Quattro gearbox exchange...

Posted: Sat Dec 13, 2008 9:31 am

Hi all,
I've been turning the old grey cells over and thinking about potential fuel saving measures...
My thoughts are based on the fact that diesel engines have a tendency to run at much lower revs than petrol (duh...) so I'm thinking that exchanging my gearbox for one from a petrol engine to one from a diesel engine would give me a higher final drive ratio, and thereby lower the revs at cruising speeds.
I was thinking about seeing if I can get ahold of a 2.5tdi box (eg an A6 quattro) to exchange for my 2.3 7a box, based on the theory that them both being based upon the 5 cylinder block they would mate up correctly.
Obviously the clutch would be uprated to tranfer the lower engine torque rate to the higher gearbox torque rate and possibly the flywheel as well, dunno yet...
Has anyone done this on here yet? Or know of anyone who has? Anyone think of any potential problems or pitfalls?
Thanks
Simon

Posted: Sat Dec 13, 2008 9:46 am

there's a website somewhere that lists the fear ratios I think, for the different gearbox codes (try a search for gearbox ratio's in the A6 forum- sure it's there)

You'll need the matching rear diff as well if it's quattro... (so it matches up)

Posted: Sat Dec 13, 2008 10:03 am

Spend the money on LPG instead.

Posted: Sat Dec 13, 2008 10:25 am

Thanks for the replies guys
yeah I was expecting to change the rear diff as well cos otherwise the center diff would be working overtime with different final drive ratios front and rear, but thanks I'll check out the tables Smile

Thanks for the idea, but I'm not really interested in the LPG route, I wanna keep the car otherwise 'normal' just liked the idea of raising the final drive ratio toget more mpg 'cruising' as I say, I still want to be able to drop a gear and 'give it beans' as and when the mood takes me...

Posted: Mon Dec 22, 2008 11:06 pm

i don't wanna poor cold water on your idea but if it was that easy to up the mpg in top gear wouldn't all manufacturer's just have a really high ratio top gear ? i know it sounds plausable but i think your petrol powered engine will struggle in top gear of a diesel box due to the lack of torque so what your saving in reduced revs your loosing in the engine trying to push harder.. i could be wrong i mean BL (British Leyland ) used to use overdrive boxes for cruising