On the Nature of Rear Ends

The reason that the limited-slip differential I scored at the junkard wouldn't work is complex. . .

For the benefit of potential non-car-person readers, I'd like to explain.  If nothing else, this is a good exercise in remembering this stuff. Anyhoo. . . With these F-bodies, you'll run into two main types of rear axle assemblies:  The 10-bolt, and the 12-bolt.  There are quite a few subdivisions of those two types.

12-bolts are generally stronger, and preferred for cars with more horsepower than sense.  I believe that the early Z-28's had 12-bolts, and probably some (but not all) of the SS models would've had them too.  Certainly all the big-block Camaros would've had them.  12-bolts are getting pretty hard to come by in the junkyard, I guess.  I've never bothered looking for one.  I'm not going to get into 12-bolts, because I simply know less than nothing about them.  I do know that a brand-new one can cost as much as $2,000.  Which is a little stupid, if you ask me.

There are two kinds of 10-bolts, one with an 8.5" carrier, and one with an 8.2" carrier.  My understanding is the 8.5" carrier is the next best thing to a 12-bolt. And can be built up pretty respectable.

NOTE:  You want to know the basic difference between a Chevy 12 and a 10 bolt?  count the number of bolts on the back of the pumpkin.  It's just that easy.

My 10-bolt is the 8.2".  Of course, the poor little thing had almost the smallest available engine (the 235 was smaller, but just).  So there's really no need for a bulletproof rear axle setup.

Now the funny part.  In the 3rd Generation F-bodies (1983-1993), there was born the 7.5" 10-bolt.  Which is why that limited-slip differential wouldn't work in my rear end.  It's just too small.  They also gave birth to a 9-bolt rear end.  But since that's plainly odd-number blasphemy, we won't even acknowledge its existence.

Of course, this is a super-dumbed down version of the real story.  In reality, General Motors created a bewildering amount of variety in these old parts.  Back in the day, the way you bought a car was by going through a big hairy form, and checking all the boxes of the stuff you wanted on the car.  They were customized in a way that simply isn't done anymore.  These days you get one or two choices of package groups, and very few real options.

What that means is that there are literally thousands of variations in the old stuff.  And they can be very subtle.

More importantly, what am I going to do with the rear axle in my car?  Probably nothing.  If I could find a limited-slip that would work, for cheap, I'd grab it up.  But I'm not about to dump down $500 for a new differential.  Not yet, anyhow.  I'll continue to troll the junkyards.  If I get lucky and find a positraction 12-bolt, I'll be snatching that thing up.  But I ain't holding my breath.

If I do end up swapping in a late-model 4200 Vortec--which is sincerely the route I'd like to go--then I would likely have to upgrade that rear axle to handle the extra power.

It's pretty interesting.  There were 8 different factory engines available for the Camaro in the '68 model year:


L26                230cid                    140hp
L22                250cid                    160hp
LF7                327cid                    210hp
L30                327cid                     275hp
L48                350cid                     295hp
L35                396cid                     325hp
L78/79           396cid                     375hp
Z28                302cid                     290hp

You can see, my 250 is nearly at the bottom of the heap.  However, if I were to swap with a 2006-2009 LL8 (the 4200 Vortec), it's about 254 cubic inches, which is super close to mine.  But those engines are conservatively rated around 300 horsepower.  Which would put me performance-wise right between the 350 and big-block 396.  AND solidly drop me into the 25 to 30 mpg range, which no stock first-gen F-body can touch.  Not even the wheezy L26.

Getting 300 horsepower out of my L22 is certainly possible, but would end up being far more expensive and time consuming than the 4200 swap (at least by my estimates)  Plus, it would still never touch that fuel economy.

Want another funny bit of trivia?  A 2012 V-6 Toyota Camry is rated at 268 horsepower.  Not much down from the top of the heap 396 in '68.  Except that the 396 probably never got more than 8 or 10 miles per gallon.