Which Diff

[Ozzy]

I have a 1600K supersport. The previous owner had the supersport conversion, 6 speed box and lightened flywheel fitted at CC. No mention of the diff on the paperwork. The car drives ok, but I sometimes get the feeling that the gearing is a little long legged. Particularly when accelerating in 2nd from about 20/25mph the pickup feels a little lacking. Sometimes from about 20mph I have to slam it back into first, if I really need to go quickly. Is this normal? I don't know if this will help, but the car reaches about 40ish in 1st, 70ish in 2nd, 85 in 3rd. Is this normal for the six speed box. As its supposed to be close ratio, should the diff have been changed?

 

Checked the speedo reading today using GPS, and the speedo reads 5mph too fast. Is this a clue?

 

Any thoughts would be appreciated.

 

Oz

 

Still Shaking.

[Mick Day]

Most likely a 3.92:1

[Richard Price]

Ozzy,

my spreadsheet recons that you should get:-

1st 2nd 3rd 4th 5th 6th

43.04 57.60 72.81 87.70 102.45 115.77

 

If you have a 3.9 diff, 13" wheels with 168/60 tyres and rev to 7K in each gear.

Your figures don't look right for a six peed box at all.

[Ozzy]

Richard. Thanks for the reply. Car sits on 15" 195/50 tyres. Actually, just thinking about it, as the speedo reads fast I'm probably nudging 40 in 1st, 65 in 2nd and around 80 in 3rd at approx 7200rpm. However, as I mentioned earlier, I still feel that response from around 25 in 2nd is a bit sluggish. Does anyone know which diff should be fitted with the 6 speed box, and how do you tell which one is on the car???

 

Oz

 

Still Shaking.

[Richard Price]

Ozzy,

OK, 195/50 15" and 7200rpm, witha 3.92 diff would give:-

1st 2nd 3rd 4th 5th 6th

46.18 61.81 78.14 94.12 109.94 124.24

and with a 3.62 diff would give:-

1st 2nd 3rd 4th 5th 6th

50.0 66.9 84.6 101.9 119.1 134.5

Does anyone know which diff should be fitted with the 6 speed box, and how do you tell which one is on the car???

Standard fitment with a 6 speed 'box would have been a 3.62 diff. But if the car has been converted from 5 speed to 6 speed, it may well have the original 3.92 diff ratio.

To check the ratio, jack the car up with both rear wheels off the ground, mark the position of both road wheels, and the diff input flange. Then turn the diff input flange 3.62 revs and the road wheels should have turned one whole revolution.