R32 Help Needed On Deciding Diff Ratio

[Irwin123]

Hi all,

I have an r32 gtst with rb25 motor and gearbox making 300rwkw. My plans are to soon go on e85 with cams and bump this up to higher 300's. Anyway that is not the query,

Recently did a track day at Symmons plains in TAS and found myself maxxed out in 5th gear down the straight, it still has the std r32 4.3 diff ratio. From my research i have semi concluded that a 3.9 would pose the best of both worlds for me as a 4.1 doesn't make a huge difference although it will help.

Only problem being a 3.9 is posing very hard to find in TAS, i have had my eye out for a few months. A r33 (4.1) diff had come up for sale which i purchased for $50, so my plan was if i found a 3.9 diff (sr20det auto) that i would put the 3.9 in the r33 diff and have two diffs that i can change simply by swapping diffs. (3.9 will more than likely be 6 bolt shafts whereas r33 should be able to swap straight in?)

I know that a 3.9 would make it feel more laggy, but it is a rediculously responsive setup currenty, and i will probably get some better fuel economy with a 3.9 for light driving A taller ratio will also help reduce the wheelspin which would be nice.

My question is what is your opinion on ratio? Has anyone changed from a 4.3 to 4.1 or 3.9? Does it make it feel alot slower or does the longer gears outweigh for the less torque applied with a taller diff? Do many of you use 5th gear at the track?

Any help will be much appreciated as i keep wasting time researching and for no gain..

Thanks, Chris.

[GTSBoy]

I put a 4.11 into my R32 when I dropped the Neo in. So I've never had the RB25 with the 4.3 diff, but I can tell you that I wouldn't really want any shorter gearing than it has now. 4.3 would only be desirable for drag racing. By the same token, I wouldn't necessarily want to go taller than I have right now. But mine is just a streeter so maintaining response is obviously important. For your track car you will be keeping it on the boil anyway, so probably wouldn't lose anything in real terms.

[KiwiRS4T]

A 4.1 diff gives me a theoretical 296km/hour at 7000rpm in 5th (roughly - best I have achieved on a track is about 250) so a 4.3 would bring that down to about 282km/hr . Therefore in your case a 4.1 diff should gain you nearly 20km/hr which should be plenty for most circuits without making your car seem "laggy" and would enable you to hang on a bit longer in each gear should you choose. And as you have observed 4.1 diffs are very common and available in 6x1. 5x1 or 3x2 bolt patterns.

[KiwiRS4T]

FWIW my memory let me down. 4.1 diff is actually doing about 40km/hr per 1000rpm so 280 at 7000 rev/min. Should still be enough for most tracks.