Wetbike Exhaust System Engine Base
Flow Path Optimization

Introduction

The WBW exhaust system is designed as a complete flow path from the engine to Pump:
Engine → “FlowMax” Plate → Exhaust “BaseMax” → Expansion Chamber “ExpansionMax” → Exhaust Base “ExitMax”→Pump(acts as waterbox)

In a two-stroke engine, the exhaust system functions as a dynamic flow and pressure system. For optimal performance, exhaust gases must maintain smooth, continuous flow from the engine through the expansion chamber and out the exit. Because the engine acts as a pump, any restriction in this path—especially at the beginning or end—limits total system performance.

Stage 1

Flow Area #1 – Banana Plate “2.90 in²“- The WBW FlowMax Exhaust Plate Is made from Billet 6061 Aluminum and corrects the transition between the engine and expansion chamber. It increases flow area to approximately 2.9 in² while maintaining a smooth curved flow path.(especially when port matched). OEM flow is 2.20 in² and thus a large restriction in the flow path. The part matches OEM surfaces closely so port matching is not required (but recommended). This ensures exhaust gases enter the diffuser with minimal turbulence and consistent velocity, allowing the expansion chamber to function as intended.

Flow Area #2 – Exhaust Base Inlet – 1.9″=2.84 in² – After exiting the FlowMax the exhaust enters the Exhaust Base. The flow areas on this piece are decent. You’ll notice it matches closely the flow area we designed the WB FlowMax Plate. There isn’t a ton of room here for improvement. it is possible to port/ bore this part and the FlowMax Plate area to achieve an equivalate of 2″ ID = 3.14 in² with some effort.. This would be beneficial for 86mm big bores and DT85 Engines. we recommend port matching the flow plate and just opening this part up a hair to 2″ if possible(not a huge deal if not)

Flow Area #3 – Diffuser – Expansion Chamber -1.9″ =2.84 in² The OEM Diffuser is adequate for a lot of riders. it doesn’t limit flow and can be port matched to the Exhaust Base. If you’re looking for the next step in performance look not further than ExpansionMax. This pipe like design will help direct some of the sonic waves back to the engine and maximize this area. We maxed out all dimensions without restricting flow (with a Tall Can). ExpansionMax is thus a nice upgrade for this area and has matched performance of a WPS pipe in some applications. We recommend you port match the 2″ ID of our diffuser to the exhaust base for maximum potential. If you have a WPS Pipe those are perfect to use as well and they come standard with a 1.75″ ID exit!

Flow Area #4 – Exhaust Base Exit – 1.75″ = 2.40 in² The OEM size of this part is 1.5″ = 1.77 in² which is a major restriction BaseMax is a service based part that requires a core be returned. We take your core and bore the Exhaust outlet to 1.75″ and port blend it into the rest of the base. This keeps the flow reduction close to 15% which is vital for making maximum horsepower. From here the exhaust enters pipes with a 1.75″ ID which perfectly matches our goal. For comparison the flow reduction if you left the 1.5″ restriction would be an almost 40% reduction in flow.

Flow Area #5 – Exhaust Exit – 1.75″ = 2.40 in² The OEM size of this part is 1.5″ = 1.77 in² which is a major restriction as seen above. ExitMax is a service based part that requires a core be returned. similar to what is done with flow area #4, we bore this part to 1.75″. This part requires port matching to the engine mounting plate. The bored ExitMax functions as a template for porting, all you need to do is blend the base plate into match the bored part. We offer port blending as a service as well.

When properly configured:
FlowMax (2.9 in²) ensures smooth delivery into the pipe
Diffuser (~2.84–3.14 in²) allows proper expansion
ExitMax and BaseMax (2.4 in²) provides controlled discharge

In short, the above flow areas and parts will bring your wetbike to a very healthy level of performance. Keep reading If your are looking to go further: for a 3 cylinder, big bore, or stroked build. Maxing out the flow areas further will increase performance.

Stage 2 – Maximum Performance

Further analyzing the table above shows us that our restrictions are at the Exhaust base exit and engine plate exit areas. Bored to the maximum size of 1.75″ these areas are ~17% less flow area than the 1.9″ diameter area of the inlet and FlowMax banana plate. Both of these parts must either be avoided or replaced to gain back the 17% flow areas. This is no easy task.

Flow Area #3 – Expansion Chamber – Diffuser – This part is the key to gaining the 17% flow area back. It will be no easy task. We will look to the WPS pipe or other custom pipe for the performance we require. We must Chop off the exit side of a WPS pipe high enough to achieve a 2″ ID exit diameter. Then we must replace this section of U pipe with one that is 2″ ID. Once this is accomplished we must insure the rest of the pipes are also replaced with 2″ ID pipe. We have now matched flow area to our ported FlowMax and Exhaust Base Inlet (2″ ID)

Flow Area #5 – Exit Max 2.0 – We designed this part to maximize the area and base exit. This takes this area to the maximum. Made from Billet 6061 Aluminum ExitMax 2.0 Replaces the OEM or bored ExitMax from above to achieve a 2’ID flow path into the pump. Due to the offset between the internal flow path and mounting surface, the design incorporates a blended elliptical transition. It requires port blending to the Engine mounting plate to achieve the desired flow and smooth transition.

By acheiveing a full 2″ID or equivalent flow area from the engine to the pump we allow the engine to make as much power as possible. For comparision a Kawasaki 650cc engine used a 1.5″ ID exhaust standard and a Yamaha 701cc uses a 2″ ID exhaust standard. Wetbike engines are 800cc for a 60Hp.

When properly configured:
FlowMax Ported (~3.1 in²) ensures smooth delivery into the pipe
Pipe Ported 2″ (~3.1 in²) allows proper expansion
ExitMax 2.0 (3.10 in²) removes final restriction

Summary

The WBW exhaust system addresses both the entry and exit restrictions present in the OEM design.
By increasing flow area, maintaining smooth transitions, and applying correct two-stroke exhaust principles, the system provides a clear two-stage upgrade path:
• Stage 1 – 1.75″ Minimum Flow — practical performance upgrade
• Stage 2 – 2.0″ Minimum Flow — maximum flow configuration

System Flow Areas
Typical cross-sectional flow areas in the Wetbike exhaust system:
Component – Diameter = Area
OEM Exhaust Outlet – 1.5″ = 1.77 in²
OEM Banana Plate – N/A = 2.20 in²

<The OEM banana plate (~2.2 in²) and OEM exit (~1.77 in²) represent the primary restrictions in the system.>

Below are our Parts (Areas >/= 2.4″²):
Exhaust BaseMax – 1.75″ = 2.40 in²
Diffuser Entrance 1.9″ =2.84 in²
WBW FlowMax Plate — 2.90 in²
Ported Diffuser- 2.0″ =3.14 in²
WBW ExitMax 1.75″ = 2.40 in²
WBW ExitMax2.0 2.0″ =3.14 in²