The table below, kindly reproduced from Terry Griffin, lists some possible ratios you may find.
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So if you are looking at buying a replacement box, you may be able to work out the basic types, but if someone has changed the ratios, without looking inside and counting teeth you may be in for a surprise, or may not know what you have in your car at the moment? Ever wondered why someone in the same car may get up hills easier, what gears ratios do they have?
Some gear boxes have interchangeable parts which can lead to confusion when buying a gear box. On the three speed box there are three different tops or gear levers (ignoring the length of lever) which are all interchangeable. But imagine the confusion if you buy a 3 speed box with the wrong cover with the reverse in the top left and the gearbox with reverse in top right.
A similar confusion exists with the 4 speed box where, whilst the 4 speed constant-mesh has its own top, the 2 and 3 sync boxes have interchangeable tops. This interchangeable top is useful if you are just looking to buy a top, but if you are buying a 4 speed box with sync, do not be fooled by the markings on the top, take the top off and check all the sync gears. The number stamped on the top cover in front of SYN indicates the number of syncs, but it may not be the original cover.
If you are hoping to buy a 4 speed box with sync on 2nd, 3rd and 4th gears, take the top off and have a look inside. The sync gears are easy to spot, there could be 2 or 3 depending on which version you are looking at.
See image below, courtesy of CA7C, (with selector removed) - the sync are the straight teeth rings, 3 rings so sync on 2nd, 3rd and 4th in this example.
Returning
to the question of “How many different gear boxes are there?” taking into
account Terry’s table with 6 different ratios for 3 speed boxes and 4 speed
boxes the number may well be nearer 50 with combinations of numbers of gears,
gear box tops and prop shafts. I have also read that it is possible to convert
the reverse gear on a 4 speed box into a 5th gear! But as any gear box you buy
may well have been tweaked over the years, so unless buying form a known seller
in addition to asking what the condition of the gear box is and having a look
inside, ask what the ratios are? What is the best gear box or ratios for my car? That is a very big question and in discussions with other A7 owners you will get a lot of different answers as it also depends not only on how you drive the car but what you use it for. For a standard road car, unless you are very unhappy, leave it as Austin intended. Some people have swapped 3 speed crash to 4 speed sync but this does require a number of changes to the car, some of which are quite drastic. Upgrading from a 2-sync 4-speed box to a 3-sync 4-speed box is very straight forward and may already have been done to your car.
In a road car, the ratios will be more difficult to assess any improvement and it is best to find someone who has already changed the ratios in a similar car and has wanted to seek the same improvements. Note top gear is still top gear and the car will not have a higher top speed, it is all to do with how quickly you get to top and when you have to change gear. In simple terms, on the flat, a closer ratio is better, on hills not so good.
If you have rebuilt a gear box changing the ratios is quite straight forward and the alternative ratios are available form Andrew Bird who, if you have not rebuilt gear boxes, will provide this service and can be found at Austin Seven Gear Box Services.
Douglas Alderson DA7C