|View single post by NigelK|
|Posted: 01-04-2015 03:44 pm||
|There seems to be some degree of consensus on both sides of the pond that the W58 gearbox does not work well with the taller 3.44 diff fitted to JH5s and GTs.
This is directly relevant to me at the moment, as my GT engine is being upgraded from factory standard to 907/912HC hybrid next month, and the workshop is trying to persuade me to ditch the Getrag 235/5 and upgrade to the W58 at the same time.
I am not keen on this for a number of reasons - i) I like the quirkiness of the dogleg 235/5 and its close ratios, ii) I don't want to have to incur even more expense by changing to a 3.73 diff to make the W58 work properly, and iii) I am concerned about the amount of modification required to make the W58 fit (my mantra is that all upgrades must be fairly easily reversible to factory spec).
On the other hand, the 1:1 5th gear in the 235/5, even with the taller 3.44 diff, is a real pain on the motorway, as 70mph = a rather noisy 3500rpm.
From a technical perspective, fitting a standalone overdrive unit would be the perfect solution, giving me a true overdrive top gear for the motorway while keeping the 235/5 close ratios for the winding stuff. But there simply isn't enough room in the transmission tunnel for an overdrive unit. I've thought about fitting the ZF S5-18 gearbox from the Lotus Sunbeam which has a dogleg shift pattern and can have a proper overdrive 5th gear, but it's very expensive in the UK and I think substantial modifications to the transmission tunnel would be needed to make the shift mechanism fit. I am sure fitting more soundproofing to the car would help reduce noise at cruising speed, but this would be dealing with the symptoms, rather than the cause.
More recently, I've been thinking about fitting the Getrag 265 and bell housing from the Lotus Eclat S2.2, which while rare is obtainable in the UK. While this is not a dogleg gearbox, it does have fairly close ratios 1st - 2.96, 2nd - 1.93, 3rd - 1.39, 4th - 1.00, 5th - 0.81 (NB these are different from the 1st and 2nd gear ratios for the 265 box in its BMW and Jaguar incarnations).
While these ratios are still fairly tall, they were considered to work pretty well in the Eclat S2.2 even with the optional taller 3.73 diff. My GT post engine upgrade should have more torque and bhp than the Eclat (which had the 912LC engine fitted). Plus I'm still on 185/70/13 tyres i.e. smaller than the 205/60/14s fitted to the Eclat. Relative to the W58 box, the gear change revs drops for the Eclat 265 are also fairly consistent and closer to the peak bhp (6500rpm) to peak torque (5000rpm) difference for the 912HC engine.
So I am hoping that the Eclat 265 gearbox could work well with the 907/912HC hybrid engine and 3.44 diff. I would also hope installing this box would be fairly easily reversible (if I wanted to go back to "matching numbers", or if my calculations are wrong and the 265 gearbox proves too tall in practice).
Any thoughts / comments would be greatly appreciated.