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900s have overdrive Posted by CMyles [Email] ![]() ![]() In Reply to: 1:1 ratio is as good as it gets, ZAAB ![]() |
The classic Saab 900 five speed transaxle DOES have overdrive in as much as one definition of OD is "higher output RPM than input RPM". In old sports cars (for instance) or my neighbor's Dodge truck, OD is provided by a separate unit added to the back of the transmission that's electrically engaged but many transmissions (and transaxles) have the OD gearing built in like our c900 five speeds. The purpose of overdrive, whichever form it takes, is to increase the RPM of the driveline between the engine and the final drive (the differential). In our 900s the clutch shaft is the "input" because it rotates at engine speed (with the clutch engaged) and the pinion shaft (which is also the mainshaft) is the "output" because it's the last element before the final drive ratio occurs. In fifth gear the lower sprocket and pinion shaft are locked together and rotate at the same speed BUT there is a ratio increase between the upper and lower sprockets so there is an overall increase between engine RPM and pinion shaft RPM, increased I/O ratio = overdrive. Fourth gear is nearly 1:1, it varies a little depending on which gears and primaries are in there. Saab made five different sets of primary sprockets numbered 4-8 with 4 having the least increase and 8 having the most. Since they are all interchangeable "Go fast" guys want the #8 set but they are hard to find.
posted by 198.233....
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