Diff Swap For Rb30det?

[brent32]

sorry what i meant was i can only just get away with stock diff with 4.3 gears on my setup but a less rev happy RB30 would feel more 'out of sync' with the standard 4.3 diff and as you said bubba your 3.9 diff swap was totally worth it and made the car drive better because of your engine characteristics.

My last question about changing ratios again was just because i was thinking that if your power band moves up the rev range 3.9 might feel to tall if you loose some bottom end torque.

[bubba]

My last question about changing ratios again was just because i was thinking that if your power band moves up the rev range 3.9 might feel to tall if you loose some bottom end torque.

It probably would but I'm not spinning the thing as high as the RB20 and peak power with the new turbo shouldn't be as high as the RB20 was so should still be fine.

Big duration cams and a bigger turbo again (T04Z) and you'd probably want the 4.3's to help get the thing on boost..

If the 3.9s are a bit too tall to be able to drop 4th with the new turbo I might have to go back to 4.3s (got a real itchin' to show up the v8 boys in the burnout comp at motorvation next year lol)

[unique1]

i got rb26/30 running stock gtr box and 4.3 diff ratio and im getting 2600rpm on 100kmh in 5th gear. It seems to be too many revs for the cruising speed and i would prefer it lower, somewhere in the 2000rpm range would be nice, its got a baby turbo (3076r) so boost response is not a problem at any rpm.

how do i work out what ratio i need to acheive 2000rpm in 5th?

[bubba]

How different is GTR 5th ratio to 25t 5th ratio? as I'm pretty sure I was up around 3000rpm in 5th with the 4.3s.. or maybe it's the RB20 boxes 5th that is a different ratio (3.9s went in when I still had RB20 box)

If our 5th ratios are the same then you're going to want at least 3.9 which gives me 2250rpm or 3.7's would probably give you pretty close to 2000rpm.

[unique1]

5th gear on the gtr box is a 0.752 ratio, unsure on rb25 or 20 boxes.

going by this websites calculator for speed/rpm and gear ratios...

http://www.osella.com.au/gear-ratios.htm

if i put in my current ratio and tyre height etc with the 0.752 ratio and 4.3 diff it says 2682rpm at 100kmh

if i put in the other ratios these are the results at 100kmh...

4.3 = 2682rpm

4.11 = 2564rpm

3.9 = 2433rpm

3.7 = 2308rpm

3.5 = 2183rpm

3.2 = 1996rpm

thats with 235/45/r17 tyres (643mm tyre diameter)

Edited January 20, 2009 by unique1

[psi]

Gearing can be interesting sometimes..

When we were playing with different cams in the speedway car (RB30), we found we could 'tune' the diff to suit the torque curve of the camshaft.. the smaller cams come on earlier and tended to run out of puff before the end of the straight, so we would gear it up. With the bigger cams, midrange torque was down a little but top end was great, so we fitted a lower gear set..

I remember pouring over the dyno sheet of different engine combo's and cross referencing the info, we would record peak and average RPM (with a little black box) and transfer this info onto the dyno pages..

We were aiming to have the 2 and 3 gears (we changed them around a bit, 2nd from a VL, and 3rd from a pintara i think) 'overlap' in the meatier part of the torque curve. we could then use the diff gears to move the shift points up or down in the rev range.

This meant that if we were stuck in traffic, we could short shift up or down a gear and use the torque to help pull around a slow car..

So, the diff ratio, the torque curve and the gearing in the gear box all made it possable to make a useable package.

In our case with a turbo road car, the torque curve is largly dictated to use by our turbo choice, gearbox gears are pritty much fixed and we have an oppertunity to change the diff ratio...

I started this thread to see what others had used, how it all translated into the real world and use the info as a good starting point.

does that help anybody? haha..

[bubba]

Just when you got me hooked you went nowhere!

If we can find out what the ratios of 25 and 20 boxes are, we can start to put them together, attempt to roughly compare each others setups and hopefully end up some what more enlightened!

[psi]

I was digging in some old notes from the car, we actually used two sets of 2nd gear! One set out of a VL and the other is unknown (I didn't add that bit!)

Gonna try and put more of it in words.. the jist of it anyhow

look at your torque curve, take note at where in the rev-range it starts and finishes.. can also be good to look at the HP.

if 2nd finishes at say 6000rpm, and when you pull 3rd it is already in the meat of the torque curve say 3500, then the car will have power everytime you change gear.. if you change at 6000. bring 2nd gear change back to 5000 (short shift) and then 4000 to see where the overlap happens, you will notice that 3rd will also get lower in the rev range. It will eventually get to low and 'fall' off the torque curve. This is the point that you want to avoid..

This is the plot for the setup you have now, then if you fit a taller ratio, the gearing overlaps will be spread apart, a very low ratio will eventually enable you to jump gears.. 2nd to 4th.

If you know all your gear ratio's (box and diff), and rolling tyre distances, there are formula's to calculate it all out with ease.

and while your there.. the same basic formula will be able to estimate your 1/4 mile times, and at what revs you go over the line!

Now, I'm not racing my car, I'm after less revs @ 100km/h, which should give me more fuel ecconomy. The same formula is used..

I got my copy of the info from an Edelbrock site years ago. If I find it, i'll post it up.

does that make sense?

I was only asking because I was curious to see how others found the set up in the real world with traffic lights and other road users..

(too lazy to look for all the ratio's and spend half an arvo crunching the numbers!)

[MBS206]

PSI: For working out the best fuel economy, you will need to know what your cars drag coefficient through air is at 100km/h.

This can be easily found out, by looking at your ECU and finding out what RPM you're sitting at, then seeing how much throttle you're applying and wack the car on the dyno, apply the same amount of throttle, and hold it at that RPM...

That's how much power you need to apply to keep the vehicle cruising at 100km/h...

Then you'll need to look at your torque curve...

And some funky trial and error on some calculations to get to a point, where at 100km/h, you're still making the same amount of POWER at the wheels, but dropping into a region that has a better DC...

Be aware, when doing a DECREASE in diff ratio, you CAN increase fuel economy, if the motor begins to labour...

[RRR31]

I have seen the ratio calculator somewhere for tyre sizes to diff ratio comparison from memory the ratios go as follows.

Standard 225/50r16 for a r32 gtst = 4.3's

225/55r16 = 4.11

235/60r16 = 3.9

Might work out cheaper than a diff change only thing is it will put your speedo out.

[GT-RZ]

Anyone know the top speed AVG for the 4.36 ratio? Even with all the torque the more response the better seems to chime in my ears... But then i don't want first to feel like a waste of a gear...

[Alf]

What ratios are available for a 32 gtst diff apart from the std 4.36, or do I need to look at replacing the diff itself to suit the ratio I'm after?

[bubba]

Anything from an S13 will fit straight up too. R33 gears will fit but the crown wheel bolts are a different size.

Best changing the whole diff though so you don't have to fk around with removing/reinstalling the pinion gear.

I got my 3.9 gears from an auto S13, I had my 2-way installed into the S13 diff by my diff guy then swapped the whole housing over with the one already in the car.

[toffy]

From what I've seen, R33 VS R32 manual, = same diff ratio.

This is from multiplewebsites I've read this...

Also, if you're revving an R33 manual to 6500RPM, and want to get it out to that same speed as what you'd normally be at at 7000RPM, you want a 4.1 diff.

my eyeballs have told me different , every r33 i've worked on that was factory manual has had a 4.11:1 ratio , and auto r33 the 4.36 , the same as all r32 (all r32 4.3)

*i was about to talk about gearbox gear ratio's , but psi has already (makes sense btw)*

Edited August 12, 2009 by toffy

[Alf]

Anything from an S13 will fit straight up too. R33 gears will fit but the crown wheel bolts are a different size.

Best changing the whole diff though so you don't have to fk around with removing/reinstalling the pinion gear.

I got my 3.9 gears from an auto S13, I had my 2-way installed into the S13 diff by my diff guy then swapped the whole housing over with the one already in the car.

Was the 2 way an SR20DET item or was it listed for an SR20DE, I'm having trouble finding one for the DE diff.

[bubba]

Was the 2 way an SR20DET item or was it listed for an SR20DE, I'm having trouble finding one for the DE diff.

The 2-way is an R32 GTSt item.

[aDrew_C]

I've got a 4.375:1 diff in my car and it's quite a bit too snappy for even the RB25. Was very necessary with the RB20, but it's just too lively with the stock turbo. With an RB25DET 5 speed I'm getting 3100rpm for 100km/ph which doesn't line up with what people on here are saying. It's not a daily anymore so this doesn't piss my off like it used to driving on the highway, but still, 1st gear is almost useless and if I put my foot down i'm in 5th wthin 10 seconds and wishing there was a 6th.

If I ever go 3ltr I'll definitely be going with a 3.9 ratio. Luckily for me the R200 longnose was in many cars with taller diffs such as the Z31 and earlier. Pretty sure auto Z31's were 3.9:1.

[Alf]

The 2-way is an R32 GTSt item.

Did you use the 32 GTST pinion gear as well?

[bubba]

Did you use the 32 GTST pinion gear as well?

Nope, used the S13 pinion and crown wheel, in the S13 housing with the R32 GTSt Nismo 2-way centre.

[GT-RZ]

slightly off topic but does anyone know how to tell if a r34 diff is viscous or helical??? I don't know if the thing was from an auto or manual and a quick spin of the drive shafts tells me the ratio is around 4.11:1 but i hear the vicious ones were 4.8 or something so too hard to tell. Any markings as such to give the game away?