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1989 v10 blazer axle ratio swap

I'm just hoping to head off his next thread asking why his 700R-4 no longer has OD etc... :1zhelp:

3.42's should be just fine for a 30" tire like the original poster is running. I think too many guys expect 2017 performance from their 198X Blazer or pick-up. It's apples and oranges. Have a little more patience, or are you racing it? It's not a 2017, but it's not a model T either. I've had my share of squares, none were "fast", but all were more than adequate for regular driving. By regular driving I mean not try to keep speeds at or above 80 and 85 mph. If that's regular, a square isn't for you IMHO. The aerodynamics aren't there to support that, and in order to overcome that you'd need double the power it has to keep those speeds up easily...enjoy the fuel bill that comes with that. These trucks do just fine up to 70 mph, comfortably and with decent economy. If you want more, get a '17 Silverado...
 
I understand that towing in overdrive is more stress on trans but that trans can and will be built to take anything that can be dished out to it not only that but our interstate is mostly flat and boat is only about 1500 lbs. That being said I do want input that is why iam on a forum so dont withhold info because to save my feelings I wont get butt hurt if you tell me I cant do something. Iam also probably going to switch this truck to a nv4500
 
Jamcc85 said:
I understand that towing in overdrive is more stress on trans but that trans can and will be built to take anything that can be dished out to it not only that but our interstate is mostly flat and boat is only about 1500 lbs. That being said I do want input that is why iam on a forum so dont withhold info because to save my feelings I wont get butt hurt if you tell me I cant do something. Iam also probably going to switch this truck to a nv4500
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Switching from .70 O/D (700R4) to .73 O/D (NV4500) is almost half as much gear reduction as switching from 3.42 to 3.73. Neither one is a huge jump, but it's worth taking into consideration before you tear into both axles.
 
I'll give my input on towing: 1500 lbs on level ground? I wouldn't sweat that very much. Hauling 8000 pounds up 5% grades? That's what finished off my 700R4. At some point there's a line between the two, but it's kinda blurry as to where it is. YMMV.

I'd add the larger cooler (as GM did eventually) and then grab a temperature gauge to monitor temps while driving.
 
The thing I figured is that most of the time I will be towing on flat ground and on occasion it shifting into drive for hills even if I have to backoff or manually shift into drive for hills and windy days. And if it starts to get bit then I drive in drive and not run 80.
 
Dude, it's your truck. Hell drive 90 in second towing a barn...
 
Lol no worries I am thankful for your knowledge like I said it may not work if it wants to run hot then I will have to shift to drive but it is not much weight and not much wind resistance and it is the only trailer I pull if it was an enclosed trailer that would be a whole different thing. So if it will run most of the time in OD with out downshifting except at hills and without getting hot then it should be good. Right worse thing that happens is I birn out a trans that already has over 100000 on it.
 
Jamcc85 said:
I understand that towing in overdrive is more stress on trans but that trans can and will be built to take anything that can be dished out to it not only that but our interstate is mostly flat and boat is only about 1500 lbs. That being said I do want input that is why iam on a forum so dont withhold info because to save my feelings I wont get butt hurt if you tell me I cant do something. Iam also probably going to switch this truck to a nv4500
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Fair enough. Here's my take on it then. With your tire size you are very close to a stock K30. They typically use 4.10 or 4.56 gears with an SM465 or T400. Read no overdrive. You have a very high geared overdrive. I would go at least 4.10. I have 35s with a 700R4, 4.10 gears and a big block and it's way to high geared for me. I would like 5.13 for my app. The problem with towing in OD is if it starts hunting (down shift/up shift) a lot you will fry the 2-4 band in that trans. If you notice when it shifts into 4th it is a long slow smooth slide into OD. With a towing load that is killer on the stock size band with stock apply pressure. You need to do one of two things. Either tow in 3rd or get the diff gears low enough that it doesn't hunt. An oversized 4th apply servo will help also. That will get the shift a little quicker but will increase holding power on the band quite a bit.

As far as towing at 80+, well, a K5 is borderline dangerous at that speed anyways and throw a trailer on behind... I'm glad you live in another state than me! LOL.

Hope you're not butt hurt. :biggrin:
 
That is what I figured as long as it is not hunting between gears iam going to try and find a set of 410 did they even come in a 12 bolt rearend I believe it to be a 8.5 inch. And as to running 80 mph that truck is perfectly stable to me who has been running at these high speeds for almost 20 years our speed limit just changed from 75 to 80 last year. That being said I dont need to pull at 80 that is just my goal. But if I cant pull in od then iam going to leave the gear ratio because it pulls ok in drive might go to 373 if not able to pull in od to still keep rpms down in drive because iwm not going to run 65 also iam flying around with idea of pulling motor and trans out and replacing with 5.3 4l60e
 
A lot of 700r4 failures are from excessive heat. The heat comes from rapid shifting between D and OD (gear hunting) and the torque converter locking and unlocking. If you can keep the heat down the survival rate of a 700r4 greatly increases.

I will also recommend towing heavy loads in D, but will also say that from real world experience a 700r4 can survive towing in OD. A friend raced midgets on the USAC tour and travelled all over the Midwest using 700r4 equipped Suburbans. The first one was a mid-80's model with a modified 350 and built 700r4. The trailer was a 18-20' tandem axle enclosed trailer hauling the race car, spare parts, tools, etc... with an estimated weight of 6-7k. Not sure how many miles but he ran that setup for years going to a race out of town almost every weekend with no failures, only retiring that Sub because it started to rust out. He then bought a '90 or '91 1500 Sub and ran it stock with only an extra trans cooler for many years with no issues. The 350 TBI blew a head gasket and the body rusted out on it before the trans had any issue. He always ran in OD, but would also drop it into D pulling grades to prevent the gear hunting issues.
 
Jamcc85 said:
That is what I figured as long as it is not hunting between gears iam going to try and find a set of 410 did they even come in a 12 bolt rearend I believe it to be a 8.5 inch. And as to running 80 mph that truck is perfectly stable to me who has been running at these high speeds for almost 20 years our speed limit just changed from 75 to 80 last year.
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You will be able to find a 12 bolt rear in 4.10/4.11, or at least should, based on the fact they were made. I don't know as a 10 bolt (8.5") rear was available with 4.10's. Front 10 bolt or Dana 44 will be easier, although not sure how common in 6 lug.

IMO there is no question the trucks can run at 80 for prolonged periods of time. It's trying to stop and maneuver it in emergencies at that speed, especially when surrounded by vehicles that have ABS.
 
I am hoping that I can find complete diffs so I dont have to deal with clearances and shimming but I will if necessary.
 
Lol none of you obviously live around here traffic is not congested on our interstate or hwys most of the time you are by yourself and I do not believe in antilock brakes but on dry pavement in any other conditions they suck I have driven multiple vehicles including new ones that I had to use the parking brake to stop because the antilocks were not allowing to stop at all. Most of you probably have a speed limit of 65 or even 55 so you cant fathom doing 80 which is just fine lets just make sure we are not getting stuck on you think iam driving to fast because iam slower then most. As far as maneuverability that truck is just fine we all no their is usually not a good outcome when doing a emergency maneuver other than a simple lane change at anything greater then 55 and pulling a trailer their is no emergency manoeuvring. We have tractor trailers doing 80 mph weighing 80,000lbs plus driving on the same road which has no emergency manoeuvring. Let's not worry about how fast iam going. I just want my truck to tow better the way I drive it now.
 
From my experience a 4.10 geared 1/2 ton rear axle, either 10-bolt or 12-bolt, is pretty rare. I remember years ago hearing about factory 4.10 geared 12-bolts and scouring junkyards and classifieds for a long time looking for one and coming up empty. Not saying they are not out there but seem hard to find. I would think 12-bolts in general would be hard to find nowadays since they haven't been in production for 30-40 years now! For a front D44 or 10-bolt there were a lot of 4.10 geared version from the factory but in 8-lug. If you have a 6-lug version already it's not a big deal to swap between 8 and 6 lug on a front axle.
 
I don't even know if the later 6 lug 14SF's commonly got 4.10's. 8 lugs I'd expect yes, but 6 lugs perhaps not?

Could always regear, but it's hard to justify when you can probably still find complete 3.73 axle pairs for less than $500. I wouldn't in this case, but I could see why someone would take the cheaper, easier option.
 
Out of all the GM trucks I've owned or seen,bought & sold,only one,a 1976 K5, 2wd I bought nearby for a friend ,ended up having a 4:10 ratio 12 bolt in it..truck boogied too,had a nice low mileage 350 and TH350 in it..

I'd guess 85% of all 1/2 ton 10 or 12 bolt rears had 3:08's in them,the remaining 15% had 3:73's or 2:73's,and 3:42's...4:10's were like !%..seen one P-10 step van that had 4:56's too,with a 250 six & powerglide..

My '69 GTO had a 10 bolt with 4:10's in it...seemed like more cars got them than trucks too..
 
The most common ratio I see is 3.08, followed closely by 3.42. Those two ratios seem to account for 90%, at least in the areas I lived in. I've seen three Blazers with 2.73's. 3.73's are hard to find, in fact I've never seen a set in 1/2 ton yet. 4.10's are Unicorn gears too in 1/2 ton.
 
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