MG Sports Cars

I am an amateur mechanic at best, but I have a question I am hoping will not make me look too stupid. I have a Jag IRS from a late eighties XJS with a 288 gear ratio. I guess I really have two questions, the first is how have you secured the IRS frame to the MGB body to ensure it will not move in ways we don't want it to move? I have thought about building some trailing arm type rods to secure it by tying it to the old leaf spring mounts, has anyone done this?

My second question is, is the 288 gear ratio going to cause me problems since ideally it should be 307?

Oh and one more question, should I be concerned that the wheels can drop enough (when the car is on jack stands) to bind the u-joints? Should I build something to limit the distance the wheels can drop?

Any help will be greatly appreciated.

Thanks

Henry, Need to know what kind of car and trans you are using. If MGB you would be fine with the GM V6 T5 that has the 4.02 first if your motor has adequate power. Limiting straps or bump stops are a good idea. Check out this board on the Roadmaster and 340 build thread.




Edited 2 time(s). Last edit at 01/14/2014 05:28PM by mgb260.

I am using the Buick 215 and TR8 transmission combo. Is the rear end gear ratio too far off to work well?

Here's a link to a very handy gear ratio calculator (in Microsoft Excel) by Dan Masters:
[www.britishv8.org]
Put various parameters (e.g. tire diameter, gear ratios, etc.) in and it will help you relate engine speed to road speed.

2.88:1 might be okay, if you like loping along and if you put a big value on fuel efficiency. Partly it's a question of preference because gear ratios are always a compromise. Have you already chosen a camshaft? Cam specs have bearing on what speed your engine wants to run.

I have a 2.85:1 first gear and a 0.73:1 fifth gear, and I settled on a 3.27 rear axle ratio (in a Ford axle). If anything, that's a little conservative. ~3.40 might suit me better, overall. I like the 3.27 for long road trips, but its a big handicap in an autocross, etc.

There must be other Jaguar ratios besides the two you mentioned.

The 2.88 is a pretty tall gear, I'll not kid you. It might work but it depends on a lot of other factors like your overdrive ratio, tire diameter, engine torque curve and driver habits. Even with all of those going your way it's not going to perform as well as something a little shorter unless you change something else like going to a direct drive transmission. However, there are a few drivers I know who would be perfectly happy with that ratio. They would also be running a stock camshaft on a high compression engine to get the wide and low torque band needed.

The good news is that you can have any ratio you desire in that center section along with any differential (posi) type. The bad news? It's a lot of work (not necessarily cheap) to set it up correctly. But wait, you said late 80's yes? Inboard or outboard brakes? That makes a huge difference.

Jim

I may be wrong but I thought the Jag diff is a DANA 44?? So gears should be easily obtainable. However, I would say worry about the gears last since the rest of the work surrounding this would be the harder issues to tackle.

There are a couple of different ones used. Up to about '86 for sure or possibly '88, Jag used the Salisbury hogshead based on the Dana44. After that all bets are off. The outboard brake design, used with the XJ40 and later XJ6s I believe as well, used a different hogshead and information on it is not widely distributed as the earlier one is.

The Salisbury unit was based on the Dana44 design and licensed from Danacorp but used different bearings and ring gear bolts. The parts can be interchanged by using the right bearings and sometimes spacers. Bottom line, Dana44 gears and differentials can be used in the Jag housing. Acquiring a spare housing is an effective means of preparing for a gear change. There are three ring gear carriers (and possibly 4 if you get into the higher numerical gears). Jag used the PowerLok differential which is a pretty good unit but does wear over time, reducing the initial drag on the clutch plates. However the clutch plates are all steel. With the proper lubricant they wear slowly, but they do wear. For this build using standard tire sizes, a gear ratio of 3.23, 3.31 or 3.54 would be the right range, with possibly a 3.73 for performance work and larger tires. For most the ideal would be split about evenly between the 3.31 and 3.54 with a fairly typical overdrive ratio around .7-.8

The Dana u-bolt style pinion yoke is one more reason to use Dana gears as it costs around 20 bucks where the Salisbury equivalent is 80. (You get rid of the noise from movement of the bolted flange yoke. The movement isn't much, but it doesn't take much.) Dana gears run about $150 (plus inflation) and a posi housing (for ratio change) is about the same. But the internal parts vary somewhat so it's better to get the complete posi, which for a Dana PowerLok is still under $500 I believe. The changeover bearings are about the same as the Jag bearings, both a bit more than for Dana. You can spend $600 on this rear axle just in bearings and seals so it's always best to start with one in good repair.And in most cases it is cheaper to shop around for the Jag RE you want rather than change the internals later, even if you are only buying the hogshead. You can do the install, then eventually find a 3.31 or 3.54 posi hogshead. They aren't common and usually not cheap, although I did find one from a dealer with low miles that had a broken cross shaft in the diff, taken out as warranty service. Took a chance on it for about 5 bills I think and made out on the deal. A little re-stacking of the clutches and it was good to go, although I still went with the Dana gears and yoke.

In the end, the difference in the ride with this RE is nothing short of amazing and I don't think we've seen any indications of wheel hop or other bad behavior (MG-Roadmaster). Road holding ability is considerably better than with the original as well, especially on uneven pavement. We did have one broken coil-over because the subframe bushings compressed and let the shock rub the bodywork. Other than that it has been flawless.

Jim

Mine has inboard brakes. I actually have some Dana 44 gears in 3.07 which is said to be the ideal ratio, but I have never worked on a diff before . Guess I need to do some research to figure out how to swap the gears and posi. I have a mild cam in the Buick 215 and had it decked to up the compression, so 3.07 may work.



Edited 2 time(s). Last edit at 01/19/2014 09:18AM by Henshouse.

Said by who? And for what drivetrain combo?
It is only ideal if in overdrive the engine is somewhere in the lower quarter of the torque peak. Drop below that and both performance and mileage will suffer. Many people are much happier with the 3.23, 3.331 or even 3.54 ratio. Tire size, overdrive ratio, and engine characteristics all play a part. Are you running a stock cam? If not be wary about that 3.07 ratio. You can pick up a good serviceable set of OEM Dana44 gears on ebay for around 50 bucks in about any ratio so having the gears on hand is not an excuse. The carrier size split is between 2.88 and 3.07 so you will need a different series carrier in either case so that is not an excuse either. Do your math. Don't just take rumors as good advice.

Jim