Great question! There are several basic differences. Here is my main article that I am pulling from.
Firstly, straight-cut gears produce less side-to-side torque, allowing the engine to deliver much more torque through them. Here's an image that illustrates the side-to-side torque of helical gears:
As the gears move around, they also push apart from each other, which generates an increase in friction and reduces the quantity of power that can be transmitted through them. A direct quote:
The main advantage of using straight-cut gears is that they produce no axial load. This ‘thrust force’ is generated by the sliding contact between the teeth of helical gears. This lateral force is applied to the input shaft of the gearbox, which in front-wheel drive configurations will then convert through to the driveshafts. This greatly restricts the amount of torque that can be applied through the gears before failures on other components occur.
So straight-cut gears effectively allow much larger powertrains to be placed in a vehicle without risking the output shafts and other bearings tearing themselves apart, producing a larger safety factor within the transmission itself.
Secondly, straight-cut gears are more efficient. Unlike in helical gears, the energy is being applied directly and immediately. On every revolution, there is essentially one contact point between the gears for every tooth. Contrariwise, on helical gears, the contact points are constantly changing, generating an increase in friction.
Thirdly, straight-cut gears are easier to replace and are less heavy. Another direct quote:
Straight-cut gears are also much easier to assemble and produce less-catastrophic failures when they do go wrong due to their simple structure. Heavy-duty transmission casings and shafts have to be used with helical gears for the transmission to cope with the additional axial load, therefore straight-cut gears save heaps of weight which is extremely important in a motorsport setup.
One is then forced to wonder why anyone would use helical gears. It basically comes down to two things:
- Helical gears allow for a smoother transmission of power, giving a more pleasant ride. From the article mentioned above:
The operation of the gears is also fairly brutal when straight-cut; the gears appear to ‘slam’ into place after each change and can make pulling away smoothly or getting a decent launch difficult.
- They generate less noise, also enhancing consumer experience. From designworldonline.com:
The noise level of helical gearing is approximately 10-12 dB (A) lower than spur gearing. That means 16 helical gear units generate as much noise as a single spur gear unit