Trading that Automatic for a Manual Transmission
Improved fuel economy and improved performance are two good reasons why a car owner could decide to replace their automatic transmission with a manual one. The actual process of doing that conversion can be relatively simple or quite complicated depending on what the details for your specific vehicle. Factors which have an effect on this are the age of the vehicle, whether a manual transmission was ever offered in the vehicle, if the intent is to go with a transmission that was standard or if the desire is to really increase performance with a different one than the car ever came with.
Older rear wheel drive cars or trucks probably provide the easiest patient to deal with. The hot-rodding community has been swapping transmission and engine combinations around since the '50's, and it is relatively common to decide to drop in the latest six-speed from a 'Vette or Camero into a number of different vehicles. Even taking out the old automatic and replacing it with the appropriate manual transmission for that car is just a matter of researching the needed parts, collecting them and digging in.
Cars with automatic transmissions come with a light-weight 'flex plate' which provides the bolted mechanical connection between the engine and transmission. Switching to a manual transmission requires that the flywheel for a manual be installed in place of that flexplate. This is necessary for any conversion of this type, since the flywheel provides the mounting provisions for the clutch - the manually operated disconnect device needed for a manual transmission. The entire clutch linkage, pedal and associated brackets are also required to support the conversion. Another complication in most cars id that while the automatic transmission makes the connection with the flex-plate, the nose or input shaft of the manual transmission requires that the rear end of the engine crankshaft have the necessary bearing installed. Some engines have cranks that are not machined for this possibility at all, which would drive an engine swap as well.
Newer vehicles with engine and transmission electronic control modules bring in a totally different level of conversion. The electronic signals that control shift behavior and coordinate the engine calibration with that of the transmission are often different, so specific investigation of these items is required even if you are simply replacing the automatic transaxle in your car with the manual one that was installed in the same model.
The opinion of this shade-tree mechanic (and automotive engineer/manager) is that if you are a performance enthusiast and are making the swap for those reasons, the effort is probably justified. If you are trying to squeeze a few more mpg out of your Chevy Cavalier and are considering this swap, you would probably be better off going out and selling or trading your automatic-equipped car for one that has the transmission that you prefer. I have described the main points and somewhat glossed over the details like tearing up the area under the steering column for the clutch pedal and linkage, etc.
But always remember that the devil is in those details.
