to upgrade to a 2 speed i think you need a longer crank shaft for the longer clutch bell " also it was me that took the disc out of the muffler. i never put it back. it goes faster, and makes more noise. it is a whole different car without it. way faster. i don't think you will blow you engine without it. i haven't. i think it is just a noise suppressor. it restricts flow, and thats a bad thing.