More specificaly, is there a haussdorf non-discrete topology on $\mathbb{Z}$ that makes it a topological group with the usual addition operation?


Question: What is meant by Engine Life Factor(ELF). Should I consider it while buying a vehicle? If so why is it not widely quoted?
Answer: ELF or Engine Life Factor is a calculated number:


ELF = 100,000/Max RPM x CR 
where: RPM = Revs per minute and CR = Compression ratio


It is supposed to indicate the longevity of an engine by reducing the "number" by how fast the engine is able to run and what its numerical compression ratio.

I see some major flaws in this thinking. 

First, it doesn't take into account how the engine is built. For instance, the difference between a turbo charged engine and a normally aspirated engine. In order to get the performance out of a purpose built turbo charged engine, you lower the compression ratio to allow the turbo to produce more boost with less fear of pre-detonation. This allows for great HP output with out the damaging affects of the dread "knock". Yet, a turbo charged engine will absorb a greater amount of abuse. You can usually bet the turbo engine would not last as long as a normally aspirated one. 

Second, this also does not take into account maintenance of an engine. An owner which takes care of a car's engine by utilizes premium filters and lubricants, performs all maintenance on-time, and doesn't abuse his vehicle can expect the engine to last an exceedingly longer time than someone who doesn't perform the same maintenance regime and abuses it. This major factor is nowhere factored into this equation (with no real way to do it either). (NOTE: I'm sure I could come up with more reasons, but think this is sufficient for showing my point.)

Realistically, this is just an arbitrary number which might be assigned to an engine based on some very unrealistic factors which have nothing really to do with longevity. While having no proof, this is probably a "real" reason why manufacturers don't utilize this number.