Final drive and power issue - Question

[Chuckcruzin1986]

On my 1986 325es, the final drive is 2.79 and I was thinking of swapping out to a 3.79.

A good friend of mine who is technically inclined + plus + said that with the swap out I would have the equivalent of about 20% more horsepower, faster acceleration through-out the full run from 0-100 mph on the track :)

I am not saying he is wrong, but I simply can't get my head around this and here is why:

Lets say I am in 2nd gear with the 2.79 final drive and running 200 hp and 200 foot pounds of torque at 5000 rpm.

Now if I had the 3.79 and lets say in 3rd gear, running at 5000 rpm, wouldn't I have basically the same acceleration?

What am I missing?

Chuck

[jlevie]

What you have to keep in mind is the torque multiplication of the final drive.

[NJ2.5rser]

Think of how the gears on a mountain bike operate (but reversed?) When you have a smaller sized gear (higher number on bike,2.79 diff) with less teeth you'll have less power upon initial acceleration however you will be able to reach a higher speed. When your your in a bigger gear (say 1st on the bike, which has more teeth, (3.79 Diff)) when you start pedaling you can take off like a bat out of hell but your top end is limited forcing you to switch to the next gear.
The mechanics behind differentials are still confusing to me after 7+ years so using an example of riding a bike or such is how I compare it. Hope it helps.

[The Dark Side of Will]

Lets say I am in 2nd gear with the 2.79 final drive and running 200 hp and 200 foot pounds of torque at 5000 rpm.

Now if I had the 3.79 and lets say in 3rd gear, running at 5000 rpm, wouldn't I have basically the same acceleration?

(Without looking up the specific gear ratios) This is correct.

You'll have better in-gear acceleration, but each gear will run out at a lower speed. Because of the Eta's low RPM powerband, you may actually get slower overall. This has been demonstrated many many times by guys who put gears in their '80's 5.0 Mustangs. With the engine's low RPM powerband, they actually got slower overall.

You'll also take a gas mileage hit.