Well, Im replacing my crappy olds 350 engine & tranny combo in my 80 coupe deville.........For a 500 caddy and a BOP th400. Both top notch and the engine has been really tuned good. I was wondering if the drive-shaft and differential would need anything to withstand the higher horsepower and perfmormance. Or did caddy make it good enough from the factory?
Thanx for the tips..........Jake

Depends, are you planning on hooking up? If you have the 7.5" axle, (some do, but I don't know if 80 cad did), and you drop on slicks or regularly beat it, the axle won't last 100K miles. Remember, this is the SAME basic axle in the 82-04 F Bodies, so YES they will take some power, dont' kid yourself when everyone tells you how weak they are. Take note of how mnay axle failures you have ever see due to high power in cars that have lots and these "weak" axles. It has lethargic gearing most likley, so that won't help. If you build for it, then you can make them work for you.

BUT, being a 80 Coupe, it very likely has a 8.5" or even a 8.875" "P" axle, both which will take a ton of abuse, so don't give it a second thought....The type "P" axle doesn't have any performance gear sets, the 8.5" is much more desired.

Driveshaft likely should be fine. I have never heard of anyone lunching a driveshaft for putting too much power into it.

I'm not sure about the 80's, but I found a service car with a tow package (posi) that took my '71 from something like 2.56 to a 2.93, and it all went right in my existing housing - awsome change. Service cars are a blessing from GM when it comes to rear gears, but good luck finding one with the ever evasive tow-package. I found mine by complete accident, and didn't know it was a posi until I had it in the shop (the yard obviously didn't know either, or I would have paid more than two bills for it). I know it's not an answer for exactly what you asked, but it's one of those things that's good to know when looking for this stuff. Hope that helped. PRG04

The '80 axle will last unless it gets abused. Abuse would be burnouts around corners and other show-off type stuff. The problem you will find with the axle is that it cannot use all the torque from your 500. Look for a posi.

Like my LT1, I can't even think about launching it will more than 1000 rpm against the converter, else smoke.... OR TC kills it all and really messes me up.

I can't imagine how hard it would be to launch a 500 in a light car.... But with some slicks on a track, mind bending....

Eva Longoria's rear end is the best!!!

You have a 8.750 inch rear end, I don't think that you will have a problem with it. I am running the same engine and rearend and have not had any problems in two years. I have a 2.28 ratio and am looking for more kick, so I bought a 3.42 out of a 4100 car. Your ratio is somewhere from a 2.28 to a 3.08. You could do the same if you are looking for more bottom end. The rearend you have is interchangable up through 83, I think. You want the one with the cut out cover, not the round one. If someone has a limited slip from a 3.42 car I would be interested in buying one, or trading for my 2.28 posi.

That 2.28 posi is hard to find. I've only seen one in 26 years of playing with these cars. It's on vacation out in the garage now. :)

I have heard of the Pontiac axle (Type "O" IIRC) called a 8.875" ring and a 8.75". Did they shrink the ring gear for 77-up? Diff gaskets from 71-84 are the same on D Bodies, and same as 71-76 B/C bodies. I have a 1978 FSM from Chevy calling the axle 8 3/4".

GM has been known to increase ring gears from time to time, (7.5 to 7.625, 8.5 to 8.6, 8.2 to 8.5, etc). Maybe they did shrink a gear once....

Does the axle look like this?

I have two factory service manuals from 80 and 82. They both specify a 8.750 ring gear for all cars. These are what are called the "cut out cover axles". The Hollander interchange books say that the 77-83 axles are all interchangeable. I know for a fact that the 77-79 axles have a bigger ring gear, I believe that it is 8.875. I also know someone that said he tried to swap a 77-79 axle in a post 80 fleetwood and the mounting brackets were different. I haven't seen this for myself to verify. From reading this forum there seems to be a lot of misinformation and confusion on rear end interchange and gear ratios. I have spent hours and hours recently to try and find out what will work. I was chasing Caprice rear ends to start with because I thought that they would swap into my 82. This was not the case, this was for the round cover rear end only ( 7.5 inch). I believe all 77-83 cars are Cadillac only rear ends. In 84 I believe that Cadillac started sharing rear ends with other GM divisions. If anyone has more concrete info please post. If you want a gear ratio over 3 to 1 then it looks like the best years are 82-83. The commercial rear end was a 3.08, the V6 a 3.23 and the HT4100 a 3.42 and all still 8.750 ring gear. For those of us that have built our old big blocks, these rear ends are desirable if you have increased the rpm range of your motor with new valve springs and a more aggressive cam.

I just pulled my carrier out of my axle and it looks like 77-79 does have the same size ring gear as the 80-83. I stand corrected. I know I heard of a 8 7/8 ring gear from somewhere? The carrier looks the same also, so I'm going to try and reuse the posi from the 77. Does anyone know if I can use a posi carrier from a 2.28, switch the ring gear and use it for a 3.42? I'm going to see if it will setup.

look at the part numbers for all the other axle stuff. Things all change when you go above 2.56. I'm pretty sure it won't work.

I am suspecting that with the engines getting weaker in 1977 as compared to the 1971-1976 cars, they may have found some minor cost reduction and fuel economy improvement in the slight shrink of the ring gear.

OR

In my FSM's (1976 Olds which has great detail and I have 77 and 78 Chevy, but those have no info on gear ratios), no gear over 3.23 ever existed. It goes down to 2.41 for 1976, but as we do know, 77-up "P" axle got the 2.28. Maybe the reason to go to the 8 3/4" was to allow room to go to the 2.28 ratio. Might be to make the ring smaller to allow for a larger pinion gear.

You cannot use anything from a 8.5" 10 bolt ("B" axle") in the "P" axle. They are different.

You have to swap the entire axle assy to make it work. I would recommend the upper and lower trailing arms as well. Do some measuring to ensure it will fit (mounting point widths, etc).