Originally Posted by FocusWRC

Hi

1st is the real problem with the IB5 box it's so short. Ideally a nice 2.8:1 first gear would be spot on ( thats something I'm working on ) Just to show what a set of ideal ratios would be here are the raios that are homologated for the 06/07 Focus WRC and the Group N Fiesta ST:

Focus WRC Group N Fiesta
1st: 2.84:1 1st: 2.83:1
2nd: 2.06:1 2nd: 2.07:1
3rd: 1.52:1 3rd: 1.64:1
4th: 1.22:1 4th: 1.35:1
5th: 1.00:1 5th: 1.15:1

If it was me I would be using the ST ratios ( BTW I have some Fiesta ST gear sets being crack checked and shot peened if you are interested? ) as they are closer to ideal competition ratios.

Unfortunately you won't be able to mix and match layshafts and gears from both boxes as teeth profiles will be different, and unfortunately the gears on the layshaft are not removeable i.e the shaft and gears are all one piece.

Just to confuse matters for what you are using the car for the series 1 final drive will be better rather than the ST one. I'm sure there will be a final drive in an IB 5 box better than the 3.8:1.

If you can afford it, I would go with a plated diff as opposed to a stock RS Turbo ( I know money is probably an issue!! ) As this can be set to a known pre load and have suitable ramps fitted for your application. The Turbo diff isn't tuneable unless you have the break off point adjusted, but a plate diff would be miles better!!