JimmyBoy

iTrader: (1)

This is something ive wanted to try and get a bit of a clearer understanding of for a little while now, as it's something im not too sure about.

My understanding is (rightly or wrongly?) that the final drive ratio ultimately relates to the number of turns the motor makes to the number of turns the wheel makes?

ie. the final drive ratio of a S2 is 3.82, so the engine will do 3.82 revolutions to the 1 turn of the wheel, am I understanding this correctly?

The final drive will dictate your maximum speed in each gear, in top gear the standard ratio of 3.82 gives 153mph at 6500rpm (195,45,16 profile tyres) while a lower ratio of say 3.4 in top gear would give you a speed of 172mph at 6500 rpm (195,45,16 profile tyres).

So presumably it is a bit of a balaning act between, how much acceleration you want, and how much top speed you want. I assume BHP is going to play a big part in your maximum speed as otherwise you could put the final drive on a standard RST (132bhp) to say 3.1 and theoretically achieve 188mph (not going to happen - the horsepower just insnt there!), and also the shed like aero dynamics of the S2 isnt really going to do you any favours!

So, assuming you had the horsepower (say 300+ bhp), what would be a good final drive ratio (does it come down to your driving style? ie is it going to be a track car / road car) and am I grasping the concept???

I would be interested to know what the final drive ratio's of christians S2 was, and what sunny's 179mph FRST final drive was.

Sorry if I went on a bit there!

JesseT

You have to take into account that the fifth gear ratio is not 1, but rather for the S2:

1st 3.15
2nd 1.91
3rd 1.28
4th 0.95
5th 0.76

So the total ratio at 5th will be 0.76*3.82 = 2.90

The available BC gearbox final drives are 3.58, 3.82, 3.84, 4.27 and 4,29.

Once one has enough power (200+) the close ratios don't affect the acceleration too much anymore. It might even become the opposite.

Anyhow a 195/50-15 tire will have a rollout of:
3.1416*15*0.0254m+2*.50*0.195m = 1.810 m
and at an engine speed of:
6500 rpm = 39000 rph
will have a theoretical 5th gear speed of:
(39000(1/h)/(3.82*0.76))*1.810m = 243400 m/h = 243.4 km/h

JimmyBoy

iTrader: (1)

So if you was having your gearbox rebuilt, you could only have one of the above final drive ratio's ? and not any old figure ?

Also, if you was getting your gearbox rebuilt, are all the gear ratio's (ie 1st, 2nd, 3rd etc...) fixed? ie you cant change 1st gear from 3.15 to 3.08 or something ?

Thanks for your reply!

DannyDK

The best one for high top speed is the xr3i mk.4 gearbox, and then fit a LSD to it, the you should be on your way, thats what im gonna do

Oranoco

I think you will find the only difference between a 3i box and an S2 box is the LSD so if you use a 3i box and sepnd a wedge putting the LSD in you will in effect have a stock S2 box And it won't be any tougher. The standard S2 ratios are good for 150mph, how quickly do you need to get to Tesco's anyway?

JimmyBoy

iTrader: (1)

hahahaha

It's more out if interest than anything really, and also I at some point in the near future will hopefully be getting a Stage 2+ bernie box, and was just after some opinions so I could make a better educated decision on what final drive ratio may suite me better.

The other thing that crossed my mind was the actual ratio's of the individual gears, as ive seen some high powered S2's and 1st, 2nd and sometimes 3rd gear are almost useless as the wheels just spin. I dont know if there is any real solution to this, other than not having such a heavy right foot.

JimmyBoy

iTrader: (1)

I second what Oranoco said, and also cant see this being true.

The final drive on the mk4 xr3i is 4.27:1 as opposed to the RST's 3.82:1. So in the case of the car being identical in every way, the standard RST gearbox will give you 153mph @ 6500 rpm, whereas the xr3i gearbox would give you 137mph @ 6500rpm.

Thanks for your replies!

NUTS RuS

iTrader: (35)

I have a 4.7 final drive (well 4.69:1 actually). It won't even do 120mph, but then again I've never needed it too Your "balancing act" theory is correct

0-60 times can easily be reduced with careful consideration to gear/ratio selections.

DannyDK

The final gear ratio in an xr3i mk.4 box is the one that will provide you with the highest top speed, just look in your Haynes manual, there you can see that the S2 has the box with the uppersit

DannyDK

The xr3i mk.4 have 4.27:1
S2 have 3.82:1

DannyDK

Hmmm, everybody i know says the same as i do(easy to say i know )but my experience with both cars(have had several of both)is that the xr3i mk. is heavy compared to the S2. I know a guy who had an S2(before he smashed it completly)and it drove about as you say 225km/h, he then gor hold of an xr3i mk.4 box and then it drove 235 km/h before the rev limiter stoppet the accelaration, so i dont get it that it should be the same, why have different ratios if the outcom is the same

DannyDK

So is it the lower the numbers the heavyer the box?

Tim

iTrader: (1)

The problem is the crownwheel on some boxes have less holes than an s2 has so its being able to find the right combination most of the long boxes will be very weak good for topspeed but nothin else as the lsd will not fit or so im led to believe.
Even a standard ratio box will give good topspeed if you have enough power and revs.

Tim

iTrader: (1)

you want the 3.58 final for topspeed.
1.6 cvh with carb

JimmyBoy

iTrader: (1)

Im just curious about if you was to say have this final drive ratio, its is going to give you a higher top speed, but how would this affect the driving at lower speeds? Would it make it a bit more of a pig to pull off in 1st? or would it really not make too much difference ?

CliffRS4

iTrader: (1)

Interesting thread this

Charlie Chalk

iTrader: (1)

DannyDK

My theory is that it would be a litle heavyer in all the gears, not much but just eneough to give you more top speed. My ERST goes all the way up to the rev limiter in 5th gear, so i should be able to have a higher topspeed with another box. By the way, was i right when i asked that the lower the number the heavyer the box?

JesseT

I just bought an old Orioin mk1 Diesel box with the 3.58 ratio. I'll take the final drive ratio from that and put it inside my S1 gearbox because I'm sick and tired with the 4.27. My Orion had a 3.84 ratio which was much better than the S1 ratio. It also makes a huge difference to the economy if travelling at high speeds with 5th gear.

Cabstar

Hi, i am driving with the gearbox from the 1.6 cvh on my Rs Turbo, i am realy surprised, how smooth the car drives under different loads, and the box has done about 320.000km with out any problem

JimmyBoy

iTrader: (1)

Like I said earlier, is the only a pre-set selection of final drive ratio's you can have? or can you have any you like?

Thanks!

JesseT

You can have any one you can find. Ford only made the ones I mentioned and then quaife at least offers some really short ones with straight cut gears for rally and sprint use.