Diff Ratio

Discussion in 'SN95 4.6L Mustang Tech' started by Paul1981610, May 27, 2014.


  1. Paul1981610

    Paul1981610 Member

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    When counting rotations of drive shaft and wheel on a 01 gt does the entire rear need to be lifted because of he limited slip. I raised passenger side and got 2 1/4 driveshaft turns per wheel rotation no way that's right
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  2. Three50won

    Three50won Member

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    Raise both wheels off the ground. If you spin one wheel and notice the other wheel spinning in either the same direction or the opposite direction then you have a posi. So try your rotation count again. If you still are unsure, then try doing 10 rotations (for a posi), count the number of shaft rotations, then divide the number of shaft rotations by 10. So if you turn the wheel 10 times and the drive shaft turns 37 times, then you have 3.73s. Always make sure the tire ends in the exact same position it starts. If it is an open diff (one tire spins while the other does not spin at all) then you can do two tire spins, count then number of shaft rotations, then divide shaft rotations by 2 (number of tire rotations).

    If this still gives you an unsure answer then drive your car at 60 mph (on your speedometer) while noting the RPMs and timing yourself. See how long it takes you to travel one mile. If it takes 60 seconds then your speedo is correct. Post what the RPMs were and we can help you figure out the gear ratio. Also, do you have a stick shift or an auto?
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  3. Gearbanger 101

    Gearbanger 101 we love us some gays up here Super Mod

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    If it's an open diff, one tire spins in one direction, the other in another. If it's got a "Traction-Loc" (not Positrack....that GM speak :fuss: ) then both wheels will turn in the same direction. Even a severely trashed traction-loc or limited slip rear end will have both wheels going in the same direction with the rear end off the ground.

    The rest of your info regarding determining gear ratio is correct though.
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  4. Three50won

    Three50won Member

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    I could have sworn an open diff has one tire rotating while the other does not vs a limited slip where one tire rotates and the other either rotates the same direction or the opposite...when the engine is off. For instance, if the wheels spin in opposite directions while the engine is off, this means that when the car is running (engine and trans spinning), both wheels will have power (and both will spin in the same direction) although one will have less power than the other. An open diff has only one wheel spinning at all times (engine on or off). Correct me if I'm wrong but I could have sworn that's how it works.
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  5. Paul1981610

    Paul1981610 Member

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    3:55 is the answer. Lifted both sides 3 1/2 turns per 1 tire rotation. Both tires were spinning same direction does that mean traction loc
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  6. trinity_gt

    trinity_gt Advanced Member

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    In an LSD both wheels are mechanically connected via clutches (cone, plate or otherwise) or through a gear mechanism (Torsen). In any case, if the driveshaft is free to rotate, an LSD car will have both wheels turn in the same direction when one is turned by hand. An open-differential car may or may not (you may see both turn in the same direction, only one turn or one turn in the opposite direction) which has to do with friction, gear oil viscosity etc.

    If you lock the driveshaft (e.g. put the transmission in the 1st gear position) and tried this again, you'd find that when turning one wheel by hand the other will turn in the opposite direction. In an LSD car you'd find a considerable resistance to the rotation -- the driveshaft is locked so you're working against the limited slip preload friction -- but the other wheel would also turn in the opposite direction.

    The only difference between an LSD and an open differential is the degree to which the two axles are tied together. They both have differential action. Compare this to a spool, where both wheels get equal torque and always spin in the same direction but have no differential action so the tires scrub (and the car can be downright scary) in corners.

    Nope. An open differential car will theoretically transmit power to both wheels under ideal traction conditions. Torque is transmitted to both axles from the differential case via the cross shaft, spider and side gears. It's just that this mechanism is apt to take the easy way out and just have the spider gears "walk around" the side gears under conditions where both tires have differing traction. When conditions aren't ideal, the tire with the least amount of traction gets the torque. If an open-LSD car has one tire on ice and the other on a prepped drag surface, the ice-wheel will spin all day and the car won't go anywhere. This can even be affected by suspension loading and drivetrain torque reactions (which is why many cars spin the same tire in a straight line...)

    In an LSD car both wheels get torque, up to a point. The traction difference in a straight line or around certain corners is basically less than the torque bias tying the two axles together so you get a "two wheel peel." But when you go around corners, you get the wheels spinning at different speeds. Under conditions like this it's still possible to get a "one wheel peel" even with an LSD.

    The LSD is just distributing the torque to both wheels more evenly than an open diff. A car with an LSD in that ice/prepped surface scenario would move -- albeit slowly -- from a stop.
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  7. Three50won

    Three50won Member

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    Thanks for clearing that up. I appreciate it. I get soo confused when it comes to transmissions and differentials. I'll probably forget this all and fall back to the same line of thinking in a few days, lol! Thanks again!
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