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Need help with exhaust system (acoustic/valves/pulse)

  1. Sep 18, 2015 #1
    Need some advice regarding my exhaust, Im building a dragbike.
    Vtwin Engine 45degrees (Harley Shovelhead)
    I whant to build a 2-1 (both cylinder connected to one pipe) I´ve learned that the primary should be at same length (driveabilty and performance) , If I dont do this, whats the effect, how to calculate it? I dont want to mess up a good Enginebuild with a terrible exhaust. Why I want diffrent length is that I can rebuild my existing ones.
    Would be great if anyone whould help me out ;-)
    best regards
  2. jcsd
  3. Sep 18, 2015 #2


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    To keep this as short and simple as possible, there is a pressure wave cycle created in each primary associated with each cylinder exhaust flow and the timing of this wave cycle is determined by the the primary's length from its cylinder's exhaust port to the exhaust end of the secondary and as the wave travels from from one primary's cylinder through the secondary it assists in the scavenging of the exhaust gases coming from the other engine cylinder. At certain rpm range of the engine this effect will increase the effective power of the engine. This is known as exhaust tuning on 2 cycle engines. In order to maximize this effect, the length of both primaries must be the same so that the pressure wave cycle time for both cylinders will occur at the same rpm range.

    I hope this explanation helps you understand the issue; but, if you have any questions or need for me to clarify something please post to this thread and I will be glad to help.
  4. Sep 18, 2015 #3


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    This is an addendum to my above post.
    What I should have respnded above, and I failed to do, is recommend above was that you contact a registered Harley Davidson dealer and discuss your issues with them.

    At the same time one online site that extensively covers this subject, including the associated equations, and is a "must view" for you is:

    My above posts covers the basic reasons for making the primaries the same length and the basic principals; but, it is of a general nature and does not cover all of the issues related specifically to Two Stroke engines where the exhaust tuning is particlarly critical because it not only scavenges the exhaust gases from the cylinders but also simulataneously draws the fresh air/fuel charge from the crankcase into the cylinders.
  5. Sep 26, 2015 #4
    As to exhaust tuning a Harley is a very difficult animal. However a drag race engine is quite easy. As already addressed the pipe time affects the scavenging of the engine. The negative pressure pulse is most significantly generated at the first significant change in size. It is a simple Calc to find the length for a specific rpm. Secondary scavenging is produced when the exhaust pulse from one cylinder passing the outlet of the outlet of another. This is not possible on a Harley. The 45 V configuration products am uneven exhaust pulse pattern. It is easy to have one cylinder Ajay in scavenging the other. It is impossible for this to work in a reciprocal fashion as the individual pipe lengths would need to change with every firing.

    I would instead look at what is commonly called s Zoomie. In the drag race world these are the single pipe single flange exhausts. By maintaining consistant diameter and adjusting the length it is possible to tune the peak torque rpm via the primary reflected wave. To get the most effect the ends should be square cut.
  6. Sep 27, 2015 #5


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    Thanks for your post, my above post was based upon a general knowledge of exhaust tuning, but was severly limited by my lack of experience with motorcyles and the lack of any experience with applying to 2 cycle engines as well. My backgroumd and knowledge on the subject is with 4 cycle auto engine intake tuning where the determination of the sonic velocity based upon close to atmospheric temperature and pressure air with a full throttle opening is easy; as opposed to determining that value for a cylinder dicharging a mixed exhaust composition gas and exhaust temperature and pressure depending upon the engine rpm.
    At the same time, on my first application I used exactly the same "try a tube length and see where it helps" method you state above; and, I only researched the theory and learned the principles and equations later.
  7. Sep 27, 2015 #6
    JBA, I am only to a limited amount saying "try it and see." What is challenging is that the 2 cylinder 45 vee causes the exhaust pulses to be out of synch. Best scavenging is when the cylinders can be timed to pulse exactly out of phase and help draw gasses. With your motor the firing measured in crank degrees of rotation is Cyl 1 + 315 o Cyl 2 + 405 o Cyl 1 etc. The odd timing prevents good coordination.

    When you only look at primary scavenging the formula is easy. P = ( ( 850xED) / RPM) -3 Where P = primary tube length, ED = 180 + # of degrees before BDC the exhaust valve opens, RPM = the engine speed that you are tuning for. ( note these variables are for inches)

    Once you have a length a pipe can be built (there are a few subtle tricks of the trade that we can discuss outside this thread). Here is the try it and see part. In general the shorter the tube length the higher the RPM the reinforcement will occur, conversely longer tubes are lower RPM. This effect will mostly overcome the mechanical parts of cam, valves, intake path, etc. The primary scavenging will drive the torque curve peak up or down. The effect is very similar to advancing or retarding cam timing. I would recommend in similar fashion to timing build the engine and get the bike on the track or at the least onto a Dyno. Armed with good info you can push the peak torque up or down within reason to get better hook ups

    I recently posted a link in another thread. Here it is again for your reading. You may find some good stuff also on intelligently setting up your ride.

  8. Oct 12, 2015 #7
    Hi the Harley forums should be able to give you some info. Generally the best advice is given by the people who sell exhaust tubing. Someone like Burns stainless or SPD will give you a recommendation if you buy the tubing from them You may be able to find someone who specializes in building Harley systems and has done dyno work. Harley may also be worth calling. If you put the bike on a chassis dyno to test your design you will most likely be upset with what you find. Make sure you bring carb jets and a timing light. IF the headers make a difference you should also see different a/f ratios at the same rpm with the different systems. You will then need to correct/ change the a/f ratio to get a good comparison. To make things worse- you could see one system wanting different ignition& fuel # then the other system. In the end you may gain 5 hp
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