Sean,

My old Saph 4x4 with the Paul Bailey 400 BHP engine was used mainly as a road car, but I did few track days as well.

With the standard diff ratios it was "OK" on the track (17" wheels) but when I blew up centre diff (cracked the casing in half) I got Steve Scott to fit a 3.9 rear diff and centre diff step-off/chain (3.9) and also to fit a plated front diff.

For road and track use it TOTALLY transformed the car. It now had much sharper acceleration an ALL gears.........The car was perfect for Castle Coombe, just pulling 6.4 k RPM in fifth at the crest of Avon Rise (145 MPH indicated).

Another huge benifit was that on the road in fifth gear (in a race situation lol) it became a single gear car from 90 MPH onwards..........I.E. on fast A roads I just kept both hands on the steering wheel, enjoying punchy 5th gear acceleration all the way to the rev limiter...........That car surprised many a M3/Scooby/EVO 6 owner, even on motorways