I am considering putting in a SLP 12 bolt 4.11:1 rear axle. Would I gain torque (seems reasonable)? Does anyone know of a formula to calculate the torque I would gain?
Posted by 2002Z4CSS (Member # 1393) on :
Rear gear change should not effect the engine making more torque. What ever hp and torque your engine makes is all it will do and the gears will just change the rpm band where the hp and torque come in. If you have an automatic, a torque convertor will multiply torque. Posted by Bill Mason (Member # 1807) on :
Agree with Greg that a rear gear change will not affect the power and torque made by the engine. For that matter, neither will a torque converter.
However, IMHO, a rear gear change will affect the torque applied to the rear wheels....up to point.
At launch a higher numerical rear gear will apply more torque to the rear wheels than a lower numerical gear. In other words a 4:10 rear gear will apply more torque than a 3:42 rear gear to the rear wheels. The difference or increase in torque is the difference between the two gear ratios. A 4:10 gear will apply approximately 20% more torque to the rear wheels than a 3.42 rear gear.....at the same engine RPM and same transmission gear.
However, once you shift up through the transmission this torque multiplication is complicated by the gear ratios in the transmission. For the same transmission gear, the higher rear gear ratio will apply more torque to the rear wheels. It's just that you can vary the overall gear multiplication between the engine and the rear wheels by varying what gear the transmission is in at any given speed.
Also, at any given speed, the higher the rear gear ratio, the higher the engine speed for the same transmission gear. But, the engine speed issue can be mitigated by shifting to the next higher transmission gear. This transmission gear variablility is what I meant about torque multiplication at the rear wheels up to a point.
Posted by poSSum (Member # 119) on :
You won't see anything on a dyno. Some claim the numbers go down slightly as a quirk of the dyno, and you may loose some with the 12 bolt because everything is beefier.
What the 4.10's do is get you into the powerband quicker. The seat of the pants says it's like a new car. Posted by DaddySS (Member # 848) on :
Think of it as using a 20% longer handle to do work. The handle end travels farther and gives you more leverage. In this case, the engine will rotate 20% further to turn the rear wheels. Also if you look at your dyno graph, a 20% increase in RPM from 2500 is about 3000, so your torque number for that engine speed is what you will be applying to the rear wheels at the same speed that were traveling at 2500 before the gear swap.
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