back pressure?

[Gisser]

How can restricting exhaust flow be good?  It can't, plain and simple. 

Ever hear of the Yamaha EXUP valve?

Yes, it improves scavenging at all rpms by changing the exhausts sonic properties.  Without such a system you can only tune an exhaust for one rpm.  Ideally you'd have a long narrow exhaust that got short and big as the revs increased, but that's not really possible...  Although there are intakes with multiple runners that work off that principle.  It's all a compromise.

But EXUP improves scavenging by using back-pressure to prevent overscavenging (sucking intake charge out the exhaust during valve overlap at low RPM).  Or so the story's told: 

The Exhaust Ultimate Power Valve (EXUP®) alters the back pressure in the exhaust system at various revs.  At lower revs the valve restricts the exhaust gas flow improving the scavenging of the motor and increasing torque. Above 6000 rpm, the valve is completely clear of the gas flow and the system performs like any other non-EXUP® equipped exhaust.

Poster is inquiring about an R6 can which, if stock, should work fine, but the aftermarket crap do seem to overscavenge (all that annoying popping).

[ben2go]

How can restricting exhaust flow be good?  It can't, plain and simple. 

Ever hear of the Yamaha EXUP valve?

Yes, it improves scavenging at all rpms by changing the exhausts sonic properties.  Without such a system you can only tune an exhaust for one rpm.  Ideally you'd have a long narrow exhaust that got short and big as the revs increased, but that's not really possible...  Although there are intakes with multiple runners that work off that principle.  It's all a compromise.

But EXUP improves scavenging by using back-pressure to prevent overscavenging (sucking intake charge out the exhaust during valve overlap at low RPM).  Or so the story's told: 

The Exhaust Ultimate Power Valve (EXUP®) alters the back pressure in the exhaust system at various revs.  At lower revs the valve restricts the exhaust gas flow improving the scavenging of the motor and increasing torque. Above 6000 rpm, the valve is completely clear of the gas flow and the system performs like any other non-EXUP® equipped exhaust.

Poster is inquiring about an R6 can which, if stock, should work fine, but the aftermarket crap do seem to overscavenge (all that annoying popping).

the gs doesn't have any overlap in the cams,at least not enough to cause over scavenging.it takes a pretty big cam to cause that problem.one with a lot of lobe lift and tight duration.

[makenzie71]

oh my god...I love how every redneck has an idea about how back-pressure is important or how back-pressure is heinous and none of them agree but everyone's right...I REALLY love how every time "back-pressure" gets brought up there's at least two people with opposing views who pop up that claim they know what they're talking about because they majored in fluid dynamics or some shaZam! like that...especially when I know that if you talk back-pressure to anyone who has a real background in fluid dynamics and mechanics they'll slap the shaZam! out of you...

Without getting too in-depth, back-pressure is completely irrelevant.  If you spend time really thinking about whether back-pressure is hurting you or not, you should retire your tools and stop modifying things.  What's important is volume and velocity.  You want to move the absolute largest volume of atmosphere and fuel through an engine at a given time as possible.  Back-pressure is something that you have to take into account when building or modifying an exhaust because IT IS EVER-PRESENT!  So I guess you can think it's important...or that it's bad...or that back-pressure is where babies come from...it's there regardless of whether your "thinks he's a NASCAR driver" neighbor or your uber-educated doctorate holding fluid dynamics professor says it's important or not.

[NiceGuysFinishLast]

Regardless of how it ran.. I bet val's bike sounded like poop with no exhaust.

[Egaeus]

I'm suffering from a bad case of back pressure.  I think it was the Indian food.