Bite the bullet or modify?

[UC645]

So I’ve had a spare 8.8 housing since June. Plan was to modify it with 9 inch ends, narrow it to G-body width, brace it, and upgrade to 31 spline axles.
BS’ing with my foreman, he brought up just stabbing a 9 inch in it, and after some math, I arrived at these numbers for cost:

Modified 8.8: around $2,500 depending on parts
9 Inch: $2,600 to $3,000 depending on center section

I’ve got a local shop that can narrow the 8.8, so that doesn’t need shipped. But with prices so close, is it just worth it to drop the 9 in it? The car’s intended use is more of a universal setup until I figure out what I like more, so that’ll be mainly many street miles, auto x, possibly some road course, and drag strip time. I’m not running a huge amount of power currently, but that’ll change in the future.

 

[86LK]

I'd go with the 9. mod work is already done and I would guess more options and cheaper options down the road

 

[Rt Jam]

I'm not one to chose one over the other from 1 or 2 factors. Big ones like strength and cost are hard to argue with but in this case both are real close.

I just brought this up in another thread and will reply 2 minor points that are often not considered.

A Ford 9" gets alot of it's strength from it's pinion having bearings in front and behind it. Problem with behind is it's in the same space a big differential wants to be. Not an issue if running a spool but you need a differential for autocross and street. So this leaves you with a tiny differential. Pic on the right.

Another reason a 9" is strong is they place the pinion gear low. This essentially is a worm drive. More tooth contact but it does make it not as efficient. Efficiency does not come from ring gear size. A fast car needs to chose a stronger rear over a broken rear but efficiency should take precedence on mild builds. 8.8's are efficient.

Basically any choice should be well reviewed, no one particular rear is better in every category.

 

[81cutlass]

If you aren't going to bolt drag radials on and hit the track or launch on a trans brake, or do burnout competitions, there is a very low chance you will blow your 7.5" up.

8.8 is a great DIY rear, if you are paying people to weld or cut or assemble, it's better in the long run to drop in a 9" or better yet D60.

A Strange 60 G body is actually cheaper than a 9" ford.

 

[UC645]

I'm not one to chose one over the other from 1 or 2 factors. Big ones like strength and cost are hard to argue with but in this case both are real close.

I just brought this up in another thread and will reply 2 minor points that are often not considered.

A Ford 9" gets alot of it's strength from it's pinion having bearings in front and behind it. Problem with behind is it's in the same space a big differential wants to be. Not an issue if running a spool but you need a differential for autocross and street. So this leaves you with a tiny differential. Pic on the right.

Another reason a 9" is strong is they place the pinion gear low. This essentially is a worm drive. More tooth contact but it does make it not as efficient. Efficiency does not come from ring gear size. A fast car needs to chose a stronger rear over a broken rear but efficiency should take precedence on mild builds. 8.8's are efficient.

Basically any choice should be well reviewed, no one particular rear is better in every category.

Yeah, that post spurred this whole thought process along. With the power I’m currently making, and can make with the 350, I shouldn’t need to worry about breaking a carrier unless wheel hop becomes a factor. However, with Eaton not specifying what weight that center is good for at that claimed 650 hp limit, that’s a crapshoot.
I’ve seen a forum post claiming about a 18hp loss going from a 8.8 to a 9, with less of a loss at lower speeds. No factors or points of measurements to put that on a scale though.

Still, I doubt I’d break it with the potential my setup currently has.

If you aren't going to bolt drag radials on and hit the track or launch on a trans brake, or do burnout competitions, there is a very low chance you will blow your 7.5" up.

8.8 is a great DIY rear, if you are paying people to weld or cut or assemble, it's better in the long run to drop in a 9" or better yet D60.

A Strange 60 G body is actually cheaper than a 9" ford.

This car will see track time, I’m just unsure of how much that’ll be. Figured it’d be better to just get this out of the way first.

The car’s already got an 8.8 in it with SN95 disc brakes that’ll get sealed over to either rear axle choice. I’ve got arms, upper IC brackets, and adjustable lower arms for an 8.8 from Trick Chassis. Need to call them to see if the 8.8 uppers can be substituted for the arms included in their raised ear 9 inch kit.

With the options packages from Quick Performance, Strange’s 60 is right in the middle of these options in terms of price, that’s unpainted, but clutch posi and 35 spline axles that I’ll never break is tempting….
But when it comes to gear ratios, the 9 inch’s adaptability is a big factor in this decision, power loss or not.

Hmmmm…..

 

[Sweet_Johnny]

The members who have commented before me know their stuff. For those new to swapping diffs or just hungry for knowledge, here are a few relevant links for your reading pleasure.

Malibu Racing G-Body Rear End Swap Guide: https://maliburacing.com/g-body-rearend-swap-guide/

GM 10 Bolt Guide @ Chevy Hardcore: https://www.chevyhardcore.com/tech-stories/drivetrain/10-bolt-chevy/

How To Swap Different Rears Into G Bodies, from this very forum: https://gbodyforum.com/threads/how-to-swap-different-types-of-rears-into-g-bodies.63755/

Bullet Proof 10 Bolt? On ThirdGen.org: https://www.thirdgen.org/forums/transmissions-drivetrain/397601-bullet-proof-10-bolt.html

Beef Up a 7.5 Rear @ Classic Oldsmobile: https://classicoldsmobile.com/forums/drivetrain-differentials-6/beef-up-7-5-rear-125382/

ZR2 S10 8.6 Rear in a G Body Build, from this forum: https://gbodyforum.com/threads/zr2-s10-8-6-rear-in-a-g-body-build.63106/

Differential Knowledge, Opinions, and Ideas by Dominic Orlando, hosted on Angelfire: https://www.angelfire.com/my/fastcar/diffey.html

What Makes The Ford 8.8 So Sought After, Reddit:

https://www.reddit.com/r/projectcar/comments/eib6n9

1981 Cutlass Rear End Swap, Reddit:

https://www.reddit.com/r/GBody/comments/etkthy

There was a great How-To on ws6transam.org on building a truly great 10 Bolt, but the domain has died.

I also recall someone basically cutting off the G-Body tubes to retain the lower control arm mounts, brakes, and axle length and mating them with truck center sections. One set of tubes slid inside the other with only minor modification. The resulting hybrid utilized a thick plate sandwiched under the rear cover that extended upward and forward to provide mounts for the upper arms. That's what I hope to go with when I finally get to that point.

 

[Built6spdMCSS]

Same discussion we had on that other post and me as well as another guy with cars that make power seem to have no issues running a 9" with apparently our tiny little Differentials.

First time I've ever heard that criticized in my life. I would like to see a mechanical engineering degree, alloy list and stress analysis tests of these differentials for that criticizm to be validated.

Plot twist...tiny little differential means less rotating mass..

 

[Built6spdMCSS]

The car’s already got an 8.8 in it with SN95 disc brakes that’ll get sealed over to either rear axle choice. I’ve got arms, upper IC brackets, and adjustable lower arms for an 8.8 from Trick Chassis. Need to call them to see if the 8.8 uppers can be substituted for the arms included in their raised ear 9 inch kit.

So if you have that in the car already, why something different?

 

[565bbchevy]

I’ve got a local shop that can narrow the 8.8, so that doesn’t need shipped.

I cost me $2,400 to build my QP 9" for my Regal but I bought the brakes, housing and third member separate which saved me a lot on shipping since the brakes and third member were shipped free and the housing and axles came from UPS instead of a freight company a complete rear would need, plus I wanted to weld the rear brace on and paint it while it was just the housing.
I don't really need the strength of a 9" for my current set up but I couldn't see dumping any money into my 7.5" to get a posi and the gear I wanted, plus I am set if I decide to throw something with more power in it in the future.

 

[UC645]

So if you have that in the car already, why something different?

Honestly? Wheels and the wrong measurements.
Current wheels are past where I wanted them because I had the wrong axle width measurement on hand and didn’t catch it. Can’t sell ‘em for what I paid, so buying another set just to go back to the specs I have now seems pointless.

I wanted to modify an 8.8 from the beginning but wanted to be cheap first, and making this rear end work properly will run me about $1,200 in parts, with no modifications I want.

Just seems pointless to throw that money at the wall for something that will get replaced further down the line while having a compromise I’m not comfortable with.