Engine and Transmission Tech

tips, technology, tools and techniques related to vehicle driveline components

Go to Thread: PreviousNext
Go to: Forum ListMessage ListNew TopicLog In


Brownwood
Keith Montague

(176 posts)

Registered:
01/25/2014 08:08PM

Main British Car:


MGB V8 Rear Diff
Posted by: Brownwood
Date: June 05, 2015 11:06AM

OK, guys, I have researched and asked some questions but I am still not sure what to do for a rear differential. I recently lost my mild 5.0 motor, so now I have ordered a crate motor. I am looking for specific recommendations now so here's the motor specifics.

My crate motor will be an aluminum head 306 that is guaranteed to dyno at 370HP. I assume that will mean 300 to 325 at the rear wheels but perhaps that is optimistic.

I am currently running a stock 1980 MGB rear axle and love the 390 gears so I will be looking at something in the 3.73 range. I also want something that doesn't require major mods to the car, a bolt in if you will. I have a budget of $1500 for the rear axle, but I am unsure what to get. A lot of folks recommend the S10 axle but the 7.5 axle would seem to be weaker than a well set up stock MGB axle.


JWD
Jim Durham
Gig Harbor, Wa.
(103 posts)

Registered:
01/22/2013 11:43AM

Main British Car:
1980 MGB Ford 302 (398.9 HP, 383.2 TQ)

Re: MGB V8 Rear Diff
Posted by: JWD
Date: June 05, 2015 02:16PM

"A lot of folks recommend the S10 axle but the 7.5 axle would seem to be weaker than a well set up stock MGB axle."

Regardless of what some people believe, the ring gear diameter is not the end all indication of rear end strength. Far from it. Talk to someone that builds rear ends for a living like Randy's R&P, Jaws Gears etc. They see/build more rear ends in a week than the "internet experts" do in a lifetime. One thing to consider is that regardless of RWHP, traction is going to be an issue and without traction, parts don't break.


Brownwood
Keith Montague

(176 posts)

Registered:
01/25/2014 08:08PM

Main British Car:


Re: MGB V8 Rear Diff
Posted by: Brownwood
Date: June 05, 2015 02:49PM

Thanks for the advice Jim. I definitely will have some issues hooking up but plan on 265 or evn 275 wide tires at some point. I also have a traction bar set up. It hooked up reasonably well with the 275HP motor and 245 wide tires on a single trak stock rear diff (if you consider 100 feet of wheelspin hooking up :) ).


DiDueColpi
Fred Key
West coast - Canada
(1365 posts)

Registered:
05/14/2010 03:06AM

Main British Car:
I really thought that I'd be an action figure by now!

authors avatar
Re: MGB V8 Rear Diff
Posted by: DiDueColpi
Date: June 05, 2015 10:31PM

Thank you Jim!

All you have to do is look at Perry's axle.
Crazy power through an axle that is "supposed to blow up"
Granted it's heavily modified, but the weak link clearly wasn't the ring and pinion.
A LSD and possibly some stronger axle shafts might be a better investment.
If you change to a different axle you'll have to spend the money on that stuff anyway.
Makes sense to me, but I'm Canadian, so...You know.

Live like you mean it.
Fred


JWD
Jim Durham
Gig Harbor, Wa.
(103 posts)

Registered:
01/22/2013 11:43AM

Main British Car:
1980 MGB Ford 302 (398.9 HP, 383.2 TQ)

Re: MGB V8 Rear Diff
Posted by: JWD
Date: June 06, 2015 01:40PM

"Makes sense to me, but I'm Canadian, so...You know."

I was born in Burnaby, B.C. and lived in Surrey until I moved to the U.S. with my dad. Most of my relatives still live there. Beautiful country, especially the further north you go. I spent a summer in Blue River. The mosquito's were the size of small birds. No repellent worked but a shotgun helped a bit.


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(6468 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: MGB V8 Rear Diff
Posted by: BlownMGB-V8
Date: June 07, 2015 10:22AM

I have plenty of respect for axle experts when they know what they are talking about. There are a lot of so called experts who do not, however. Over the last 40 years I've run into more than a few of them. And regardless of anything else, basic metalurgy has not changed, and the bottom line is, more metal is going to be stronger, more rigid, and resist wear longer.

I'm not saying you can't put 700hp through a 7-1/2" ring gear by any means. It's like race engines. How often do you want to tear them down? Every race? Every season? Never? It makes a big difference in how you build it. Same thing with axles. If you replace the axle every season or two you can afford to use lighter weight parts. But do you really want to do that with your street car? Not me. I want to put it in once and forget it, beat on it like there was no tomorrow and not worry it is ever going to fail in any way.

I have personally flat worn out an 7-7/8" R&P set (OEM axle) in my MGB. And broken spider gears. The R&P gears do not catastrophically fail, but the extra horsepower CAN put enough pressure on the gear teeth to make them wear a great deal faster than what is considered normal for a street car and start growling. On the track this would not be a concern.

But don't let me influence you. I'm sure you will be most happy with your light weight axle and your big engine. Who knows, if you don't drive it very much it may never be a problem... for you. But there is a good reason the OEM's only put the 8" under Pintos and such.

Jim


MGBV8
Carl Floyd
Kingsport, TN
(4511 posts)

Registered:
10/23/2007 11:32PM

Main British Car:
1979 MGB Buick 215

authors avatar
Re: MGB V8 Rear Diff
Posted by: MGBV8
Date: June 07, 2015 12:34PM

Except when they were backing up a 289 in early Mustangs? ;)



BOOTLEG
Gary Rosema
Wisconsin
(47 posts)

Registered:
11/18/2013 07:26PM

Main British Car:
1978 MGB Rover 3.5L

Re: MGB V8 Rear Diff
Posted by: BOOTLEG
Date: December 16, 2015 07:42AM

I'd like some commentary on recommended diff lube for my new 3.31 gear set. Dick Luenning recommends 140W but that may come from all his race time on the track. I bought a quart yesterday but thought I'd get some opinions before I pour it in. Also besides weight, anybody using synthetic?


MGBV8
Carl Floyd
Kingsport, TN
(4511 posts)

Registered:
10/23/2007 11:32PM

Main British Car:
1979 MGB Buick 215

authors avatar
Re: MGB V8 Rear Diff
Posted by: MGBV8
Date: December 17, 2015 05:19PM

Some run 75w90, some use 75w140. Some run this in either flavor.

[www.royalpurpleconsumer.com]


Sorry, only registered users may post in this forum.