G80 & GT4 for 2004 AWD Astro Conversion

[forrestmccarthy]

I just got some bigger tires (255/70r16) and my van is now a bit sluggish on steep climbs. I’m considering changing my gearing from 3.42 to 3.73 and adding a G80 limited slip differential while I’m at it. Seems to me, if I can find the right donors, the simplest thing is to swap out the entire axle assemblies.

For potential donors, am I limited to 2003-05 AWD Astros with the stock tow package (G80, GT4) or can I expand my search to earlier Astros or even other Chevys with the G80 LSD and GT4 gearing?

While I have the van lifted and pulled apart, I’m considering the 4X4 conversion with a NP233C transfer case.

While this is my first post I’ve been a voyeur for several years. I’ve learned a lot from this forum and I know some of you have already been down this bumpy road. I don’t need to reinvent the wheel. Expertise and advice is most welcome.

[Astrophysics]

Hi,

The GM 7.5 ". 10 bolt rear differential is used in Astro. You can have a Rear End Specialty shop change the ring and pinion to get the 3.73 ratio. 4.10 ratio is also available. Some forums on S10 Blazer say it is safer to use the 3.73 instead of 4.10 ?

The front diff on Astro is a bit harder to re- gear single axles need to be pulled and diff has to be opened to get to ring and pinion gears. Also , the Astro front diff is 7.2 " GM.

My 2003 Astro has 3.42 ratios. My tires are standard load p245/75-16 cooper Discovery AT3,which are about 30" diameter.

The 2 speed t case mod to np233 or np231 is nice because with low range 4 lo in the dirt you get the extra 2.72 : 1 gear reduction from the t case which makes the tall heavy tires move ok at least in 4lo.

AP.

[MountainManJoe]

For potential donors, am I limited to 2003-05 AWD Astros with the stock tow package (G80, GT4) or can I expand my search to earlier Astros

I believe that in 2003 GM changed the rear axle from a 5-lug drum system to 6-lug with disc brakes. Maybe not a show stopper, but having different wheels sounds really inconvenient. As far as I know, Astros have a unique track width and probably the differential location too. Again, maybe not a show stopper, but the sliding door limits how wide you can go. It seems to me that the donor route is by far the easiest and most economical, but I think you will have a hell of a time finding 4.10s, but the availability will also depend on your area.

A low range TC might be a good option. It all depends on what you intend to do with it.

[forrestmccarthy]

While I do a lot of desert/mountain off road driving, I also spend a lot of time on the interstate system getting there. For that reason I don't want to bump up to the 4:10. With 30.1" tires and a 3:73 I would be near the factory engineered RPMs that I had with 28" tires and the 3:42 (GU6) . My speedometer might even be accurate, though I'm expecting my mpg to drop yet again.

In the winter I pull off my large cargo rack and the van transitions from an overland vehicle to a mobile backcountry ski hut (previous photo). Currently the van handles snow and ice with precision and grace. It has been my experience that an AWD with an open differential performs better at higher speeds on slick surfaces than a true 4X4. I am concerned about switching to a G80 and 4X4 for winter driving.

I'm equally concerned that when I head to Utah next month with the 30" tires and cargo rack I'm going to have trouble getting over any steep bits. Same goes for this summer when navigating old mining and logging roads in the mountains. For the summer season the G80 and 4X4 conversion seems like the direction to go.

Need to find the right balance.

The photo below is from last summer with my cargo rack and 28" tires.

[MountainManJoe]

I had 30's on 3.73, and I found I was coming up short on torque and getting stuck in sand, snow and steep inclines. The transmission would just slip and I went nowhere.
In your case the TC might be better than re-gearing. You would get low range/more torque for offroad, and still get good highway MPG in high range.

AWD performs better in snow, and behaves better on-road, so you would sacrificing a bit of handling there. Actually, I don't even know if that's what you have. You wouldn't have problems with the G80 on the highway though. It has a limiter which turns it into an open diff above 20mph.

[Captn. Crunch]

I like my 3.73’s, they are a nice compromise. I’d like to make a few suggestions if I may. Forget the G80, they are outdated and fundamentally flawed. They don’t work until you already have wheel slip, engage with a clunk due the pawl style actuator, then disengage at 20-25 mph. I went with the Detroit Truetrac. It’s a Torsen style carrier. It works all the time and doesn’t require rebuilding ever. So get a set of gears, good ones. Don’t go cheap here or the whine will drive you mad.
For the front go to car-parts.com and search for front axle assy. for chevy S-10 Zr2. They all came with 3.73’s. Find one with manual transmission and three button tc and it’s probably a cast iron diff vs. our aluminum.
That’s what I did. I was so thrilled with how invisible my Truetrac was, I put one in the front diff also so I’m sporting twin torsens with a two speed tc. Quite impressive traction and handles like an Audi Quattro - well a 2 ton Quattro

[forrestmccarthy]

My 2004 Astro is an AWD with the standard GU6 and open differential. It has the 6-lug configuration with disc brakes.

Capt. Crunch's suggestion of twin torsens with Detroit Truetrac differentials and a np236 transfer case is intriguing yet likely out of my league budget wise. While this would be an awesome configuration during the snow free season, I'm also concerned about winter driving. Additionally, I'm not tooled (nor skilled) for doing most of the work myself. I'll be farming out the heavy lifting to my local auto shop that is conveniently located just two blocks away. While they like working on custom vans, they are less excited about rebuilding differentials.

That said, I found a reasonably priced 2004 Chevy Blazer Zr2 S10/S15 Front Axle Assembly with low mileage. It's an NP1 (three switch), AT (GT4) -- I prefer an AT so I can drink my coffee while driving Given that I can find an appropriate rear with compatible GT4 gearing, other than rigging a stick and the other typical mods for an Astro 4X4 conversion, what else would I need for the front? Would the assembly be plug and play with my current stock front axles?

[Captn. Crunch]

The Zr2 front diffs are identical except for the pass. side axle. Our vans have a one piece while the S-10 4x4’s use a two piece. There is a collar on the two piece axle that is vacuum controlled. When 4wd is selected, this collar is pulled on and connects the two pieces of axle to engage the rt. front wheel. You can leave it as it is and buy an aftermarket cable assy. your van will be 2wd until you pull the knob. You can put your tc in low range and leave the front end disengaged for a 2wd creeper gear handy for the boat ramp. Or you can swap over your vans one piece axle. There is no difference between a manual or automatic transmission equipped trucks front diff except for the material they are made of. Almost all are aluminum save the few trucks that where manual transmission and three button electric tc. These were cast iron.

[Captn. Crunch]

Too bad I’m so far away. I’m junking my 99 with 3.73’s and I have a used torsen in the shed. If you can find a rear end locally, you can do the Truetrac install yourself. As long as you don’t touch the pinion gearyou can take the carrier with the ring gear out, swap the ring gear over to the Truetrac and re install. Just take care to put everything back EXACTLY the same way it came apart. Shims and bearing caps is really all there is to it. Once you pull the pinion gear you’re committed to crush sleeves and the whole nine yards.
After re-reading the above posts I need to clarify one point. The Zr2 diffs are identical but the drivers side axle should be measured before reusing as I doubt the lengths will be the same. The Zr2 guys used our one piece pass. side axle as an upgrade to eliminate the weaker two piece setup so I know they are interchangeable

[MountainManJoe]

Just a few corrections.

Capt. Crunch's suggestion of twin torsens

I think Cap was referring to just the rear. I don't think there are Torsen diffs that will fit the stock IFS front axle. There are some shitty lunch box lockers, but I would stay away from those.

Torsens are great. If you can afford that route then do it. It will probably end up costing significantly more than a used G80. I've had good experiences with the G80. It has gotten me through a lot of bad situations. Regarding the clunking: It does not engage with just a pawl. It does has a clutch pack. Still, you have to be sensible with it. It's not an "offroad" locker so you can't hammer on it. I think it's good value for money.

The AWD transfer cases housing is a magnesium alloy. Strong and light.

There is a thread somewhere in the forum about installing an SUV front axle with disengagement.

[Captn. Crunch]

Front torsen GM 7.2”. https://www.ebay.com/itm/MFactory-GMC-S ... 1844126253

Front limited slip (clutch style) call Jeff @ http://www.engineered.net/links.htm

Rear torsen GM 7.5”-7.625” used https://www.ebay.com/itm/93-02-Camaro-F ... SwDk5UKviE

Rear Govlock refurbished. https://www.ebay.com/itm/GM-7-5-7-6-Eat ... Sw~OVW1Mvd

Cast iron front diff 2003
Front Axle Assembly
Chevy Blazer S10/S15 high wider performance package (opt ZR2), transfer case opt NP1 (3 switch dash control), MT (3.73 ratio, opt GT4)-125K 125,500 8T1551 $155 Ayas Auto Solutions USA-NC(Greensboro) Request_Quote 336-900-0591 Request_Insurance_Quote
Live Chat

[MountainManJoe]

Oh cool. Isn't technology great LOL.

Good to know, because that 7.2" torsen would work in my Savana van. Thanks Cap.

[Astrophysics]

Excellent info!

Good job everyone.

If money was no object, I would get Advance Adapters, Atlas , 4 speed transfer case.

Then you could have a manually controlled twin stick controlled gear driven (instead of chain) transfer case. The twin stick allows 4 wheel or 2 wheel drive
Then select ranges such as
1.0 to 1
1.5 to 1
2.72 to 1
3.5 to 1

Use tc ratio of 1:1 for easy highway
1.5 :1 for hilly highway
2.72:1 for low range dirt low speed
3.5:1 for ultra low range off road hills

AP

[MountainManJoe]

$3600 so it'll be worth as much as the van LOL . Those beasts weigh twice as much as the 136 though.

[WE7X]

Buyer be ware!
I would suggest you be sure to get some sort of a guarantee and a right to return for any reason, if you buy a used diff. I bought a couple that looked good, and felt good in my hands; but upon having a rear end shop inspect them up, finding a significant amount of wear that would otherwise be hard to discern without the knowledge and experience of a good shop.
It was a lot of wasted work to get them out of the vehicle, and then find them too worn to be worth installing.