Modified engine with which rear end?

[SoFlaFiat]

Planning on dual carb, head work, 421 exhaust, haven't made final decisions on domed pistons but want opinions on rear ends.
I have several, including a 3.9, and everything I have read so far points to that for good highway driving. Problem is, I will use the car around town a lot as well. It's a 2L block and head. Opinions?

[SLOSpider]

1800 head on your 2.0 block and wont matter what rear end. I like the ability to run 87 so no dome for me.

[So Cal Mark]

if it were mine I'd use the 4.3, but then I want the best acceleration. It doesn't matter if the engine is at 4k at hwy speeds

[twblanset]

I have a similar engine with the rear axle from an automatic (I believe it's a 3.6). That's taller than your 3.9, and I really like it on the open road for cruising. The off-the-line acceleration isn't at all impressive but it's not bad. I'm going to get a 3.9 and try it just to see how I like the better acceleration, but in it's current set-up I get really good mileage and I can cruise easily over 70 and I can go over Donner Pass to Reno in 5th gear.

[Tobi]

There is no point on getting more power out of the motor and then go ahead and waste it on a long rear end unless there is an Autobahn close by!

[Daniel]

if it were mine I'd use the 4.3, but then I want the best acceleration. It doesn't matter if the engine is at 4k at hwy speeds

I'm with mark on this!

[aj81spider]

It's a personal decision, but what you're going to use the car for will play a big role. I personally find that highway driving with a 4K drone is very fatiguing. If you're doing a lot of highway driving a lower RPM will make it more comfortable (as will a wind screen!).

[MIGHTY 8]

By the way, How can you tell by looking at one to know what type it is?

(I've since replaced mine twice, so it's not original to the car.)

Thanks.

[carl]

I have a 71 124 coupe and a 73 124 spider, both with a 2L convertion. I installed a 3.9 rear in both cars. With all the torque available, the original rears were too short....if you pull away from a dead stop in 2nd gear then you have what amounts to a fours speed. Fiat installed the 3.9 rear on it's 2L spiders so drivng down town, down the highway or at a track day is just fine.

carl

[vandor]

I have a 3.9 and a longer 5th gear in my Spider with a 2 liter. Since I use my Spider to drive to events that are 3-4-5 hours away, or even 2 days drive away, it was very important for it to be quieter on the highway. With the original setup it would turn nearly 5000 rpm at 80mph!
It really depends on personal preference and what kind of driving you will be doing.

[Tobi]

I have a 3.9 and a longer 5th gear in my Spider with a 2 liter. Since I use my Spider to drive to events that are 3-4-5 hours away, or even 2 days drive away, it was very important for it to be quieter on the highway. With the original setup it would turn nearly 5000 rpm at 80mph!
It really depends on personal preference and what kind of driving you will be doing.

There we go!
So instead of putting money in a motor and than install a longer rear end to get the rpm's down, how much would it be to have a longer fifth?

[vandor]

The gears to do that simply don't exist. The ones I have came from a rare version of a Polish made Fiat and took me 3 years to find. I have tried to adapt/modify other gears, but so far have not been able to make anything work.

[Daniel]

The gears to do that simply don't exist. The ones I have came from a rare version of a Polish made Fiat and took me 3 years to find. I have tried to adapt/modify other gears, but so far have not been able to make anything work.

Hey Casaba

What model fiat was this gear from? and why didnt you make copies lol whats your RPMs looking like now?

[divace73]

I have thought bout this dilemma for a while and have been looking into the electric overdrive units some of the English cars had and see if it is somewhat adaptable....I love acceleration but really dislike the high RPM drone on freeways.
I use to have a lazy 3.4 in my car which was great on the freeway and didn't know what I was missing until I installed the 3.9.....with torsion diff
I have a 4.44 pinion and ring gear with another torsion centre ready to go for the smaller circuits and hill climbs, but really dislike to think of the freeway drive to these locations.

Incidentally we have a racetrack here in Sydney formerly known as Eastern Creek and have been told I'd run out of RPM with a 4.3....
The 3.9 seems to be a good compromise.

BTW I am running 195/55 15's for tyres

[vandor]

>What model fiat was this gear from?

I do not know actually, other than it was a Polish made Fiat. So likely it was a 125 or Polonez. Friends in Europe got the gear set for me.

>and why didnt you make copies lol

I'm trying...

>whats your RPMs looking like now?

These are 0.80:1 ratio, so 10% longer than stock. So the highway rpm is 10% lower. What was 4000rpm before, now is 3600 rpm.