I've got an 89 Formula that came with the 10 bolt 2.73 (2-series carrier) open differential. I'd like to go to a 3.42 posi setup. I have two main questions. First question is whether it's better to replace the gears with something like this and get a posi unit and keep the rest of the rearend? Or, find a complete 9-bolt 3.42 setup from a salvage yard somewhere and use it?

If I decide to do option 1, will the gear setup I linked from SLP work, or will there be a problem with the carrier, or does the Zexel-Torson unit replace the carrier? Thanks for the help.

Those gears are fine, except they're about $50 more from SLP than from alot of other places.

A posi unit IS the carrier. So, the $400 thing is a complete carrier, 3-series. It is the correct one for those gears.

Since your car is a 89, there's a chance (slim, but there's a chance) that you have 28-spline axles. However, there's at least a 80% chance that your axles are 26 spline, in which case that carrier won't work with them. You would need either a 26-spline carrier, or new (or at least other) axles with 28 splines, to use that particular SLP carrier.

Yes I know their web page says 82-88, and 89-up; don't believe it, until you look at yours.

Yeah, I figured SLP was expensive. If I get them, I'll get them from someplace else. Is there a way to tell if I have a 26 or 28-spline axle without tearing the differential apart? Maybe an RPO code?

Nope... no other way.... they were the same RPO code, they just had a stock of old ones on the shelf, and used them up until they ran out, before starting to use the new ones. Different ratios and brakes got swapped at different times during the model year. 2.73 drum seems to be one of the combos that they must have had ALOT of 26-spline units on hand to exhaust during the 89 production run, so that's one of the ones that seems to be least likely to have 28s.

Only way to know is to look. I sure wouldn't start buying parts until I knew what I had though, if it was me.

I should've known this wouldn't be simple. So assuming it is 26-spline, would it just be better to find a complete rearend setup from a yard with 3.42's? My concern is I've heard those weren't that strong to begin with, let alone one that's 10+ years old. So even if I find one, I'd probably want to put in the ZT unit anyway.

Just pull an axle and look. It's not hard or anything. Beats spending a bunch of money at the boneyard that might not be any different from what you've already got.

Yeah, I'll definitely check before I buy anything. What's the consensus here on how much power a 10-bolt can handle? From what I've read 400hp is quoted as the upper limit. Eventually I'd like to get my motor up around that. Would it be worth it to just upgrade to the 9-bolt or 9" Ford in the first place then?

There isn't really any power limit persay but there is a traction limit. Traction combined with high power levels kills our drivetrian components.

My 89 is 3:42 26 spline FYI