Need Help On Diff Ratios- Thinking Of Changing 4.11 To 3.69

[conan7772]

Currently driving an RB25/30 R33 GTS-T with 310 rwkw and 990NM,

stock viscous diff now inadequate for that power.

Need to go a mech LSD, found one salavaged in a diff from an S15. Nismo GT Pro 1.5way.

There's a huge difference in ratios, R33 is 4.11 and the S15 is 3.69.

The car currently does wheel spin quite severely once the GT3076R hammers on.

I'm wondering if the 3.69 is too tall or will it be perfect for my setup.

Certainly can afford to reduce a fair bit of torque.

What sort of rev drop would I expect, say at 70 kph in 4th.

Would it make fuel consumption better or worse as a daily driver.

What about 0-100 kph

Cheers.

[bradsm87]

3.69:1 is def too tall. Just swap the Nismo centre into the R33 diff. If really want a bit taller ratio, 3.9:1 out of an S14/S15 auto would be the furthest i'd go.

[Beer Baron]

yeah I agree 3.9:1 would be as far as I'd go too. 3.7 with the current ratios will have you reaching some astronomical speeds in each gear though. 1st will feel like 2nd almost. punch it in to one of the many drag/gear ratio calculators out there.

[juggernaut1]

I have an s15 with 290rwks with stock diff ratio (3.69) and find its about right with that power........1st gear is useless still but second gear it starts getting traction (depending on air temp) and third is fine.

I also have a Nismo close ratio box so 1st 2nd and 3rd are taller than the stock 6 speed box so they may be similar ratios to your 5 speed.

To give you an idea.....my second gear takes me to around 125 to 130 klm's hr at 8000rpm....and on the dyno my 5th (your 4th) tops out at 225klm's (at 8000rpm) as confirmed by my speedo and also by the dyno chart.

Edited September 11, 2009 by juggernaut1

[bubba]

S15 manuals are 6spd though aren't they? that would help with the taller final drive ratio.

Personally, I went from the stock 4.3's to 3.9 (auto S13 gears) and it suits rather well. Probably wouldn't want to go any taller though..

[WHITER33GTS-T]

What about with the added torque of the 3lt...?

I am after another diff for my 33. I have a 26/30 and C4 auto so want something to swap over for cruising the streets and not reving its arse of and keep the 4.11 for the drags.

[juggernaut1]

S15 manuals are 6spd though aren't they? that would help with the taller final drive ratio.

Personally, I went from the stock 4.3's to 3.9 (auto S13 gears) and it suits rather well. Probably wouldn't want to go any taller though..

Yep ...6 speed as mentioned in my post...I also mentioned that I have taller ratios with the Nismo box....I also run taller aspect ratio tyres.....so overall I have quite tall gearing in the lower gears...which the pissy 2 litre SR has no trouble coping with.

I had a 4.11 diff in another car (changed from 3.54)...sure in gear acceleration was like it was hit up the arse by a truck...... but I was that busy changing gears...I'm sure the mechanical torque advantage was outweighed by the extra time lost in more frequent gear changes.

As mentioned these guys have 3 litre bottom ends ...the torque of which should more than cope with the 3.69 ratio....however.....this would also be subject to what wheel/tyre sizes/apect ratios (read rolling diameter) you are running too.

Edited September 12, 2009 by juggernaut1

[usmair]

can some please explain this tall diff business for the newbies out there

[juggernaut1]

taller diff ratio = lower numerical diff ratio = higher speeds for each gear.

example 3.69 is a taller diff than say a 4.11.

3.69 will haver higher speeds for each gear than 4.11 ratio and will have higher top speed than the 4.11 if the engine has sufficient torque.

as a broad generalisation bigger engined cars have taller diffs because they have the torque to use the taller diff ratio.....but depending on tyre/wheel diameter

Edited September 12, 2009 by juggernaut1

[conan7772]

I have an s15 with 290rwks with stock diff ratio (3.69) and find its about right with that power........1st gear is useless still but second gear it starts getting traction (depending on air temp) and third is fine.

I also have a Nismo close ratio box so 1st 2nd and 3rd are taller than the stock 6 speed box so they may be similar ratios to your 5 speed.

To give you an idea.....my second gear takes me to around 125 to 130 klm's hr at 8000rpm....and on the dyno my 5th (your 4th) tops out at 225klm's (at 8000rpm) as confirmed by my speedo and also by the dyno chart.

130 kph in 2nd gear ! That's mad.

Good point with the gearboxes, S15 is 6 speed, didn't know that.

Still a hard decision to make without having driven one.

Try before you buy would be nice.

Currently with the 4.11, 1st and 2nd have always been too short for me even with the RB25 with 185 rwkw.

3rd has always been nice and long.

Now, with the 3.0 it still breaks traction heavily in 3rd, say already rolling at 70 kph, then floor it, engines ramps for about 2-3 secs then turbo hammers on, then sideways as hell.

But if throttle is controlled better, accelleration is constant and endless, always seem to run out of road.

So thanks for the comments, I'll stick with my current ratio or maybe a touch taller.

I'm not looking at overtaking the Veyron.

[juggernaut1]

Lol ..stock s15 does over 230klm's .....I want to be at least faster than that.

Edited September 12, 2009 by juggernaut1

[bradsm87]

Lol ..stock s15 does over 230klm's .....I want to be at least faster than that.

That's power limited, not revs limited I'm pretty sure. I think when I still had the stock diff in my S14 which is also 3.69:1, it did about 245 at about 6200rpm then backed off lol.

Edited September 14, 2009 by bradsm87

[juggernaut1]

Yes you are correct - hence with the extra power = acceleration and speed increase. I didn't want to end up being slower with lower ratio diff like what happended with my previous car when swapped to 4.11 diff with twice the rwhp....obviously gearing was too low for the power.......revved out past peak hp in 5th gear at Barbagello.

Esentially my point being if you have the torque use the gearing to your advantage but it depends on what your using your car for too.

[bubba]

Ok I'll see what I can remember (haven't driven the car in months )..

RB25 turbo gearbox

3.9:1 diff gears

235/40/18's

2nd gear would get me about 110kmh but that's shifting between 5500-6000rpm (due to power nosing over).

5th gear 3000rpm = 140kmh

5th gear ~2500rpm (I think) = 100kmh

Once my engine is back together, I will have a much better idea as I will be able to wind it out to redline.