are GM 7.5 rears really bad? - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Transmission - Rearend
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 05-04-2012, 10:32 PM
Augusto's Avatar
From the middle of the world
 
Last wiki edit: How to rebuild an engine Last photo:
Join Date: Mar 2006
Location: Ecuador, South America
Age: 51
Posts: 1,329
Wiki Edits: 6

Thanks: 5
Thanked 3 Times in 3 Posts
are GM 7.5 rears really bad?

if they are not good, why they make lots of aftermarket gear sets for them?

I have an '81 Malibu that came with 2.73 or so and want to install a 4.10 gear set.

Will the rear end with 4.10 gears be more strong than it would with a lower ratio? it makes sense to me since there will be more mechanical advantage and will have less resistance to turn, therefore will last more.

am I correct?

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 05-05-2012, 12:02 AM
WDCreech's Avatar
Grumpy Old Goat Herder
 
Last photo:
Join Date: Apr 2006
Location: Anchorage, Alaska
Age: 71
Posts: 1,334
Wiki Edits: 0

Thanks: 3
Thanked 35 Times in 33 Posts
Quote:
Originally Posted by Augusto
if they are not good, why they make lots of aftermarket gear sets for them?

Will the rear end with 4.10 gears be more strong than it would with a lower ratio? it makes sense to me since there will be more mechanical advantage and will have less resistance to turn, therefore will last more.

am I correct?
First; They maye them because ther a lot of Chevies out there and people buy them.

Second; the 4.10 is a lower ratio than a 2.73, just like low gear in a TH350, 2.53:1 is lower than high gear, 1:1. Will the 4.10s be stronger? I doubt it. It still has only a 7.5" ring gear. The 7.5 won't last if it's subject to any kind of thrashing. Even an 8.2 is considered inferior to a lot of other rears. I understand that the 8.5 is quite durable, but I can't confirm that because I run a Ford 9".
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 05-05-2012, 01:29 AM
cobalt327's Avatar
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 60
Posts: 5,037
Wiki Edits: 1616

Thanks: 128
Thanked 598 Times in 547 Posts
Quote:
Originally Posted by Augusto
...Will the rear end with 4.10 gears be more strong than it would with a lower ratio? it makes sense to me since there will be more mechanical advantage and will have less resistance to turn, therefore will last more.

am I correct?
In this case, I'm thinking not. Because the pinion gear is a smaller diameter, it seems to me that there's be less tooth surface area engaging the ring gear, so there will be a higher average loading on the pinion gear teeth.

But whether that's the case or not, it's really a matter of getting traction or not getting traction. If traction is sufficient, the 7.5" pinion/ring gear will fail sooner than a larger set of gears (all else being equal)- regardless of the ratio.

As far as there being good aftermarket support for the 7.5", it was installed into millions of vehicles, some quite sporty. Not everyone is going to want or need a stronger diff than the 7.5", so there's a good selection of gears available for that segment.

On the other hand, because there are so many vehicles that came w/the 7.5" rearend that have been modified, the aftermarket also has a good selection of replacement rearends: Ford 9", 12-bolt, and Dana types, along w/a lot of info on swapping in other rearends.

It's a damn shame GM used the 7.5" instead of the corporate 10-bolt 8.5" rearend in almost all of the G-cars and and 3rd. gen F-cars. If not for that, these cars would be even better for hotrodding.

Last edited by cobalt327; 05-05-2012 at 01:37 AM.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 05-05-2012, 11:47 AM
Registered User
 
Last photo:
Join Date: Jul 2004
Location: At Speed
Age: 51
Posts: 1,399
Wiki Edits: 0

Thanks: 2
Thanked 41 Times in 39 Posts
Lower gears will make it blow up faster.
The gears, diff and case are so small that when loaded, the gears are able to separate which results in the teeth popping off. More load from pinion gear to ring gear will make it worse.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 05-05-2012, 01:09 PM
BOBCRMAN@aol.com's Avatar
Member
 
Last photo:
Join Date: May 2002
Location: Holly, michigan
Posts: 8,244
Wiki Edits: 0

Thanks: 30
Thanked 298 Times in 278 Posts
In reality (at least around here). The gears aren't what fails. Tho, I suppose they would under huge doses of torque and tire..

I see more broken 7.5 axles than center section parts. Thats the weak point..
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 05-05-2012, 01:58 PM
Augusto's Avatar
From the middle of the world
 
Last wiki edit: How to rebuild an engine Last photo:
Join Date: Mar 2006
Location: Ecuador, South America
Age: 51
Posts: 1,329
Wiki Edits: 6

Thanks: 5
Thanked 3 Times in 3 Posts
well, I have a rear from a 1978 C10, it should be a 8.5, right?

I would have to add the tabs and stuff to adapt it to the G body's 4 link, not much of a big deal, if this is a much better option I think it's worth the hassle.

I also have another 8.5 from a 1970 camaro, but is missing the pinion yoke and I haven't found a replacement yet.

are both the camaro and c10 rears the same? wich one would you reccomend?

it cost the same the 4.10 gears for the 7.5 and the 8.5, so makes no sense spending in something that may break, the engine is a 144 B&M supercharged 355 SBC.
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 05-05-2012, 03:43 PM
Registered User
 
Last photo:
Join Date: Jul 2004
Location: At Speed
Age: 51
Posts: 1,399
Wiki Edits: 0

Thanks: 2
Thanked 41 Times in 39 Posts
78 C10 should be an 8.5.

70 Camaro is probably a 8.2 Chev. If it is an 8.5, it will use the same yoke as the C10 axle, with the correct u-joint installed in the driveshaft

The 8.5 is alot stronger than the 7.5.

I usually see a few broken 7.5 per week. Most of them already have aftermarket axles, so I cant say much about the durability of GM axles.
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 05-05-2012, 03:54 PM
Registered User
 
Last photo:
Join Date: May 2009
Location: S/E Michigan
Posts: 650
Wiki Edits: 0

Thanks: 0
Thanked 9 Times in 9 Posts
The C10 axle would need to be narrowed to fit the Malibu. That means new axles, cutting and welding the housing correctly.
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 05-05-2012, 07:03 PM
Registered User
 
Last photo:
Join Date: Mar 2010
Location: Western NY state
Posts: 60
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Some of the last several years of the S10 used a 8.5 axle in a 4x4 version that will be about the right width for a G body.The other option is finding a Grand National Buick axle. Or go for a somewhat expensive. converted 9 inch Ford.
A properly set up 7.5 will take more abuse that you think....but it will break eventually from that abuse.
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 05-05-2012, 07:29 PM
cobalt327's Avatar
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 60
Posts: 5,037
Wiki Edits: 1616

Thanks: 128
Thanked 598 Times in 547 Posts
Here is a thread I bookmarked earlier (my dd is a '80 Malibu wagon).

Links given to me by SSedan64:
http://www.maliburacing.com/tech_rearend_swaps.html
http://www.maliburacing.com/rearend_guide.html

Links for putting a 8.8 FoMoCo rear end into a Malibu: http://www.maliburacing.com/forum/vi...hp?f=4&t=72070

Avoid the GovLoc diff.
Reply With Quote Quick reply to this message
  #11 (permalink)  
Old 05-05-2012, 09:35 PM
Augusto's Avatar
From the middle of the world
 
Last wiki edit: How to rebuild an engine Last photo:
Join Date: Mar 2006
Location: Ecuador, South America
Age: 51
Posts: 1,329
Wiki Edits: 6

Thanks: 5
Thanked 3 Times in 3 Posts
definitely both my C10 and Camaro rears are 8.5 units, according to the id pictures they are not 8.2's

the C10 has 5.5 bolt pattern and the Camaro's 4.75

the C10's is newer and looks in better shape, can I use the Camaro's axles in the C10's housing, so I won't have to drill new holes?

also a very important question about the width, I don't mind if the rear end is wider, I can use different wheels to compensate somewhat, but the real problem may be: IS IT VERY IMPORTANT FOR DRAG RACING TO HAVE A NARROW REAR END? or is it only a matter of fitting very wide slicks under the fenders?

I can't use a very narrow rear anyway, the frame rails will not let me, unless I start doing some serious work, but if it is a must then I will do it.

thanks for the help, the links have enough reading material to keep me bussy for hours.
Reply With Quote Quick reply to this message
  #12 (permalink)  
Old 05-05-2012, 10:38 PM
cobalt327's Avatar
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 60
Posts: 5,037
Wiki Edits: 1616

Thanks: 128
Thanked 598 Times in 547 Posts
A couple thoughts...

IIRC you are building a SBC turbo drag car. Depending on how hard this engine combo hits the tires and how good the traction is, there could be a need for more strength than a stock 8.5" diff can handle- but the basic 10-bolt 8.5" rear end (with mods as needed) has been used behind some damned gnarly engines (single-digit et turbo Buicks, for instance).

The rear end width isn't that important at least not until you start getting up there in power and traction. In cases like very high hp drag cars, often the rear ends are narrowed to fit enough tire under the vehicle w/o needing a lot of positive wheel offset. Having excessive offset can cause the wheel to flex, giving traction problems as well as the chance of breakage. A side benefit of narrowing the rearend is it will add some strength because the axles are shorter, but I wouldn't say narrowing a rearend that can fit enough tire for the job will be an especially useful modification- unless the wheel offset becomes too severe.

Guys will weld the housing to the tubes, swap out the housing ends for Ford-type bearings to get away from the C-clips, etc. All things that may or may not be needed, depending on things like the weight, manual or AT, size of the tires/traction, slicks or treaded, and of course the engine output.

If this is a track-only deal, I'd suggest you look into a full spool for it, along w/stronger axles (w/more splines and larger wheel studs) if it turns out the stock 8.5" diff/axles/rearend housing isn't enough.
Reply With Quote Quick reply to this message
  #13 (permalink)  
Old 05-05-2012, 11:50 PM
Augusto's Avatar
From the middle of the world
 
Last wiki edit: How to rebuild an engine Last photo:
Join Date: Mar 2006
Location: Ecuador, South America
Age: 51
Posts: 1,329
Wiki Edits: 6

Thanks: 5
Thanked 3 Times in 3 Posts
yes, the engine is a supercharged 355 SBC w/manual transmission

I'm removing absolutelly all the stuff that is not necesary, this includes glass, doors (only the skin remais) interior, etc etc, I'm making it very very light.

this will help the rear end survive some abuse, less inertia means more longevity.

also I will be running street tires, P305/50R15 rears, no slicks.

I want to run a spool also

I was concerned about the rear end width because I tought wider tracks made the cars wander down the strip, not having to narrow it (for now) will let me concentrate on other important aspects of the build.
Reply With Quote Quick reply to this message
  #14 (permalink)  
Old 05-06-2012, 10:24 AM
cobalt327's Avatar
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 60
Posts: 5,037
Wiki Edits: 1616

Thanks: 128
Thanked 598 Times in 547 Posts
Quote:
Originally Posted by Augusto
...supercharged 355 SBC w/manual transmission... *SNIP* ...I was concerned about the rear end width because I tought wider tracks made the cars wander down the strip, not having to narrow it (for now) will let me concentrate on other important aspects of the build.
Ahh, that's right- roots blower, not a turbo.

The narrowed rear track WAS found to help early front engined rails track straighter. Those rails had a low c/g and roll center that didn't present an immediate danger for having a roll over problem, so the need for a wide rear track wasn't there.

IMHO the need for a narrow rear track isn't really a concern in your case being as how you are using a rear suspension and are not so severely over powered and under "traction-ed" as those rails were.

This is an article by Billy Shope of Ramchargers fame. He knows a LOT about the subject.
Reply With Quote Quick reply to this message
  #15 (permalink)  
Old 05-06-2012, 10:52 PM
Augusto's Avatar
From the middle of the world
 
Last wiki edit: How to rebuild an engine Last photo:
Join Date: Mar 2006
Location: Ecuador, South America
Age: 51
Posts: 1,329
Wiki Edits: 6

Thanks: 5
Thanked 3 Times in 3 Posts
thanks a lot for the link, there's a ton of useful info in there, specially the calculators are incredible, you can clearly see the effect of making changes.

also I found out that my C10 is actually a 12 bolt, camaro's still 10 bolt.

I guess my best choice is the C10's, is a lot wider though, I will have to make it shorter somehow, also will have to build some sort of brackets or something to bolt the upper links, something will have to be welded to the tubes and run up over the pumpkin.

here are some pics of my rears:
Attached Thumbnails
Click image for larger version

Name:	DSC00432.JPG
Views:	57
Size:	295.6 KB
ID:	65243   Click image for larger version

Name:	DSC00436.JPG
Views:	51
Size:	258.8 KB
ID:	65244  
Reply With Quote Quick reply to this message

Recent Transmission - Rearend posts with photos

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Mopar Rears rj57 Transmission - Rearend 2 04-28-2008 06:22 PM
are they posi rears Sheir-Ron Transmission - Rearend 21 07-19-2003 08:50 PM
83 monte rears furfman Transmission - Rearend 3 06-22-2002 04:44 AM
REARS ARI Transmission - Rearend 1 01-04-2002 09:52 PM
rears ramjet1320@msn.com Transmission - Rearend 0 01-04-2002 07:49 AM


All times are GMT -6. The time now is 10:42 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.