421cc Twin-Cylinder Dragster Engine Development

This project is about engine development of a 421cc twin-cylinder two-stroke dragster engine intended for drag racing. As a recent graduate of high school (2026) I have developed a strong interest in mechanical engineering and motorsport. The goal of this project is to develop a drag vehicle and gain experience in mechanical engineering. I also intend to study mechanical engineering at university in the near future, where I will also apply to participate in formula student. I am developing this engine as an individual and all economic factors are provided by myself.

Example of drag bike

All measurements in CAD are derived from existing components I have at hand.

Components i have at hand

LAYOUT/CONCEPT

The general idea is to convert a manual two-stroke engine derived from a Yamaha YFZ350 into a CVT transmission system, in this case the Minarelli Horizontal CVT transmission system. Given the YFZ350 engine originally uses a manual transmission integrated into the crankcase, this project involves a custom design of a crankcase to accommodate the CVT transmission. The YFZ350 crankshaft does not provide mounting for various variator components as well, which means a custom axle adapter has been designed as well. The concept I’m pursuing is to heavily modify the existing CVT case and develop a custom crankcase to be bolted onto the CVT case.

General assembly

The YFZ350 engine has the ignition on the left and the driven axle on the right, whilst the CVT is only suitable for left side use, I have designed the custom crankcase where the crankshaft is flipped upside down, providing the driven axle on the left side, where I then can comfortably mount an ignition system on the right. 

OEM crankcase for YFZ350 My crankcase design

Given the CVT system is not built to withstand such power derived from YFZ350 powerband, various reinforcements have been made, e.g. reinforced axle adapter and CVT cover integrated with bearings to further resist radial forces. 

Variator axle assembly

Individual components

Crankcase:

The custom crankcase consists of two parts, upper respective lower crankcase half. This means the crankcase is split horizontally. Given this project is on a tight budget, the halves have been deliberately designed to be as simple in geometry as possible and it has also been decided to cut out any unnecessary steps in manufacturing, e.g. the crankcase ports have been simplified to reduce CNC machining steps where I then intend to port match myself. I have also decided to not include threading within manufacturing to further reduce steps, given I’ll thread myself.

Crankcase halves

Bolt on adapter:

To mount the engine assembly onto the CVT transmission system, an adapter is required. The idea is to drill a hole into the existing CVT case and then insert an adapter which has a circular bolt pattern which will then be TIG welded onto the CVT case, which allows for the mounting of the engine to be bolt-on.

Bolt on adapter

Custom variator axle:

Since the YFZ350 crankshaft is not designed around a CVT transmission, an adapter is required to mount the Variator assembly. The custom axle is to mount onto the existing axle using the existing straight key slot and further reinforced against axial forces by set screws. The custom CVT cover has integrated bearings that hold the custom axle using a custom nut, which further reinforces both radial and axial forces.

Custom variator axle (see "Variator axle assembly" picture for a more detailed description)

CVT cover:

The custom CVT cover is designed to have integrated bearings to provide further reinforcements and stiffness. The original CVT cover doesn’t provide this feature. The major cover components consist of cover plate and two bearing housings which are bolted onto the plate. I have deliberately decided to manufacture the CVT cover in separate simplified components because it reduces waste and possibly cost, also it allows for modularity in case, if something breaks.

Custom CVT cover

Frame mounts:

To mount the engine onto the frame, custom reinforced bolt-on frame mounts are needed given the original mounts could snap and had to be removed, given they were integrated onto the Minarelli crankcase halves. The reason the frame mounts are bolt-on is because it allows for modularity and it also simplifies the geometry potentially reducing manufacturing complexity and waste. 

Frame mounts

Lower crankcase bracket:

A bracket is required to mount the ignition baseplate as well allow for the mounting of a torsen arm. The bracket is bolted onto the lower crankcase half, where it would be significant reduction in waste rather than integrating it into the crankcase half. It also allows for modularity, where if the bracket breaks it could easily be replaced rather than replacing the entire crankcase half.

Bracket

3D printed intakes:

Given the design, the angle of the intakes is different than OEM YFZ350 engine, which means custom made intakes are required to compensate for carburetor angle. I have designed a two-component intake system, which is intended to be SLS printed in both TPU and PA12. The intake consists of a baseplate in PA12 which is port matched to the reed valves to improve flow efficiency. A TPU pipe is then mounted onto the baseplate with an interlocking mechanism ensuring it won’t fall off, the TPU pipes are then further secured by hose clamps. Given this design, it also allows for full 360 degree turning of the TPU pipes whilst at the same time does not interfere with the flow.

Intake

Manufacturing support

The project is funded by myself, and all components are out of my pocket. I have designed all specialised components myself but lack tools to manufacture them myself. All the specialised components are quite a big chunk of the budget and support would allow me to distribute the budget in different fields. Given I document this projects on social media and usually get around 50-100k views per video, I think showcasing your manufacturing ability could be of interest to PCBWay given my viewers are in the specific niche of two-stroke tuning. I also think, since this project uses various materials and manufacturing methods, it would showcase your broad range of materials and manufacturing methods.

Social media