Which Differential - Choosing the Right Differential

[spike_esq]

There is so much experience within the membership which might help me choose the right and most suitable differential to use with the FIAT 2.0l engine. The rear wheels are 14" on a standard minor rear axle. Presently we have the 4.22:1 ratio diff fitted and during recent trails we just feel the engine is over revving whilst traveling at 70 mph. Perhaps a 3.9:1 or even maybe the 3.7:1 ratio might be a better option. I don't imagine either of these would truly effect significantly performance from stationary start. Welcome any comments to help ensure that the correct partnership of engine and differential came be achieved. Welcome all your feedback. Your experiences in helping to choose the right combination are greatly appreciated. Many thanks. Best Regards......spike.

[warb21]

I have a standard 1098 so can't speak for your engine. But assuming the principle is the same....
I changed from a 4.55 (thinking it was a 4.22!) to a 3.9 diff. There is a difference at low speeds, a general loss of what one might call 'nippiness' and climbing hills is harder work, but overall perfectly fine. At high speeds e.g. 65mph the revs feels much more comfortable though it's still a tad too noisy to listen to Radio 4 ;-)
Search for the thread entitled 'what a difference a diff makes' where I described more.

[warb21]

And here's that thread: http://www.mmoc.org.uk/viewtopic.php?f=4&t=55942

[panky]

I've got a 3.7 driven by a lightly tuned 1098. The acceleration has dropped back to what it was with the standard engine and 4.22 diff but now 60mph is equal to just over 3000rpm I'd worry about the poor old half shafts though with the 2litre up front, maybe a swap to a Ford axle would be a better choice.

[Chipper]

I'm frankly amazed you've been able to drive any appreciable distance at all, if asking a standard 1098cc Minor rear axle to cope with the torque of a 2-litre Fiat twin cam!

Mine struggles to cope with the almighty power of a standard(ish) 1275cc MG Midget engine - 3 broken halfshafts and gearboxes and one whiney diff, so far, since fitting the engine 60,000 miles ago!

When I had a 2-litre Fiat twin-cam engined Minor saloon, the previous owner/builder had fitted a 3.9:1 MGB 'tube type' axle, which looked like a nice sturdy item, but even that seemed a bit low-geared, particularly on the motorway - you could stick it in top (5th) gear at anything above about 30mph, and just waft along on a wave of torque.

I gather that most of the Fiat 131/Mirafioris that many of the 2-litre Fiat twin cams were purloined from had a 3.9:1 diff or thereabouts, but of course, that was a slightly heavier car with a very boxy and unaerodynamic design.

The other snag was, the wheel PCD of the MGB axle was 4.5", so I had to source a pair of the ultra-rare Marina 575/8 cwt van front hubs to fit to the front so that all four wheels had the same PCD.

[Monty-4]

If you have that much extra power then the lowest diff ratio possible would be best (3.7), no? If it doesn't destroy itself of course.