Aftermarket exhaust -- what other mods will I have to make?

Discussion in 'Mechanics Garage' started by Lazy in AZ, Aug 11, 2010.

  1. Lazy in AZ

    Lazy in AZ New Member

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    Q: If I put aftermarket pipes or a high intake air box on my bike, will I need to rejet my carbs or make other major modifications to the engine to make it run better? Is there a difference whether it is a 2 stroke or a 4 stroke?:unsure:

    The best answer is that when you replace the stock exhaust designed for that bike with one that isn't specifically designed for the engine then you run a high risk of the exhaust 'waves' running back into the cylinder and stifling the incoming mix.

    Here's how it works:

    The air/fuel mix enters the intake valve at the same time that the exhaust valve is entering its closing phase which helps to pull the fresh mix into the cylinder. This is called scavenging and is very effective at higher rpms, so engines more tuned to racing with a large valve overlap, that’s why they do it. Now, on the other side is the exhaust system. They are designed with bends, curves, flat spots, baffles and what not specifically to control one thing. It's not the noise, that the mufflers duty. It's not the emissions - that would be a catalytic converter or your fuel mix adjustment.

    The exhaust pipes duty is this:

    Think about what happens when you ignite a pocket of properly mixed fuel. The resulting ignition would produce a small explosion that would move whatever wasn't nailed down (piston, connecting rod) and then find the easiest way to neutral air (atmosphere) which would be the exhaust valve in this case. There is one by-product that is usually overlooked on a motorcycle when we think of exhaust. We know about the actual exhaust gases, and the heat, and the noise... ah, the noise.

    Most people will tend to believe that the noise factor of the bike is achieved with aftermarket pipes. Not so, simply remove the exhaust and you can disprove that theory. Add on sleeves only amplify or change the pitch of the noise that the engine already makes. The noise comes from the cylinder during ignition and travels out with the gases.
    :scooter:

    Now... that said, what is noise?

    Sound travels in what form? You got it, sonic pulses or waves. These waves travel through the exhaust pipe and ricochet along the walls of the pipes, which as I stated are designed with different textures. There is a specific point in the pipe where it narrows or changes direction or pitch abruptly. This prevents some of the waves from exiting the pipe and thus sending them back to the exhaust valve. When timed properly, these waves will arrive at the valve when it's closed and normal function will result. When improperly timed, the waves will force exhaust back into the open valve and into the cylinder, slowing down the incoming mix and choking the engine.:sick:

    There is a reason for this method. Yes, you could just simply run straight pipes and be very loud and look very cool. But most engines on motorcycles are designed for racing applications or high end running, and even the straightest pipe isn’t going to stop the sound waves from backing up in the pipe and ricocheting back to the valve.

    The power curve of a motor puts most of it's horsepower near the peak of its rpm's and when it is in that range then the wave factor in null and straight pipes would be the best because the air flow through the pipes will negate the sound factor. But if you don't have any restrictions then the flow is constantly at it's peak and your throttle isn't, which means basically that at low and mid range it's gonna bog and sputter, but the moment you hit high range rpms it's going to pull the handlebars out of your hands and slam your ass on the ground.
    :vtr2:

    Most manufacturers include a control valve in the exhaust system now that electronically monitors the amount of sound wave refraction that it sends back to the exhaust valve. The lower the rpm, the smaller the valve opening and the more reduced the exit hole is. This maintains a steady amount of pressure back to the valve at variable rpms and allows the engine to run at its peak horsepower through a wider band.

    It's all about the control of flow through the cylinder, not the Venturi. When you tinker with the exhaust setup, you're ultimately going to affect the fuel mix and air flow requirements because you've messed with the amount of mix being allowed to flow into the cylinder for combustion. Some aftermarket systems don't need any adjustment because they were designed for that engine and simply look different outside but do the same exact job inside. Others require bigger or smaller jets in the carb, higher or lower fuel injection mapping, a larger air flow via a new filter mechanism... even cam timing adjustments. It all depends on the dynamics of the system you are installing.

    So, why re-jet the carbs or increase the air flow and even reset the cam timing?

    It boils down to this. If you restrict the exhaust and don’t have the sound wave issue under control, then you only have one option. Increase the flow of the mix incoming at the intake valve and make a bigger boom to force the exhaust out of the cylinder. You really can’t do this effectively without changing the cylinder width and piston size (and for those of you who defend multiple point spark plugs – you can’t make the fire any hotter than it already is by adding an extra spark! :crazy:). But, we can make slight modifications that will at least get the engine for the most part back to its original power rating.

    Know this, if the exhaust system you install needs additional tuning elsewhere in the system to achieve proper flow, you'll know it when you ride it cause it'll feel like your carrying a thousand pounds on your back seat. You might even be able to tell just by it's idle characteristic cause it'll sound sick as hell (and not in a good way).

    What modifications need to be done on a bike when installing aftermarket pipes will depend entirely on the type of bike and what pipes you are installing. Each one is going to have a different reaction, so it’s hard to say. If the pipes are not specifically designed for the bike you are installing it on, then modifications will be required to get it back to factory power and beyond. Some aftermarkets will even tell you what mods to make before or after install to prevent poor performance.

    The bottom line is that unless is it designed for the bike you are installing it on, in order to see any horsepower or performance gain you have to make some internal modifications, install a Power Commander and have it run on on a Dyno to get the proper mapping for the current condition of your bike - especially if it's got some mileage and wear on it.

    I know that it’s hard for most mechanically inclined individuals, including myself, to fess up and take our bike or car to the shop to be done right. It is definitely worth your while when considering an exhaust change (especially on a newer model bike) to take it to a performance shop and have an evaluation done FIRST. For a price, they will give you a game plan on how to best time and tune your engine to fit any exhaust system you may have your eyes on, and give you the best scenario depending on the amount of money your willing to dump into it. AND, if they have one I’d highly recommend having them run it on a DynoJet. Those things are awesome and tell you so much about what your engine is actually doing while it’s running. It’s like hooking your bike up to an EKG and getting a visual on its heartbeat, rhythms and the little fluctuations that can easily be missed by even the best of technicians.

    I got most of this from Theory notes and being in Clinic 9 - Engine Performance and DynoJet. My Instructor, Manny had this to say about it:

    “You can not superceed the research and engineering that the manufacturer has put into the engine design by simply replacing the stock exhaust on your bike and expecting higher performance without making serious modifications to the internal functionality of the engine.”

    Manny, by the way, has nearly 15 years in the field and was the Lead Builder on several championship winning AMA teams and also has experience in Moto GP, so I take his word as gold.

    Hope this helps!
     
  2. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    You're the man Lazy! This is something I've been looking for a good answer to for a long time. For some reason, I never outgrew the "why" phase of my childhood.
     
  3. pjvtec

    pjvtec New Member

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    :hangme: Wow, That's why too long to read. I would recommend a Dynojet Jet Kit, richen up the fuel mixture screws to 3 full turns out from lightly seated.
     
  4. Deadsmiley

    Deadsmiley Member

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    1470 words to be exact. :tongue:
     
  5. Lazy in AZ

    Lazy in AZ New Member

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    "The minute we stop asking "Why?" is the minute that we are informed - rather rudely - that we really have no clue what the :censored: we are talking about." -Mr Boyle (my Vo-Tech Electronics Instructor)

    It's stuck with me since I was in high school and it's very true. The only stupid Q's are the ones that you don't ask, Tink. Got any more for me just fire away.
     
  6. Deadsmiley

    Deadsmiley Member

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    I hope you don't think I was dissing you. I merely asked myself, "How many words are too much? Where is pjvtec's word limit?"

    We now know that it is somewhere south of 1470 words. :cool:
     
  7. Lazy in AZ

    Lazy in AZ New Member

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    There are NO simple answers when it comes to the internals of an engine, PJ. And I'm hoping that your not thinking that simply installing a kit and turning out the fuel screw a half a turn is gonna fix every bikes running problems?
     
  8. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    Must be a joke or 'tardedness :crazy:

    "the question is too long to read, but let me tell you the answer is blah blah blah :puke:"
     
  9. Lazy in AZ

    Lazy in AZ New Member

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    :nelson:
    no harm done. I'm normally long winded both on and off screen, and PJ obviously hasn't read any of my other posts. Otherwise he'd have been expecting it already.
     
  10. pjvtec

    pjvtec New Member

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    I like the fact that the info was provided. It even sounds like the info came from an MMI grad. I am also an MMI grad. I completed the basic course, YamaPro, F.A.S.T Suzuki course, Honda Program, and Dynojet Dynamometer Training. Once in the field, I went to Advanced Electrical at the Honda Training facility in Ohio.

    My opinion is this. There are ton of ways to do this. How much do you want to spend for a few extra ponies. My favorite is to dyno your bike on a dyno that has a air/fuel sniffer. You can do more air/less air testing, but that takes so much time. A sniffer will tell you where in the power curve your bike is running lean or rich. Then you'll now where to adjust. Maybe pilot jets, needles, needle jet, main jets. I've done this enough. I like DynoJet jet kits over Factory jet kits.

    Good luck with your tuning.
     
  11. invisible cities

    invisible cities New Member

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    Factory Pro is a better way to go (for the VF500F), IMO.

    The Dynojet kit (for the VF500F) requires drilling the vacuum slides the FP kit does not.
     
  12. invisible cities

    invisible cities New Member

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    A first rate post.

    This answers a bunch of questions I have had on exhaust systems for V-Four engines.

    Many thanks for taking the time to write this down.

    Much appreciated!
     
  13. nessus

    nessus New Member

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    very good writening....i feel so match smarter...

    i just bought a GSX 750 R (i still have my viffer and she stays) and he has power commander and aftermarket pipes. Dynojet was my next idea and your text saying im in right way...
     
  14. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    :bump: :bump:
     
  15. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    another bump for an important post
     
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