All HE pre facelift V12s had the higher geared 2.88:1 differential and the GM400 3 speed autobox. Togther with the Michael May designed "fireball" head, this combination gave far better fuel economy than the pre-facelift so-called "flathead" V12 engines.
For post-facelift V12s (and I have an idea not necessarily all of them, as I have vague recollection that the earlier facelifts with the 5.3 engine did not) had a 4 speed GM autobox and, because of the higher ratio in top this enabled, the diff ratio was changed to a less high ratio 3.something:1 - which gave a lower first gear (better acceleration) and a higher top gear for better economy.
The 6 cylinder cars had a lower diff ratio, essentially for the same reason (ie their 4 speed ZF boxes had a better spread of ratios).
So the choice comes down to the balance between acceleration and economy; if you swap a lower ration diff into a 2.88:1 pre facelift HE V12, you will get better acceleration off the line as it will be lower geared; but worse fuel economy and a lower top speed (with the same GM400) as it will run out of gears. The four speed V12 facelift box bridged this problem with, in effect, an extra higher ratio.

 

My understanding is that they also have a lock up torque converter which would improve efficiency as well.

 

That is not correct, bellhousing is integral with transmission case and is Jag specific bolt pattern.

 

Although the AMC pattern looks very much the same as the V12 one does.

 

i have heard the Jeep 400 trans has a Jag bolt pattern?

 

No. Jeep (AMC) is a unique bolt pattern, and tilted 4 degrees for the transfer case.

The V12 bolt pattern is unique to Jaguar, no other TH400 variant will bolt to a V12 without an adapter plate.

 

That is why I said LOOKS the same .

 

This tranny is a PITA to clean up!

All my normal cleaning methods have only removed a fraction of the ***** stuck on the outside. Will probably go to a CO2 cleaner to get it cleaned up properly.

 

Great job with the pic's and discussion.


When Jag went with the TH400 it was the best automatic transmission in the world. many still think it is.
The T700, 4 speed had not been developed yet so they stuck with the 3 spd until the 90's IIRC.
For fuel economy I would like to have the 4 spd but as you noted the 2.88 rear end compensates for it to a degree. Still I'd rather be running somewhat less than 3000+ rpm at 80 mph.

 

Can you use over cleaner ( sodium hydroxide) to clean off the oil? Usually works well.

 

I'be used brake cleaner, carb cleaner, engine cleaner, combined with a high pressure cleaner etc... I have only managed to remove grime on the immiadiate surface. I want it totally clean though. CO2 blast ing is supposed to be the best way...

 

Take it to an automotive machine shop and ask for it to be hot tanked.

 

Got a tip: I'll use some oven cleaner on it first. If that doesn't help, then I'll search for help Should do the trick though!

 

I have used Easy-Off oven cleaner on everything except the oven, great stuff.

 

Oven cleaner did the job quite well! Will use that more frequently as it costs less than brake cleaner and works better with less effort. Just need to wash it with water afterwards.

 

Looks good. Cleans more than just ovens.Any time I can't clean something I grab Easy-Off oven cleaner.

 

Not me



"Better" can be debated.

The important word being "choice".

In the car hobby world ....most any kind of car....we have lots of choices for modifications and modernizing. Sometimes people act almost as though you're committing a sin (or being an idiot) if you don't go that route.

There's no *obligation* to modify or modernize . There are various considerations: budget, intended use of the car, expedience. Even sentiment comes into play....a fondness for the original design even if it's old school. Nothing wrong with that.

For example....

There was a time in my life when I did lots of highway driving. Frequent weekend jaunts of 500-700 miles. Nowadays, though, I seldom travel the open highways so low cruising RPM and highway-speed fuel economy are not important considerations for me. Thus, keeping the TH400 and going with a 3.31 differential works well for me....even though some people look at me as though I just landed from Mars when I tell them what I've done.




IMO, the one of the down-sides to the old TH400 is that it's quite parasitic.

Decades ago, in my hot rodding days, some guys went with the TH350 trans instead. It could easily be built to take quite a bit of torque and abuse....but you 'saved' 10-15 horsepower versus the TH400

Very true.

When I had an XJS I used a shift kit, kickdown mod, and 2500 rpm stall speed converter. It was almost like driving a different car; gave me the improvement I was looking for without changing transmissions.

None of this is meant to disparage a decision to go with a more modern transmission, mind you. Not at all. It's undoubtedly the right choice for some people. Perhaps even for me....under different circumstances.

Cheers
DD

 

Doug, very wise words. After seeing how cheap torque convertor upgrades are, I'm considering a different one. Though then again, I'm not making this a race car

 

There's only one advantage to the higher stall convertor-- to let the engine increase RPM more easily. I found this to be an advantage in moving a 4100 pound car off the line with 2.88 gears. If that's not a concern then I wouldn't make a change.

But, even if staying with Jaguar-spec stall speed (2100 rpm, as I recall) you still want a premium grade torque converter if you intend on exploring the 6500 rpm redline that the V12 is easily capable of. A generic rebuilt converter intended for Dad's old Buick may not hold up.

When I ordered a torque convertor from TCI https://www.tciauto.com/ the guy I spoke with was familiar with Jaguar spec convertors and explained some of the differences.....although the details escape my mind at the moment. Particular attention to balancing, special welding....stuff like that.

High RPM durability is something to consider with a trans swap as well. When I was considering a TH700 conversion for my XJS I spoke with a few trans specialists who had reservations in this regard, ranging from 'sceptical' to 'a disaster waiting to happen'. Of course there are numerous TH700 upgrades to assuage any concerns but you're adding costs on top of the conversion kit, the transmission itself, and the pre-installation overhaul.....as I can't imagine anyone would go through the trouble of a conversion but install a used transmission in "as-is" condition.

Cheers
DD

 

Took all the gears and drums apart today. Lucky I didn't just fit it, like I was told to do. The clutch plates are all VERY worn. Heck, one disc part has even snapped. I would have probably only had a very short bit of joy with this as is. The gears (planet and sun) are fine. I assume that one of the former owners had either not changed the ATF and/or driven it with a low level. The clutches are down right burned... Not a big issue though, as I will be replacing them anyway.

Never trust a transmission you have not tested yourself!