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

[balmy]

No. The same revs almost exactly.

[The Breaker]

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

[david painter]

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.
Dave

[PHUQ]

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+.

[marina12]

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.

[locost_bryan]

These are the standard Marina diff ratios :-

Annotation 2022-11-01 145057.png (3.81 KiB) Viewed 6935 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 (13.79 KiB) Viewed 6938 times

[Mandator]

OK Left field.
But years ago Commercial operators used to fit auxiliary boxes in the transmission to lower the revs at the diff (or indeed speed up by reversing the box). The auxiliary would be engaged by a switch on the dash, so presume electric solenoid engaged?
Presumably this was before, or a cheaper alternative to, Eaton 2 Speed axles.
Could this be achieved on a Marina without cutting chunks out of the Trans tunnel or floor?

[KwS]

OK Left field.
But years ago Commercial operators used to fit auxiliary boxes in the transmission to lower the revs at the diff (or indeed speed up by reversing the box). The auxiliary would be engaged by a switch on the dash, so presume electric solenoid engaged?
Presumably this was before, or a cheaper alternative to, Eaton 2 Speed axles.
Could this be achieved on a Marina without cutting chunks out of the Trans tunnel or floor?

I imagine they're fairly large units. If you've been under a Marina recently, there is stuff all room for anything else under there without cutting. High revs in a classic like the Marina are kinda just something that I've accepted as part of the charm.

[MarinaCoupe]

You have to be careful with 5-speed gearboxes. Some like the Ford Type 9 has a true overdrive 5th gear overdriven by 18%. The 4th gear is 1:1.

Others like say the Suzuki gearbox has 5 gears, but 5th is 1:1 and there is no overdrive.

[david painter]

It is possible to obtain the above but were very rare i think some early Volvos used a overdrive in the propshaft I think it would be a lot of work and be easier to install a overdrive 5 speed gearbox or something like a Dolomite or Spitfire OD box. Whatever route you take there will likely be some cutting and fabrication involved
Dave