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Adding Overdrive

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71tr

71tr

Jedi Warrior
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As though a complete frame-off rebuild isn't enough work! I'm considering the addition of an overdrive unit to my stock 4-speed TR6 transmission. Any thoughts or experiences to share? How about A-type vs J-type overdrives? What are the technical aspects of such an endeavor?
 
I did the same thing you're contemplating about 7 years ago on my 70 TR-6. I have written an article explaining my trials and tribulations on the VTR website. I'd highly recommend it as the car is much quieter aat highway speeds, and having a 7 speed is great! I put in an A type overdrive. I understand the J type is more reliable, but I have't had any problems with mine. Let me know if you need any additional info on this. I have several sources that might be able to help land you a rebuilt unit, either type.

Mark
 
You have a number of choices with the OD. A Laycock J type, D type or A type will fit. The J-type is original to the car and may be had with up to 28% reduction in the final drive ratio. That's big, really makes the car a highway cruiser (for as much as a TR-6 CAN be one, anyway)!

I installed a professionally rebuilt Jtype (28%) in my GT6, couldn't have been more pleased with it. Adds a whole new dimension to the car.

Do it, you'll be glad. But, make sure it's in good condition, and check the oil often.

Good luck, let us know.
 
Should'nt a 71' have an "A-Type" and not the "J-Type". I thought the "J-Type" came out a little later than 71',...like 73' I believe?
 
Stinky has a good point regarding which overdrive unit is appropriate for a '71. I believe the primary difference is in the mounting as later sixes had a different transmission mount system. I'm not concerned with this as I've located a good article on retrofitting the transmission mount to accomodate either unit.
 
Another point to consider before deciding on which unit to go with is the type of transmission you have- whether it used imperial units or metric. Anything prior to transmission serial number CD20282, is imperial, and therefore a J-type won't fit unless the mainshaft in the stock transmission is swapped out for the proper metric one, which means a transmission re-build. Also, in previous posts on this subject, I discovered a company Quantum Mechanics that specializes in od units, both in rebuilding and in conversions. They have all the necessary parts you would need to convert your present stock tranny to allow for the use of a J-type.
 
You're right, either A or J was used in '71. It sort of depends on your concerns about originality vs. availability vs what fits most easily into your car (rear mounts are different, but can be adapted as you noted).

J-type tend to be a little less expensive and a little more easily found. A-type can be faster shifting into OD. Most folks seem to feel the J-type solenoid is a better setup, the A-type's is a little primitive, fussier to adjust properly and more exposed to the elements.

In fact, there are two different A-types. The early one has a larger accumulator piston and really snaps into OD quickly. Generally that's the most desirable for racing, plus it's rarer and so it's usually the most expensive. The second A-type was put in the cars mid-TR4 production, and is simply a little softer shifting thanks to a smaller accumulator piston.

There are several versions of the J-type, too, but I don't know the details.

Plus, both A-type and J-type were widely used in other cars, mainly distiguished by the rear mount arrangement. Those may or may not be easily adaptable to your car. On the other hand, it might be cheaper to get one out of a Volvo sedan, for example, if it can be made to work.

With any of the ODs, you might want to budget in the cost of a rebuild. There have been improvements in components that will make either type OD more reliable and durable.

For example, the A-type originally used a mechanism that didn't protect against reversing with the OD engaged, which can damage the OD and/or the gearbox (properly set up inhibitors will prevent this from happening, unless the OD sticks "on"). More modern parts available for the A-type eliminate most concerns. The J-type already uses the more modern design. Also, clutch compounds have vastly improved and there are some other possible internal upgrades.

Most A-type are 22% reduction, but can be modified to 25 or 28%. Most J-type are 25%, a few are 28%, but either can be modified to 22% if that's preferred.

The first two digits of the part number stamped on the Laycock plate attached to the OD tell what reduction it was set up to provide when manufactured. "22xxxx" equals 22%, etc. However, any old OD might have been modified to another reduction during its lifetime.

Particularly the stronger shifting early version A-type and any of the ODs on a modified, higher output motor, might be too strong for aggressive use of 2nd gear OD. One key area of concern is the diff, particularly the IRS setup due to the mountings. In fact, many TR6 only came with 3rd/4th gear OD. It's relatively easy to install a second inhibitor switch to enable 2nd gear OD on any gearbox not fitted with it. Likewise, it's easy to remove 2nd gear OD inhibitor, to prevent using it if it's a concern.

I'm installing an A-type in my TR4 and really looking forward to using it, especially with a 4.1:1 diff and 60 series tires.

Cheers!

Alan
 
I struggled without an overdrive on my 73 TR6 and finally took the plunge about a year ago. Extremely glad that I did. I went with Quantum Mechanics. The hastle was extreme in that I had to build a crate for my 4-speed so that John could modify it and attach his newly rebuilt J-type to my unit. The problem got magnified exponentially when the unit that he sent DIDN'T WORK. Pulling a tranny out of a TR6 is a monumental feat. I quickly ran out of friends. To John's credit, he was a perplexed as I was and certainly made it good, but I had to do all the work over. The second setup now works like a charm. I did have the aftermarket steering column control switch crap out on me once (Taiwan, that's all that is available from any vendor) but other than that the system works great now. Oh, and as if I didn't need other problems, my speedo didn't work properly at all so I had to send it off to Nisonger for a rebuild and recalibration. All is well and I can cruise at 70 while the big six just purrs. I would have to recommend Quantum, except for the one major boo boo. He to this day doesn't know why the other rebuild didn't work in my car. Go figure. Good luck.

Bill
 
If you get used to using your clutch when engaging the overdrive, it will outlast everything else on the car, which was learned from racing 35+ yrs ago.
 
Hi again,

Bill's post reminded me, the column-mounted OD switch never worked for me. Instead I got a momentary push button from Radio Shack for $2 and did some surgery on one of those after-market, leather-covered shift knobs that was gathering dust in the garage. So the OD switch is right there on the shift lever.

I believe Racestorations is offering a shift knob with an integral OD button, if the Radio Shack route isn't your cup of tea. Or, someone told me that the Stag had a button-on-the-shift-knob setup, might be adaptable to TRs if you can find one.

In addition, I'm going to try Revington TR's "Black Box" OD controller. With it shifting is said to be more like a 7 speed (or 6 speed if 2nd gear OD isn't used). The car isn't back on the road yet, so I can't say how I like it.

Cheers!

Alan
 
As if an extra problem needed to be added on to this issue, the overdrive switch that comes with the conversion (and is the only one out there) mounts on the steering column and works like this: down-on, up-off. Wrong. My original manual that came with the car states the opposite. Sooooo, I did a little grinding on the innards of the switch and made it work right, up position-overdrive engaged. Had to rebend the arm a bit to clear my left knee, but it works well, till this one breaks. Like the idea of a shift-mounted switch although I would think that it might inadvertently be actuated just by resting one's hand on the knob. I think the old Healeys had a dash mounted overdrive switch, if my memory serves me.

Bill
 
Just dawned on me! I guess all I had to do was reverse the wires to make it work opposite. Oh well, at least the wiring colours are still right.

Bill
 
Bill,

I made the same modification to my OD switch. UP should be ON!! As for places to get a reliable rebuilt unit, I can think of two: My friend Mark at British Auto or British Miles. I bought mine from them (British Miles) and it has worked like a champ for the past 7 years. I've watched Mark rebuild A and J overdrives and its like watching a surgeon at work!! Good luck!

Mark
 
Aside from the article on BuckeyeTRiumphs web site, you may also purchase a premade bracket from Rimmer or Quantum(I think?) that bolts to the Early frame mounting points. The SPITFIRE o/d mount is also used...should come with the bracket along with the studs. I did this set up, but I wasn't pleased with the clearance of between the bracket & the sump cover of the o/d unit(j-type on early frame)....too close for me. Don't forget, you can also find a late frame & get the mount out of it. I suspect the method described on the BuckeyeTRiumph site is the cheapest & I'm told it works really well.

The frames were changed for the '73 model year so all earlier cars would've used the A-Type.

Also, if you use a j-type from a Volvo(240) you may be looking at coming up with a different output flange as they used different sizes...and lengths. You may also have to drill the output flange...if it is the correct size/length. Also, you may have to change the speedo drive gear as well as re-calibrating your speedo.

You will definitely have to change the main shaft so while you're in there you may as well rebuild the gearbox....oh maybe the mainseal on the back of the engine, the sump gasket...uh oh...it's starting to add up.

Todd Bermudez
CD198L
CF25768UO
 
Alan,
I had a three speed O/D transmission (a Ford) set up with an "on off" switch for control. It seemed that first gear over drive was a little taller than 2nd non O/D. Assuming a 28% overdrive can be obtained in all 4 TR6 gears, do the gears have sequential gearing or is there over lap?
 
Hi,

There might be some overlap if you were putting OD on 1st gear, but that's not possible on the TR gearbox. You can only install it on 2nd, 3rd & 4th.

In fact, a lot of folks don't bother with or avoid 2nd gear OD (might damage the drive train if slammed into OD, especially with a modified high output motor and the later IRS cars). Although it was on the earlier cars with A-type OD, Triumph didn't make OD available on 2nd gear in many of the TR6 with the A-type or J-type. I understand they again made it available later in TR6 production. In other words, they only offered OD on 3rd and 4th.

But, it's a relatively easy modification to add 2nd gear OD to an OD-equipped box that's not set up for it (drill the top cover, install the switch, modify or replace the OD wiring harness with the two-switch type).

28% reduction seems pretty uncommon overall, especially on A-type. It can be custom set up on a J-type or A-type. I believe 25% reduction is more common with J-type, 22% is typical for A-type.

This is all with respect to TR gearbox/OD units. The sedan gearboxes and ODs I don't know about, although many can be adapted for use in TRs. But, besides mounting and case length issues, who knows what gearbox ratios and what OD reduction might be found in the Triumph sedans (plus Volvo, which used the same OD for a while, and not doubt some other manufacturers).

I can only speak for TR3/4/4A in that 22% gives a nice set of ratios in the 7 speed or 6 speed arrangement, positioning the OD gears nicely between the non-OD gears. I don't have the specifics handy, but the factory service manual spells them all out, both in terms of ratios miles per hour/rpm. 25% or even 28% would be particularly nice on top gear out on the highway, especially with a 4.1, 4.3 or even 4.5:1 diff and/or smaller diameter tires.

Hope this helps!

Alan
 
For the life of me, I cannot figure out why I would want to engage overdrive in third gear (J-type, TR6) and even less why or how anyone would use overdrive in second as in the A-types. I use overdrive on the highway to get better mileage, lower the wear on my engine and hear myself think. What gives with Triumph (or anyone for that matter) offering this option? Sounds dumb to me.

Bill
 
Hi Bill,
I use third & third over occasionally when driving in heavy slow & go type traffic. Too lazy to use the shifter & clutch repeatedly. Just a matter of convenience.
D
 
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