Jag IRS or Ford 8" or 9" rearend set up

[Neil Verity]

I'm new to the British V8 forum but love the concept.  I'm working on a MGB V8 (modified Rover 4.0 with Buick 300 crank and heads) that will be a dedicated Auto-X and track car.  I have raced an MGB in GP and HP in SCCA for almost 8 years so I know the mgb part, but not the V8 part.  Question:

I originally planed on using a Jag rear end with IRS for the V8 MGB project (sounded good at the time).  Having now acquired one, I now realize that it weighs a ton.  Thus, I'm thinking I need to substantially lighten it or change over to either a 8" or a 9" ford rear end (which I presume is lighter).   FYI, In SCCA EM auto-X trim the minimal weight limit is 1700 lbs so that is my target weight.  The Goal is to have > 250 hp to begin with but over time get it into the > 350 hp range once I/car progress from Auto-X to Track days.  I would like to build the rear end/axle right the first time to handle the expected higher stress.

What is the weight difference between a Jag XJS IRS (all complete, DANA rear end....in its "cage" as it came off the car),  8" and  9" ford rear end with cut down axles/tubes?  Are gear sets available (3:90. 4:10, 4:30 etc..) for the 8"?  Is an 8" strong enough to withstand 350 hp and hopefully a robust torque curve?  

Any thoughts on which set-up to use will be appreciated.

Thanks

Neil

[BlownMGB-V8]

First off, yes the Jag suspension is heavy. But not extremely so and most of it's weight is sprung weight so very helpful in terms of ride and handling. It can be made considerably lighter. But first off, using the entire assembly is perhaps not the best way to go about it. The cage adds unnecessary bulk and weight, both bad. The lower control arms are the next thing to look at as they are extremely heavy for the job they do, and can be fabricated and made significantly lighter. Same for the half-shafts. In fact, it would be possible to use aluminum tubular half-shafts and drop quite a bit of weight, half of it unsprung. The same could be done for the lower control arms. Single coil-over units instead of the dual shock assemblies give another weight advantage. It may be possible to use Wilwood brake calipers or another lighter weight unitized caliper and parking brake to reduce weight even more.

So rather than starting with the total weight of the stock unit, it might be better to begin with the component weights. I can probably get you some of those in the next week or two, as we assemble the suspensions for both the MGB-Roadmaster and my 340 MGB roadster. If you haven't looked at those threads there is a lot of good info on IRS. Do a search to narrow it down to manageable proportions.

Jim

[Neil Verity]

Thanks Jim,

Good advice.  I have skimmed the other threads but will go back, re-read and pay more attention.  If you get component weights please post.  I will do the same, but I'm a few weeks away from getting to the rear end (still working on Trans/engine location).  

Thanks

Neil

[MGBV8]

More 8 inch Ford info:

http://www.customclassictrucks.com/hotnews/0505cct_ford_8_inch_rearend_engine_tech/index.html

http://www.mustangandfords.com/techarticles/127_0310_ford_differential_builders_guide/index.html

http://www.hemmings.com/mus/stories/2006/06/01/hmn_feature19.html

http://www.fordmuscle.com/blog/8-inch-or-9-inch/112192

http://www.mustangmonthly.com/projectbuild/mump_0009_ford_8_inch_rearend_build/index.html

http://www.mustangmonthly.com/howto/41618/index.html

[Neil Verity]

Hi Carl,

Thanks for the links.  Lots of good info.  Sounds like a modified 8" may be the way to go (lots of options, gear sets etc...).  In one of the articles they mention an approximaetly 40 lb weight savings over the 9".  I think I can eliminate the 9' and decide between an 8' and a lightned/modified JAG IRS.  I did read somewhere the that 8" could not be cut down (axle or tapered housing) but perhaps aftermarket folks have resolved this issue (if it is an issue?).

Thanks again

Neil

[Moderator]

Did you see the axle photos and captions in this article?
Les Gonda's MGB GT V8 race car (71 photos)

If you don't have a local shop that you trust to narrow an 8", one option would be to send it to Currie.

I would recommend using new axle shafts... Moser Engineering offers various options for that.

[Edd Weninger]

Curtis,

You don't have to send a rear end to Currie, they have a huge stock pile (literally about 20' high) in back of their shop and probably have anything anyone would need.

Save the shipping cost.

[MGBV8]

I did read somewhere the that 8" could not be cut down (axle or tapered housing)

Actually, narrowing the 8 inch is no different than the 9 inch. It is the 28 spline axles that is the problem. The early 28 spline axles are tapered & can only be shortened a little. The later 28 spline axles (somewhere around 67-69 & up) are straight.  

Like Curtis, I would just order new axles in the correct length.

[t.lay]

Pre 67 ford 8" housings have tapered axle tubes. They also have less webbing on the third member so it's structurally a little weaker than later chunks - basically get a 67 or newer 8" and it's not an issue. Standard/replacement build parts for an 8" are good to about 300 ft/lbs of torque. Currie has an aluminum third member that can take quite a bit more and cuts some additional weight I believe. Hot rodders have built 8" to take big torque.

[NixVegaGT]

Just to stir the pot a little how about a Toyota rear axle. It's got the same dropout center section like the Ford 8" / 9" and is a bit lighter. Just tossing out options. Also the Mopar 8 3/4" is pretty tough.