiting gases is high so having restrictions in the pipe will reduce torque. We want a clear wide path.
At lower RPMs, the velocity of the exiting gases is lower and if the path is too wide, exhaust scavenging cannot occur properly, a phenomenon where the tail end of the exhaust pulse is actually helping to drag the piston along the way. So, if a flapper introduces a kink (same volume of air passing through a smaller pipe), the velocity of the exiting gases will be increased enough to implement a proper exhaust scavenging correctly.
 

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Discussion Starter #8
Conclusion : better to keep stock exhaust if planning to do street riding mostly because of better low-RPM torque.
 
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I couldn’t stand my bike sounding like a Chinese scooter with the stock exhaust, even with the valve open all the time mod. Personal choice I suppose. The stock SC77 exhaust sounds much better than the SC59.
 

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Discussion Starter #10
I couldn’t stand my bike sounding like a Chinese scooter with the stock exhaust ...
[...]
ha ha ... is Vespa Chinese-owned now?
 

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What possible changes could the 3rd party vendors do to improve performance from the headers to the muffler? widen the single pipe after the 4 pipes transition into it? Just curious ...
Each Cylinder header pipe inner diameter is larger allowing more free flow and volume. Same as what is done with aftermarket headers on any engine.
For a street bike I say save your money or spend it else where and keep the stock. Same advice I have for velocity stacks.
Track only bike just the opposite
I would however suggest a slip on exhaust for various reasons. Look, sound, increased flow, weight savings.
 

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One reason not mentioned yet is to help with idle quality and prevent loping/hunting. This is notorious on 2008-11 bikes where just about every slip on exhaust causes idle to bounce between 1100-1300 and oddly enough idles perfectly if you cover the exhaust with your hand.

No amount of fuel or ignition adjustments can fix the issue. Only way is to either replace the headers (as another user reported), put in an exhaust silencer (HELL NO), or raise the idle to about ~1400 (what I chose) to allow the engine to idle smoothly. The 2012-16 models have a heavier flywheel so it's less of an issue.
 

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I had an idle search issue initially on my 2015 with an aftermarket air filter and Taylormade slip on but once I started autotuning it went away after like the third tuning session
 

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I’m not an engineer, have not played one on TV, and have not stayed at a Holiday Inn recently. But it seems to me that with catalytic converters in place it’s hard to imagine that the exhaust needs any additional resistance to create more torque.

Its an interesting theory, but not one I’ve ever had much belief in it, at least on a mildly modded production vehicle. And I have been around quite a few modified cars and bikes being dyno’d before and after and saw no loss of low or mid range torque when a less restrictive exhaust was added, including my own Ninja 1000 (not the one I have now, a 2011 I had).

I suspect it’s more about emissions and allowing the catalytic converters to heat up at part throttle. But again, I’m no expert and freely admit it.
 

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What possible changes could the 3rd party vendors do to improve performance from the headers to the muffler? widen the single pipe after the 4 pipes transition into it? Just curious ...
A lot. The race kit specifically calls for after market tuned headers to be used on the SC77 and SC82 whether SuperStock (which use 100% stock engine) or SuperBike which really only permit blue print, balance and head porting.

Akra Evo race pipe headers are an 8 - 10hp bump out of the gate on an aggressively tune - and the entire exhaust system including muffler, mounting hardware and flanges is only 7lbs.

Just as many reasons to go aftermarket full exhaust as to not (y)
 

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Anything done to an engine to flow more air is going to move the peak power and power curve towards the top end of the rpm range. PERIOD!
The loss of back pressure or air restriction is what causes the loss of low end torque but increases top end HP. When this is done and the AFR and timing is properly tuned the impact is lessened but still moves the power curve towards the top end.
Each component installed to increase air flow through the engine impacts the result to a degree. So say if you just add a slip on the effect will be minimal compared to adding higher flow air filter, full exhaust system, velocity stacks, and ram air intake tubes.
 

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Conclusion : better to keep stock exhaust if planning to do street riding mostly because of better low-RPM torque.
On a stock tune SC77? Probably.

On a tuned SC77, low rpm throttle tuning will more than make up for any drop in torque.
As it is in stock form the throttle body butterflies are barely open in 1st gear below 6,000rpm no matter how wide you crank the throttle which is the real reason for the lethargic response.

Anything done to an engine to flow more air is going to move the peak power and power curve towards the top end of the rpm range. PERIOD!
The loss of back pressure or air restriction is what causes the loss of low end torque but increases top end HP. When this is done and the AFR and timing is properly tuned the impact is lessened but still moves the power curve towards the top end.
Each component installed to increase air flow through the engine impacts the result to a degree. So say if you just add a slip on the effect will be minimal compared to adding higher flow air filter, full exhaust system, velocity stacks, and ram air intake tubes.
True, but remember in the fly by wire world the low speed low revs throttle behavior is already artificially nerfed which only adds to the result.
It is possible to mitigate this lethargic feeling simply by allowing what low end torque there is to be more readily accessed with throttle tuning.
 

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Discussion Starter #19
Most people who put Yoshi slip-on ended up installing a PC5 and used their custom-map specifically created for Yoshi slip-on. That stabilizes the Idle and other peculiarities that came with replacing pipes. My current setup has a Yoshi slip-on and PC5, which the previous owner installed, and I don't experience any Idle issues.

But, ... I don't like the sound of the Yoshi. I prefer the slicker and more subdued OEM sound, which reminded me of the old police Honda ST1000 cruiser.
 

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Most people who put Yoshi slip-on ended up installing a PC5 and used their custom-map specifically created for Yoshi slip-on. That stabilizes the Idle and other peculiarities that came with replacing pipes. My current setup has a Yoshi slip-on and PC5, which the previous owner installed, and I don't experience any Idle issues.
Have a R77 slip on on my 09 1krr and with autotune, Dynojet map, custom dyno tuned map all it did was lope. The "tuner" added 7% of fuel in the idle range and that did help somewhat, but it still hiccups once in a while.

Had a RS5 on my 07 CBR600RR and that loped too. Went back to stock exhaust on that.
 
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