Good morning. I have an '01 with auto that came with the 2.73 rear diff ratio. The car is having the torque tube replaced, and the trans/diff are out of the car. Seems like a good time to address the rear gears. It has been suggested that I swap the 2.73 diff for the optional 3.15 or even the 3.42. I have some questions about how that can be done, and what to expect performance wise.
1. It's my understanding that the taller gears will not fit in the 2.73 case. Is this correct?
2. If the taller gears do not fit the 2.73 case, will a 3.15 or 3.42 case be a simple bolt on?
3. What would be the performance gains from the 3.15, and what would have to be adjusted / calibrated for speedometer accuracy?
4. What would be the performance gains from the 3.42, and what would have to be adjusted / calibrated for speedometer accuracy?
I have seen used 3.15 diffs on the auction site ranging from $300 - $600. My stock diff is going to be getting new seals while it is out of the car, and I'm wondering if it is worth it to get the taller gears and have that diff serviced in place of the old one.
TIA!

 

No u can't put 3.15 or 3.42 into a 2.73 case. You could but you have to modify the speed sensor location to make it work. bs. Then you have to tune it for shift points. Lot more money. Just buy a different a 3.15 or 3.42. Now thoughts are interchangeable.

 

You can certainly use a higher ratio in the 2-series case, but it requires a ring gear spacer. You will probably spend a lot more money getting one rebuilt than just buying a whole new unit used.

 

I swapped in a 3.42 into my car that came with a 3.15 gear
I was quite dissappointed in the results Very little performance gains and 1/4 mile ET
I would always recccomend a STALL converter over a gear change any day---
Not a reworked stock converter as they don't hold up (these usualy only go as high as apprx 2600)
But a new small diameter converter of at least 3000 RPM This will make your car a beast eveywhere !!
Someone else may chime in on going from a 2.73 to a 3.42--

 

That is my question - will a used 3.15 or 3.42 diff bolt right up where the 2.73 is? I have no interest in getting the stock one rebuilt.

 

I'm not gonna mess with the torque converter right now.

 

It is my understanding that there are 2 sizes of carriers - 2.73 and the 3.15 / 3.42. I'm want to know if an entire diff assembly with the 3 series gearing would bolt up in place of the stock 2.73 with no modifications.

 

No other mods needed. You will need a new seal and O-ring for the separator plate when you transfer it to the new diff

 

Are u talking about bolting up the the tranny?? The hell with the 2.73 transaxle. But the one on Ebay. Save a lot of headaches

 

Yes - taking the 2.73 diff off of the trans and putting on a 3.15.

 

yes you can install the other diffs, you just have to transfer over the plate and o-ring.

 

Decided to go with the 3.15. Found a used one on the auction site with 45000 miles. Listed at $422; I offered $350 and they accepted. Should be delivered to the shop by the 11th. Looking forward to the improved off the line performance!

 

You will have to get a tune for it to work correctly. Make sure you get somebody with experience who knows what they are doing.

 

What specifically will need to addressed? Speed sensor? Speedometer?

 

 

SOME FACTS AND WARNINGS!!!

If it were ME, I would at least disassemble the differential and inspect the clutches and clutch belleview springs.. The OEM C5 Belleview springs are well known to crack & break apart. I would also install new differential output shaft seals in the side covers. Higher mileage differentials can have worn clutches and or broken springs.. The center section of the differential can be removed and reinstalled without disturbing the gear set up alignments.

There is a bulkhead sealing plated in the front of the differential (between the differential and trans bell housing) that will need to be transferred to your New Diff if the one that you get doesn't have it. The bulkhead sealing plate is only used in the AUTOMATIC (A4) cars. The O ring on the plate and lip seal inside the plate should also be replaced with a new one.

YES!! You will need a PCM tune to readjust your transmision shift points.

 

There are several parameters that will need to be changed. That is why you need to have someone who is experienced. There is probably someone in your area who can do the tuning. ECS will do a good job but you will have to remove your PCM and send it to them. A good shop that does dyno tuning will probably be able to help you. This is not an uncommon upgrade.

 

I went from 2:73 to 3:42 in my 98 C5. I l
lve in the mountains the road to town is awesome, 35 miles of easy, new road, lotsa curves. the car seemed always to be floating unless I downshifted. The 3:42 really brought it to life. Im sure Im sacrificing freeway RPM though, but pretty much stay in the hills and really like it!

 

Oh, and it hauls ***** now!

 

You cannot re gear a 2.73 to a lower gear ratio like you want However you can install a whole different rear end from a 3.42 take off diff from a Z06 This is what i did--- no issues other than a 3.42 diff only came on a manual trans and you will have to use your old trans plate from your auto trans diff in order to make it bolt on PLUS you will need to have the trans tuned as far as shift points are concerned Both at P/T and more importantly at WOT If you don't make the WOT shift point correction your trans may not shift at WOT and hit the safety rev limiter