Mystery Diff????????

[snaky]

Hi Chaps,
I've a problem with identification of the diff in my car. Long story but I'm fitting an electronic speedo running off magnets/sensor mounted on the prop.
I've got it all sorted except for the ratio of the diff for the calibration calculation.
The tabs on the diff read:- BP- & 7.5
So, has anyone out there got the faintest idea what ratio this diff is?
The diff is a Jag unit, driven by a T5 on the back of a 3.9RV8.
Thanks in advance for any help you can offer.

Ian.

[kdavies3]

Two choices here I reckon.
Remove the diff rear cover and count the teeth of the crown wheel and pinion gears.
Or Jack up the rear end and carefully measure the number of turns of the prop to one rev of the wheel. Not entirely accurate but will give you a good idea, as there aren't that many ratios around, the normal ones are 2.88, 3.07, 3.31, 3.54.
My guess is for your car you ideally want a 3.54:1.

http://animatedgif.net/flags/-flags-uncat/Wales.gif

Kev Davies
Dax, 383ci Chevy Stroker, Tremec TKO
South Wales

:tu :tu :tu

[snaky]

Thanks for the tip Kev,
I've just got in from the Garage and the little sucker is a 3.31:1 and it's a powerlock. So bit of a bonus coz I'd no idea when I went out there.
I will soon be able to meter the velocity on the speedo rather than judging my speed by the progress of vehicles around me.
One other thing, you suggest an ideal ratio of 3.54:1 on my car, would this explain why it tends to run out of steam in 5th gear?
Regards,
Ian.

[TINKA]

Hi Ian, I am running an RV8 3.9L with a 3.31 diff with no problems at all. The 2 real choices you have really for a RV8 are the 3.54 or 3.31, so you should not really be having any problems on that front. I think you will find with most Cobra's using a RV8 are not going to accelerate in 5th, that well in comparison to the other gears,as this is really a cruise gear rather than for accelerating.

I have an electronic speedo and the same diff and I am using 16" wheels. My speedo is from ETB and if this is the same setup as you the number you put in for calibration is 2510.

Hope this helps Martin :tu :tu :tu :tu :tu

[snaky]

Thanks for the info Martin.
I'm using 15" wheels with the 235/60 R15 98W tyre on them. My speedo was obtained thru Europa and the calculation works out pretty similar.
Then You cross-reference with a binary table to set the pins in the back of the speedo. It must be easy if I can do it!!
I was out in the beast today picking up some new tyres and 5th gear seems much improved over the last time I drove it. Could be I'm just being tarty about 5th running out of steam!!!
Regards,

Ian.