4 stroke exhaust sytems

Discussion in '4-Stroke Engines' started by Quenton Guenther, Apr 16, 2009.

  1. Quenton Guenther

    Quenton Guenther Motored Bikes Sponsor

    Tuning the exhaust system can be an important component in achieving maximum power when optimizing the performance of a 4 stroke motor.

    The RPM range that a straight pipe produces maximum power is very narrow. The best way to improve the performance of a straight pipe is to "tune" the exhaust length and I.D. to the required power band.

    Exhaust systems for motorbikes can be very beneficial to the owner. Motorbike exhaust systems can improve the sound, improve hp and torque, are lighter and many look much better than stock pipes. First is the duty of directly tuning the engine exhaust pulses. The diameter and length of the pipe is very critical to keep the speed and volume of the exhaust correct. The diameter and volume are also critical for best power results.

    This post describes the principles involved in using tuned exhaust systems on four-cycle engines. Properly tuned systems can increase the charge of fuel delivered to the cylinder and a related phenomenon can aid in the extraction of exhaust gases from the cylinder.The overall situation is rather complex but in the end, a compromise must be made between maximum horsepower and the width of the power band.

    When I was racing 4-stroke motorcycles in the 60’s, 70s, and 80s, I used exhaust tuning to increase horsepower by cutting the pipe. In order to gain the maximum power the pipe had to be cut to a certain length, which is critical.

    Because the pressure waves in an exhaust system travel at the speed of sound, I calculated how long it would take a sound wave to move from the exhaust valve to the end of the pipe and back again [round trip]. Next I needed to know the approximate RPM at which I wanted to run at top end, and then determine how long the exhaust valve would be open from beginning of travel to end of travel.

    When it was right I would have an exhaust pipe that would carry a positive pressure wave of exhaust pulse down the pipe to the open end. There it would collapse and create a negative pressure wave that would return back up the pipe. If the negative wave arrives back at the exhaust valve just before it closes, it will suck more of the exhaust gases out of the cylinder. This lowers the pressure inside the cylinder and makes the next intake stroke more efficient.

    When the intake valve begins to open and the exhaust valve is still off its seat it is called valve overlap. The valve overlap allows the negative exhaust pulse to actually pull more fresh mixture past the intake valve and into the cylinder. The exhaust gases rushing out are further assisted by the piston pushing up on the exhaust stroke. This forms a stream of hot gas in very rapid motion away from the cylinder. This stream of hot gas has inertia and it will tend to continue moving in the same direction out the exhaust pipe even after the piston stops pushing it. This creates a region of reduced pressure in the vicinity of the exhaust valve. By opening the intake valve just prior to top dead center, while the exhaust valve is still open (overlap), the gases going out the exhaust pipe will begin pulling the new intake mixture in behind them. Or, the intake stream will try to flow into the region of reduced pressure behind the exhaust stream, if you want to look at it that way. So overlap merely takes advantage of the inertia of the exhaust gases and the low-pressure region that it produces near the exhaust valve at the end of the exhaust stroke.

    The principle involved here and why the exhaust systems are engineered for specific lengths if we want to make use of the pulses within the pipes. The shape of the exhaust pipe is that of a constant diameter pipe [not a “grooved, rough, irregular interior], on a 4-stroke engine. That shape is an important part of the exhaust tuning, so if the pipe I.D. has "ruffles" or "valleys", or "ridges", it will seriously effect the pulsing of the system.

    The length of the exhaust pipe, prior to the muffler, is another important factor due to the speed of the pressure waves mentioned earlier. The length must be such that the exhaust pulse can travel to the end and be reflected back up the pipe to arrive at the exhaust port just before the valve closes. Higher RPM will require a shorter pipe and lower RPM a longer pipe to produce the maximum effect at the RPM we want to use for maximum horsepower. It's not practical to vary the pipe length, so we compromise with some specific length that will deliver the best overall performance just above the desired cruising range.

    The Max continuous RPM for the HS or Honda is 8000 RPMs, and I calculated the exact distance from the exhaust port to the end of the pipe is [not going to tell] inches. Technically, that is a centerline distance and not just the measurement of the exterior surface of the curves on the pipe. The shape and distance is critical and can't be ignored or changed without changing the power band numbers.

    So when people change these exhaust systems on their own, they are unknowingly changing the design power curves. Unless they know exactly what they are doing and why, they will likely end up with less power, or unpleasant operating characteristics that are no longer suited to what they are trying to use it for.

    This post isn't intended to bash anyone, however bad information on exhaust systems can cause great harm, and I suggest if someone doesn't have a clue it might be wise to use caution before causing the destruction of someone's motor.

    Although a few have voiced complaints about EZM taking the time to test and get it right, please understand that most of the problems with motorbike kits & motors have been caused by lack of proper testing and the rush to market.
    As an example the flex pipe used on Motorbike Mikes EZM prototype doesn't work as well, is louder, and has already started to rust, and he spent a lot of time and money on his system during earlier testing for EZM. His test system won't be an EZM product, and he will be installing the EZM certified flex pipe as soon as the next shipment arrives in CA.

    Hope this information is helpful and
    Have fun,
    Last edited: Apr 16, 2009
    breaksalltherules likes this.

  2. uncle_punk13

    uncle_punk13 Guest

    Good to know... Always the wealth of information Q., thanks, and I guess I have more math and work to do... :whistling:
  3. wavygravy

    wavygravy Guest

    old race trick on the old english bikes when i was twiddling around the track was long straight pipe capped, then drill a hole or 2 at a time until she started, then keep drilling till she reved a bit , then take her to the track for final flow testing. it worked in a pinch!
  4. Quenton Guenther

    Quenton Guenther Motored Bikes Sponsor

    Hi Rif,

    I already did all the math on the EZM flex pipe system. I am going to Dawson Springs, KY in a few weeks to run my proto-type on Kenny Thomas's "Roller Road". Kenny has been kind enough to set up a special 50 CC class and will even give out certificates on each run. I am not going to compete, but rather collect data on a pure stock EZM motor. If time allows I will test with the stock pipe and also the EZM flex system.
    As many already know, we had to slow down the production EZM kit because of our drive system, and the EZM flex exhaust system also boosted the top speed and required us to re-visit the drive ratios again. In order to stay legal in most states we needed to keep the top speed around 30 MPH with a heavy rider, sadly our drive system works so well we had a difficult time limiting the top speed.

    Because of clutches, gears, and "heavy" grease, many systems need to maintain ratios near 20 X 1 [18.75 to 22.75 X 1] to develope enough power to reach speeds of 30 MPH. EZM had to "load" the system to keep the speed near 30 MPH, we released the proto-type kits with a ratio of 15.65 X 1 to cause the motor to run out of power at higher RPMs. Here are a few numbers that might be of interest to many, the EZM motor is only turning 6000 RPMs @ 30 MPH, whereas motors using a average ratio of 22 X 1 has the motor "singing" at 8500 RPMs @ 30 MPH. At 8500 RPMs the motor is way past the torque curve and drinks more fuel, and is much harder on the life of the motor. When the flex exhaust sytem is added to the EZM motor system the midrange and top end increase slightly. During testing we found many "optional" pipe systems actually lowered the power, reduced the MPG, but did look "cool". In some tests we found the motor to run hotter, bogg during acceleration, and flatten out on the top end.

    When testing exhaust systems our concern was the possibility of motor damage at higher RPMs. During a motorcycle race in Hamilton, OH in the 70's I managed to destroy a 250 CC 4 stroke Ducati by installing a homemade exhaust sytem. The Ducati had managed to survive over 50 races without any problems, but in less that 10 laps [5 miles]my new exhaust sytem managed to put a hole in the piston large enough to put a "dime" through. The reason the exhaust system "trashed" my motor was due to the many "ridges" inside the pipe due to welding, so I quickly learned the I.D. of an exhaust pipe had a major effect on jetting & flow. We discovered similar results recently when we started testing flex pipes [natural gas pipes], and found the motor to start losing power when hot on the top end.

    Hi wavygravy, I too raced english bikes [Triumph & BSA] and found the short pipes with the reverse cones to work best on the short tracks, and the longer pipes to aid on the 1/2 and mile courses. When I raced Harleys I found cutting the pipes at a 45 degree angle made a major difference.

    Have fun,
    Last edited: Apr 17, 2009
  5. MotorBicycleRacing

    MotorBicycleRacing Well-Known Member

    4 Stroke Exhaust
    The one (Evans) on the right is RATRODER's

    Attached Files:

    Last edited: Apr 17, 2009
  6. nsideus

    nsideus Guest

    The Evans is one sweet bike. I like how the gearbox is polished also nice pipe but and we Quote here "when people change these exhaust systems on their own, they are unknowingly changing the design power curves. Unless they know exactly what they are doing and why, they will likely end up with less power, or unpleasant operating characteristics that are no longer suited to what they are trying to use it for.

    This post isn't intended to bash anyone, however bad information on exhaust systems can cause great harm, and I suggest if someone doesn't have a clue it might be wise to use caution before causing the destruction of someone's motor."
  7. Quenton Guenther

    Quenton Guenther Motored Bikes Sponsor

    Hi Forbisher,

    Sure do like the pipe on that AJS. Wonder if the pipe on the Evans adds more top end?

    Good looking exhaust sytems.

    Have fun,
    Last edited: Apr 17, 2009
  8. jbcruisin

    jbcruisin Member

    I know that my flex pipe gave me more power on my 49 cc Huasheng.
  9. Old Bob

    Old Bob Member

    oh boy, regular asphault ripper you got there
  10. andreasklapp

    andreasklapp New Member

    as much as exhaust matters, intake does,too. you have to tune both together for the desired power band. there are some calculators on the internet that will give you the numbers. four stroke motors create a fuel cloud above the carb (standoff). so you need something that will maintain this standoff, as well as correspond with the exhaust system. here is one of those calculators:
    and another:

    there are others out there if you look up "intake runner length calculator" etc.