Hey I have always wondered a couple of things and I finally thought of asking on here..

What do gear ratios like 1.43 mean? (that may not be accurate..) Like how in a 5spd they are all different. Can someone explain that? Also, in a Bimmer auto with the sport package it gets a "performance rear axle" and it changes the numbers....what do those numbers mean and what do they relate to? How would a change in the rear axle make it faster? Also, someone said that the compression is lower in a 3rd gen than a 4th gen. What does a compression ratio mean? I've always wondered and now I can finally know. Thanks

 

A gear ratio of 1.43 means the input shaft has to turn 1.43 times for every 1.00 turns of the output shaft. It is a way to quantify (express in numbers) the amount of mechanical advantage provided by a mechanical mechanism. Any number greater than 1.00 is called an "underdrive"; any number less than 1.00 is called an "overdrive".

A higher numerical gear ratio (such as 4.11) compared to a lower numerical gear ratio (such as 2.73) provides faster acceleration but a lower maximum speed. It's a tradeoff.

Compression ratio is a way to describe how much the unburned fuel/air mixture is "squeezed" before it is burned. Visualize one cylinder in a motor with the piston at Bottom Dead Center. The volume above the piston crown is the volume of the cylinder plus the volume of the combusion chamber. Call that number A. Now visualize the same cylinder with the piston at Top Dead Center. The volume above the piston crown is the volume of the combustion chamber. Call that number B. The compression ratio is A divided by B.