Which Overdrive Would you Choose?

Lately, I've been noticing a little more grinding in my transmission when shifting into 2nd gear. I have suspected for quite awhile that the synchro's are probably due for a re-build so last year I bought a J-type overdrive unit from a dude in New Zealand that included the correct mainshaft and adaptor housing in the hopes of having the o.d. installed when I have this re-build done. If I'm going to be in there, why not have it done? The transmission itself is a J-type from a late model TR6 that the p.o. installed a year or two before I bought the car (TR250), which about 11 years, or 25,000 miles ago.
So last week I contacted John Esposito at Quantum Mechanics about getting his take on what I plan to do. He thinks I would be better off with the A-type because it is a much more robust unit, being better suited to the big TR's and, in his words, J-types "are good for Spitfires and barely adequate for the GT-6." Now I know J-types have been used on TR-6's during most of their production run and I have a hard time believing they were not up to the task, but then again, John knows his stuff and I certainly can't dismiss what he says.
Price-wise, if the o.d. unit I bought is in good shape, then I will save several hundred dollars by re-building the transmission and having it installed compared to having my transmission converted over to accept an A-type which John would supply.
Given the choice, what would you do? Has anyone ever had a J-type O.D. go bad on them because their car had too much power? Afterall, we are talking Triumphs here!

Since you already have the J-type on hand,...plus the car already has the J-tpye gearbox installed, I'd just go ahead and use the J-type.
For originality sake it would be nice to go with the A-type. But given the cards you've been dealt, it just makes more sense to me to go with the J-type.

Did Triumph not change the "specification" on the TR6 with the J-type so that OD could be engaged only in 3rd and 4th gears? I seem to recall that happening, and it would make sense in that eliminating the 2nd gear OD would then NOT overstress what John E. is saying was a weaker OD unit.

Presumably the decision to go with the J-type was a typical "commonisation" (read: cost-cutting) measure imposed by BLMC at the time...or perhaps Laycock was discontinuing other units and it was necessary? Just guessing....

Also, don't the two types require different rear mounts?

Mine had the J type conversion when I bought it about 25K miles ago. No issues have arisen.

There was mention made above of a j-type transmission or gearbox. This is unclear as there is a j-type overdrive unit and an a-type overdrive unit but the transmission itself is simply a non-overdrive TR6 gearbox, correct? If you already have the j-type unit and the correct mainshaft for the conversion then stick with it, I don't think converting everything over to an a-type is going to make any difference with the exception of the mounting points for the overdrive unit on your frame. The j-type overdrive will require you to fabricate a mounting adapter to fit it to the early style frame, not a big deal.

I had John install a new J type in my 74 TR6 several months ago. John and I spoke at length about the A versus J and he most definitely recommeneded the J over the A. Please be sure that you two are on the same page, as I had several phone conversations with him and have his email stating J over A before taking it to him to have the work done.

Hi,

Whew! This *is* a hotly contested issue.

Money aside, some like the A-type better, while others prefer the J-type for some of its characteristics. All I can say is "it depends".

It really does depend upon how the user wants to use the OD, what their goals are, for installing one.

Both are reasonably strong units, probably with an edge going to the A-type (like John says, and he should know!).

Both can be set up with 22%, 25% or even 28% reduction.

The A-type, and particularly the earlier A-type, use higher hydraulic pressure and jump into OD operation much more quickly. For performance work, i.e. vintage racing, A-type is generally prefered.

The J-type is probably more than adequate if the goal is pleasant highway cruising, thanks to lower rpms, and the OD is usually only engaged on top gear.

J-type might be the winner in another respect: The A-type's operating linkage is more exposed and vulnerable. The A-type's linkage adjustments also can be slightly difficult and finnicky. And, the J-type solenoid might be more reliable (not to say that the A-type's is particularly bad).

The higher pressures in the A-type might lead to a bit more tendency to leak, if that's of any concern (Hey, we're talking LBCs here! They all leak, don't they?).

Original A-type could be pretty easily damaged if the unit were ever engaged in reverse. The isolator switches normally would prevent this, but it might happen if the OD stuck "on", for example. During modern rebuilds, it's usual for changes to be made that make this sort of damage much less likely. There are also common changes made to improve clutch centering.

Modern clutch materials are much improved and available when rebuilding either A- or J-type. For overall parts availability, the J-type is probably a bit better.

Yes, Andy is correct. 2nd gear OD was initially dropped when the switch was made to the J-type. But, it was later re-introduced and it's actually pretty easy to set up 2nd gear OD on either type. Whether or not this is desireable is up to individual preference. 2nd gear OD can be hard on either OD, probably is more-so on the J-type. In either case, 2nd gear OD should be used carefully.

If money is an issue, A-type OD tend to be quite a bit more expensive, since they are rarer and more sought after by vintage racers. I can't be sure, but it seems J-type were more widely used on other cars - Volvos, etc. However, if a non-TR unit is used it will probably need special mounting or modification to fit.

With either one, as mentioned, various reductions can be set up. A-type typically came with 22% and J-type with 25%. 22% gives very good gearing in between 2nd and 3rd, 3rd and 4th. 25% or even 28% might be prefered for highway cruising.

In conjuction with OD units, rear axle ratios were often changed. 3.7:1 was standard with non-OD gearboxes on the earlier cars, while 4.1:1 was usually fitted when an A-type gearbox was installed. Later cars that had a J-type might be fitted with 3.7:1 or 4.1:1, where 3.45:1 was the usual US spec to TR6 (3.7:1 was usual for UK "PI" spec). I'm not certain about TR250/5 ratios. In any case, 4.1 will give the best acceleration, but at the cost of additional rpms when highway cruising.

Someone recently mentioned that the J-type doesn't require a relay, like the A-type does. I haven't confirmed this and it's not a major issue anyway. I know with the A-type, the OD switch/lever on the steering column can be a problem. If it gets grounded accidentally, the OD solenoid can remain engaged all the time, probably burning up the solenoid. Don't know if this is also a possible problem with the J-type, though. Speaking of the OD switch, I personally don't like the idea of having to reach to the lever/switch each time to engage/disengage the OD, so am using a button on the gear shift lever instead, in conjunction with Revington TR's logic controller. I'll let you know how I like it, once the rest of the work is done and the car is back on the road. Of course, someone looking for originality, or just using the OD for rpms reduction at cruising speeds, might prefer the original style lever switch.

So, depending upon what you want the OD to accomplish, your choices of OD unit, how it's set up, and its related accessories might be quite different.

Thanks to everyone for their views. I just want to make a couple of comments before allowing this thread to be put to rest.
It is my understanding that transmissions prior to CD20282 are refered to as A-types and those after as J-types. I may be wrong, but I believe that's how Roger Williams refers to them in his restoration books. And to convert an A-type transmission to a J-type O.D. unit requires much more than just changing out the mainshaft, and vice-versa with a J-type transmission to an A-type O.D. unit.
As for my correspondence with John, I have two e-mails from him where he clearly and unequivocally states that he feels the A-types are more robust and therefore better suited to the larger TR's than the J-types, which really surprised me to say the least. That's why I wanted some comments from those who have J-types. As I said in my earlier post, John has had many years in this and his advice cannot be discounted. I can't explain the difference in the advice he gave Paul and me and I hate to say this in an open forum but maybe there is some salemanship going on. He certainly wouldn't be the first to be guilty of this. However, I am going to send him my J-type O.D. unit to test to make sure it is ok to use and if it proves ok than I will be using that and not going with the A-type.

I think we may be mixing terminology here. The CD20282 tranny number was the cutoff when Triumph switched the bearings for the input shaft, pressed fit vs free cage. I'm not so sure it correlates to the switch from a-type to j-type overdrive units. For instance; Quantumechanics offers three types of overdrive mainshafts 1) a-type shaft for pre CD20282 trannys 2) a-type shaft for post CD20282 trannys 3) j-type shaft for ? (likely only post CD20282 trannys). Maybe the true limitation here is mounting a j-type overdrive to an early tranny without making internal gearbox changes. My point is, the correlation between early and late trannys and a-type vs j-type overdrive units is not 100% positive. IMHO

Abrs 53,

Could you please post the two e-mails from John? They may be interesting reading.

Thanks.

Alan

Alan,
To print both entirely would take up too much space. Here are excerpts from each.

"...If it were my car, given the engine modifications to increase the horsepower, I would not go with the J type. The A type is stronger, although more mechanical and prone to temperamental behavior (after all, it was designed in 1948!). I would also consider a 5 speed transmission conversion. We will be offering this option next month using the English Ford type 9 (Sierra) transmission as seen on my website for the MG conversion kits. This would put a modern 5 speed transmission in place of the flawed Triumph transmission and antique overdrive technology..." excerpt from e-mail dated 11-9-05.

"Dave - The J types are more reliable and shift smoother than the A types, but the A types are stronger. If you look at the internals, you can see why, everything is bigger in the A type than the J type. We only build A type racing overdrives for the big TR's. The J type is good for the spitfires, and barely adequate for the GT6..." excerpt from e-mail dated 11-11-05.

Dave,

Thanks.

Alan

Strange ... J-types were also used behind the Stag OHC V8, where most people consider them much stronger than the earlier A-type. And at least my J-type has much beefier looking gears than my A-type. The J-type also has a pressure oiling system that is active all the time.

Still, my choice would be for the A-type for it's instant engagement. The J-type has to build pressure when you flip the switch, which takes a couple of revolutions. The A has an accumulator that holds pressure all the time. But for an IRS car, you probably want the later A-type with the smaller accumulator, as original for the TR-250.