I have an 87 SC with 2.8. It has the 3.42 rearend. I want a 3:73, maybe even with posi. I found 2 camaros right up the road in a junkyard. One is an 86 Z-28 with a 305. One is an 84 or 85 4 cylinder. My question is this. Do all z28's have the 3:73 posi? Also, do all 4 cylinders have the 3:73? I think I read somewhere that the 4 cylinders all had 3:73's, and my friend tells me that he thinks but is not sure that all z28's had posi 3:73. Please help me on this one asap, i am itching to drop one or the other in my car. By the way, the z28 has drum brakes, that's the first thing i looked for.

VIN on z28: 1G1FP87F2GN119754
VIN on L4 : 1G1a?8722el254670

Thank you in advance for your help!

 

Instead of the VIN's you should have gotten the codes from the right front of the axle tubes. That tells what the rear ends actually are. Most 4cyl's got 3.73's but a few other ratios were available. Some '86 Z-28's got 3.73s. With a 305 it could very well have a 3.73 posi. '86 was around the time things started getting better. Early '80s cars seem to only get a posi when special ordered with one. 3.73's,3.23s (most likely), and 2.73's were available in '86 for a Z-28. Go down and test the axles and rotate them around while counting the yoke to find out what the ratios are. I would grab the Z-28 axle first if it has a posi. To test that rotate one wheel with the transmission in neutral. Other wheel rotates same direction, posi. If the other wheel spins backwards, open diff. Drum brakes are not a turn off, a disc brake rear is just more likely to be posi. The 4cyl almost undoubtably is open diff.

 

With an older car in the boneyard you can't assume it has the rear it came from GM with. I would mark the drive shaft yoke, jack it up count turns of the wheel versus driveshaft turns and that'll give you the ratio.