Controversial issue -- switching out transmissions

My series one xj6, had the chance to do either, the car was an original 33,000 mile car and drovre it on weekends out east. Although my ETYPES I swap, change and have fun, Id stick with the original. Yes its slower on the shift and no overdrive gear but, A series one xj6 was just done right for the time, if the rest of the car hasn't been messed with.
gtjoey1314

Remember the basic toughness is there. Piston speed is the biggie and the same stroke was used on the 3.4 which sat literally for 24 hrs at full throttle on the engine brake in testing.

The 4.2 has an issue at top revs with crank harmonics that affect the middle bearing, but you don't hold 5000 for long. I had a 4.2 with Webers and cams that kept pulling like a train beyond 6000, so much so that I got a rev limiter and set it to 5750. Never missed a beat, as you can hear ( http://www.youtube.com/watch?v=h5HwL15NGbg ), but it would have needed new main shells ever 10,000 miles instead of every ten years, probably.

Since most street engines simply won't pull that well to those revs, durability of a broken-in factory-built or correctly blue-printed engine is not an issue, more or less whatever you do, if you keep on top of cooling, lubes, maintenance, and torqueing things down. Just my opinion of course, no free rebuilds here... :-)

Remember the basic toughness is there. Piston speed is the biggie and the same stroke was used on the 3.4 which sat literally for 24 hrs at full throttle on the engine brake in testing.

The 4.2 has an issue at top revs with crank harmonics that affect the middle bearing, but you don't hold 5000 for long. I had a 4.2 with Webers and cams that kept pulling like a train beyond 6000, so much so that I got a rev limiter and set it to 5750. Never missed a beat, as you can hear ( http://www.youtube.com/watch?v=h5HwL15NGbg ), but it would have needed new main shells ever 10,000 miles instead of every ten years, probably.

Since most street engines simply won't pull that well to those revs, durability of a broken-in factory-built or correctly blue-printed engine is not an issue, more or less whatever you do, if you keep on top of cooling, lubes, maintenance, and torqueing things down. Just my opinion of course, no free rebuilds here... :-)

Very descriptive, John :-).

My 4.2 experience is limited to just a bog-standard Ser III XJ6 but I remember 4000 rpm being a a sweet spot, yes. Redline was 5000 but there was no advantage, acceleration-wise, to revving beyond 4500. You don't go any faster and that last 500 rpm always seemed a bit brutal.

Cheers
DD

That's good to know Pete. Thanks
4000rpm is right in the sweet spot for the Jag XK. At least as far as the cylinder head fluid dynamics are concerned. The bottom end has a long stroke however. It's designed very well with proper British toughness but I always start thinking about all those parts spinning around when the tach starts heading toward the top end. I think of those old movies where the steamship captain shouts "Give me flank speed" into the voice tube and then they cut to a shot of the engine room and the steam engine connecting rods are flying, smoke is coming from the boiler and guys are wiping sweat from their brow - then they show the steam gauge pushing the red line.

According to Jaguar brochured the UK/ROW Series 1 XJ cars had 3.54 gearing, manual or auto (3.77 for manual overdrive), whereas US cars had 3.31, which is in direct contrast to the E-type situation where pretty much only the US cars got 3.54.

It's true that a 4.2 in good condition will sing along at 4000 all day long. It can even be quieter/smoother around those higher revs depending on resonances.

Pete

Thank you all so much for these contributions. What a wealth of knowledge! Work on the car is delayed because of the snow and so still at the stage of finding out precisely what is the issue. I'll post an update soon. Thanks again!

Graham

Edited on 2014-02-07 9:55:27
Some thoughts on this while I have morning coffee.
I've done the 700r4 conversion on two ser3 cars. One was a six cylinder car and the second is a V12.
As Doug pointed out, your 1970 xj6 has a BW8 transmission and not a 66. Both are decent transmissions in my opinion and can go for years without any problems. The 8 is 1950's tech. It's simple in design with no special features. Its a little over tasked with the heavy XJ saloon. Common failure modes are slipping bands due to the high torque forces needed to launch 4000 lbs from a standing start. The BW12 (which is in my 74 V12) is stronger but not very smooth. The 65 and 66 are smoother but not as heavily built in my opinion. The biggest issue that I know of is the rarity of hard replacement parts on these old Borg-Warner's which sometimes complicates a "simple" rebuild. Dick's Coventry West may be able to help with that.

Your series one has lower final drive gearing than the series three has. 3.05:1? 3.31:1? 3.54:1? Not sure but none are as tall as the 2.88:1 found in the series three cars and the XJ-S. The lower gearing helps with starting the car from a stop but you trade higher highway rpm for that. High rpm on the highway isn't such a bad thing with the Jag XK engine. It likes to run that way with its hemi head. However, when driving a stock XJ at speed, I always think it wants another gear. The 700r4 fourth gear is 30% overdriven. That takes your 3750 rpm on the highway back down to 2600 or so. You notice that at the gas station. If you want your XK to breath, you can always spool it back up to 3700 and roll down the road at 100 mph in overdrive. The Jag saloon with overdrive would do well over 150 mph in overdrive theoretically. You'll never see it. The car runs into an aerodynamic wall at around 130 and you need a lot more horsepower to overcome that.

The 700r4 (4L60) is very strong. It is used in a variety of GM cars from Camaros to Suburbans and Silverado pickups. It has no problem whatsoever pulling a Jag saloon. It's not even trying.

There are always issues installing a non-stock drivetrain. The 700r has some advanced features like the TV cable and a variety of valve body and governor configurations. You need to do a bit of homework on it before buying. There have been enough conversions done that the knowledge base is there. You have to get the shifter sorted and there is a list of little things like speedo drive, reverse lights, neutral safety, torque converter lock-up, etc. The engineering has been done. I use the kits from Johns Cars.

Bah! Make a virtue out of a vice and fit a Jag manual to really wake the car up!

If your *1970* XJ6 has the BW66 then it isn't entirely stock to begin with! It would have a BW8 or BW12....as the BW66 was used quite a few years later on the Ser III cars.

I'm reluctant to say anything contrary to Dick's remarks as the depth and breadth of his experience is so great....but, truth is, I don't think I've ever heard of anyone else being unhappy with the decision to use the 700 trans. Good reviews, and even rave reviews, are the norm as far as I can tell.

Cheers
DD

The conversion takes a lot of tweaking to make it work right. You also have a overdrive 4th gear and the stock 2.88 rear end is to tall to work with it in most cases. The model 66 when set up properly matches the car really nice and unless you go on extended expressway drives and want lower revs, I would suggest you stick with the stock setup. Also, the GM conversion uses a solid rear block mounting instead of the Jaguar Spring setup. This transmits a lot of noise into the car. The shift lever assembly needs to come out of a later model XJS HE (81-87) so the gear selector will match. I did this years ago to my 1985 and if I had it to do over, I would have stuck with the stock one.