Here is an 8-speed sequential gearbox, which is part of my series of orderly gearboxes with slideshow instructions on YouTube.
About this creation
This is probably the smallest ever 8-speed sequential gearbox. It is based off of my close-ratio 4-speed, which is very small itself. Because it's ratios are so close together, and because it using a two-speed to multiply the four-speed to an eight-speed, I used a two-speed with a wider range than the entire 4-speed, and the shifting order is as follows, if the numbers stand for the 4-speed and the L/H stands for the 2-speed: 1L, 2L, 3L, 4L, 1H, 2H, 3H, 4H. There is a built-in stepper, as well!
You can see my slideshow instructions at: