Differential swap / bought it, work and install now

[SirRobyn0]

Ok So I have another rear differential thread, but it's several years old so I'll give a quick recap and then tell ya what my plan is. So my truck is an 84' C20 14 bolt semi-float with 3.41 gears, 305 and rebuilt 700R4 with some beefing up. 3.41 gears with a 305 isn't the best at all and over drive use is limited to on the highway and flat ground. Towing my trailer (18 foot open trailer, occasional 6 - 7K load). I mean the truck will pull it even over the pass but it's not easy on the truck or me. So I'd like 4.10s or even 4.56's. I'd kind of like to go to the full floater, as well just because I do put weight in the bed from time to time and I'd prefer the heavy dutyness of it.

So there's a guy about an hour and a half north of me with a 14 bolt full floater out of a 78' C20. Other than the parking brake cable difference from the 70's models to the 80's models (I'll need to get new parking brake cables. It should be direct bolt in. That's a plus for me. Guy is selling it with the leaf springs $200. Rear diff is not locked up or anything like that, but he doesn't have a clue what the ratio is.

So my plan is at minimum to open the thing up find out what the ratio is and plan for it to need bearings and brakes. If it does not have a ratio that suits me, I'll do the gear change myself, but it will be out of the truck and I can take as much time as I need to fiddle with the thing and get it all set up correctly. If I'm lucky it'll have a ratio that will be good for me and I won't have to mess with that part of it. Which leads me to my question. What do ya'll think would be the most common ratios for a 78 C20? I haven't a clue about engine or transmission on the truck it's coming out of but my stab is it'll either have 4.10's or 3.73's.

I can't get up to see it until next weekend so I'll let ya'll know if I bring it home, but honestly at that price even if it needs to be fully gone though it's a deal at least for around here.

 

[bucket]

Yes, 3.73 and 4:10 seem to be the most common by far.

Iirc, '83/'84 is the year split between parking brake cable setups. You missed it by just one year, lol.

 

[PrairieDrifter]

Aren't the 3.73 and 4.10 carriers different? Or was 3.73 the cutoff?

Under 3.73 is a different carrier than 4.10's and up. Can't remember if 3.73 was the split though.

 

[Frankenchevy]

I thought it was 4.10 and down were the same.

 

[Craig Nedrow]

my 86 is 4:10

 

[SirRobyn0]

Ok thanks guys. I think this differential might work out well for me.

Also @bucket No worries about the parking brake thing. My cables are bad anyway and I've held off a few years not knowing if or when or what year differential I'd end up swaping in. So ultimately I'd just end up swapping to the older style cable setup.

 

[bucket]

I thought it was 4.10 and down were the same.

Yep. I haven't looked into it in several years, but I believe you can get the commonly used gear sets for either carrier.

 

[SirRobyn0]

Ok well I'm going to go look at the differential tomorrow at 11am. So I know how to get a rough calculation of ratio by turning the pinion and counting, but and I'm not sure if I will get to pop the cover before I buy it, I'd like to but we'll just see how it goes. Is the ratio commonly printed on the ring gear somewhere I might be able to see it?

 

[Ricko1966]

Ok well I'm going to go look at the differential tomorrow at 11am. So I know how to get a rough calculation of ratio by turning the pinion and counting, but and I'm not sure if I will get to pop the cover before I buy it, I'd like to but we'll just see how it goes. Is the ratio commonly printed on the ring gear somewhere I might be able to see it?

There will be a code stamped on the axle tube. The ring and pinion will have markings on it,not usually the ratio though. Normally the part number,a matching number so the ring and pinion stay as a set,like if one says 263 the other will be 263 so you know they are a set. And sometimes a date code. And 4 more numbers you will use for division to get your ratio. 41÷10=4.10 manufactured 03of70

 

[SirRobyn0]

Well I went and looked at it this morning. It's out of a truck that was running and being driven 4 months ago, but he said the transmission was going and then he got hit. So that's good news as far as the differential hasn't been sitting in the mud for years or something silly like that.

I took the fill plug out and go a little sample of fluid, and didn't find anything terrible. The guy took my offer of $150 and took it home. Also he left the leaf springs on it, and as long as they are the same length I'll use his as it has more leaves.

Here's some pics.

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So the bad news is that when I got home I jacked up it up and put it on blocks. I got about 3 3/4 of a turn of the pinion for every turn of the axles. I did it three or four times to be sure I was doing it right so that means she's got 3.73. Damn I was really hoping for 4.10's. So I'll have to decide if I want to swap it to 4.10's or 4.56's or leave it alone. I'm not sure I'd be fully happy with 3.73's on the other hand that's what's already in there. The semi-float currently in the truck is 3.41's.

We tonight I will sit down and run the ratio calculator and also check to see what it will cost to get me a set of gears. At least if I decide to re-gear it, it will be off the truck and I won't have to worry about getting it together in a certain time frame so I can drive it.

 

[SirRobyn0]

So this isn't really a question or a statement, it's more of a thought process. I've already thought it out and read it so I know at least part of the answer for me, but given that the 31" tall tire is factory, and even my tires which are a bit wider than factory and still the same height so perhaps this information will help someone else, or maybe you'll disagree with my thought process and share yours.

So this is a speed / RPM chart for the various common ratios, or at least the ones that concern me atm.
Current gears at 3.41's so I'll start with that one. Mainly we are concerned with direct drive but I have included 2nd gear to see the RPM change at speed for a downshift like getting slowed down climbing a hill. Anyhow.
3.41
65MPH - 3RD 2,400 RPM
60MPH - 3RD 2,200 RPM
55MPH - 3RD 2,000 RPM
60MPH - 2ND 3,600 RPM
55MPH - 2ND 3,300 RPM

3.73 (the ratio my differential has in it)
65MPH - 3RD 2,650RPM
60MPH - 3RD 2,450RPM
55MPH - 3RD 2,250RPM
55MPH - 2ND 3,600RPM

4.10
65MPH - 3RD 2,900RPM
60MPH - 3RD 2,650RPM
55MPH - 3RD 2,450RPM
50MPH - 3RD 2,200RPM
50MPH - 2ND 3,600RPM

4.56
65MPH - 3RD 3,200RPM
60MPH - 3RD 2,900RPM
55MPH - 3RD 2,700RPM
50MPH - 3RD 2,500RPM
45MPH - 3RD 2,250RPM
45MPH - 2ND 3,600RPM

Ok so it's easy for me to see from this chart that keeping the 3.73's are out. It's not enough. As ya'll already know no matter what I choose everything is a compromise. I kind of feel like the cruising RPM in third towing should be at bare minimum 2,500RPM. And not over 3K. Not saying that the engine wouldn't run along happily at 3,500 but I sort of feel like that's great for a hill climb but not really necessary at other times. 4,10's put me in this range for towing at around 60 - 65mph (speed limit for towing on the highways around here is 60mph or lower in some cases.) 4.56's are pretty much going to cap me at 60mph, though I could do 65, 4.10's would allow 70. It's all a compromise. With the hills around here 4.56's look real attractive for towing.
Now I'm going to throw in my second but not quite as big of a concern. Running empty in overdrive.
3.41
80MPH - OD 2,050RPM
75MPH - OD 1,900RPM
70MPH - OD 1,700RPM

3.73
80MPH - OD 2,250RPM
75MPH - OD 2,100RPM
70MPH - OD 1,950RPM

4.10
80MPH - OD 2,450RPM
75MPH - OD 2,300RPM
70MPH - OD 2,150RPM

4.56
80MPH - OD 2,750RPM
75MPH - OD 2,600RPM
70MPH - OD 2,400RPM

When running empty and looking at that it's no wonder why the engine likes to run at 80mph or higher on the highway! Even though I like to run at 75!

I can see why 4.10's are kind of considered the gold standard in these trucks. No not as good a towing gear as 4.56's but not as limiting in direct drive either. I honestly don't think 4.10s will hurt my empty freeway mileage much if at all, but 4.56's certainly will. If I still had the bird farm and was running all over the state for feed I'd go with the 4.56's, but now.... I just really don't know how much towing I'd do and if it would make the 4.56's worthwhile or if they'd just become a PIA. I think because of the 700R4's low first gear that I don't think I'd have to worry to much about it short shifting 1 - 2 even with 4.56's so there is that. Although delayed a couple years now I still plan to replace the 305 with a 350.

So I will probably order parts on Monday, that gives me until then to decide if I'm going to go with 4.10's or 4.56's. I think I'm leaning towards 4.10's