I will be running a Sierra 3.64 diff with Type 9 on a ST170 Zetec, and am wondering how this gearing will work in a Seven.

My particular Type 9 has the following gearing:

1 3.36:1
2 1.81:1
3 1.26:1
4 1.00:1
5 0.83:1

Some have suggested that the diff might be too tall, commenting that a 3.9 or 4.1 would be more appropriate for a Seven. Others have suggested that 1st gear will be too low and that I should consider contacting BGH for a taller first gear.

I know that many are running the T9 and Sierra diff combo, so would appreciate experienced opinions as to how the car fits into the real world as a driver. My planned use is primarily as a road car, but I will definitely autocross the car and may do some road racing.

Thanks for any input.

Shaun

people may have good advice, but in reality i think you need to drive the car and see for yourself what suits you. Id keep on as planned, and have a spare 3.92 diff that you can swap in in half an hour if required. Pick the diff for the event, so to speak. I doubt that you will need to change first, but in the unlikely event that you really want to, you can always pull out the box.

based on 205/60/13 rear tyres and redline of 6500rpm i make the max speed in each gear:
1st:36
2nd:67
3rd:96
4th:121
5th:145

a bit high
with a 3.92 it would be

1st:32
2nd:62
3rd:89
4th:112
5th:135

If you go the BGH route then the costs will mount up quickly! No point in opening the box without a full rebuild, ratios and the lubrication mods they can do to improve durability.

I found 1st/2nd on a standard GBX to be quite short when using the 3.92 final drive but that FD ratio was the best choice for fast road use. I now have a 4.44:1 FD and a BGH close ratio GBX. Great on track and short blasts on he road.

Hmm ST170 engine has 145 ft-lb it isn't exactly lacking in grunt so certainly try the 3.64 FD first.

Thanks for all the replies. Sounds like I should try the 3.64 rear first.

With regard to BGH, I can't see doing too much with them as I am in Canada and shipping of a complete gearbox would be horrendous. I was just thinking of them as a source for a taller first gear.

Using a ratio calculator so kindly provided by one of the list members here, and using the info provided by cossy, it looks like I might be okay as is. I guess we'll see...

Thanks again.

Shaun