3.90 OR 4.30 REAR END

81SPIDERMATT · Post by **81SPIDERMATT** » Fri Aug 18, 2017 11:53 pm

81 Spider. Blew up my rear diff and have the option of staying 3.90 or going 4.30 . I am leaning to the 4.30 as most of my travels are mountain driving and seldom hit the highway. what say the braintrust??

I know that the 4.30 rear end will need drive shaft replacement, shock mounts are different but I can use the same shocks, not sure about the panhard bar or the short trailing arms ? I think the long ones are the same

anything else needed for the 4.30 rear end swap? thanks as always for any advise or tips... Matt

nelsonj · Post by **nelsonj** » Sat Aug 19, 2017 12:56 am

If I ever had the choice, I'm going with a 3.9. My engine revs too high at freeway (plus) speeds and I would really like to drop that high end down. The ideal solution is the 4.3 with a lowered 5th gear ratio - but good luck making that happen.

Peace.
Out.

RRoller123 · Post by **RRoller123** » Sat Aug 19, 2017 5:53 am

I think 5th is much too low on these cars (on my '80 anyway) and the engine revs way too high on the highway. And first is too low as well, it barely gets the car moving. It is a funny collection of gearing. I think I could use only 2,3 and 4 and just forget 1 and 5 and most driving would be fine. Hahahah! I would go for the taller diff.

klweimer · Post by **klweimer** » Sun Aug 20, 2017 10:44 am

I would find one of those calculators on line to see what the difference in rev's is at your desired road speeds. Might be a lot of work for a couple hundred RPM. I've been happy with the 3.90 ('82 FI).
Kirk

81SPIDERMATT · Post by **81SPIDERMATT** » Tue Sep 12, 2017 6:14 pm

4.3 was the final decision .... could not be happier ... ripping the mountain roads is where it really shines ....Matt

phaetn · Post by **phaetn** » Fri Sep 15, 2017 10:17 am

Do you mind if I ask where you sourced it?

I am looking to replace my 4.3 diff and avoid going to 3.9 which is what most vendors offer. I am intrigued that you had another option.

Cheers,
phaetn

aevansgatech · Post by **aevansgatech** » Fri Sep 15, 2017 2:00 pm

Any pre-conditions to the original diff going?

81SPIDERMATT · Post by **81SPIDERMATT** » Fri Sep 15, 2017 10:31 pm

aevansgatech wrote:Any pre-conditions to the original diff going?

sliding down the dirt back roads on to pavement is what I am sure had something to do with it .... age ... mileage .... driving style .... but the mechanic buddy that cracked her open said it was on of the worst damage he had seen

phaetn ... pm responded to

aevansgatech · Post by **aevansgatech** » Mon Sep 18, 2017 2:24 pm

81SPIDERMATT wrote:
aevansgatech wrote:Any pre-conditions to the original diff going?

sliding down the dirt back roads on to pavement is what I am sure had something to do with it .... age ... mileage .... driving style .... but the mechanic buddy that cracked her open said it was on of the worst damage he had seen

phaetn ... pm responded to

Not sure if I should say sorry or congratulations.... sounds like you have fun with it

phaetn · Post by **phaetn** » Tue Sep 19, 2017 1:28 pm

Now that the car is back on the road after having the head machined and valve seats cut I can do a test run on the highway.

Going 100km/h (62.5mph) in 4th gear I was pulling 3900rpm. I guess given that it's not overdrive then that means that I have a 3.9 diff rather than the 4.30:1 that originally came with the car. Yet I don't have the split case, but the older style.

I guess that means the p.o. put in a new pinion and ring gear. Would everything still fit the same way in an old case?

Cheers,
phaetn

81SPIDERMATT · Post by **81SPIDERMATT** » Wed Sep 20, 2017 12:19 am

I have no idea .. I will pay attention to rpms and speeds tomorrow and get back to you ... Matt

aj81spider · Post by **aj81spider** » Wed Sep 20, 2017 8:59 am

3900 RPM at 100 kph doesn't necessarily mean you have a 3.9 rear end. It depends on the size of your tires.

An example: 205/50-15 tires have a circumference of 72.8 inches.
At 62.5 mph you are traveling 66,000 inches per minute.

66,000/72.8 is 906.6
906.6 x 4.3 is 3898 rpm or almost exactly the 3900 you are seeing.

I don't know what size tires you are running, but you could certainly see 3900 rpm at 100 kph with a 4.3 differential.

81SPIDERMATT · Post by **81SPIDERMATT** » Thu Sep 21, 2017 8:30 pm

60 mph turning 3900 rpms ... 14 inch tires .. 4th gear ....Matt

phaetn · Post by **phaetn** » Sat Sep 23, 2017 1:13 am

Thanks, gents.

Good point, aj81spider. Doh! I had forgotten about my tires. I used to run 185/70-13s and now have 205/50-15s. According to TireSize Calculator the bigger tires are almost a half percent smaller in dia. and circum. (-0.4%), though I know tires can actually vary by manufacturing batch.

You're right on. I'm still 4.3. Trust the math. Jeez, I'm an epistemology teacher, I should know this... Sometimes my brain gets clouded by the allure of buying new Fiat bits.

Cheers,
phaetn

81SPIDERMATT · Post by **81SPIDERMATT** » Sun Sep 24, 2017 10:01 am

[quote Not sure if I should say sorry or congratulations.... sounds like you have fun with it

[/quote]

haha ... don't know how I missed this comment... yes I do have quite a bit of fun ... Matt

3.90 OR 4.30 REAR END

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