Overdrive for V8s

[David C]

I just stumbled upon a web page for Cabmasters in Lancs advertising that they have 46RH transmissions in stock. No idea of price but they seem fairly hard to find now, particularly in the UK. My research suggests that these should be suitable for late 408 through 410 and both types of 603 - but please don't take my word for it. PATC in the US makes a kit to attach them to the big block engine in the 411 but there may be issues with balance of torque converter.

[Green411]

Hi David
Have just seen this, it's extremely useful - thanks. Such a swap is on my (long!) list of jobs to do on my car.
I've emailed them asking for a price and will report back.
Cheers
Mark

[David C]

I will be fascinated to hear. You should be able to buy a used transmission in the USA for between $500 and $1000. Easy to spend another $2000 on rebuild of torque converter and transmission. About another $1000 to crate, send within the USA and get it back to the UK. Add 20% import duty and you could be about $5000 in with loads of phone calls and emails to set it up. That should be a good starting point to assess the value of the option from Cabmasters that I think was advertised as rebuilt.

Alternatively, get a used transmission and ship back to run as is and you might get away with about $1500 but costs to rebuild will probably be higher in the UK. It will also be more difficult to find someone who knows what is required for the rebuild.

It's worth considering the stall speed of the torque converter. Many second hand units are likely to have been rebuilt by now and a good few people are likely to have gone for a higher stall speed to go behind a tuned engine. This may not be what you want in a Bristol. It's a fair chunk of cash to sort out so worth factoring in to the total.

[Green411]

Sorry for the slow reply, which you will correctly interpret as 'I thought I'd posted this but have just discovered I hadn't actually done so.....D'oh.....'
I emailed Cabmasters about the A518/64RH, their chap Brian says £2k + VAT, only 2 left, buyer collects from just north of Manchester. Their photos suggest it comes with a torque converter. Their replies were quick and friendly. They are buyers of surplus OEM stock so may not know the full ins & outs of all their items so I think that if you want one then do plenty of research to make sure it'll work with your engine, adaptor plate notwithstanding. They are advertising them as suitable for some diesel trucks plus Aston Martins. So are they a diesel trans or a petrol car trans? According to Herman at www.bigblockmopar.nl a diesel transmission has heavier duty internals which Herman thinks will be bad for economy in a petrol car.
Having said all that the photos show a part number of P52119161AB which when I searched on it comes up with Dodge Dakota/Ram 2wd truck transmission at www.2040-Parts.com in NY, USA. Allegedly brand new OEM, $777! It's so unfair.....

[David C]

At that price I would say it's a no brainer. According to my research the internals between petrol and diesel models are interchangeable, mainly the number and thickness of clutches.
I would imagine the benefit of the lockup would far outweigh any losses due to heavier duty internals.
My research suggests that the A and LA engines are internally balanced and should bolt up directly. The wide block engines will need an adapter and probably a different flexplate to balance the engine.
Please note that I'm not an expert on this and don't have a lot of practical experience but I have spent a good few hours on my own project so I think I'm qualified to suggest a few questions to ask those who are genuinely clued up on the subject.
For anyone seriously considering upgrade to overdrive the major considerations are cutting a chunk out of the crossmember on the car and making a new mount, shortening the prop shaft, upgrading transmission cooler pipes from 5/16 to 5/8 and probably fitting a new cooler or rebuilding the radiator to a higher spec. Also some work on the gearbox tunnel but I can't help with details of this as I found a smaller 42RH that I hope will not need any modification.
Control of the unit looks relatively straightforward with parts costing £100 to £200.

[David C]

I have just noticed that you say the Cabmasters unit is 64RH. This requires more investigation before rushing to buy.. 46RH seems fine but I don't know about the 64RH.

Edit - I have double checked and can't find any reference to 64RH so assume this was a typo.

[Green411]

D'oh......sorry!

[Green411]

Points noted about dimensions and trans tunnel. I too would be reluctant to bash or resize the trans tunnel in any way, and there's got to be enough room for the linkage and pipes. I found a site that gave specs for modifying the crossmember and shortening the prop if fitting a 518/46rh. But like you, a smaller trans appeals. Dont want to cause heart attacks anywhere but I know a GM TH200-4R has a good gear spread, is relatively small & light, has a mounting point close to the 727's and should be relatively findable and affordable vs Mopar stuff. Also are rebuildable to take big power. Not that I'll need that, but it'll need to handle big-block torque. At this stage I'm just not sure what to do.

[David C]

One of the regular visitors to this site has a 46RH that was fitted to his 410 by a previous owner. I haven't inspected it but I don't think the tunnel modification was too severe and he seems very happy with it.
Given that there seems to be an opportunity to grab a 46RH in this country for very reasonable money I would suggest you give it some serious thought but don't take too long about it if there are only 2 units available.

[AndrewA]

Hi - it's me David is referring to - I thought it encroaches a bit as the 410 seemed not terribly roomy down in the footwell but now having driven another (with an original gearbox and not noticed that to be any roomier) I reckon any encroaching is relatively minimal.
My car is going away to have gearbox looked at in a few weeks.
If it has to come out - which I'm hoping it doesn't - I'll be able to provide more info.
Just out of interest am I right in believing the gearbox has to come out either out the front with the engine attached or it can come out through the car?
Thanks
Andrew

[David C]

The gearbox comes out through the car.
It would be a good plan to get underneath as soon as you can and soak the screws that hold the tunnel in penetrating oil. Yours will probably not be too bad as the tunnel has been out within 20 years. A few of mine were a bit reluctant to be disturbed after 53 years.
If the gearbox does have to come out please take lots of pictures of tunnel, chassis, transmission, pipes etc.

[AndrewA]

I'll do my best re the photos etc but it's all happening three hours away at these people Home
They had two Bristols in and a Gordon Keeble when I last spoke and came highly recommended - they also fit aftermarket overdrive conversions by the look of their website.
Happy Christmas
Andrew

[Sam410]

Originally Posted by AndrewA

Just out of interest am I right in believing the gearbox has to come out either out the front with the engine attached or it can come out through the car?

When SLJ took out my gearbox for overhaul they took out the engine and gearbox together.

[AndrewA]

Thanks for that - I guess I'll find out on Monday!

[David C]

I have just been asked for details of fitting an overdrive gearbox to a V8. A fair bit of the information will be elsewhere in the messages but I have tried to bring it together as follows:


I fitted an A500 42RH transmission to a 410 that I think was originally in a Dodge Durango. Another 410 has an A518 46RH, probably from a Dodge Dakota.

A500 and A518 are series numbers with subsets 40RE, 42RE, 44RE, 42RH etc and 46RE and 46RH. The higher the number the stronger the transmission. For a 411 or 603 you are probably better off with a 46RH. The RH part is very important as it is hydraulically controlled as opposed to the RE which is electronically controlled. The RH models can be simply automated with mechanical switches but the RE models will need a separate electronic controller to operate overdrive and lockup. My conversion uses two pressure switches to enable overdrive and lockup, a vacuum switch to disengage overdrive and a half throttle switch to disengage lockup. PATC in the USA provide a kit of switches but I managed to source some in the UK which I think make for a neater and more convenient installation.

These transmissions bolt directly to a 410 - although I had to trim the bottom corner off the block to get the starter to fit - but will need an adapter for the bigger block 411. They should also bolt directly to the small block 360 in a 603 but that needs confirmation.

The crossmember needs the top half removed in the middle. I had a thicker 6mm plate welded in place to restore the structural integrity and then adapted the Dodge mount to fit.

The vertical rod for the shift lever needs to be further from the centre line of the transmission than the original. On the 410 with 46RH it was canted over but this made adjustment a little tricky. My gearbox tunnel was significantly modified to keep the shaft vertical and works well mechanically but is not so neat. I think the ideal solution is between the two choices.

The speedometer cable is an easy fix. Ready made short extensions are available but I chose to fit a right angle drive which works perfectly with the original cable.

The original cooler lines are 5/16" but the overdrive gearboxes increased this to 3/8". However the pickups they were used in were nearly twice the weight of our cars and regularly used for towing. I have retained original lines and cooler and added an increased capacity finned sump but am fitting a transmission temperature gauge to confirm whether this was the right decision.

The replacement transmission will need to come with a lockup torque converter. This introduces the choice of stall ratio which caused me some worry. The unit which came with my gearbox is made for 2300RPM which I thought a bit high but it seems to work well.

The prop shaft will need to be shortened and balanced as the overdrive transmissions are about 10cm longer than the 727. I had this done by Propshaft Services near Heathrow and it was reasonably priced and quite a quick turnaround.

There is only one overdrive ratio available as far as I know. This is 0.69 : 1 and results in a very relaxing 1600RPM at 65MPH with a 3.07 rear axle ratio.

The 42RH has a lower first gear than the 46RH or the original A727 so it's a win /win with sharper take off and considerably lower cruising revs.

It's a bit early to give an accurate indication of fuel saving but first signs are promising.

Having located a donor transmission there are a few minor modifications that are worthy of consideration if rebuilding the 'box.
Sonnax make a manual valve that allows the torque converter to fill in Park. It is suggested that this is fitted along with a lube pressure regulated valve.
The plastic pistons fitted to these transmissions can fail with time. Billet aluminium servo and accumulator pistons seem a good plan and the accumulator springs are known to fail so should be replaced with new.

[Green411]

Thanks David - v useful detail. Sounds as if you did it yourself?
I'm still havering about my own 411, needless to say. But my latest thoughts are that the GM200-4R that I was focussing earlier on has the same drawback as the Mopar transmissions in that the crossmember causes a big problem and needs modification.
I found this site https://m.roadkillcustoms.com/automa...on-dimensions/ which gives dimensions for various transmissions (you have to scroll right to see the measurements) but it shows that the GM TH700-R4 has a trans mount in virtually the same distance from the block face as the TF727, and is 4" shorter overall. It's based on the medium-duty TH350 3-speed so should be no bigger, and possibly smaller width/depth than the TF727. (Am struggling to find info/dims for transmission case width and depth). There's the thickness of the adaptor plate to consider, but the same applies to any o/d auto fitted behind a 383, be it Chrysler , GM or anything else.
So am wondering if the 700-R4 tailshaft would fit over the top of the big round cross-member just like the TF does, plus a relatively simple mod to the mount. They have hydraulic controls, with upshifts controlled by a throttle-valve (TV) cable, which would need careful initial setting up and adjustment to get the pressure right - as do the rods on a Chrysler. It's GM, so readily available and relatively inexpensive. Same issues as any transmission re TC lockup - kits are available
But, unlike others on the forum, I haven't actually done anything about it....

[David C]

I did most of the conversion myself but had a workshop do the welding and fitting for two reasons. Firstly I'm not as young as I was and I'm not strong enough anymore to wrestle the transmission on my own.
Also, however good or bad my welding may be my insurers could be forgiven for having reservations about DIY. There are times when it is prudent to have someone else to blame.

The cutting and welding itself is pretty straightforward. I hope John Tullett will forgive me for naming him as someone who has come up with a simple plan involving a removable section that allows transmission removal from underneath or inside the car. For no other reasons than conservatism and lack of structural engineering expertise I decided to leave the crossmember as intact as possible in my conversion and make life far more difficult for myself.

The part of the project that took me most time was the vertical shaft for the gear linkage. Per my previous post I ended up with a beautifully engineered vertical solution that takes up more tunnel room and took hours to craft in glass fibre. John assures me that an inclined shaft can be made to work well and requires far less work. Up to the individual to decide.

Given that there are now a few V8s with Mopar overdrive transmissions and most of the issues have been addressed I'm inclined to suggest you go for the known Devil that is the 46RH. It may also be easier to get signed off by your insurers as it's a newer version of the original. Per another recent post there is a bellhousing made by Ultra-Bell to attach the 46RH to big block Mopar without an adapter plate although it looks as though it needs some fairly serious metalwork.

One fairly major reason to go for the GM option, however, is that you have fitted a Sniper injection system. As I understand it this will operate an electronically controlled GM 'box so you will have an easier time finding a suitable unit to fit. The Holley terminator will control a Chrysler 46RE but as far as I'm aware the Sniper doesn't yet.

[AndrewA]

Not much to add to David's post except to sing praises of conversion.
Mine was done 20 years ago by previous owner who did a lot of continental touring. I've achieved 28 mpg on long runs at circa 65-70 mph and 25 mpg when in the 80-95 range, in Europe this summer.
I've got 2.8 at the back so at 65 mph I'm 1400/1500 rpm.
I don't think it's blunted performance - it acquitted itself well at recent sprint at Southport.
Mine's on manual switches so overdrive is on a stalk off the steering column and lockup on another separate switch. It sounds a faff but in practice one just flicks in an out of overdrive as required and I only engage lockup on motorways/ long runs.

[Green411]

Thanks David and Andrew - v interesting - and depressing in Andrew's case!