New Rear Diff

[Gyuri69]

I lost an argument with a Concrete truck a few years back.
And I have been without a beloved Spider for far too long!!!!!
I want to build my new fiat more for fun in the city or track days and I plan to go all out!!!!

I will be rebuilding the engine and I'm shooting for about 130-140 hp
Wondering what diff to put in......4.3???
Should I change to a 131 Transmission?
And if so.......where do I find them

[AndyVAS]

Are you talking flywheel or wheel horse power? Not sure if you have a plan for your power desires but give me a call at the shop and I'll gladly go over it with you to help ensure you end up in the ballpark.

For just fun times and hammering on the car, the 4.30, 4.10, and the 3.9 will all work well. The taller gears will extend your shift points. Having more usable 2nd, 3rd, and 4th will serve you well. I would personally go with the 3.90 gears.

The 131 transmission would be overkill but as the saying goes, excess is success! You will need to get the shift extender for it so the knob ends up in about the same place. It fits with only minor sledge hammer use.

[gm404]

I have up on ebay the pumpkin type pre 1978 diffs all the time, rebuilt items with common ratios 3.9, 4.1 and 4.3. All factory tools and adjustment rings available, so I build them up from scratch to look and run like new.
This is a 4.1, sold so far only one to Switzerland
http://www.ebay.com/itm/382260253149
Have one 3.9 ready with new LSD locker
http://www.ebay.com/itm/382260260737
Minor items, mainly new stuff, I list sometimes as well.
http://www.ebay.com/itm/382260234465
I have also one last used 4.44 main gear pair on stock and can get that anytime new from Russia.

To get different (shorter) axle ratio for the later type inseparable axle with rear mounting cover, Poland is the go-to place, as I heard. I was looking last night how i could get hands on an used axle. Poland it quite close to me. I am interested mainly due to the longer 11/41 ratio they has on some Fiat-related Polonez and FSO trucks. Would love to unite it with the early pumpkin style diff.

[blazingspider]

+1 on gm404's rebuilt diff's!! I just installed one with a 3.9 ratio in my 77 and am very pleased with the unit!! This guy knows his stuff!!

[DieselSpider]

I have been looking for a better diff for my 78 which currently has the front load diff that I believe originally came with it and is now mated with a 4FB1 Isuzu Diesel and MSG5 Transmission.

Number of gears: 5

Gear ratios (overall):
I 3.76 (12.63)
II 2.18 (7.32)
III 1.36 (4.57)
IV 1 (3.36)
V 0.86 (2.89)

This engine gearbox combo was originally mated to a 3.36 rear axle with P 155/80 R 13 tires on it in a 1980's Chevy Chevette.

Is it possible to more closely match the original Fiat diff to this ratio and potentially reduce rpm by about 20%? I believe it currently has the 4.1 or 4.3 which is too truck like with the 5,400 rpm Diesel and MSG5 transmission. Currently you are in 5th gear by 32 mph just lightly cruising around. 1st is usually only good to about 15 mph max with normal shifts at about 2 mph and you can almost skip it altogether and start off in 2nd if your not facing up hill. You are out of 1st in a heartbeat especially since the engine has been turbocharged.

[AndyVAS]

The lowest ratio they make for the early diff is 3.9:1. The late diff from an automatic car was 3.58:1 but those are pretty rare.