Update #11 - Rear axle assessment

Update #11 - Rear axle assessment #

I’m trying to identify what type of rear axle this car has. I need to decide whether it’s good to restore and keep or whether it needs to be replaced. There are some minor alterations here and there, but, as best as I can tell, most of this car is original. So, my first guess is that this rear end is original to the car.

There are several different types of rear ends that this car might have come with. The diameter of the ring gear in the differential is used to distinguish them. E.g. ’the 7 inch rear end’, ’the 8 inch rear end’, or ’the 9 inch rear end’. There is a lot of talk online of the 8in and 9in rear ends. Both seem to be used on the V8 versions of this car. My car however is the less popular (at least, less popular with people looking for muscle) inline 6 engine.

I started by looking at some images online pointing out identifying features of the 8in and 9in versions. Neither seemed to look like what I have. Then I noticed that there is a little metal tag with some codes stamped in it. It’s in the fourth picture below. It reads:

3.20 5GB 910

Looking this up in the Chilton manual, I was able to confirm this as the 7.25 inch rear end. The type is ‘conventional’ (i.e., not a limited-slip) and the ratio is 3.20:1.

If I’m going to make any changes to this car which mean more power going to the rear end (which is likely), I want to make sure it’s strong enough to handle it. We can get a rough baseline idea of what sort of power the rear end can handle from the original engine itself. According to Wikipedia, this 200 cu engine put out 120hp (or 89kW if you prefer) at 4,400 rpm when it rolled out of the factory. My understanding is that this measurement was taken from engine alone, i.e., with no other load on the engine, like transmission, or AC. So, lets take off around 20% and call it 100hp (72kW) actually hitting the rear end.

I can’t seem to find any specifics on what the maximum this rear end can handle is though. This thread on the ford six forum seems to suggest that they can take 136 hp (and some abuse) at least. That’s about 101 kW. I won’t be doing burn outs or anything, I just want a decent amount of power for motor-way acceleration.

However, the bible for this engine - The Ford Falcon Performance Handbook - says:

These rear ends are adequate for stock sixes, but if it needs a rebuild, consider replacing it with a heavier duty rear. They should only be used for restorations.

I.e., don’t put more power into these rear ends. So, it’s hard to be sure what the best thing to do here is.