HELP!!! Westfield diff problems
Discussion
Help please! I've recently put a 2007 ZX10 engine in my westy and although the acceleration is phenomenal, it tops out at 112mph (accurately measured). On pretty much every track this is way too low, and I need to increase ideally to about 130mph which I think is a good compromise and should suit most of the UK non GT tracks. Its really frustrating as I've just started the track day season so I need to get this sorted quickly.
The only diff I could get my hands on at the time of the conversion was a sierra 3.38 (i was looking for a 3.14 but found it impossible to get one), however even with a 3.14 I've calculated it would still only top out at 120mph. I've worked out that to hit 130mph I need a 2.9 diff, which it seems I can get from a 1989 onwards mini or metro 998cc unleaded. Is my only option to use this entire diff and housing, make a new bracket to support the diff housing and then weld the ends of the metro driveshafts onto my existing driveshafts? This all seems like a lot of hassle - surely there are other people putting modern litre bike engines in without going through all this? Apparently the gearing on a new litre bike engine is very different to the earlier blade / R1 engines most people historically had fitted onto their westies etc, hence the diff problems and the need to change the existing diff ratio.
Thanks in advance!
The only diff I could get my hands on at the time of the conversion was a sierra 3.38 (i was looking for a 3.14 but found it impossible to get one), however even with a 3.14 I've calculated it would still only top out at 120mph. I've worked out that to hit 130mph I need a 2.9 diff, which it seems I can get from a 1989 onwards mini or metro 998cc unleaded. Is my only option to use this entire diff and housing, make a new bracket to support the diff housing and then weld the ends of the metro driveshafts onto my existing driveshafts? This all seems like a lot of hassle - surely there are other people putting modern litre bike engines in without going through all this? Apparently the gearing on a new litre bike engine is very different to the earlier blade / R1 engines most people historically had fitted onto their westies etc, hence the diff problems and the need to change the existing diff ratio.
Thanks in advance!
JT71 said:
is there any way of getting someone to build a crown wheel and pinion on a 2.9 ratio that will fit into my diff casing? surely this has got to be the simplest way?
Simplest yes, but it sounds as if you'd need to get a new one cut, and that is hideously expensive. I would think it's easier/a better idea to swap the diff for a version that can be easily changed in the future.I've got a BMW diff which is IIRC 2.56:1, they also come in 2.9 somehting and 3.1 something but they are massive, very heavy, probably an extra 7 or 8 Kg on top of the Sierra 7" diff. Also need different mounts or you can weld a couple of brackets onto them to make them fit in a Sierra carrier. Driveshafts are huge too.
The 2.56 and 2.9x are from the E36 TD and TDS diesels, I think the automatics. A good BMW breaker will be able to give you more information.
If it was me, I'd stick with the good wheels and tyres and go for the Freelander diff, even if it does mean having the chassis welded. Best option, lighter, better availability and cheaper LSD.
The 2.56 and 2.9x are from the E36 TD and TDS diesels, I think the automatics. A good BMW breaker will be able to give you more information.
If it was me, I'd stick with the good wheels and tyres and go for the Freelander diff, even if it does mean having the chassis welded. Best option, lighter, better availability and cheaper LSD.
You could do a couple of things and retain the Sierra diff.
1) Do what I did and fit a custom crown wheel and pinion. I bought a 3.076:1 ratio from Kam diffs (it was about £600 but has cured my top speed issues).
2) Fit larger wheels, or tyres that are slightly larger at the rear. Moving from a 195/50/R13 to a 205/50/R13 would make a difference, as would moving to a 195/60/R13.
If you increatse the overall rolling radius, you will increase your top speed.
Hope this helps.
1) Do what I did and fit a custom crown wheel and pinion. I bought a 3.076:1 ratio from Kam diffs (it was about £600 but has cured my top speed issues).
2) Fit larger wheels, or tyres that are slightly larger at the rear. Moving from a 195/50/R13 to a 205/50/R13 would make a difference, as would moving to a 195/60/R13.
If you increatse the overall rolling radius, you will increase your top speed.
Hope this helps.
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