Changing diff gears...

[B and C]

Looking at the hub it looks like a semi-floating hub. Mine has a full floating hub and it sticks out past the wheel and has bolts holding a center cap on.

 

[highdesertranger]

yes that is a GM 14 bolt semi floating rear axle. to bad it's not a full floater. highdesertranger

 

[MrNoodly]

Whether it's too bad depends on how the OP wants to use his van.

 

[highdesertranger]

well the reason I said "to bad" is the full float is much more heavy duty and much easier to work on. highdesertranger

 

[wcurtin1962]

http://www.drivetrain.com/parts_catalog/overdrive_units/

Just a thought...................

 

[KingArthur]

Resurrecting this again... As you can tell I move slowly on this stuff. Mostly due to winter being a terrible time to do anything constructive up where I'm at.

I suppose today I'm going to start thinking about cracking open the rear diff to get a look at what gears are in it. Will report back here soon (maybe).

Going to be posting elsewhere on this forum because I'm feeling that I'm about ready to start working on this old beast and get it somewhere that I want it to be.

 

[KingArthur]

According to my calculations based on 10 teeth on the pinion and 41 teeth on the ring, this differential is at a 4.10 ratio.

That explains the 7ish mpg!

What an absurd gear ratio for a vehicle like this. I suppose this is our tax dollars at work.

Although I can't fault the absolutely heroic amount of RTV that was used to seal this diff back up. Guess they had plenty of that!

I suppose now I can calculate engine RPM? There's no tach but I know gearing and tire size. So that's enough to figure it out right?

I'm somewhat glad that we're changing these gears out. There's about a pound of metallic sludge sitting on the magnet on the bottom of the diff case. I suppose that's normal?

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[B and C]

4:10's don't sound so bad. I have 4:56 gears but have overdrive. Mine gets 11-12 MPG depending.

Sure you don't have a major miss in the engine or a slipping trans?

 

[highdesertranger]

I wouldn't count on a gear change alone to improve the mileage much. at what speed are you getting 7mpg? that thing has all the aerodynamics of a brick. you will get your best fuel economy around 40-45mph.

I also believe there is an issue with your engine, that 6.2 should be getting at least 12mpg. even with 4.10's.

highdesertranger

 

[KingArthur]

4:10's don't sound so bad.  I have 4:56 gears but have overdrive.  Mine gets 11-12 MPG depending.

Sure you don't have a major miss in the engine or a slipping trans?

I don't think so. To be honest I'm not exactly what sort of fuel mileage it's getting. It just feels like I can watch the mileage go down. Later this week I will go out and test to see for certain what I'm getting.

 

[KingArthur]

I wouldn't count on a gear change alone to improve the mileage much.  at what speed are you getting 7mpg?  that thing has all the aerodynamics of a brick.  you will get your best fuel economy around 40-45mph.

I also believe there is an issue with your engine,  that 6.2 should be getting at least 12mpg.  even with 4.10's.

highdesertranger

I'm going to say it's 7 combined. It's definitely under 10. I will test later this week on flat ground with a combination of highway and stop/start miles. I agree that the best fuel mileage is going to be at lower speed because of the cumulative effects of poor aerodynamics. That unfortunately can't be helped much.

It just seems to be such a mismatch of gearing for me. If the van had overdrive it would make sense, but at 50 miles an hour in drive the engine is absolutely howling. If I had a tach I could figure out where were at RPM wise, but it just doesn't seem good or efficient to me. The engine produces its peak torque at 2100 RPM and is certainly using less fuel at that speed rather than wherever it's sitting now. Ideally I'd want it to sit at cruising speed in drive at about 2100 RPM would I not? So I would think it would be smart to gear the diff to achieve that.

After reading up on the 6.2, I did learn that GM originally designed it for military/marine use and therefore created it unconventionally as diesel engines go in making the stroke shorter than the bore. This allows the thing to rev higher a bit more safely, but torque curves don't lie.

I mean, I'm not sure what other experience, but I find it sort of absurd that the van shifts from 1st to 2nd at something between 5-12mph. Regardless of the load I have in it. That can't possibly be right can it?

 

[Amboman]

Buy a TinyTach make sure it's for 4 stroke. You just run a small wire on th the unit around the closest plug wire to the cab & it tells you the rpm & the hours when you shut it off, very handy. I have a HD Dana rear axle with 355s that would do you well. Tiny Tach link https://www.tinytach.com/

 

[B and C]

That is the range mine shifts at and about 35 into 3rd and 45-50 for overdrive.

 

[Weldman]

Reading this and I see no mention of a turbo, is there one in there? If not I would look into that next, they are gutless without one (IMHO) from experience driving the M998 versus M1097...

 

[KingArthur]

Reading this and I see no mention of a turbo, is there one in there? If not I would look into that next, they are gutless without one (IMHO) from experience driving the M998 versus M1097...

No turbo. I think doing a turbocharger conversion would be more complicated than just putting in a different engine no? I'm not really into putting a turbo on an engine that was never designed to have one. I know since it's diesel it's more reasonable to do such a thing, but I'd rather not have to go to all the trouble to make a dog meow if you know what I mean. If I can get the fuel mileage I'm looking for without swapping in a Cummins thats plan A.

 

[Weldman]

No turbo. I think doing a turbocharger conversion would be more complicated than just putting in a different engine no? I'm not really into putting a turbo on an engine that was never designed to have one. I know since it's diesel it's more reasonable to do such a thing, but I'd rather not have to go to all the trouble to make a dog meow if you know what I mean. If I can get the fuel mileage I'm looking for without swapping in a Cummins thats plan A.

Putting in a turbo would pay for itself over time and you can basically put a turbo on anything. Well if you ever feel the need to scratch that itch here you go...
Banks SIdewinder

 

[Amboman]

In IDI diesels Ford used before the Powerstokes, the last year 1994 they added a turbo & it added 5HP & NO Torque & a lot of things to go wrong & lots of cost.

 

[Amboman]

Here's some good reading for you, looks like you want a newer 6.2 & if it has a Serp belt it's the best ones just like ords IDIs. Here's the link http://www.trucktrend.com/how-to/engine/0554dp-62l-buildup/ Sorry about the Tiny Tack I never owned a GM diesel only 5.9 Cummins 12 valvves & my IDI ambo.

 

[B and C]

If you plan on driving in the mountains any, a taller gear will give you problems going up steep grades. I know I have been on a few steep ones where I was in low gear doing 30 MPH. Of course my rig weighs about 9700 Lbs.

 

[KingArthur]

Well I just called my local (well fairly far unfortunately) junkyard and they have two 1 ton GM 14 bolt axles. I think before I buy the ring and pinion gears and, as I've now learned, 200 dollars worth of bushings and bearings plus some tools I don't have, I'm going to see if I can just swap on a different rear. I would imagine this would be less complicated.

They didn't know what ratios were in the rears. If I'm lucky the stickers with the rpo codes will be present. I'm looking for something close to 3.42 to achieve around 1900-2400rpm in drive at 60-65mph. I do have access to a press so I could do the gear swap myself, but after reading about it, it's not easy on a full-float. And there's less information available for the semi-float which is harder to work on and I'm not sure I want to mess with it.