Gear ratio change?

[midlandtech]

I've been wanting to do this for awhile. Need to find the right price as I'm sure shipping would be expensive

[alexandre]

I'm running a 3.46 from an auto 325. PO's bill shows about 250$ for the diff from their local yard and 2h labor for install.

I love the setup, it fits the 330 motor and 6 speed beautifully - although I've never driven a 3.07 ZHP so I can't compare.

[BCS_ZHP]

EDIT: Didn't intend to insult anyone's intelligence with the below post, just decided to add it after there seemed to be some earlier questions in this thread on what a differential gear change would do and how it would affect the car's performance.


So our manual transmissions have the following gear ratios:
1-4.35, 2-2.5, 3-1.66, 4-1.23, 5-1.00, 6-0.85

Now couple that to your stock 3.07 differential gear ratio and you get final drive ratios (multiply each transmission gear ratio by the differential gear ratio) of:
1-13.35, 2-7.68, 3-5.10, 4-3.78, 5-3.07, 6-2.61
What this means is your engine has to make 13.35 rotations when in 1st gear to rotate your rear tire once, 7.68 engine rotations in 2nd gear to rotate your rear tire once, and so on.

Now do the same exercise for a 3.38 differential gear ratio which was in any 330 with an automatic:
1-14.70, 2-8.45, 3-5.61, 4-4.16, 5-3.38, 6-2.87

So after a swap to a lower differential gear ratio, you have to make more engine rotations in any of the 6 gears to get the same one rotation of your rear wheel. At any given speed, more engine rotations means you're higher up the rpm band. Climbing up the rpm band is where your HP and torque start to increase (there is a limit up near the top of the rpm band where both HP & torque gains flatten out).

So changing to a lower differential gear ratio changes your final drive ratio and should move you up the power and torque band sooner. How much you want to jump is up to you, some guys even go above the 4.00 ratio level to get even more power & torque sooner but as BP & others noted it negatively affects fuel economy.

On fuel economy, how much will it negatively affect it. All things being equal, a 3.38 or 3.46 differential gear ratio change should decrease your fuel economy about 10-12%. There are more intricate calculations you can use but try this one:

3.38 - 3.07 = 0.31, then 0.31 / 3.07 = 10.09%; 3.46 - 3.07 = 0.39, then 0.39 / 3.07 = 12.7%

And use the same for figuring out your new cruising speed rpms. If your car cruised at 55 mph at 2500 rpms, after the differential gear ratio change your rpms at 55 mph should be about 10% higher or around 2750-2800 rpms.

Hope that clears up some of the mystery behind a differential gear change. Drag racers do it all the time, we used to do it back in the day on the street with the 60's/70's muscle cars too.

And costs, I picked up a used 3.38 differential for $200 and new output seals & diff fluid from ECS for another $40. Just waiting for Stuart to get his lift installed so we can make this swap happen the easy way, he tells me its about a 2 hour evolution to swap the diff on our cars.

[az3579]

Also note that by cruising at higher RPMs, you're wearing your engine a slightly more accelerated rate as well. The difference between the stock diff and the lower ratio diff is negligible with daily driving, but it is still a difference nonetheless. This is one of the reasons why race engines' lifespans are measured in hours not by miles; they are constantly revving off the charts. A street car's engine will typically last longer the lower you keeps the revs on average. That's one reason why in the E30 world, the 325e's are slow as balls and only rev to 4.5k redline, but they last forever (mileage in the 300's and 400's is not unheard of). Same story with diesels; low revs but are bulletproof.

Personally, unless I'm at the track, I'd prefer to keep the revs as low as possible so 1) I don't have to visit the pump more often, and 2) my engine lasts longer when I'm not pushing it. Unless you auto-x frequently, I personally don't see too much of a benefit unless you don't care about either of those things. In that case, it sure is a helluva lot more fun with a (different) diff!

[derbo]

Reason I put it is was for LSD and the upp gear ratio. Its going to be funnnn

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[az3579]

Reason I put it is was for LSD and the upp gear ratio. Its going to be funnnn

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Well now that you have a douche buggy, you don't have to worry about wearing out your ZHP nearly as fast.
(self proclaimed douche buggy, before anyone bites my head off here!)

[derbo]

its okay. I love my douche buggy. Remember that article about Prius drivers and bmw drivers are the worst dbags. I bet I get double since I own both. Hahahah

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[az3579]

But you're neither... right? lol

[derbo]

But you're neither... right? lol

Two wrongs make a right?

two negatives together make a positive?

LOL

BP has good points on the gear ratio change. Higher RPM definitely will wear the engine faster. How much? Can't really say, but I highly doubt its enough to make a problem out of it.

[Hermes]

Engine wears out, put in S54. Problem solved