Thread: 4.11 Diff Ratio Conundrum

Originally Posted by goingbush

Someone is pulling your leg. straight cut diff gears have not been produded for at least 110 years only some very early veteran cars had them

The 4.11 gears will be helical , most likely Ashcroft. May be Quafe ATB - but that wont be making any noise.

After testing, I am convinced that the diffs are 4.11 but as to straight cut, who knows and why would you do that? If I can get access to a fibre optic camera I could go in through the fill or drain plugs but the result would be academic as knowing would not make all that much difference except explaining some of the noise and vibrations. Both Quaife and Ashcroft are reputable so no real difference there. FWIW if I had my wish, it would be helical cut.

something to think about changing gear ratios (whether diff or transfer). My experience, I have a 130 puma 2.4 which is 1.22 (ish) HR. I run 315/75 (35s) and to compensate for the bigger tyres i fitted 4.11s. 4.11s reduced my gearing by 16% from 3.54s. With the larger tyres, the net effect was about 8% in the kph readout. It used to sit about 2400rpm at 110kph. I ran that for 5 or so years and i do at least 1 big trip a year with long driving days for the trip. I decided to change my HR ratio to 1.1:1 to bring the hwy rpm back to what the factory gearing and tyres was (about 2200 rpm at 110 kph). In my head, it would reduce engine noise and modest fuel economy. What transpired was farther from my expectation and i would not do it again. I robbed 9% torque multiplication from the driveline by dropping to 1.1:1. The engine now struggles to pushing a heavy 130 with 35s. It will cruise on the flat holding speed, but any hill that has a gradient or is on the long side the speed falls off real fast and i'm searching for 5th. The fuel economy is no better and in some respects, could be worse. i now have to get into the right foot moreso than before and the engine has to work harder to do the same amount of work at the wheels. Turns out, that 9% gearing is significant how the engine performs on the hwy to keep the 130 chugging along.

Weight is not as much of a problem with a 90, but if you feel the engine is on the lower end of the torque range a change to the gearing might drop you out of the fat of the torque band and have you hunting for a lower gear moreso than before. To every action, there is an equal and opposite reaction.

Also, the Ashcroft 4.11s use less tooth count to get their ratio, which results in louder gear noise. My gears, both being of the less tooth count approach, are noisy cruising on the hwy. I have found that the transfer ATB also has gear noise. Overall my driveline is noticably noisier than a stock set up.