Club Cobra - Anyone done a Jag rear gear ratio swap?

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Anyone done a Jag rear gear ratio swap?

I'm thinking about switching out my 3.5:1 diff gears (Jag IRS) to fit a lower ratio around 3.9 or 4:1.
Has anyone had this done? We're there any pitfalls and what did it cost?
Any recommendations for shops (ideally in Brisbane)?

Thanks

Craig

I just put a set of 3.73 in my Jag rear that had 3.23. I switched from a Jag/Spicer carrier to a Dana 44 carrier as it was easier to find gears and the carrier. To go up to the ratio you want, you would have to change the carrier and go with Dana 44 gears. The difference is the size of the bolts that hold the ring gear in place. Dana uses 3/8" and Jag uses 7/16". You can drill them out one way, but you would have to use spacers to go the other way. Not fun. Not sure if you can find Jag gears or not. Somebody told me they are making them again although they are not cheap.

Jag makes the carrier break between 3.54 and 3.76 ratios. If you want to put a gear numerically higher than 3.54, you need a carrier. Dana makes the break between 3.73 and 3.91. You would need the 19 spline style for the stub axles. Dana makes both the 19 and the 30.

I replaced the carrier, bearings, ring and pinion, and you have to replace the front coupler flange as the Dana 44 gears use a different spline pitch. Also, the front pinion bearings are a bit odd when you use Dana 44 gears. Nothing that cannot be done.

I have a complete Jag center section with a 4.56 gear if you want to rip your head off...LOL It worked great with the 6 speed in the car.

are you drag racing? why go higher than 3.73?

If I were you, I would leave the 3.50 gears alone. I went from 3.31 to 3.73 and it just spins the tires all the time. With the Tremec 5 speed I cruise at 80 wit 3,100 rpm in 5th with .68 overdrive. I drive it a lot and get 15 mpg on a long cruise or rally. With the 3.31 gears, I could get more than 20 mpg. I wish I had gone with 3.5 rear gears because every time I fill the tank it's $75.

Craig, Go here and read every page.
DazeCars, Mustang IRS or Independent Rear Suspension page II

Page 4 has some interesting info re Dana 44 parts and Jag Diffs. ( some well kept secrets)

The site will give you a good idea in what's involved in stripping and reconditioning the Jag diff.

Also good info on shortening the drive shafts and LCA.

And after you've read all that you can go here and see what other people are doing with the Jag diffs.
http://irsforum.boardhost.com/viewtopic.php?id=242

EDIT: As to a possible repair shop to change the ratios, I know years ago diff lapping at Kedron was the place to go but I'm told the old guy who did the Jag stuff is no longer there. Also they seem to have a set starting price for Jag diff repairs. $1000.00 is the minimum I'm told.

craig
I agree with REBEL 1 I had my diff in Kedron diff lapping & would highly recommend them
thanks for the chat Saturday ,,wife loves the video
rob

Guys thanks for the information and links. From what I've now seen and with an indication of price, it all looks like a bit of a worry.

To explain, I've been playing with tyre sizes and diff ratios and overdrive gear options and a lower diff ratio change seemed viable. With the rebuild cost, ring and pinion and also the carrier, you've convinced me to look at simpler and less expensive options.

Great result!

Rob - likewise, it was good to chat the other day. I had no idea that we stood there beside my Cobra yapping for nearly 5 HOURS!!! I didn't even offer you a beer. How rude of me!

750hp
Denis welsh motorsport in the U.K make new gears.
I purchased a set a little while ago and they are quality.
The gears were mated with an auburn gear LSD clutch pack (see the IRS forum as previously stated) and all seems to work well.
Spiteri's in Melbourne bolted it all together although they did have to play with it a bit to get it right but that was because it was the first time they had done that conversion.
I am running around 600HP through it and no worries,(back off murphy).
Problem these days is all the jag gears you get second hand are usually well past their prime and were not designed for the amounts of power we are putting through them,
the gears from Welch are.
My thought at the time was do it once by doing it right.