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custom fab'd pipes and performance

Discussion in 'The Garage' started by antblyme, Nov 29, 2010.

  1. antblyme

    antblyme XS650 Member

    This is my 1st XS650 build. Looking to get some opinions on this.......Made my own pipes from a mixture of 1.5" OD bends and stock XS pipes that I have laying around. The pipes are mocked up and at the head, there's still about 8" of that inner tubing still there. So it basicall goes from 1.25" to 1.5". Question is should I remove that 1.25" and run 1.5" the whole way......or maybe that 1.25" might help? thanks
  2. Ghetto

    Ghetto Bob it!

    It should help with back pressure in theroy.
  3. jamesgs4

    jamesgs4 fuck this, let's ride!

    The back pressure myth is just a lean condition caused by better flowing exhaust compared to the air/fuel mixture entering the combustion chamber. Fix the lean condition by balancing the air/fuel to the correct mixture and all will be well.
    B_Crabaugh likes this.
  4. XSLeo

    XSLeo XS650 Guru Top Contributor

    I plan on building a set of 1.5 inch headpipes. I have the ends off a stock set of pipes. I cut the pipe off just at the first bend. I then cut the outer pipe off at the holes about 1/2 from the weld. I cut the inner pipe to about two inches long from the gasket flange.
    I will weld 1.5 inch pipe to the bit of outer pipe I left on the pipe. The rest of the headpipe will be 1.5 untill under the engine. There I will up the pipe to 1.75 so the mufflers will seal better.
    The stock flange and the bit of 1.25 pipe will act as anti-reversion valves, torque cones, what ever you call them.
    Jamesgs4, The back pressure is a myth, the head pipes if to large slow the exhaust velocity. Like the 1.75 pipes on a 650. If the velocity drops to much the scavage effect of the exhast is lost. With out scavanging the cylinders won't fill with a fresh charge of air/fuel. This looses power. Thats why the open headers, drag pipes, big pipes Run so bad. No low end power They need tpo's and ego's that Mike's sells.
    You can build a set by cutting up a stock set of headers. The 1.5 inch pipes don't really need them. I'm using them so I get a good seal to the head, don't like exhaust leaks.
    Last edited: Dec 6, 2010
    B_Crabaugh likes this.
  5. gordonscott

    gordonscott XS650exhaust.com

    Bingo! This man is dead on correct!!!
  6. gordonscott

    gordonscott XS650exhaust.com

    It’s the length and diameter of the pipes that pull the remaining (10% or so) spent gases out of the cylinder that the piston can not push out on it’s own.
  7. jay760

    jay760 XS650 Junkie

    With all the information on xs's, there seems to be very little on exhausts, with size and length for torque and power, especially with the differance in characteristics between a standard 650 and a bored out engine.
  8. gordonscott

    gordonscott XS650exhaust.com

    Hi Jay, There is tons of info on pipes! Back in the 70's it seemed like everyone was running an xs on the track. There isn't much happening today that already hasn't been done 30+ years ago with these bikes. Get your self a copy of the "XS Perfomance Guide" on 650 Central.com. I have done some dyno testing myself and come up with the same results they did 30yrs ago using some of the same pipe specifications . Cases where comming apart at 70+ hp so there is a limit to what you can do with these engines.
    HrdnSwede likes this.
  9. jay760

    jay760 XS650 Junkie

    Found the link to the guide eventualy, is it worth the money? what is it paperback or download?
  10. Jack

    Jack XS650 Junkie Top Contributor

    Is back pressure really a myth? There are those with defined opinions who would argue other wise. I guess Super Trapp got it wrong with their tunable disc mufflers which was designed to adjust the outlet volume to vary back pressure to bias power performance in the mid range or for maximum top end performance. The trick to any engine in allowing it to breath better is looking at reducing back pressure but without hurting performance,it's all a balancing act. What works for your engine may not be correct for another.
  11. gordonscott

    gordonscott XS650exhaust.com

    Destroying a myth.

    Some say that "an engine needs backpressure to work correctly." Is this true?

    No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.

    The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.

    The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.

    The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.

    Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

    Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque. - Adapted from Thomas V.

    I'm not starting a pissing match Jack, I know your a smart guy (not Sarcasm) but I can pull 20+ more of these articals off the web right now. I'm not a scientist so I just regurgitate information that I read and go with Acoms Razor. By the way Jack long time, hope all is well. P.S. Roy sold all his xs's and probably won't see him on here to much anymore.
    B_Crabaugh and HrdnSwede like this.
  12. i have custome pipes on my bike and the bike runs great...
  13. Tech7

    Tech7 XS650 Junkie

    Scott is absolutely right. Internal combustion gasoline engines are designed to operate at 14.7/1 air fuel ratio. This is ideal for performance and efficiency. I know this, I am learned in this as an auto technician. The problem is, we have motorcycles that are basically 30 years old. And we keep fucking with them. There are so many schools of thought on tube length/diameter/carb size/ jetting, etc. It could make your head spin. Believe me. We have been getting 400 HP out of a 2 barrel SB Chevy all year. Broke one piston though. In my opinion, run a pipe/carb combination that is ridable and is reliable. Let's face it, the XS 650 could get smoked by any halfway decent modern half liter bike anyway. No doubt, the XS is a great motor, but..Just shoot for reliable and cool. I'm thinking 1-1/2 max diameter, a little baffle and a little on the rich side to keep from a burnt valve or a holed piston.
  14. Teebs

    Teebs King of the Ass-Hats

    GS, you think you can just start throwing things like "facts" and "sound reasoning" around like that and then say you're not looking to start a pissing contest??? I call "bullshit".

    Actually, it was way too long. I didn't bother reading it. I figure if anybody needs that much space to explain something, they have to be full of shit.

    Ridemall likes this.
  15. Jack

    Jack XS650 Junkie Top Contributor

    Hey Scott , Hope all is well with you also! There's no pissing contest on this end,just years of wrenching dating back to the late sixties and my advise to you is don't allow your mind to become corrupt with all the BS on the internet that BP is bad for an engine performance,it can either be your enemy or your friend,just depends on what the engine is intended for.Yes there are a lot of variable complex factors involved here like cam timing,piston speed,port velocity,muffler design,pipe volume,pipe length ,etc,etc. You need to ask yourself why is performance enhanced with the usage of AR inserts at the exhaust port or when reducing header pipe volumes, or why the Mac 2 into 1 headers perform better with a choked down collector volume,it has nothing to do with the correct A/R either. Once you answer these questions, you'll develop a better understanding where I'm coming from with my opinionated views.

    Me smart????????? No, I'm no where in the same league compared to the real gurus,just a little wiser from tuning my engines for a particular purpose.

    I sure hope Roy is OK,if your see him,give him my best regards and like him I'm stepping back from the XS too and I wish you the best also Scott.......Later!
  16. jay760

    jay760 XS650 Junkie

    Why dosen't anybody come up with facts on size/ length on 2-1 / 2-2 pipes instead of theories, whenever it's anything to do with the engine there are lot's of people to give advise but as soon as it comes to the exhaust there seems to be a big void, with conflicting theories.
  17. gordonscott

    gordonscott XS650exhaust.com

    Actualy we (Jack and I) are not too far apart on this belive it or not. We dissagree on a few things but aggree on a lot. I don't like the term "backpressure" it implies restriction and this is where I get missunderstood a lot. I have dyno tested my pipes and was very surprized at the results on some of them. For example my 2into1 open header produced more power at 4,000rpm then my 1+1 glasspack hi-pipes? Same bike, no changes other then bolting on a different exhaust. These are roadbikes (most are not running on the track) and yes performance should be set up for useable rpm's not out there in redline land. Lets dummy this up a bit, I said earlier that a good performing exhaust will pull the remaining spent gasses out of the combustion chamber that the piston can not push out on it’s own. If you were to siphon gas out of a tank the old school way (with your mouth) you would want a regular size hose to do it with. If you where to do it with a vacuum line it would take forever to get the gas out of the tank. If you where to do it with a 2†diameter hose your lungs would give out before you would get any gas out. The right tool for the job, and the job of an exhaust is to remove the spent fuel in the combustion chamber; there is NO reason to keep it there, that’s it! Now Jay, I have some basic facts about tube diameter and such, so if you want me to give away the store here you go... I have had best results with 1 1/2" tube, no surprize there! On length I've seen the best performance gains on "MY" stock motors with jetting change and pod fillters to be 42" in length at 550ft above sea level (up or down a few inches in length won't matter on the street). This is with "open pipes" things change with mufflers and stock jetting. On a stock bike with stock jetting and stock air cleaner I got a 3+hp gain doing nothing more then bolting on my 2into1 open header, and this gain was at 4,100rpm. And it was only a 1/2 point off on the sniffer! (lean)

  18. gordonscott

    gordonscott XS650exhaust.com

    If you are going racing yes! Download :wink2:
  19. gordonscott

    gordonscott XS650exhaust.com

    :agree: except for the baffle :D If you want speed then forget about the XS :laugh: I feel the need I jump on my Cyclone M2 :thumbsup: p.s. showing off now :D


  20. gordonscott

    gordonscott XS650exhaust.com

    Ok lets move on to the "best oil" to use :laugh:

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