Converting a 4 speed gearbox to a 5 speed (1800)

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Re: Converting a 4 speed gearbox to a 5 speed (1800)

Post by balmy » Thu Oct 27, 2022 10:48 pm

No. The same revs almost exactly.
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Re: Converting a 4 speed gearbox to a 5 speed (1800)

Post by The Breaker » Sun Oct 30, 2022 10:24 am

Is there an add on gear box availble that would give you a few higher gears

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Re: Converting a 4 speed gearbox to a 5 speed (1800)

Post by david painter » Sun Oct 30, 2022 2:07 pm

Th only bolt on gearbox options from the BL range would MGB 4 speed overdrive and Rover 2000SD1 LT77 5 Speed both as far as i am aware require modifications to the Transmission tunnel the other option is Triumph Dolomite 1850 and Spitfire 1500 single rail gearboxes again 4 speed overdrive these i understand need the 1st motion (primary input) shaft changing ant the Transmission tunnel modifying. By the time 5 speed gearboxes had become the norm BL had moved on to an almost total front wheel drive layout Jaguar and Land Rover a side the Commercial option would have been the 2000cc Sherpa petrol again basically an SDI 2000 Gearbox probably with different ratios. Going for a higher numerically lower final drive will reduce the revs per mile in all gears a 5-speed gearbox has a typical gear ratio of 0.90 to 1 If the existing 3.63 final drive was changed to 3.27 forth gear would be almost the same revs per mile as fifth gear on the standard 3.63 Final drive but would be more sluggish in the lower gears.

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Re: Converting a 4 speed gearbox to a 5 speed (1800)

Post by PHUQ » Sun Oct 30, 2022 8:25 pm

Gearboxes are- generally- fairly fixed units. You can't just bolt another gear in there any more than you can turn a 4 cylinder engine into a 5 cylinder by nailing an extra cylinder on the back!

An overdrive goes on the output end of a gearbox and gives you the option of either a "straight through" drive or engaging a higher ratio gear to "step up" the output speed. Basically it's a separate 2 speed gearbox (one speed being straight through) that goes on the back of the main gearbox. However, I guess to keep the length down, the overdrive versions of the Marina box (Triumph Dolomite/ Spitfire) are actually a different output shaft and tailhousing etc so it's much easier to swap the whole gearbox than try and fit an overdrive to the original gearbox. But you'll still need to make the transmission tunnel bigger to fit it in, have a custom shorter propshaft made etc. So no cheap or easy magic trick here either. A good Triumph overdrive box is probably £500+.
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Re: Converting a 4 speed gearbox to a 5 speed (1800)

Post by marina12 » Sun Oct 30, 2022 8:49 pm

Bit off topic. Never went down the diff/ gearbox route. Sort of lived with a 1300cc 50mph car with the potential to do up to 70 mph sparingly when their contemporaries seemed more capable at motorway speeds. Drove a 1800- The torque was great. By the time they ended things like 1300 5 speed MK1 Astras, VW Golfs and MK1 Fiestas all rather showed our cars’ heritage. They seemed happier at the slower speeds. Still great cars and never failed to get me to where I was going and on occasion helping their contemporaries that had ‘sat down’. Back on now.

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Re: Converting a 4 speed gearbox to a 5 speed (1800)

Post by locost_bryan » Tue Nov 01, 2022 12:04 am

These are the standard Marina diff ratios :-
Annotation 2022-11-01 145057.png
Annotation 2022-11-01 145057.png (3.81 KiB) Viewed 1577 times
These are the speeds for each combination. Changing from a 3.63 to a 3.27 drops the revs by 10%. As a comparison, I have shown the Ford Sierra 5th gear, which drops the revs by 18%.
Annotation 2022-11-01 125932.png
Annotation 2022-11-01 125932.png (13.79 KiB) Viewed 1580 times
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