Matt George Posted April 30, 2012 Share Posted April 30, 2012 Hi all,I've just fitted a rebuilt 2500 engine into my 2000 saloon, and am now pondering what to do with the diff. Currently still on the original 4.1:1 item, which is fine around town, but not so good on the motorway. At 3000rpm - still running in, so not gone above 3k yet :( - you're doing 65-70mph, which isn't really great, given I'm setting up the car for long trips trips down to Le Mans, RBRRs etc.I have a recon Chris Witor 3.45:1 diff that I got cheap a while back, so was planning to put this in, but am now wondering what effect it it will have on initial acceleration, hence my consideration of a 3.7:1 diff. Anyone else got any thoughts on the better of the two? The car also has a gearbox from a late 2500S fitted, with a 28% overdrive, so I'm thinking that combined with a 3.7:1 diff would be a good option? Be great to hear what other people reckon either way.If it makes any difference, the engine has had the head skimmed, plus a PI cam, HS6s, full extractor manifold and Witor sports exhausts system, which obviously has a bearing on the driveability of the car too.Cheers,Matt Link to comment Share on other sites More sharing options...
CRAJ Posted April 30, 2012 Share Posted April 30, 2012 3.45:1 would be what a 2500S would have as standard with the 28% o/d, this will give you 70mph at about 2600rpm, does on ours anyway.The upgrades you have to the engine will just make it more able to pull that gearing, a standard S can run out of steam a little on long motorway drags, especially if you are doing less than 60-65.Colin. Link to comment Share on other sites More sharing options...
andy thompson Posted April 30, 2012 Share Posted April 30, 2012 3.7 Diff in 2500 with 28% overdrive works fine - especially for lots of fast road - mountain work like 10CR - also a good diff for towing compared to the 3.45.If you have one of each why not just try them both? I ran a 3.7 in my tuned PI and it did a 14.8sec 1/4 mile :) obviously a lot of other things helped too LOLYuo "might" lose a bit of economy but that's all down to the right foot really - the OD should make cruising pretty similar - as good as an early 2.5 with 22% A Type for example Link to comment Share on other sites More sharing options...
thescrapman Posted April 30, 2012 Share Posted April 30, 2012 3.45 I suspect. Nice long legged cruiser. Link to comment Share on other sites More sharing options...
Matt George Posted April 30, 2012 Author Share Posted April 30, 2012 lagerzok wrote:3.7 Diff in 2500 with 28% overdrive works fine - especially for lots of fast road - mountain work like 10CR - also a good diff for towing compared to the 3.45.If you have one of each why not just try them both? I ran a 3.7 in my tuned PI and it did a 14.8sec 1/4 mile :) obviously a lot of other things helped too LOLYuo "might" lose a bit of economy but that's all down to the right foot really - the OD should make cruising pretty similar - as good as an early 2.5 with 22% A Type for exampleAndy - problem is I only have the 3.45:1 just now, so wanted to test the water for opinion on both before trying to source/swap for one.I think I'll probably go with the 3.45:1, and when the engine is nice and loosened up and able to rev more freely, it will be better able to pull the taller diff. Should be a lot of fun by then :)Cheers,Matt Link to comment Share on other sites More sharing options...
andy thompson Posted April 30, 2012 Share Posted April 30, 2012 3.45 Diff works fine too - thats why the factory chose it - 3.7 works better in a tuned PI if you want to pull 125mph (whistle) as lets the overdrive top work as a top gear not an overdrive. You can always fit lower profile tyres if you want lower the gearing slightly ..... Link to comment Share on other sites More sharing options...
Nick Jones Posted April 30, 2012 Share Posted April 30, 2012 I find that my PI would happily pull a taller gear - that's with 3.45:1 and 22% A type OD. I've yet to have a properly healthy engine too........Should you need to re-home the 3.45:1 diff for any reason - mine howls........ ;)CheersNick Link to comment Share on other sites More sharing options...
estately Posted May 2, 2012 Share Posted May 2, 2012 My money is on the 3.7, at the moment.I've just rebuilt my 2500 motor, replacing a 2000 in my daily driver, and initially fitted a 3.45 diff. But found it to be a bit less than I'd hoped when towing the racecar.So I put a 3.7 in and find it great. Speedo is slightly out, but I think its still comfortable on the motorway in terms of revs. And a nice increase in pulling power.I had planned to put the 3.45 back after the tow to the race meeting but I think i'll leave it in now.At the same time i made up some Datsun half-hafts and can't rave about them enough! awesome! Ken :) Link to comment Share on other sites More sharing options...
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