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Staintune Exhaust on my VTR250

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by CrayolaS7, May 2, 2011.

  1. Was reading about the exhaust that came on my bike and it is a staintune exhaust which apparently has a baffle that can be removed. I assume this will make it louder which I don't really care about I am just wondering if there is any advantage performance wise if I take it out?



    My experience is more with cars where I know that in general less restrictive exhaust is better, but don't know jack about tuning bikes. I remember reading that 2-strokes need the backpressure tuned properly, does this still apply for 4 strokes?
     
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  2. Bugger all performance difference and you'd need the carby retuned or else you'll have flat spots.
     
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  3. Staintune generally do a bit of research with their exhausts so they will be about as good as it gets for your bike. You will need the carbie tuned, but you need that anyway with a standard vtr.

    You will get a few percent and it will likely be a nicer ride all round.

    Still staintune are not cheap.

    Also get you head out of the back-pressure mentality. It's word of mouth misinformation.
     
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  4. +1 to this.
     
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  5. -1 staintune are junk for proper performance, but they do have a nice finish.
     
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  6. So just to clarify, are you talking about smokers here too?
    My old RD certainly didn't enjoy a cracked exhaust...
     
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  7. Just to clarify the exhaust was already fitted long before I got the bike, I'm just wondering about the baffle.

    Also ibast, 2-strokes definitely need a certain amount of back-pressure and it's important to quite a few other engines types. I was just asking.
     
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  8. To elaborate, it's not really about "backpressure" as such. As ibast alluded to, that's... Not really the correct answer. Backpressure is a symptom, for want of a term, not a cause.


    An effective exhaust system has to optimise scavenging of the exhaust gasses. That is, extracting all the waste from the cylinder before the valve closes, so there's more room for the fresh fuel-and-air charge to occupy.

    The two mechanisms by which this occurs are:
    - kinetic scavenging, where the momentum of each pulse of exhaust creates a vacuum behind it as it travels through the exhaust system and 'sucks' the next pulse of exhaust gas out of the cylinder when the exhaust valve opens.
    - harmonic scavenging, which is the "tuned length" part; the exhaust system is designed such that a pressure wave travelling through the exhaust gasses reflects off of a major discontinuity, returns toward the exhaust valve and presents a vacuum to the exhaust gasses just as the valve opens. (This is how 2 strokes primarily make their power, since they don't have an exhaust valve - they need the rarefaction and pressure pulses to act as a pretend 'valve')


    The catch with kinetic scavenging is that the momentum of the exhaust gasses needs to be kept high. Which means using pipes of a small cross-sectional area; smaller cross-section means higher velocities, which is more momentum.

    This is great at low revs (and why engines which are most efficient/most-torquey at low revs have narrow diameter, long-length intakes and exhausts).

    As the gas flow picks up speed (eg: higher revs, attempting to make more power by burning more fuel and air per second), the boundary layer of stagnant gas flow increases in size, choking the speed at which gas can flow through the exhaust system. That creates a tremendous flow restriction. The solution - use a larger cross-section pipe to reduce the influence of the boundary layer by (1) reducing gas velocities and (2) providing more area for the gas to flow through. Hooray! We now have an exhaust system which is optimised for 200 horsepower worth of gas flow instead of 50 horsepower worth of gas flow!

    The catch: The problem with increasing the diameter of the exhaust system is that by optimising it to flow plenty of gas, such as when the engine's at high revs making lots of power... The pipe cross section is now so big that when the engine revs are low and the gas flow rate is low, the gas velocity drops to 2/10ths of **** all, which ruins the kinetic scavenging effect.

    THAT is what someone is describing when they complain that their fooli-sik 8" diameter ricer exhaust ruined their bottom-end power. To chase a few more horsepower at the top end they've destroyed the ability for the exhaust gasses to flow properly at low-revs. The engine doesn't need "backpressure" in the form of a "restrictive" exhaust or a potato shoved in the tailpipe. It needs gas velocity to be kept at an optimal amount so that effective scavenging can occur.

    Without effective scavenging, the engine can't get all the exhaust gasses out of its cylinder before the valve closes and the "suck" part of the combustion cycle begins, with some volume of the cylinder wasted by hot, consumed fuel and air.


    NiteKreeper's cracked-2-stroke-exhaust dilemma (and indeed, anyone that's ever had an exhaust leak with a 2 or 4 stroke engine, particularly in the headers) is because an exhaust leak disrupts both kinds of scavenging. It disrupts the 'tuned length' harmonic scavenging by messing with the behaviour of the rarefactions and pressure pulses, and it also prevents a proper vacuum forming behind each pulse of exhaust gas and so kinetic scavenging is hampered.


    The point we're trying to make is that it's false (or at best, a gross oversimplification) to say that "engines need backpressure". Backpressure may be a high-revs symptom of an exhaust optimised to function best at low revs, but it's not the reason the exhaust is effective or ineffective. An exhaust can have high backpressure and still be effective at allowing the engine to breathe. Similarly, you can have a restriction-free "zoomie" on your engine with zero backpressure and have terrible performance. It's all about the scavenging and optimisation.

    (note that kinetic and harmonic scavenging play a part on the intake design side of things too. See Toyota's T-VIS and other "variable length" intakes, etc, bi-modal exhausts, etc - these all focus on trying to provide two or more 'optimal' configurations instead of just one).
     
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  9. OK Spots =D>=D>=D>=D>

    Crayola, best just leave it be and enjoy what you have.
     
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  10. but in a bit of real world experience with a staintune pipe i have this to say.

    i had a staintune slip on pipe on my 03 yamaha wr250f. it came on the bike and was obnoxiously loud. like stupid loud. so i decided to buy a baffle to stick in it and here were the results.

    it was a bit quieter, but not that much really.

    i lost about 3,000rpm of usable power, all in the top end. the bike was completely robbed of most of its power over 9,500rpm which made it completely crap. considering it revs to 13,500rpm, it was a sizeable part of the rev range. hell, it wouldnt even hit the rev limiter in over-rev when it used to just smash off it easily. i would have lost more than 10% of my power at peak rpm and even more at the limiter.

    and there were no bottom end gains. it was just dead flat. dead shit more like it.

    so i pulled the baffle. back to how it was before. but in comparison, a mate had the same bike with a 'proper performance' slip on muffler so i tried it. more power everywhere than the staintune.

    in summary, baffle in is not always the way to go. and staintune make absolute crap exhausts if you are looking for performance.
     
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  11. Good post Spots.

    Also, people also get the concept of a negative pressure wave mixed up with the concept of back pressure.

    Consider a motorway in stop start mode. You are generally moving forward but every now and then you will stop. This stop point tends to move backwards through the traffic.

    The same thing happens in exhausts and is critical to two-stroke performance and can seriously help or hinder 4-stroke performance.

    The negative pressure wave can be used to block an exhaust port when the valve is open or the piston has exposed it. This is used to run more radical timing than you otherwise could.

    Despite my analogy you do still get a negative pressure wave with limited back pressure and this is the error in peoples thinking.
     
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  12. Plus I bet it already sounds awesome.
     
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  13. Thanks for the explanation, I study mechanical engineering and do actually understand the effects you're describing. Just that my thermo/fluid-dynamics textbooks don't exactly teach you about whether or not your 250's performance is going to be effected by a particular exhaust, and I had always heard it referred to simply as "backpressure".

    For the record some of your terminology is actually a bit off :p

    Thanks heaps for your help.

    Edit: I believe what you describe as the gas flow rate dropping too low is a bit more nuanced than that. Generally speaking the faster a flow moves in a pipe, higher the pressure drop behind it, similar to a venturi effect. If the flow rate drops too low all that is left to scavenge the gasses is the pressure differential between the exhaust gas inside the cylinder and the atmosphere. By the time the the exhaust stroke starts this gas has given most of its energy to either the piston or to heating up the engine and so that pressure differential isn't that great.

    As for my bike, I took the baffle out last night and rode today, there was no noticeable different either way in terms of power. My bike is just about ready its 30K service though so is probably in need of a tune up anyway. As for the sound, it is louder but not uncomfortably so. The main thing though is it's deeper at and grumblier sounding at lower RPMs, and sounds a bit less tinny at high RPMs.

    I think I will keep it off for now.
     
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  14. Quite probably, yeah; it was late, I have a habit of speaking in generalisations and it's been a loooooooooooong time since I studied exhaust systems in-depth (Formula SAE, whee!). :)

    And you're right - fluid dynamics at the B.Eng level doesn't really get into the more transient effects because the equations describing transient flow effects get a little frightening. Make that a lot frightening.
     
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