Char1ie

iTrader: (6)

Does anybody know a resource for info on diffs and ratios in a 2WD Sapphire?

I drove at Castle Combe yesterday and my car is hopelessly under-geared there but perfect at Brands.

It is time to start investigating what is in my car, and what I can put in my car. So I need to know where to start, what markings I should look for, and how to work out the relationship between top speed engine power and diff ratio.

Thanks all,


Charlie

AndyPen

iTrader: (24)

Glad you got your drive in mate

Norwegian Wolf

Gearbox:

1.gear: 2,95 : 1
2.gear: 1,94 : 1
3.gear: 1,34 : 1
4.gear: 1 : 1
5.gear: 0,8 : 1

Diff 7,5" : 3,64

Seen quite a few changing to diffs with 3,92 ratio..

205/40-17, 7000rpm, 3,64 diff : 167,7mph
205/40-17, 7000rpm, 3,92 diff : 155,4mph
205/40-17, 7000rpm, 4,29 diff : 142,3mph

5.gear on a T5 box on the results listed above.

Char1ie

iTrader: (6)

Thanks AndyPen. My turbo let go at the end of qualifying but my Araldyte fix on the radiator held! Anyway, no race for me and lots of oil smoke from the turbo failure. Next race is at Brands 26/27 April.


Thanks NW for the diff ratios. I am running 17s and don't think I am getting much more than 110mph at the top of 5th. That comes up very quickly. I think my car was set up for hillclimbs before so may have an even shorter ratio than 4.29.

Is there a marking on the diff case that will tell me what's inside or will I have to open it up to see what the gears are? It doesn't look too difficult to remove. Is it sealed?

Charlie

Norwegian Wolf

With a 4,44 ratio you would reach 137mph at 7000rpm..
With a 5,5(!) ratio you would reach 111mph at 7000rpm..

Both results with the same tyres, box etc. as above..

Often there is a little sign mounted by one of the bolts on the casing..
But its easy to remove the casing, and the ratio should be printed on the crownwheel(?).
It's like that on org. diffs.. Think it's the same on other diffs aswell.

Char1ie

iTrader: (6)

Thanks again. My car was a hillclimb car so may well have 4.4. or 5.5.

I have been told that my diff is locked using high pressure viscous coupling. I assume that this means that the diff oil is under pressure so I am reluctant to open it.

Is there a way of determining the ratio by the relationship between input and drive shafts? 5.5 turns of the input driveshaft = 1 turn of the wheel driveshaft on a 5.5. diff? Or is that too obvious?


Thanks,

Charlie

ian sibbert

Hi Charlie,

The final drive ratio's for the 7.5" diff are usually only at the lowest 4.4:1, i'd hazard a guess to say that is probably the ratio most used in short sprint and definately on gravel spec rally car. If I were building a circuit racer i would be selecting a 3.9:1 for a good combination of decent acceleration and top end speed.

The way to check is exactly as you describe mark the flanges and count how many turns you get on the input in relation to the output. You should get 4 turns and it resting at approximately 4 - 5 oclock for a 4.4:1(if you get my drift)

The LSD part of your diff, is as you a say a viscous type, this is a pack bolted to the crown wheel through which the output flange passes, it is a seperate item to the crown wheel, pinion and sun & planet gears.

The viscous diff is a series of perforated plates suspended in a silicone gel, as they turn at the same speed, in normal straight line driving the temperature of the gel doesn't increase, and the diff is open but as the diff begins to operate (cornering etc), the temperature of the gel rises and solidifies and in turn causes the diff to begin to lock.

The gel in these viscous diffs can be changed to promote a higher level of lock usually measured in Nm, Bara or CTS both offer uprated viscous diffs.

HTH

Ian

Char1ie

iTrader: (6)

Thanks for the comprehensive answer Ian.

I will remove and investigate my diff. I'm guessing 4.4.

Then I need to think about the different lengths of UK circuits. My car hits 4500rpm in 5th at the end of the straight at Brands but ran out of grunt at Combe.


Charlie

ian sibbert

Charlie,

The 4.4:1 ratio will make the car feel very spirited off the line and as long as you dont run out of puff will perform well....if funds allow have another diff available to put in for the longer circuits. We only have 3 bolts on each of the rear lobro joints to make exchanging the diffs easier, the remaining 3 bolt holes are filled by replacing them with dowels, that way you still have the necessary strength but less time faffing around changing the diff......just make sure you re-check them at shortish intervals to ensure they stay tight....

Regards

Ian

Char1ie

iTrader: (6)

It certainly does.

Ian, the dowel system sounds good. Did you make them or is there an off the shelf dowel that you use?

Do you have any thoughts on Castle Combe and diffs? From comments I have heard before it seems a 3.9 suits the circuit. Of course it varies from car to car. Experience counts for a great deal I guess.


Charlie

ian sibbert

Hi Charlie,

I'm not particularly qualified to comment on Castle Combe, but given your present engine/gearbox spec (I assume standardish) i'd probably recommend trying to get hold of a 3.9:1 diff, you could upgrade the engine spec to let her rev harder but you'll be into serious money in no time and also threaten the other transmission components.

The dowels are available as a 909 part, but you can easily manufacture them is you know a good engineer.....

You can see them here on a pic of a front diff....



Ian