FD modification is pretty popular these days....everyone is getting a 4.7, 4.9 or even 5. So what is the limit of FD for our cars? What's the math behind it?

 

5.0 is the shortest I've seen. I would imagine that a shorter gear would NOT be possible because of clearance issues with the tranny housing.

As well, the gear on the counter shaft would be so small that it might make the shaft to weak.

Regards,

 

And a FD with ratio higher than 5 won't do any good (most likely).
You will get issues like traction, gears being too short and need to shift too frequent, terrible top speed etc. (unless you customize all the gear set ratio)

 

Just throw a 6 speed in... yummmmmy
http://www.quaife.co.uk/product/gearbox.htm#six

 

6 speed would be sick