Part 11 Back Axle Decisions
The TR7 final drive falls into four categories:
1) The TR7 with the 4 speed gearbox and matching (Marina type) back axle -1976 to late 1979.
2) The TR7 with automatic transmission and early (Marina type) back axle -1976 to late 1979.
3) The TR7 with the 5 speed gearbox and heavy duty (SD1type) back axle -a small number in 1977 and then late 1979 to late 1981.
4) The TR7 with automatic transmission and heavy duty (SD1 type) back axle -late 1979 to late 1981.
Categories 1) and 2) are not suitable for the V8 conversion as they will not, reliably, transmit the power of the V8 to the rear wheels. I believe it has been done but I would not recommend it.
Category 3) is the usual option as the gearbox requires no modifications and the back axle casing is correct for the conversion.
Category 4) is the best option as far as the back axle is concerned, as it includes the best ratio (3.08:1) for the V8 conversion. The problem is that these cars are very rare. If you are lucky enough to find one, make sure that it has the original back axle.
Basic gearbox and back axle identification.
The following are the prefix letters and axle ratios for the various 4 and 5 speed gearbox/axle combinations.
|1)||4 speed manual||CG1||CG1||3.63:1
|2)||4 speed auto||027||CT||3.27:1
|3)||5 speed manual||CL1||CH1||3.90:1
|3a)||5 speed manual||CL1||CL1||3.45:1*
|4)||5 speed auto||027M and 076N||CK1||3.08:1
Note*: Some late cars destined for Europe, USA and Canada (and possibly some UK cars) have an axle ratio of 3.45:1 with the 5 speed manual gearbox, however I have few details on this – can anyone help with any information re VIN numbers for this option?
When converting to V8 power, the output of the power unit increases from 105bhp (TR7) to at least 155bhp (SD1V8). With this increase in power, and therefore torque, it is possible to raise the final drive ratio without unduly compromising acceleration.
The obvious parallel to this is the genuine TR8. In UK spec trim the TR8 has an output of 152bhp @ 4750rev/min, an acceleration time 0-60mph. of 7.8secs and a top speed of around 120mph in fifth gear. As the V8 engine, in standard configuration, is limited to 5500rev/min, a higher axle ratio is required, thus using the3.08:1 final drive gives the TR8 a road speed of 27.6mph per 1000revs, which at maximum torque equates to about 130mph less losses due to mechanical constraints. Using the original TR7 axle with the 3.90:1 final drive, a road speed of 20.8mph per 1000revs or about 100mph could be achieved. Acceleration would be much better, but fuel consumption, top speed, engine wear and the gearbox would suffer. So the decision to use the higher 3.08:1 ratio is a good compromise. A further, even higher ratio of 2.84:1 as found in the Rover Vitesse would make the V8 a great cruising car but the acceleration would suffer if a standard tune engine was being used.