Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Pinion and Crownwheel changes

Hi All,
I was originally toying with a five speed gearbox for my Kent engined 4/4 but think the easier route may be to alter the differential ratios about to give the car more cruising legs. Currently the car has had the engine tweaked (Stage 3 head, Kent 123 cam, lightened flywheel) and puts out 106bhp. I can't find anything in the paperwork to suggest that the differential has ever been apart so I assume it still has its 4.1:1 differential from the factory. The car does 4000rpm in fourth at seventy mph (on the speedo).

There seem to be other pinion and crownwheel sets available from the likes of JB Sports Engineering etc, but can anyone in simple English explain by how much fitting a different pinion and crownwheel set will alter the ratios? Ultimately I'd like the car to cruise at 70mph doing something closer to 3000rpm. Is this realistic? I realise it will lose some acceleration in making the changes.

Someone please help the confused dunce

4000 revs at 70 = 17.5mph per 1000rpm which is the accepted measure in the UK. 17.5 is quite low. Top gear is conventionally 1:1 in 4 speeders.
At 4.1:1 when engine revs are 1000, wheel revs are 243.90 at 17.5mph
At 3.7:1 the wheel revs would be 270.27, an increase of 26.37 at 1000revs, so 10.8% which means at 4000 revs you would see a 10.8% increase on road speed, so 77.57mph.
Any lower than 3.5:1 you would feel the difference on acceleration. A five speed box - assuming fifth was an overdrive ratio and not 1:1 should give a similar outcome. In this situation, ovedrive fifth ratio's will be typically 0.87.
Going up a tyre size will also help, but you need to consult a tyre size comparison site to establish circumferences, and the difference is smaller that you will get with either of the above changes.

All that is very fag packet by the way!!!

I have my 3.73:1 diff innards stashed away in the garage. I don't know if the axles are still the same, my car is 2010 but let me know if you are interested.

Not the same axle, the one he wants is a Salisbury, yours will be a BTR.
See GoMog

Alternative ratios for the Salisbury will be pricey.

Snailmog,

In principle fitting a higher ratio crown wheel and pinion will give you the more relaxed motorway cruise you are seeking. With your fourth gear ratio you'll need about 3.055:1 for 3,000rpm at 70mph. However, if you stick to the four speed gearbox and change the back axle ratio, you need to remember that you will also raise all the other ratios as well. In particular, your first gear ratio will increase, possibly to the point that setting off on a steep incline may need excessive slipping of the clutch. Additionally, acceleration in second and third gear will be dulled and you may find motorway inclines will be a struggle in forth.

In the 1970s and 80s, when the Ford Kent cross flow engine was in everyday used, 3,500 to 3,750rpm at 70mph in top was the norm, so I suspect your 4,000rpm at 70mph was chosen by Morgan to give spritely in gear performance.

So, looking at DaveW's post and IMHO, I would think that keeping the back axle ratio standard and trying to source and fit a five speed box which keeps the same first 3 gear ratios, a 1:1 fourth, and with a 0.87:1 overdrive fifth would be a better solution.

I've considered both options for my 2 seater - i.e. Type 9 box or change axle ratios. I don't trial the car, but I would not want to aim for 3000 rpm at 70 in a X-flow with Morgan aerodynamics - it would lead to constant cog-swapping on motorways. OK, mine is pretty much standard performance.

The mods done above tend to raise the power output at higher revs, not low down torque, so the problem may actually be worse if too high a diff ratio is chosen...

It's a shame there isn't an overdrive available (together with the space for it to fit!).

Cheers,

After considering various options, I'm going down the 5 speed Type 9 route to replace my 4 speed box. The whole car needs a rebuild so it made sense to me to do this modification along with some others.

I've been following this thread with some interest, as I've contemplated similar modifications to my pre-crossflow 4/4.

I'd be very interested in hearing if anyone has actually tried installing a 3.7 gearset in a pre-crossflow car. This setup should yield 3428 RPM at 70 mph and 3094 RPM at 60. Will the stock 87 hp motor support this?

Thanks in advance for any replies,
Warren

Not that spec car, but I knew a crossflow four seater that regularly took a family of four and a trailer on holiday that axle that was probably about that ratio (+4 one). That car was ok.

Graham, it is high time that one of your British cottage industrialists relives the production of the"Running Hare" rear axle overdrive which was offered in your country in the early sixties! I am now looking for one for more than 40 years