Fiat Dino 2 Litre rear axle ratio exchange

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Dinoswede
Posts: 68
Joined: Sat Apr 05, 2014 4:22 pm
Dino: Fiat Dino 2.0 Spider

Fiat Dino 2 Litre rear axle ratio exchange

Post by Dinoswede » Sun Sep 15, 2019 2:35 pm

When cruising your Fiat Dino in 110 - 120 Km/h, you probably are in a constant search for a 6:th gear...
If you own a 2 Litre Fiat Dino, there is no "long 5:th gear option" to install, like the alternative 5:th
gear set for the 2,4 Litre/ZF Dino.
I have now finished a project which maybe could be of interest for the members of the Forum: an
exchange of the original 2 Litre Dino 8:39 rear axle ratio, to a 10:43 gear ratio.

This raises the speed with around 13,3 %, at given engine rpm, in all gears. (The ZF "long 5:th gear"
raises the speed with around 9,5 %, at given engine rpm - but in 5:th gear only, of course.)

At first, I was concerned the lower gears would be all "too high" - too much affected. Fortenately, I
soon realized that the lowest gear always is the least affected, when changing the final gearing - and
vice versa. For the detailed outcome - see the diagram table below.
001 (2).jpg

BLACK lines = 8:39 RED lines = 10:43

When driving in 1:st or 2:nd gear, you probably will not notice any changes at all. An up-change to 3:rd,
and you will notice it is "a little longer". 4:th gear now is very close to the original 5:th, and the 5:th
gear now has become the overdrive 5:th you always have been missing...
The engine handles all this effortless. Personally, I can´t understand why FIAT did not choose this (or a
similar) gear ratio for the Dinos. The car´s 0-100 Km/h performance seems unaltered, as you now can
reach 100 Km/h without a second gear change up to 3:rd gear. See the diagram table.

Top speed? I have not been able testing it, but it ought to be in the 210 - 220 Km/h range. The main
enemy here is the air resistance - when you double the speed, the air resistance goes up by four times!

The last step in this project was a re-calibration of the speedometer. This demands some special tools,
so I turned to a local car instrument shop, to have this properly done
007.JPG
001.JPG
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007.JPG2.JPG
011.JPG
003.JPG

Dinoswede

gguarige
Posts: 15
Joined: Sun Mar 08, 2015 5:17 am
Dino: Fiat Dino 2.0 Spider
Location: Sacramento, California

Re: Fiat Dino 2 Litre rear axle ratio exchange

Post by gguarige » Mon Sep 16, 2019 6:06 am

Remarkable and agreed! Look forward to hearing about your progress an where you had the final drive ratio manufactured.

Spiderman
Posts: 50
Joined: Wed Nov 27, 2013 5:47 pm
Dino: Fiat Dino 2.4 Spider

Re: Fiat Dino 2 Litre rear axle ratio exchange

Post by Spiderman » Mon Sep 16, 2019 9:23 pm

Excellent work.

I have always thought this would be the perfect solution for the 2.4 also.

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ThomasK
Posts: 105
Joined: Mon Dec 30, 2013 12:44 pm
Dino: Fiat Dino 2.4 Coupe

Re: Fiat Dino 2 Litre rear axle ratio exchange

Post by ThomasK » Mon Sep 16, 2019 9:26 pm

For the 2,4 Liter you can use a Fiat 130 Differential with Lim Slip for the same result.

Dinoswede
Posts: 68
Joined: Sat Apr 05, 2014 4:22 pm
Dino: Fiat Dino 2.0 Spider

Re: Fiat Dino 2 Litre rear axle ratio exchange

Post by Dinoswede » Tue Sep 17, 2019 11:02 am

Many thanks for kind comments. It has been a lot of work finishing all this, but also very rewarding, if
looking at the outcome. Here comes some more info:

This project actually started with a search for a machining shop, competent enough to be able fabricating
new gear wheels with a higher gearing, for the 5:th gear. (It is a complicated process making new gears for
the 2 Litre Dino gearbox, as the actual gears also include many synchromesh details and reverse gear in
the same design.) See picture below.
010.JPG
Eventually, I found one company with the right skills and machinery, but they wanted 8000 EUR for one set...
It was then I started thinking about a change of the rear axle ratio, instead.
After a lot of research, I finally identified a few different old FIAT cars, using different parts from the same
"rear axle assortment" as the 2 Litre Dino. The only car using a suitible ratio (10:43), that also would fit the
rear axle of a 2 Litre Fiat Dino, is the Fiat 124 CSA.
This car has its own IRS, but the ring & pinion gears have the same dimensions as the original Dino parts.
A major problem here is, 124 CSA was built in just over 1000 units between 1972 and 1974, and after all
years, not many spare parts are availible.
ring & pinion.jpg
So, the challenge was finding a remaining 10:43 gear set (Fiat no 4154471). The internet sometimes is a
blessing, and I was happy to find a new gear set, after have been searching for a long time!
(For the 124 CSA racers, there were both 7:43 and 8:43 rear axle ratios availible, but these are - obviously -
of no interest for Dino owners.)

Dinoswede

dinoverde
Posts: 181
Joined: Wed Nov 27, 2013 5:56 am
Dino: Fiat Dino 2.0 Spider

Re: Fiat Dino 2 Litre rear axle ratio exchange

Post by dinoverde » Fri Sep 20, 2019 6:57 am


Dinoswede
Posts: 68
Joined: Sat Apr 05, 2014 4:22 pm
Dino: Fiat Dino 2.0 Spider

Re: Fiat Dino 2 Litre rear axle ratio exchange

Post by Dinoswede » Fri Sep 20, 2019 9:16 am

Unfortenately - no. The basic Fiat 124/125 models used a assortiment of similar, but not the same rear axle parts.
The different ring & pinion gear sets, for instance, are smaller/weaker than the Dino parts. There were several
gear ratios availible - like 10:43 - but that doesn´t mean these parts will fit the Dino.
8926241.jpg
8926241.jpg (38.59 KiB) Viewed 204 times
The Fiat 124 CSA had 128 HP in "standard tune", but fully race-prepped cars delivered around 180 HP, which
means these gears (4154471) were designed to cope with this level of engine power.

Strange enough, in the 2 Litre Dino Spare Parts Catalogue, page B4.04, FIVE different ring & pinion sets are
listed. Four of these are marked with "a little star", which is said to declare these parts never was kept in stock.
One of these numbers is 4154471 (10:43) !! Note, this catalogue was printed five years before Fiat 124 CSA was
produced! I guess this indicates, FIAT actually had some thoughts about the Dino gearing...

Dinoswede

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