Make your GB350 even more Comfortable | SP Tadao’s POWERBOX Pipe Reviews and Installation Instructions

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The Honda GB350 is popular for its orthodox body design and air-cooled single engine. In order to bring out the charm of the newly designed long-stroke engine that produces maximum torque at only 3000 rpm, SP Tadao has been developing an exhaust system for about 6 months. With the completed power box pipe, we succeeded in realizing the comfort and exhilaration = feel-easy that is typical of big single engines.

POWERBOX Pipe produces tenacious and abundant torque typical of big singles


The POWERBOX pipe for GB350 was completed after testing several different thicknesses and lengths until the skilled SP Tadao staff was satisfied. Unlike the full cowl model, the exhaust pipes are all visible on the naked version, so the performance as well as the form of the pipes, such as the bend of the pipes, were carefully considered.


The OEM exhaust pipe has a chrome protector covering the rear portion where the catalytic converter is built in (image shown after removal). Riders riding with the OEM exhaust pipes may not notice this, but there is still a lot of room for growth in this exhaust pipe.


When installing the POWERBOX Pipe, remove the OEM silencer. Remove the bolt on the band at the front end of the silencer and the bolt nut on the upper stay of the main unit to pull it out backward. When installing the POWERBOX Pipe, the gasket attached to the connection between the exhaust pipe and silencer is not used, so if the gasket is still on the silencer side, remove it.


Remove the exhaust gas sensor attached to the front end of the exhaust pipe using a wrench. The trick is to loosen the sensor while unraveling it so that the sensor wiring will not be partially twisted. This sensor will later be attached to the POWERBOX Pipe.


The exhaust nut is located behind the cylinder fins, so a socket wrench should be used to remove it. Do not remove it all at once, but loosen the bolt at the rear of the exhaust pipe as well as the bolt in the middle of the exhaust pipe. Also, since there is a ground wire attached to the rear of the exhaust pipe that connects to the frame, remove the ground wire as well.


After removing the exhaust nut along with the bolt at the rear of the exhaust pipe, slide the entire exhaust pipe forward and remove it from the engine. If the mouthpiece gets caught on the cylinder head, pull the entire exhaust pipe forward slowly while shaking it slightly. Be careful not to let the removed exhaust pipe hit the front fender.

Since its debut in April 2021, the Honda GB350 has continued to enjoy strong sales in the small motorcycle category. The GB350, which debuted to replace the Yamaha SR400 when it was discontinued, has been supported by many users since its launch as a road model equipped with an air-cooled 4-stroke single engine in an orthodox-style chassis.

In pursuit of the "big-single" style engine in the midst of high environmental and safety performance requirements, the long-stroke type engine has a long stroke value relative to the cylinder bore. The long-stroke engine is characterized by its strong low-speed torque, which is in sharp contrast to the short-stroke engine that delivers power at high rpm.

In fact, the GB350's engine produces maximum power at 5500 rpm and maximum torque at 3000 rpm. Although the character of the motorcycles is completely different, the four-cylinder engine of the popular Kawasaki ZX-25R in the 250cc class produces maximum power at 15,500 rpm and maximum torque at 13,000 rpm, which is another dimension of the normal operating range when riding on public roads.

Another feature of GB engines, which are long-stroke and run without revving the engine to high rpm, is the large flywheel mass. The flywheel is a part inside the engine, and the heavier the mass, the more tenacious the engine is at low rpm, but the slower the response tends to be. In air-cooled single engines, which can be classified into sports type and traditional type, the GB clearly belongs to the latter. This is why it is important to season the engine in the low rpm range.

Although it is certainly one of the representatives of the 400cc class, for SP Tadao, who has consistently pursued a comfortable feeling, the GB350 exhaust system should have more room for improvement. Specifically, we thought that it would be possible to match the characteristics of the heavy flywheel mass by drawing out the torque at very low rpm, which is close to idling.

SP Tadao's original exhaust system, POWERBOX, was developed with the theme of both comfort and exhilaration in mind. For many riders, the importance of how much torque is produced in the first gear, when the clutch is connected, and when and how much torque is extracted in relation to the right hand operating the throttle, is a firm belief based on 30 years of experience and know-how in exhaust system development. We have a firm belief in the importance of torque production in the range where the driver's right hand operates the throttle.

The POWERBOX pipe for GB350 is a product that replaces only the OEM exhaust pipe; the silencer uses OEM parts.
Many riders may have the image of replacing the silencer when they think of exhaust system replacement, but the role of the exhaust pipe is important in order to adjust the torque line, which is the theme of SP Tadao's exhaust systems.

After repeated testing with prototypes of different pipe diameters, lengths, and internal structures, we have completed a POWERBOX pipe that produces torque characteristics that follow throttle operation with ease. We are proud to say that when the engine is shifted up early after the clutch is engaged and the throttle is opened quickly from a low engine speed in a higher gear, the stickiness of the engine and the way the torque rises are distinctly different from those of a normal engine.

Although only the exhaust pipe is replaced, it also succeeds in changing the exhaust sound, such as the sense of heartbeat and crisp pulse, and emphasizing the big-single character.

Owners can install by themselves by referring to the official installation video.


Compared to the OEM catalytic converter, the POWERBOX pipe is beautifully finished from the mouth to the silencer joint. The etched plate is one of the key points of the pipe.


Transplant the OEM ground wire to the rear mounting stay of the POWERBOX pipe. The other side of the ground wire is attached to the frame.


After temporarily assembling the POWERBOX pipe to the engine, install the exhaust gas sensor and the OEM silencer, and tighten the bolts and nuts of each part. The protector used with the OEM exhaust pipe is not reused, but the connection with the silencer is not uncomfortable.


The POWERBOX Pipe for GB350 has an attractive form that smoothly connects from the mouthpiece to the silencer insertion area.


Since the OEM exhaust pipe and silencer for the GB350S have a black finish, the POWERBOX pipe is also a black edition. While there is a black edition for GB350, there is no bare stainless steel version for GB350S, only black.


The GB350 with the POWERBOX pipe specification can be driven with a lively single sound without any jitteriness even when shifting up at low engine speeds. The feeling of exhilaration and exhilaration of the heavy flywheel rotating smoothly in accordance with throttle operation to generate magnificent torque is the very essence of "feel good".

The clutch lever is extremely light thanks to the assisted slipper clutch, which is a recent trend in new models, and the torque at the moment of clutch meat is fine and nerve-wracking in the action of shifting into first gear and starting.

A slightly more experienced rider would unconsciously raise the engine rpm to cope with this, but the GB350 equipped with the POWERBOX pipe does not require such concern. The large flywheel mass, combined with the idling rpm, allows the rider to start the engine by slowly engaging the clutch. Of course, this method does not allow the engine to get up to speed very quickly, so it is practical to open the throttle as soon as the clutch is engaged, but there is no need to use a half-clutch to compensate for the loss of speed.

Flexibility and stickiness while driving are also noteworthy. With a single engine, a jerky feeling is often accentuated in exchange for a punchy feeling, and control of the throttle at low openings can become severe. The powerbox pipe, however, provides torque at 40 km/h in 4th gear and 50 km/h in 5th gear, enabling a pleasantly smooth ride with throttle control alone.

You may feel a contradiction in the expression "single pulse feeling" and "smoothness," but SP Tadao's exhaust system development is to achieve a perfect balance of the two to achieve a pleasant "good" feeling.

The POWERBOX pipe for GB350 is made of stainless steel and is available in both an unpainted finish and a heat-resistant black paint version for GB350 and a black edition for GB350S. All are bolt-on versions and can be easily replaced by removing the factory silencer and exhaust pipe.

The OEM exhaust pipe has a built-in catalyst at the rear covered by a protector, but the POWERBOX pipe also incorporates a catalyst just like the OEM, so there is no need to worry about vehicle inspections. Also, the exhaust gas sensor and ground wire installed in the OEM exhaust pipe are transplanted into the POWERBOX pipe.

The installation instructions included with the POWERBOX can be downloaded from the SP Tadao online store, and an official installation video is also available for users who want to replace the exhaust system themselves.

We are proud that the POWERBOX pipe developed for the popular GB350 has a "comfortable" and exhilarating feeling that anyone can experience. We invite you to experience the "feel good" advocated by SP Tadao at the test ride events held at motorcycle accessory stores nationwide.