Hi,
Here is some information many may find useful.
When comparing two horsepower four-stroke motors on a dynamometer, the motor with the greatest average power [wide power band] is the one that will typically work best for powering a single speed motorbike.
Additional factors that should be taken into consideration are, RPM range, torque & hp curves.
An excellent example is the Honda GHX50 motor because of the wide RPM range. The motor produces 2.2 foot pounds of torque at 4500 RPMs, but the torque doesn’t drop below 2 foot pounds until 7800 RPMs. Very strong torque numbers over a 3300 RPM range. The motor continues to produce additional hp all the way to 7000 RPMs, but doesn’t drop below 2 HP until it reaches 8000 RPMs. The HS 142 motor is very similar but available data suggests a slightly less powerful motor [our tests indicate the motors to be almost identical, however the HS manages to rev slightly higher].
One of the biggest differences in power output and performance was the exhaust system. Most kits contain a short pipe and small diameter I.D. A few still leave the “box” style muffler on the motor, but forgot to factor in several important concerns. The “box” style exhaust system wasn’t designed to operate in opposition to wind, and would be much safer if the exhaust was re-routed [when riding the exhaust is pushed by the wind towards the rider].
Exhaust systems cannot produce additional power on their own. The combustion efficiency and engine pumping processes is influenced by the exhaust system. The design objective for a high performance exhaust is to reduce engine-pumping losses, and by doing so, increase volumetric efficiency. The result of reduced pumping losses is more power available to move the motorbike. As volumetric efficiency increases, fuel mileage also increases because less throttle opening is required to move the motorbike at the same velocity.
A lot of confusion surrounds the issue of exhaust "back-pressure". Many performance-minded people who are normally very knowledgeable still cling to the old school concept.... "More back-pressure means better performance."
Facts indicate backpressure in an exhaust system decreases available engine power because of increased engine-pumping losses. It is true that a few motors are mechanically tuned to certain amount of backpressure and can show a loss of low-end torque when that backpressure is reduced. It is also true that the same motor with reduced backpressure can be re-tuned to show an increase of low-end torque with the same reduction of backpressure [often with a change in ignition timing]. Most importantly, maximum mid-to-high RPM power will be increased with the lowest possible backpressure. Period!
The ultimate engine modification is to maximize the proper air and fuel flow into, and exhaust flow out of the engine. The inflow of an air/fuel mixture is directly influenced by exhaust flow, particularly during valve overlap (when both valves are open].
One of the biggest issues with exhaust systems is the motor needs the highest flow volume possible throughout the mid-to-high range portion of the power band for maximum performance. This is where the problem starts, larger pipes flow better at higher RPMs and smaller I.D. pipes work better at lower RPMs creating a fundamental conflict. A smaller diameter exhaust pipe will provide higher flow velocity than a larger diameter pipe. Of course the laws of physics won’t allow that same small diameter pipe to flow sufficient volume to realize maximum power at higher RPMs. This is the paradox of exhaust dynamics and the solution is usually a compromise.
The most common mistake made by many is the selection of an exhaust system with pipes that are too large in diameter, too long, or too short for their engine's state of tune. Bigger is not necessarily better and is often worse. Shorter is often worse, as is too long.
All pipes are not equal, both inertial scavenging and wave scavenging will vary with I.D., and length, often dramatically. This, in turn, causes different tuning requirements between designs. "Tuning" does not mean installing a new sparkplug. It means configuring the correct components for maximum efficiency to work as a “team”.
Longer pipes typically produce more low-to-mid range torque than shorter pipes and it is torque that moves a motorbike. Where in the power band do you want to maximize the torque? Ever notice how long the EZM flex pipe is?
· Longer pipes increase power below the engine’s torque peak and shorter pipes increase power above the torque peak.
· Large I.D. pipes limit low-range power and increase high range power.
· Smaller I.D. pipes increase low-range power and limit high-range power.
Here is where the boys are separated from the men!
Inertial scavenging………..
Wave scavenging……..
Both impact exhaust system efficiency and affect each another.
Scavenging is simply gas extraction. These two scavenging effects are directly influenced by pipe shape, diameter, and length. When the exhaust valve opens, two things immediately happen. An energy wave, or pulse, is created from the rapidly expanding combustion gases. The wave enters the exhaust pipe traveling outward at a nominal speed of 1,300 - 1,700 feet per second. This wave is similar to a shock wave from an explosion. Simultaneous with the energy wave, the spent combustion gases also enter the exhaust system and travel more slowly at 150 - 300 feet per second. Considering the energy wave is moving about five times faster than the spent gases, it will get where it is going faster than the gases. When the outbound energy wave encounters a lower pressure area such as the muffler or the atmosphere, a reverse wave is reflected back toward the exhaust valve without significant change in velocity.
The reflected wave moves back toward the exhaust valve on a collision course with the exiting gases whereupon they pass through one another, with energy loss and turbulence, and continue in their respective directions. What happens as the reflected wave arrives at the exhaust valve depends on whether the valve is still open or closed. This is a critical moment in the exhaust cycle because the reversed wave can be beneficial or detrimental to flow, depending upon its arrival time at the exhaust valve. If the exhaust valve is closed when the wave arrives, the wave is again reflected toward the exhaust outlet and eventually dissipates in a back and forth motion [pulse]. If the exhaust valve is open when the wave arrives, its effect upon exhaust gas flow is only part of the problem as it can also alter the incoming fuel charge [wave scavenging].
An exhaust pipe of the proper length (for a specific RPM range) will place the wave’s anti-node at the exhaust valve at the correct time for it’s lower pressure to help fill the combustion chamber with fresh incoming charge and to extract spent gases from the chamber. This is called wave scavenging.
When gases move outward as a gas stream through the exhaust pipe, a decreasing pressure area is created in the pipe behind them [vacuum]. Think of this low-pressure area as a partial vacuum and visualize the lower pressure "pulling" exhaust gases from the combustion chamber. It also helps pull fresh air/fuel charge into the combustion chamber. This is called inertial scavenging and it has the largest effect on engine power at low-to-mid range RPM.
And that is how it all works!!!!!!!!
BTW I have 2 different PDF on Honda and one says 2.5 HP and the other says 2.1 HP, if anyone wants a copy let me know and I will email to them.
Hi Jesse,
The flex exhaust can be ordered from your nearest authorized dealer. We are currently out of stock as it is our fastest moving accessory, but shipments are in route.
Have fun,
