Technical Gearbox swap?

[superunknown]

I've got a 150 T-Jet Dynamic.

I'm doing a lot of motorway driving at the moment and the 6-speed box is a let down. 6th is exactly the same as 5th would have been in a 5-speed car. In other words, the extra gear has been used to squeeze the ratios together for acceleration rather than providing a longer-legged gear for motorway driving.

70mph = 3,000 revs. So I'm regularly pulling 4,000 rpm and can be doing so for miles on end (obviously this is when I'm abroad, Officer ). It's not doing my ears or my wallet any good.

Looking at the stats, it would appear that the box off a 1.6 Multijet diesel would give a much better spread of ratios, with a higher top gear. The final drive ratio is 3.35 rather than 4.18. Whilst in theory this might affect acceleration, in practice 1st gear is ridiculously low and I'm not sure the close ratios really help the 0-60 time when you have to keep disengaging the engine to change gear. What's lost in gear ratios would probably be saved in lack of shifts. And anyway, it's hardly a race car, is it?

SO.... the big question. Is this do-able? Are the boxes a straight swap? Any idea of timescales or costs??

 

[RobW]

I've got a 150 T-Jet Dynamic.

I'm doing a lot of motorway driving at the moment and the 6-speed box is a let down. 6th is exactly the same as 5th would have been in a 5-speed car. In other words, the extra gear has been used to squeeze the ratios together for acceleration rather than providing a longer-legged gear for motorway driving.

70mph = 3,000 revs. So I'm regularly pulling 4,000 rpm and can be doing so for miles on end (obviously this is when I'm abroad, Officer ). It's not doing my ears or my wallet any good.

Looking at the stats, it would appear that the box off a 1.6 Multijet diesel would give a much better spread of ratios, with a higher top gear. The final drive ratio is 3.35 rather than 4.18. Whilst in theory this might affect acceleration, in practice 1st gear is ridiculously low and I'm not sure the close ratios really help the 0-60 time when you have to keep disengaging the engine to change gear. What's lost in gear ratios would probably be saved in lack of shifts. And anyway, it's hardly a race car, is it?

SO.... the big question. Is this do-able? Are the boxes a straight swap? Any idea of timescales or costs??

You need to sell the car and buy a diesel

 

[superunknown]

You need to sell the car and buy a diesel

I'm not convinced. I like the flexibility of the petrol unit, I just don't think the gearbox is well-matched for the sort of driving I'm doing now.

It's a car I've had from new, so it's been looked after and I know its history and that the mileage is genuine. Oh and I've already lost about 50% of my money so it makes more sense to keep it and benefit from the plus points, like the fact it's only 18 months old and has done less than 20,000 miles.

If I were going to sell it and take the depreciation hit, I'd be buying a 500SL or a QP IV Evo, not a diesel Bravo!!

 

[Tad-bravo]

Obviously you can't just take the gearbox off a multijet and screw it to a tjet, it's a completely different engine.

I for one am glad to have a 5 speed gearbox, the car is faster than the equivalent with 6 speed and one less gear to switch.

 

[RobW]

I'm not convinced. I like the flexibility of the petrol unit, I just don't think the gearbox is well-matched for the sort of driving I'm doing now.

It's a car I've had from new, so it's been looked after and I know its history and that the mileage is genuine. Oh and I've already lost about 50% of my money so it makes more sense to keep it and benefit from the plus points, like the fact it's only 18 months old and has done less than 20,000 miles.

If I were going to sell it and take the depreciation hit, I'd be buying a 500SL or a QP IV Evo, not a diesel Bravo!!

my car (TJet 150 Sport) does 80 at 3500 rpm and returns 40 mpg

you could possibly change the final drive or fit larger diameter tyres

 

[twincamms]

My diesel does 2000rpm at 70mph..............

 

[superunknown]

Obviously you can't just take the gearbox off a multijet and screw it to a tjet, it's a completely different engine.

Why "obviously"?? The engines may be completely different but that's not to say they aren't designed to fit a common gearbox.

Does anyone actually know, for definite, whether the boxes are common to all Bravo 198s (albeit with different internal ratios) or entirely different depending on the engine?

 

[superunknown]

my car (TJet 150 Sport) does 80 at 3500 rpm and returns 40 mpg

you could possibly change the final drive or fit larger diameter tyres

That's interesting - the rev:mph ratio appears the same, but my fuel consumption is higher than that. Even with very careful driving and the aircon turned off I've never seen more than 36.2mpg.

TBH I'm not too bothered about the fuel consumption in isolation, it's the combination of that plus the thrashy feel on the motorway and the knowledge that the engine is often doing twice the revolutions it really needs to be doing for long periods of time. I'm "mechanically sympathetic" and it just feels wrong.

That and the fact I keep going for 5th only to realise I'm actually trying to change up into 7th, which isn't there....

Changing the final drive ratio is essentially what I'm trying to do. If that means that on A-roads it essentially becomes a 5-speed, that's fine - no worse than any other car I've ever owned. I'd like the 6th to be more akin to an overdrive. The reason I'm asking about 1.6 Multijet diesels is because, if the gearboxes are a like-for-like swap, I reckon it would probably be quicker and easier to replace the whole unit (probably by sourcing one from a written-off vehicle) than to remove, internally modify and then refit the one that I have. But it all depends on whether they're a direct swap.

I'd rather not go down the taller tyres route as the suspension is already on the soft side for me. It may be getting a handling kit at some stage, which is something for another Thread.

 

[Tad-bravo]

Why "obviously"?? The engines may be completely different but that's not to say they aren't designed to fit a common gearbox.

Does anyone actually know, for definite, whether the boxes are common to all Bravo 198s (albeit with different internal ratios) or entirely different depending on the engine?

It's obvious because the engines are completely different, even if by some miracle the gears and ratios are the same (which they're definitly not), the shell of the box can't be the same.

If you want to be sure, search for the part numbers for both models and see if they're the same.

 

[superunknown]

It's obvious because the engines are completely different, even if by some miracle the gears and ratios are the same (which they're definitly not), the shell of the box can't be the same.

If you want to be sure, search for the part numbers for both models and see if they're the same.

I still don't follow your logic. There's nothing to stop the gearbox outer casings from being the same, but with different ratios internally. It makes sense for car manufacturers to do this, as it means that the mounts etc. can be standardised for the car, rather than a custom fit for each power plant. Provided the interface between engine and box is designed to be common, the rest of the engine can be completely different and it won't matter.

Here is an example of such gearbox interchangability from one of the Alfa forums: http://www.alfaowner.com/Forum/alfa...2-fiat-bravo-brava-gearbox-compatibility.html

Note that the linked thread refers to swapping boxes between different types of vehicle. I'd just like to swap between one Bravo and another, which I don't think is half as "obviously" impossible as you make out, or indeed an unreasonable topic to raise.

Part numbers won't necessarily help, because internally the gearboxes WILL be different, and I would presume they'd be numbered as such to avoid confusion. But that's the whole point, I want a different box with a different set of gear ratios. What I'm trying to establish is whether the mounts, linkages, bellhousing, clutch etc. are all common across the range or not. Because if there's more in common than not, a swap would be a lot easier than trying to redesign from scratch the internals of the gearbox I currently have.

 

[Tad-bravo]

The thread you linked is about interchanging the gearbox from the same engine shared by two different cars. It's from the same type of engine that's why the transplant is possible.

You are asking about using the gearbox off a DIESEL engine on a PETROL one, it's not the same thing.

I'm gonna stop repeating myself now, good luck.

 

[lewey]

I've got a 150 T-Jet Dynamic.

I'm doing a lot of motorway driving at the moment and the 6-speed box is a let down. 6th is exactly the same as 5th would have been in a 5-speed car. In other words, the extra gear has been used to squeeze the ratios together for acceleration rather than providing a longer-legged gear for motorway driving.

70mph = 3,000 revs. So I'm regularly pulling 4,000 rpm and can be doing so for miles on end (obviously this is when I'm abroad, Officer ). It's not doing my ears or my wallet any good.

Looking at the stats, it would appear that the box off a 1.6 Multijet diesel would give a much better spread of ratios, with a higher top gear. The final drive ratio is 3.35 rather than 4.18. Whilst in theory this might affect acceleration, in practice 1st gear is ridiculously low and I'm not sure the close ratios really help the 0-60 time when you have to keep disengaging the engine to change gear. What's lost in gear ratios would probably be saved in lack of shifts. And anyway, it's hardly a race car, is it?

SO.... the big question. Is this do-able? Are the boxes a straight swap? Any idea of timescales or costs??

The reason you have 6 speeds is that the M20 gearbox you have due to having one of the heavier FIATs is only available from FPT (FIAT Power Train) as a 6 speed. There are no other reasons IMO. If FIAT would have been able to get/make a suitable 5 speed for the right cost, I bet they would. This goes for all the FIAT/Abarth T-Jet cars that have 6 speeds. The lightest/ lower torque combinations can get away with the power ratings of the older (and more suitable) C510 5 speed box (Abarth 500, GP 120).

The problem you have is that the other 5 speed gearboxes used in the Bravo which have gearing you might like more cannot be fitted to the T-Jet FIRE block. This is because the gearbox types used were never fitted to any FIRE engine in any vehicle I know and therefore a bellhousing to make it happen is not available. As the FIRE engine was traditionally the lower power engine type in older cars, there are very few options that can take the torque in your heavy car anyway. If you were talking about a 6 speed Abarth GP, I would have said you could get a C510 from a T-Jet 120 and bob's your uncle (probably not reliable if you drive hard though).

I have a Bravo 150 engine in my Uno and use a 1.3 MultiJet GP variant of the C510 gearbox with a T-Jet bellhousing. I get away running 180 lb/ft torque with plans for well over 200 because the car is 850Kg. 2500RPM in 5th sees 70MPH. With a a box from a Bravo JTD, it could be under 2000!

 

[RobW]

That's interesting - the rev:mph ratio appears the same, but my fuel consumption is higher than that. Even with very careful driving and the aircon turned off I've never seen more than 36.2mpg.

TBH I'm not too bothered about the fuel consumption in isolation, it's the combination of that plus the thrashy feel on the motorway and the knowledge that the engine is often doing twice the revolutions it really needs to be doing for long periods of time. I'm "mechanically sympathetic" and it just feels wrong.

That and the fact I keep going for 5th only to realise I'm actually trying to change up into 7th, which isn't there....

Changing the final drive ratio is essentially what I'm trying to do. If that means that on A-roads it essentially becomes a 5-speed, that's fine - no worse than any other car I've ever owned. I'd like the 6th to be more akin to an overdrive. The reason I'm asking about 1.6 Multijet diesels is because, if the gearboxes are a like-for-like swap, I reckon it would probably be quicker and easier to replace the whole unit (probably by sourcing one from a written-off vehicle) than to remove, internally modify and then refit the one that I have. But it all depends on whether they're a direct swap.

I'd rather not go down the taller tyres route as the suspension is already on the soft side for me. It may be getting a handling kit at some stage, which is something for another Thread.

Ok a few more tips

Use premium fuel
Get the car lowered
Check what tyres are fitted, some have more rolling resistance

 

[TJetPete]

Ok a few more tips

Use premium fuel
Get the car lowered
Check what tyres are fitted, some have more rolling resistance

Good advice I've noticed that Premium fuel really does make a difference! I had to put a tank of normal fuel in yesterday and it's really not lasting

 

[RobW]

Good advice I've noticed that Premium fuel really does make a difference! I had to put a tank of normal fuel in yesterday and it's really not lasting

Tests prove 10% increase in fuel economy for premium unleaded, and it is not 10% higher cost; therefore standard unleaded is the most expensive

 

[superunknown]

Ok a few more tips

Use premium fuel
Get the car lowered
Check what tyres are fitted, some have more rolling resistance

Thanks.

It's normally fed 97-octane Shell V-Power. I might still have some bottles of Millers VSP I could add as well, though? :devil:

Lowering is on the cards as part of a general suspension upgrade, though I don't want to be scraping the nose on speed bumps so it won't be THAT low.

The tyres are the original factory fitted items (Continentals?).

The key aim of the proposed gearbox swap is to make the motorway driving experience more relaxing. As said before I don't mind high fuel consumption if the car is worth it (I have owned some fairly loony cars in my time), but I do object if the overall driving experience is also less than satisfactory.

I suspect the 1.4 petrol engines have a 6-speed close ratio box simply because they're a bit puny for the weight of the car, and all the power is at the top end of the rev range. However I don't need to drag a caravan up a 1-in-3 hill, so 1st doesn't need to be quite as low as it is, and if I want to accelerate sharply when driving in top I don't mind changing down. It's a gearbox, that's what it's for.

At the moment it will still pull away quite nicely when floored in top at 4000rpm, which is a neat party trick but actually smacks of a final drive ratio which is waaaayyyy too low. I'm happy having 6 speeds, I'd just like them all to be a bit higher. The 1.6 Multijet diesel box appeared to offer a solution (the identical reverse ratio of 1:3.545 points to some shared ancestry) but if we're sure they're truly incompatible then I'm back to swapping the Bravo for something else entirely.

I notice that no-one has categorically stated yet that the mounts, bellhousing etc. for a 6-speed 150 T-Jet gearbox are definitely different to those for a 6-speed 1.6 Multijet diesel. [I know the ratios are different: that's the point.] Lewey, your answer is very comprehensive but if I understand it correctly you're talking about substituting a 5-speed box for the 6-speed? That's not what I'm trying to do.

I understand that, if it is possible, changing the car in this way would affect acceleration and probably reduce the car to 5 gears on A-roads, as 6th would be too high to use. That would be fine by me, if not everyone else's cup of tea.

 

[enor]

Where's the torque peak of your petrol engine? Be warned that the ratios in the 1,6 MJT box are VERY high... 2000 rpm in 6th gear equals 120 km/h (75 MPH); and at those speeds wind resistance is pretty substantial - my 1,6 MJT uses 50% more fuel at 120 km/h than it does at 80 km/h which says a fair bit about the power needed to keep up with the wind.

Do you think your petrol engine is up for delivering that kind of output at such low revs? I am personally having a very hard time believing that....

 

[cainy1989]

I get about 380 miles to a tank out of my 1.4 non turbo, thats using stock 95RON, put the 97RON in and I get 420, pushed it to 475 using the shell Vpower stuff, just shows how much you do actually save

 

[lewey]

I notice that no-one has categorically stated yet that the mounts, bellhousing etc. for a 6-speed 150 T-Jet gearbox are definitely different to those for a 6-speed 1.6 Multijet diesel. [I know the ratios are different: that's the point.] Lewey, your answer is very comprehensive but if I understand it correctly you're talking about substituting a 5-speed box for the 6-speed? That's not what I'm trying to do.

Where I said "...other 5 speed gearboxes used in the Bravo...", I meant 6, but could have just said "gearboxes" really. I did answer your question in my answer. No suitable bellhousing exists to mate the other (all are a different type) gearboxes that fit to a Bravo to a T-Jet. That may change if FIAT decide to use that gearbox on a future FIRE engine on some upcoming model, but considering the lifespan of the engine, that is very unlikely IMO.