BMW: 1986 318i - high revving, ring and pinion gear, driveshaft

Question
Hi Josh,

I see you have helped many people with their BMW propblems, I
hope you can help me.

I recently had my exhaust replaced and a new cat put in and the
"DIF" replaced. However, since these have been changed I
noticed that when travelling at 80mph the revs ar running at
about 5500. I tried the take the car to 90mph and the revs
nearly went into the red - is this normal?

I look forward to your help.

cheers
alan  

Answer
Alan,
 The CAT should have no bearing on this issue, the "DIF" could.  I'm assuming you are talking about the Differential or "Rear-end".  This is where the driveshaft connects, and transfers power to the wheel(s) via a ring and pinion gear.  When you had this replaced, did you just now notice the change in speed/rpm?  Before the Diff was replaced were you able to drive at those speeds with a lower RPM?

 If this is the case, whoever changed your DIF put in the wrong ratio.  I think 3.91:1 is standard for the 318i manual transmission, but the M3 and 318i (automatic) were available with a 4.10:1 ratio.  These mean that the driveshaft will spin 3.91 times for every ONE turn of the wheels.  So if you have a higher ratio then the engine/driveshaft is able to spin higher.

 An easy way to think of this is a bicycle with gears.  If you put the bicycle into a LOW gear, you spin the pedals alot, but you dont go very fast, but it's REALLY easy to pedal.  On the oposite end of the spectrum you can put a it into a HIGH gear, where you are pedaling at a low RPM, but the rear tire is spinning really fast, but each pedal stroke is hard to push.  This is the game of gearing.  Some engines dont produce very much power, so they use gearing to help them out.  It sounds like someone replaced the differential with a 4.10:1 from an automatic (providing your car has a manual transmission).

 So If I were you I would slide under the car and check out the differential.  There should be numbers stamped on the case like "43:11" or "41:10", and those are a 3.91 and 4.10 respectively.  (divide the first number by the second: 43/11= 3.91) Those numbers are the tooth-count on the ring gear and tooth cound on the pinion gear.  43teeth on ring gear and 11 teeth on the pinion.

 Sounds like someone made a mistake when they found a replacement for you.

 Hope this helps,
 Josh