6 Speed Gearbox Conversion

[sam93]

Evening all,

A few months ago, there was a post about a highly modified GT6 which had been built by Geoff Taylor. I am particularly interested in having the luxury of a 6 speed gearbox fitted to my GT6. After many emails sent between us, the final result on the conversion is that it would be possible for most of us. However this relies on Geoff designing and casting a custom bell-housing which will mate the Mazda gearbox to the standard triumph engine. He has said that he'd be happy to do this as he loves a challenge, however due to the large financial investment required he'd want confirmed numbers in the multiples of 10. Therefore I think if there was say between 30 - 50 people interested it would be a step closer to happening. Please see below for the email responses from Geoff, which explain it better.

Please fill out the above poll !

Cheers,
Sam.

1) My engine and gearbox are 7" further back than the standard Triumph position. I think that a Mazda 5/6 speed gearbox will fit within the chassis rails without modification to the chassis if the engine is located in the standard position. You would need to fabricate a gearbox mounting and have a new propshaft made. I designed a special aluminium adapter plate to mount the Mazda gearbox to my engine. From memory it did require some modifications to the block (angle grinder not milling). The Mazda gear boxes are very cheap because so many of the rotary engines have died and made the car worthless.
I also had to move the gear lever in my install - I'm not sure where the lever comes out using the standard engine position.

As for the Diff - this does involve a lot of modifications to the chassis which was combined with changing the whole suspension design on my car.

I have attached some pictures for you. The full spec of my car is too long for an email but keep an eye out for Retro cars magazine who are doing an article about the car shortly.

2) It's been a while since I did my car!

The gearbox is out of a special edition Mazda MX5. I'm not sure if the RX8 bell housings are the same I think the starter motor position may be different.
I also forgot that I made a flywheel for it to use the Mazda ring gear. I think the std GT6 flywheel is to larger diameter to fit in the bell housing.

I can get a price for the adapter plate if you like but the conversion is not as simple as buying a kit to do the type 9. I guess I should buy another GT6 and make a kit for it - it wasn't my intention, when I built the car, to offer parts of the car on a commercial level!

3) I suppose I should call the adapter plate the rear engine plate really! It replaces the steel one that the standard gearbox mounts to on the engine. This is necessary because the Mazda bell housing is more integral to the gearbox than on older gearboxes with interchangeable bell housings.
The GT6 flywheel is to larger diameter to fit in the Mazda bell housing, this also means the standard starter motor goes in the bin to. The Mazda bell housing mounts the starter motor onto it and is very close to the original position of the GT6 starter. So you end up with effectively a Mazda gearbox, starter motor and flywheel on a GT6 engine! No the Mazda flywheel doesn't fit the GT6 crank before you ask! Hence the bespoke flywheel. It uses the standard size GT6 clutch cover and the Mazda friction disc, that by pure coincidence is the same size as the std GT6 disc. The flywheel and clutch assembly on mine weighs something like 7kgs from memory.
The problem with changing the flywheel is the fact that the engine will now be unbalanced. It is possible to match balance flywheels but it's never as good as balancing the whole rotating assembly.

Really the neatest solution for making a kit is for me to cast a bell housing that mates the Mazda gearbox to the Triumph engine. Then there wouldn't be a flywheel or starter motor issue. However casting a bell housing is not a simple process and would mean I would have to invest a lot of time and money into something that has a very small market.

I can get you a price for all the bits you'll need to do it the same way I've done it but it's not going to be cheaper than a type 9 conversion!

4) No that's fine. If enough people were interested I would consider making a bell housing - I love a challenge!

All the best.

5) Really it's got to be multiples of ten for it to be cost effective for everyone.

[Bainzy]

If you can get the gearstick in the correct position, I suspect you'd have a lot of interest? Otherwise I can't personally see it getting enough numbers to make it economically viable...

[ferny]

The final gears in the five and six speed boxes aren't that different.  The only advantage is that you'd get a short ratio box.

[Phwoar]

Are the pictures of the rear suspension/diff modifications available anywhere?

[Craig C]

My recollection of the Mazda 6-speed is that it is a very long gearbox.

Have you considered the small-engine BMW 6 speed box? Reasonably light, will take the power, plentiful and cheap and is made by the "owners" of Triumph...

Craig C

[esxefi]

the trouble is with a lot of these new g/boxes is the integrated b/housing.
it isn't a bolt on/off job.the gear clusters and gubbings are contained in part within the b/housing casting making conversions difficult in the fact that you need a conversion plate between the triumph engine backplate and the donor bellhousing.(as listed above).

what ever happened to 'modular' engineering :-/

[Nick Moore]

You might be able to get around the integrated bellhousing issue by making a new engine back plate with the shape and bolt pattern of the new bellhousing. That's not a complete solution, the flywheel/clutch depth and the starter motor position might need work. But it's easier to make a flat steel plate than a new bellhousing.

However, most of the 6 speed gearboxes on Japanese cars have a 1:1 fifth gear, with 'overdrive' only on sixth. That's to make up for the narrower power bands of modern, highly tuned engines. If you stick with your standard diff ratio in your Triumph, you'll just end up changing gear more often to get up to speed! You could change to a 3.27 diff to give each gear a wider range of useful road speeds, but I'm not sure a four cylinder Trumpy engine would have enough torque to pull in overdrive with that sort of diff ratio.

There are some gearboxes with overdriven fifth and sixth gears, such as a T56, but they're BIG. They're designed for large engines with big low-end torque.

[ferny]

I only know the MX5 stuff, but... The final gear on a five-speed box is .814 and on a six-speed it's .843
Differentials started at 4.3 and ended up 3.636

[CHRIS211083]

I think the type 9 gearbox conversion is very easy and cheap to do. The 5-6 gear isn't needed really. Four gears to accelerate is fine and with 4th being 1-1 and 5th as overdrive usually 0.82. an extra lower gear would just make 0-60/0-100 times slower.

6th is just more weight in the gearbox that wouldn't be used.

Chris.

[Phwoar]

Just to throw another variable into the equation ....... I'll be using a Toyota T50 box with the following ratios ...

First Gear: 3.587:1
Second Gear: 2.022:1
Third Gear: 1.384:1
Fourth Gear: 1.00:1
Fifth Gear: 0.861:1

Not quite as high in top gear as the type 9 but it only weighs 22Kg. (ahem, ahem, it will be mated to a 4A-GE 1600cc engine) OK, shoot me now.

Cheers,
James

[Sheepy]

James the 4A-GE is a little sreamer! Nice engines.

As for the above 6 speed, theres a lot of "if" & "maybe" so I dont think many will go for it, you need to change so many parts to make it work.
The type 9 is an easier route and they are still available.

[Clive]

And don't I wish I had picked up a couple of t50 boxes when they were cheap.....lovely bits of kit. However, I am lucky enough to have bought t9's at the right time and price, and then I thought £50 was plenty.

[ferny]

6485 wrote:

Just to throw another variable into the equation ....... I'll be using a Toyota T50 box with the following ratios ...

First Gear: 3.587:1
Second Gear: 2.022:1
Third Gear: 1.384:1
Fourth Gear: 1.00:1
Fifth Gear: 0.861:1

Not quite as high in top gear as the type 9 but it only weighs 22Kg. (ahem, ahem, it will be mated to a 4A-GE 1600cc engine) OK, shoot me now.

Cheers,
James

Excellent! That's the route I was going to go down but the gearbox option was far too expensive - I did very limited research I must add. Got any info? Start a thread, please!

[Phwoar]

4058 wrote:

James the 4A-GE is a little sreamer! Nice engines.

As for the above 6 speed, theres a lot of "if" & "maybe" so I dont think many will go for it, you need to change so many parts to make it work.
The type 9 is an easier route and they are still available.

I think it's going to be OK, overbore .040, head planed, block decked, I'm hoping for 150 BHP.

cliftyhanger wrote:

And don't I wish I had picked up a couple of t50 boxes when they were cheap.....lovely bits of kit. However, I am lucky enough to have bought t9's at the right time and price, and then I thought £50 was plenty.

Fortunately, they're reasonably plentiful here in Oz.

ferny wrote:

Excellent! That's the route I was going to go down but the gearbox option was far too expensive - I did very limited research I must add. Got any info? Start a thread, please!

I kind of got carried away and collected a few more than I need, seems I have five now, averaging about AUS$ 100 each.

Just got to put it all together with a Subaru diff and MX5 rear control arms, hubs, etc., BMW half-shafts with Lowbro 930 CVs and  bit of figuring to do to get it all right.

[Phwoar]

4058 wrote:

James the 4A-GE is a little sreamer! Nice engines.

As for the above 6 speed, theres a lot of "if" & "maybe" so I dont think many will go for it, you need to change so many parts to make it work.
The type 9 is an easier route and they are still available.

I think it's going to be OK, overbore .040, head planed, block decked, I'm hoping for 150 BHP.

cliftyhanger wrote:

And don't I wish I had picked up a couple of t50 boxes when they were cheap.....lovely bits of kit. However, I am lucky enough to have bought t9's at the right time and price, and then I thought £50 was plenty.

Fortunately, they're reasonably plentiful here in Oz.

ferny wrote:

Excellent! That's the route I was going to go down but the gearbox option was far too expensive - I did very limited research I must add. Got any info? Start a thread, please!

I kind of got carried away and collected a few more than I need, seems I have five now, averaging about AUS$ 100 each.

Just got to put it all together with a Subaru diff and MX5 rear control arms, hubs, etc., BMW half-shafts with Lowbro 930 CVs and  bit of figuring to do to get it all right.

[CHRIS211083]

Do you have the 16v red letters(earlier black letters) or 20valve(chrome letters) 4a-ge. 16v is 128bhp and needs the stock manifold and ecu to keep low end torque. The 20valve is a different beast and almost 160bhp in stock form is possible but they are a Iron block so are 123kg without bits and bobs.

Chris.

[Phwoar]

Hi Chris, it's the "smallport redtop" which was factory rated at 100 Kw (135 BHP) when new. I picked my bare spare engine up out of the back of a ute and put it on a bench so I'm thinking 123 Kg sounds a bit on the heavy side. Even if it weighs in at near that, the peripherals are lighter, all in all it should be about the same weight as the 1147cc engine. I'll be moving it back a little, compared with the standard engine location, to balance the car a little better.

The 4A-GE would be a stunning engine if it had an alloy block, it's a real pity Toyota didn't go that extra step.

Cheers,
James    

[CHRIS211083]

Hi James,
             Its 123kg without bits and bobs attached. The 1147 is heavier at about 135kg. I also have a book figure of 94kw so 126-128bhp. The Iron block is cheaper than the alloy for mass produce. The small port is a good motor but you should have gone for the 20v as it's 160bhp out of the box, has individual throttle bodies with velocity stacks as stock and is the same weight as the lower powered versions. However the Zetec se is only 82kg and 150 with throttles, you can see why I went this way. Willk be nice to see it once you have finished. You should start a thread showing the mods you needed to do. Do you need to mod the chassis to fit this engine???

Chris.

[Phwoar]

I will start a build thread when I start to fit the engine, gearbox and rear end. The chassis may need notching to fit the engine but it won't take much. I will have to modify the rear of the chassis significantly. Lots of pics when I start chopping, I promise.

I believe the smallport 4A-GE was released as a more powerful version in Australia, same as the Japanese model. There's a couple of reasons I didn't go for the 20v, the distributor hangs out of the back of the head and I would have had to cut into the firewall to make it fit, Toyota made superchargers for the 16v, I could, if the fancy takes me, bolt one on later. I think Triumph would have come up with a 16v crossflow engine in time and I think the Toyota's copy of the BDA is a more sympathetic match, more so than a "Volvo" motor.

Cheers.