Looking to upgrade my 98 auto diff I was thinking just swapping in a c5 z06 diff or would it just be better to go a different route thanks

 

Welcome!
Part of the answer is based upon modifications you may have already made to your car and/or future mods you may have planned. Also, do you currently have the 2.73 or optional 3.15 axle ratio. If you are not sure, you can check your RPO listing (either in the underside of the left rear storage compartment cover for early production models, or on later production1998 models and all C5s after, it is on the glove box door). RPO code GU2 indicates the 2.73 ratio differential; G90 (Performance Ratio), and G92 indicates the 3.15 diff.
Also, are you looking specifically for the 3.42 differential or the Z06 diff? All manual transmission C5s had the 3.42 diff. I believe all Z06 differentials had the shot peened ring and pinion which should provide strength but, again, what do your plans for your car require the shot peened R&P?
Making the move from the 2.73 to the 3.42 diff will give a noticeable step up in performance; going from the 3.15 to 3.42 diff will not give as dramatic increase.
As far as a different route, there are Forum Vendors that produce differentials that are significantly stronger than the OEM diffs, but these will be more expensive than the factory differentials.
Best wishes with finding the differential that will provide the "extras" you are looking for.

 

I currently have longtubes x pipe axle back and intake
I plan going fbo and cam and I’ll be happy I have 2:73 gears I was thinking of buying the c5z06 rear end becssue it’s a direct fitment

 

Both the C5 manual and C5 Z06 differentials will be a direct fit, and you will notice a nice difference in performance!
Sounds like a good plan.

 

One vendor to check into for a differential is RPM Transmissions. They sell various Stage packages depending on your needs.

 

I have an RPM 3.73 rear drive with WaveTrac differential. It made an old car driven by an old man more competitive on a road course track.

With an A4, you will need to reprogram the PCM if you install a lower ratio otherwise the shift points will not be at the correct RPM/ throttle setting.

 

Going from 2.73 to 3.42 is a great upgrade. I helped a friend do his and he loves the new gearing at 3.42 in his A4.

Yes, they are a direct fit as mentioned above. However, you'll need to remove the A4 diff seal plate from the old diff before installing it to the "manual" diff. The plate is only needed on the A4. Therefore a direct swap will only work once you've applied the seal plate to an MN6 trans diff for fitment to an A4.

There are a few guys that have learned this the hard way. When they swapped to an MN6 3.42 in their A4 without swapping in the plate. Then they had to take the diff back out and do it right with the plate.

Also as mentioned above you will require reprogramming post the swap for the A4 to accept the new gearing. In an MN6 car a gear swap does not require reprogramming. However the A4 does.

 

I have a 2000 vert that came.w/G92 3.15 diff. I went to a 3.42 C5 Z06 diff, and it made a huge difference. Probably more than usual, because I have a 3200 stall Yank converter, and the 3.42 makes a stall converter feel 'tighter'.....

 

Also a great point. The OP will also want a more aggresive than stock (1600 rpm) stall converter.

 

so I would just take old seal plate off stock diff and for reprogarmaming what all would I need to do I know I will need to do the gauge cluster but what else thanks for all the help

 

Yes, you would swap the plate from your current diff to the new 3.42 diff.

As far as tuning goes, A4 or MN6....I am not your guy. I've only owned MN6 cars and I've always had my cars tuned by top shops. Never did it myself and never let a monkey do it. Too many hacks causing chaos with bad tunes and breaking people's stuff.

I may be mistaken, so some others may chime in with A4 experience. On the A4 C5 you do not have to make a speedo correction with a gear swap. The tuning software will allow you to select gear ratio, which will adjust speedo. However you must do a shift point correction. You need to adjust both at wide open throttle & partial throttle shifting. If you don't tune for the change at wide open throttle your trans will not shift and hit the fuel cutoff/rev limiter. At partial throttle it will shift too late.

That's about all I know (or think I know) about the A4 when it comes to tuning.

You may want to create another post with the specific ask about tuning the diff change in an A4 if you plan on tackling yourself. A pro tuner with HP tuners software will know what to do, but I'm sure plenty of guys have done this change themselves with a simple handheld tuner like Diablo etc.

 

thanks man I already ate it I plan on returning the car when the diff goes in and apreciate all the help