I am starting on a ev conversation next month on a 78 mgb without the engine and transmission it weighs in at 1540 lbs
I am wanting to do a direct drive configuration if possible with a top speed of 85mph preferably 95mph since that’s just keeping up with traffic around here and a range of 270 miles I just had a few questions about hardware
...
what torque and would be required...
The requirement for torque depends on your requirement for performance, and assumes some specific gearing and tire size. The gearing in this case is presumably the stock final drive (3.909:1) and the tire size is presumably stock (165R14, or about 24.5" overall or 861 revolutions per mile). You can think of that in terms of actual performance (rate of acceleration), or by comparison with the original car.
The car will be heavier than it was originally. You have it at 1540 lbs without powertrain and
Automobile-Catalog has the curb weight (complete, with fuel) at 2339 pounds. It's hard to see even the MG boat anchor of and engine and iron-case transmission weighing 800 pounds, but maybe assume that the converted vehicle will weigh about the same 2340 lb (1062 kg).
The stock 1978 MGB (manual 4-speed transmission version for North America) had an engine which could produce (according to
Automobile-Catalog power and torque curves) more than 75 b-ft of torque over the entire engine speed range normally used in driving (1800 to 4500 RPM), and 63 HP (47 kW) as long as you shift to keep in it a suitable engine speed range for performance (4000 to 6000 RPM). That went through a transmission, so the torque was multiplied:
- 1st gear:
- 3.036:1, so 75 lb-ft is multiplied to 228 lb-ft
- 1800 to 4500 RPM engine speed corresponds to 11 to 28 MPH
- driveshaft speed for this engine speed is 593 to 1482 RPM
- 2nd gear:
- 2.167:1, so 75 lb-ft is multiplied to 163 lb-ft
- 1800 to 4500 RPM engine speed corresponds to 15 to 39 MPH
- driveshaft speed for this engine speed is 831 to 2077 RPM
- 3rd gear:
- 1.382:1, so 75 lb-ft is multiplied to 104 lb-ft
- 1800 to 4500 RPM engine speed corresponds to 24 to 61 MPH
- driveshaft speed for this engine speed is 1302 to 3256 RPM
- 4th gear:
- 1:1, so 75 lb-ft is unchanged
- 1800 to 4500 RPM engine speed corresponds to 34 to 85 MPH
- driveshaft speed is the same as engine speed (1800 to 4500 RPM)
So you can pick your road speed range and see how much motor torque you need (without any additional gearbox) to feed to the axle to get close (because the engine does up to 88 lb-ft) to matching the original car's performance at moderate engine speed... and the engine is stronger at higher engine speed.
Unfortunately, DC motor suppliers generally don't provide any performance specs in the range of speeds that you actually need... only from their maximum torque point and faster. You can optimistically assume that they can maintain that peak torque down to zero speed, perhaps without overheating.
With stock tire size and final drive ratio (3.909:1) 95 MPH corresponds to 5053 RPM.
