: For those of you who feel the 7.5" rear end is too weak for your Cad....



N0DIH
07-07-05, 02:01 PM
http://www.dragtimes.com/Pontiac-Trans-Am-Timeslip-6076.html

10.70's, 2700 lbs, w/100hp shot on a stock 7.5" w/26 spline axles....

Rumors are on the Suburbans with the 8.5" (really an 8.6") is that towing at 6000# towing limit often causes rear end failures. So how much towing can a 7.5" take? I don't think any towing package came with a 7.5" did it?

Has anyone on the list actually broke a 7.5" rear due to power or towing?

That said, all HT4100 RWD cars in 1985 had 3.42's std and 3.73's std in high altitude applications. So you might be able to get a 3.42 gear cheap that is a bolt in. 307 cars were common with 2.56's.

evilrussian
07-07-05, 02:54 PM
Is that your TA, N0DIH? That's one fast SOB, sweet!

I suppose I have the same axle and to tell you the truth I'm a bit afraid to even put posi in it, nevermind getting a stroker crank for my small block. Plus, my DeVille has an extra 1300 lbs, I wish it was as light as a Trans Am.

N0DIH
07-07-05, 04:13 PM
Not mine, mine is a 1980 Turbo (without the turbo engine, I lunched that in 1989!)

A friend of mine raced his 7.5" with 2.29's (14.5@106mph...) with a stout Pontiac 400 in a 3800 lb 78 GP (they aren't as light as they looked) and finally blew his THM350 which overreved the 400 and tossed a rod out the side of the block, and as it went, it took the camshaft and the next rod with it. So he fixed the drivetrain and then installed a 3.42 GN rear end, and tore the 2.29's apart. Full of stress cracks, all gears folded over, but still running, it was still working before he took it out.

They are a tough small rear end. Many 3rd gen F cars have them and routinely abuse them without much breakage.

90Brougham350
07-07-05, 06:51 PM
I'll agree with the 7.5 incher being a tough lil' sunuvabich, but then again, I'm not complaining about the 8.5 in my Brougham. Of course, a lot of the "weak 7.5 inch" talk is either rumor or marketing. I'm sure Richmond would love it if everyone thought the 7.5 was too weak to handle over 250 horse.....lol

Brian

Night Wolf
07-07-05, 07:37 PM
The Caddy weighs a little more then 2,700lbs though....

either way, I never really considered a 7.5", but it is possible....

my '79 has the stock 2.28 gears, which do nothing to help performance...

3.42, IMO is way too low for the BB Cad, I dunno, it would be winding out on the highway.....

Plus, none of these are Posi, right?

There are a few ~1983 HT4100 RWD DeVille's around here.... they have a rear end that I could use...

I still think my best bet is to find a '77-'79 limo/commercial chassis/towing package and snag the 2.78 w/ Posi and rear disc brakes, although that in itself requires a little bit of work....

So the easiest way to upgrade the gearing is to use a 7.5" from a RWD HT4100.... are there anything with 2.78-3.11 that use the 7.5"? the 3.42, IMO is just too low.... talk about winding the 425 out, but sheesh, the stock 425 will probably keep you into the seat to 100mph :)

davesdeville
07-07-05, 07:56 PM
If I stuck a 7.5" in my 75 Deville, then built the 12.5:1 compression 500 for it, with some drag radials I'd kill it in a matter of months or weeks... But if you're not making big torque or if you have regular street tires a 7.5" isn't that bad.

N0DIH
07-08-05, 01:40 AM
I am curious where everyone gets these odd rear end ratios. GM has only a "few" for the 7.5/7.625 and the 8.5/8.6" rear ends.

2.14, 2.29, 2.41, 2.56, 2.73, 2.93, 3.08, 3.23, 3.42, 3.55, 3.64 (Buick only), 3.73, 4.10 and 4.11. These being for RWD only. FWD is all messed up, I mean different....

And in my experience, no 8.5 ever got lower than 2.56. So my guess is if you have 2.28's (really 2.29's) you already have a 7.5". Unless Cadillac had some rare exclusive gearing in the late 70's that the rest of GM never got. It would make sense to have that high of a gear to help the 7L Cads get some fuel economy to deal with CAFE (which was getting stiffer each year or 2 then) and the screwy "grams per mile" emissions requirements.

Bummer, I just gave away my 3.23 7.625" in set! I never got it installed, the 7.625's are bolt in replacements for the 7.5", but you need a 7.625" carrier, as they bolt in the 7.5" housing. The axles are the same spline even, but double check to be sure. It was also called the 7 5/8" rear. I think most were under S-10's can F Bodies starting in the mid 80's (mine came from a 86 Camaro). So go pirate S10's and F bodies for gearing!

HT4100 cars = 3.42's on a 7.5" carrier
HT4100 cars = 3.73's on a 7.5" carrier, high alt
307/5.0L cars = 2.56's on a 7.5" carrier
307/5.0L cars = 2.56's on a 8.5" carrier, towing package
350 Diesel cars = 2.93's on a 8.5" carrier
350 Diesel cars = ? on a 8.5" carrier, towing package
Commercial Chassis, all have 8.5" or 9.5" rear ends.
Interchange 77-96 (later big back end wagons excluded). Check for ABS issues, 3 channel vs 4 channel if you have it.

Weakest link on most rear ends (GM) is the spider/side gears, forged ones can be had from Randy's ring and pinion.

I say a 500 and a 7.5" can cooexist nicely, providing you never hook up! Never install slicks on a 7.5" car.

Night Wolf
07-08-05, 03:06 AM
it is a 8.5" rear, that is what it says in the service manual, that is what has been confirmed as well, GM 10-bolt, 8.5"

From what I heard, from many sources, it is 2.28...... some refer to it as 2.29, not that it would make a difference, but I wonder what it really is... until then, it is a 2.28 to me.

AFAIK, Cadillac *DID* get its own unique gearing, but I have a 2.28 with an 8.5" carrier.

2.56's really do nothing to help the dog of a 307 to start with....

The Ape Man
07-08-05, 01:00 PM
I have the parts book around here somewhere. From memory: 77/8/9 Deville Fleetwood came through standard with 2.28:1 open rear which was a 10 bolt but had 12 bolt guts. IIRC the book referred to it as an 8-7/8" gear set. These were also made in a posi but are rare as hen's teeth. I have one vacationing in the garage with ALL the other junk. They are easy to tell because they are heavy.
You can go to other car lines to swap axles. No serious performance can be had without a posi. The thing is the other car lines like Chebby use a different axle flange. Sometimes they can be swapped over without screwing up the pinion setting but it takes a little skill. Station wagon or cop car axles are probably the best to look for.

N0DIH
07-08-05, 03:13 PM
Up till 76 most GM B/C/D cars used a 8 7/8" rear 10 bolt that was not releated in any way shape or form to the 12 bolt Chevy rear. It is strong, able to to 7000# with the heavy sleds that powered them, making them able to handle around 14000# gross. Not bad for a car..... I don't think this is the same rear end as the 77-82 up rear.

The 10 bolt as we know it is the Chevy 10 Bolt (designed by Chevy, not related to the old Pontiac 8.2", which IS related to the 12 bolt Chevy rear, with bolt in axles). So it sounds like Cad got an "exclusive" gear ratio. Cad was good for that....

For low buck bolt in posi, check the "Lock Right" or "PowerTrax" bolt in posi/locker unit at http://www.powertrax.com/. Might be a good way to go, I hope to do mine before winter!

Edit: I just found something very interesting. Seems there was an exclusive rear end for Cadillac from 77-82 (Non HT4100 cars). See the attached PDF. That would explain the odd ratios that are not the GM standard 10/12 Bolt (7.5"/8.5")

Years...........Rear.....Splines..Rear Cvr Bolts.....Comments
1991 - 1996 GM 8.5" rg....28....10...................
1991 - 1996 GM 9.5" rg....33....14...................Commerial Chassis
1988 - 1990 GM 7.625" rg.26....10...................w/o Towing
1987 - 1990 GM 8.5" rg....28....10...................Towing/Armored opt.
1985 - 1987 GM 7.5" rg....26....10...................w/o Towing
1984...........GM 8.5" rg....28....10...................Round cover
1982 - 1984 GM 7.5" rg....26....10...................DeVille/Fleetwood Brougham
1977 - 1984 GM 8.875" rg.30....12...................Comercial Chassis
1977 - 1983 GM 8.875" rg.30....12...................Deville cutout cover
1977 - 1982 GM 8.875" rg.30....12...................Fleetwood Broughham

N0DIH
07-16-05, 06:48 PM
Something I just found on the 7.5/7.626 (7 5/8") for the 4.1L RWD cars.

http://www.ws6transam.org/10bolt.html

They can be modified to survive 350 hp with not too much cost. So if you have a mild 425 car, this might still be a low buck way to get some 3.42's in there from a 4.1 RWD car and ensure they live.

Night Wolf
07-16-05, 09:37 PM
I am confused now...

what is the cheapest, most easiest way to get better gears on a '77-'79 DeVille then the 2.28 that were stock...

and would those 3.42's be too low of a gear for the 425? even pretty much stock? I know highway mileage will probably go down the gutter... who cares tho.... but going form a 2.28 to a 3.42 really has to do something for acceleration... but, will that cause the 425 to wind out too much, which it really dosn't like to do?

The Ape Man
07-16-05, 11:04 PM
I am confused now...

what is the cheapest, most easiest way to get better gears on a '77-'79 DeVille then the 2.28 that were stock...

and would those 3.42's be too low of a gear for the 425? even pretty much stock? I know highway mileage will probably go down the gutter... who cares tho.... but going form a 2.28 to a 3.42 really has to do something for acceleration... but, will that cause the 425 to wind out too much, which it really dosn't like to do?

The 425 will kick a$$ with a 3.42 axle ratio. The gas bill will kick your a$$. Furthermore, the engine will run pretty warm with all those R's on the highway. Other than that no problem. If actually drag racing then a 3.42 open rear is a waste of time. Get some kind of posi. Station wagon or cop car rear axles are out there.

N0DIH
07-17-05, 11:30 AM
With the odd rear end in the 77-8? cars with the 2.28's, are any performance gearsets available? It isn't a 8.5 10 bolt, it looks like it is the old 73-76 8.875 in 77-up axle dimensions. It isn't the 12 bolt, so don't waste $$ buying those gears. GM made gears, nearly $600 a set, up to 3.23's. And posi's were rare in 73-76.

So cheapest way to get 3.42's, snag a rear from a 82-85 4.1L car. (Check YOKE! It MAY be different! So you might have to change rear U-Joint) Does the stock 77-79 rear have a FLANGE or u-joint?

Time to get a beefed up THM200 4R in there with 3.42's. They can handle a ton of power if built correctly, ask any 11 sec GN owner. But beyond that they go THM400. I personally would like to build a 77-79 7L car with OD and 3.42's, it would have to be a great daily driver.

If you are boneyard hunting, look for RPO G80, that is ths posi code.

Stone stock, you don't want to hookup with a 7.5", so keep the slicks off. But beef it like that article said, and you should be ok with occaisional dragstip action. Towing may not be the best, as the heat on the ring gear can be excessive. I think towing is why GM put the bigger axles in some of the time.

I have always heard that the pinion climbing the ring gear is the problem, that is why the Ford 9in is so strong, it has a bearing on each end of the pinion, so it is super strong there. But the relationship of the pinion gear to the ring isn't optimum, so it loses power there, not an efficient gearset. The 10-12-8.8's are all more efficient. So in a nutshell, if you take a car and drop in a 9in over a 12bolt, all gearing being the same, the car will slow, slightly, bit slow down, efficiency is lost.

Honestly the Ford 9in shouldn't be too hard a swap in a 77-96 car either. I am sure one of the 70's big Fords/Merc/Lincolns would work, and many Lincoln's had rear disc. Might want to check it out for those who want to get away from the 2.28's.

codewize
07-18-05, 12:21 AM
I've been considering a Detroit Locker with 3.55's in my Brougham. Then I saw some talk of heads and a cam. Once my wife gets another car I think I can easily make the Brougham a 14 second car.

90Brougham350
07-18-05, 12:41 AM
This is a little off-topic, but I've heard from 2 different people that the 8.5 10 bolt passenger rear-end is actually stronger than the 12-bolt truck rear-end. Can anyone confirm or deny this?

Brian

Night Wolf
07-18-05, 02:15 AM
With my 2.28's, at 75mph I am turning over 2,000RPM in 3rd... that is really nice...

according to a calculator, with 3.42's, in 3rd I would be turning over at 3,000RPM at 75mph....

3,000RPM would be killing the 425, that would really not be good....

the TH400 is staying, I don't want to get involved with swapping out the trans, no to mention I just want to keep it...

I simply want the thing to accelerate better then it does now, as the 2.28 just make it bog really bad, but still not kill itself on the highway while trying to keep any decent mileage....

What rear is in a 1976 DeVille, and would that work or do I need '77+

Also, do all wagons (Caprice?) have the Posi or only some? there are lots of them there and I could check that out.... but what are the gears?

N0DIH
07-18-05, 02:12 PM
I have always heard that if you can break a 10 bolt 8.5", you can break a 12 bolt 8.875".

So if that means much.

My 1978 Chevy books shows an 8.75" rear, but does not detail which cars got it. It might be the Vette rear end for all I know. But the 8.5" was for sure in the Chevy Wagons of the year, so at least getting rid of the 8.875" 2.28's with a Std GM 10 bolt might be a cost saver.

I have a set of non posi 2.56's with carrier if anyone wants, no rear end housing, just the gears (ring, pinion, side/spider) and carrier. It came from a 1985 or so Chevy truck that we found a 1971 Camaro rear end that had 3.42's for $30. No, you can't stick 8.5" carrier in a 7.5", I looked, to small inside.

I love old magazine articles, they talk about 3.42's and 3.55's being highway cruiser gears and economy gears! They felt that spinning 3000 rpm at 60 mph was ok!!! Boy we have changed!

codewize
07-24-05, 11:05 AM
With my 2.28's, at 75mph I am turning over 2,000RPM in 3rd... that is really nice...

according to a calculator, with 3.42's, in 3rd I would be turning over at 3,000RPM at 75mph....

Go with something in between. 3.08's or 2.72's <<Is that right? 2.72?

3,000RPM would be killing the 425, that would really not be good....

Although the 425 was not a high revving engine I don't think 3k is going to hurt it. I had almost 400K on my 425 when the radiator got an unnoticed leak and blew a head gasket. Ticked me off. The car ran like a bear. Strong dependable. Your really couldn't ask for more. When it happened I was driving on the hi-way at 80 and I turned the key off as soon as the light came on. Muscled it to the shoulder but it was to late. That was a sad day for me.

the TH400 is staying, I don't want to get involved with swapping out the trans, no to mention I just want to keep it...

Great trans. Strong and long lasting. These were very sought after for drag trans because of the strength.

I simply want the thing to accelerate better then it does now, as the 2.28 just make it bog really bad, but still not kill itself on the highway while trying to keep any decent mileage....

What rear is in a 1976 DeVille, and would that work or do I need '77+

Also, do all wagons (Caprice?) have the Posi or only some? there are lots of them there and I could check that out.... but what are the gears?

The Ape Man
07-24-05, 12:05 PM
What rear is in a 1976 DeVille, and would that work or do I need '77+

Also, do all wagons (Caprice?) have the Posi or only some? there are lots of them there and I could check that out.... but what are the gears?

The pre '77 Deville/Fleetwood rear axle is a totaly different animal. It would take fabrication to make it fit under a newer model. Too much work.
They did make those with an optional posi though. Good luck finding one anymore.

Not too many wagons have posi but they were always the first place to look when boneyard hunting. It seems that the chances of finding a posi under a wagon is better than a regular car. I always look for the little metal tag on one of the cover bolts.

N0DIH
07-24-05, 05:49 PM
An Impala SS axle would work, most are posi, as are most cop cars. 77-96, take your pick. As well as any 73-77 A/G body with posi, again, a bolt in axle for you.

2.73's or 3.08's are a good choice. I ran 3.08's in my 455 in my T/A and got 11 city, 16 highway mpg. OD would have helped a lot, but it ran fine. Lean mixtures (I never got it to run how I wanted...) tended to make it run a little warm on the highway, but 75 mph was only 3000 rpm with 28in tall tires. I think 3.42's are ok too, if the engine is sound, the shorter gears are actually easier on the engine. It is spinning more, but there is more torque multiplication, so it actually has less throttle input needed. But for ever turn of the engine requires some fuel and has a % friction, spinning higher rpm will still take more fuel. 2.28's are likely too low for 55 mph cruise to be best mpg, but work ok at higher rpm.

Remember the old days, 3.55's were considered "economy' gears, and 4.11's to 4.56's were the power gears.

Night Wolf
07-24-05, 08:12 PM
which cars were a A/G body of those years?

At the local yard there are a few Impala SS and Caprice 9C1's there, most are wrecked, but not in a way that would effect the rear end...

so the rear end from any of those '77-'96 cars would be a direct bolt in for my '79? Did any of them have rear disc brakes? in that case would I need to snag the master brake cylinder or any parts from it?

N0DIH
07-24-05, 11:38 PM
73-77 Monte Carlo, LeMans, Grand AM, Grand Prix, Cutlass, Regal, Century and the like. I know the Grand AM is a G-Body, but they didn't make too many. I supect the pre 73 A bodies will work, but I do know sure the axle width changed (narrower).

Impala SS are 3.08's, 9C1's from 94-96 are 3.23's. I don't know the earlier years.

The only "issue" is the yoke, I don't have one to compare, run some u-joint part numbers at napaonline or autozone.com and see what you find. But likely they are the same. The bolt up is supposed to be the same, again, I haven't done one, but should be based on what I have read over the years and one my friend did from a 77 LeMans to a 85 Delta 88.

As for disc brakes, Impala and 9C1's are (94-96) you would need combination valve and master cyl (or possibly mod your master cyl, it can be done on some by removing the pressure valve that maintains a certain pressure for the rear drums).

Read up on the disc conversion. I have the backing plates and calipers for the old style, but Bill Harper wasn't sure if these were worth much over converting to his style. Katshot indicated these old versions were problematic with the calipers (I think sticking calipers and difficult to reset them at brake swap time, you had to crank the caliper piston back in). You can run the modern version ok, just need the master cyl. I have found the aluminum master cyl's are not a direct bolt in for the iron ones. Good weight savings. Get a hydroboost or the power brake booster that came with the aluminum master cyl. Hydroboosts are the ultimate in brake pedal feel. Like ABS cars with the ABS pump running all the time, super solid pedal.

The 94-96 (93 and before?) rear ends were 4 channel ABS, so they have 2 separate brake lines, so install there can be difficult as you need to change over lines from the old rear.

How much does that yard get for a rear end on a Impala or 9C1? I would love one for the caliper parts, but unless it has posi, I don't need the rest.

Night Wolf
07-25-05, 12:01 AM
reason why I ask about the master cylinder is because the one in my '79 DeVille is new, so I odn't want to dump it...

My father has a '73 and a '74 Grand Am, as well as a '73 Grand Prix sitting at his property, they are all rotted out parts cars, but I know the rear ends are there... the Grand Am's were nothing special, although they do have a 400 or 455 in them, the Grand Prix also has a 455, and I know the previous owner raced that car alot... I should ask him if it is a Posi or not...

but the rear ends from these cars would be a direct swap with mine? is there anyway to know the gearing on them?

I am not sure how much the yard charges, thats why I was just going to ask them, they aren' that cheap on their prices tho....

N0DIH
07-25-05, 12:10 AM
Well, I do know, 73-77 A/G were interchangeable.
77-96 B/D were interchangeable
I do know 77 A swaps in with no mods to a 85 B (personal, although not first hand experience, when he gets his computer back, I will get him to post)

So, from that, and knowing GM carried the 73-77 A/G platform to the 77 B/D, I would say with a 95% certainty. The 77 A car was one I drove known by the Missouri Highway Patrol as the "Wild Indian" and with a built 455 and 3.23's, never lost a pursuit, ever. I drove it to where it got dismantled. It was in sorry shape, but still pulled nicely. In power and when I hit the brakes!

use your control arms to be sure on placement. Odd things like english to metric changed in from time to time, so look for that.

I would check on the 73 GP and 73-74 GA, cheap parts for sure! Look for tag on rear cover for posi. GM liked 2.73's to 3.23's then, likely closer to 2.73's. Emissions was measured in Grams per Mile then (in addition to tailpipe) so they liked using as high gearing as they could get away with. And Pontiac was torqey, so they easily got away with the high ratios. Chevies were torqueless wonders, (aka, 350 small block) and needed 3.08's to 3.42's std just to keep up.