Project 318ti eta

[CirrusSR22]

So I've had a brand new 318ti 3.45 open differential laying around for a number of years. The other day I got to thinking if putting the 3.45 diff into my automatic 318ti (4.44 diff) would help my freeway fuel economy significantly without killing acceleration. Yesterday I ran the numbers in a gearing calculator and found out the 3.45 would drop my cruising RPM at 70mph from 3000 to 2350. Sounds good as my all time high MPG figure (38.1 mpg) was done at 60mph which is about 2550 RPM. My big concern with swapping in the diff was with acceleration. I'm not too concerned about quick acceleration, but more was worried about lugging the engine.

Today I thought I'd give it a try. My results.....
Acceleration: Not nearly as bad as I thought it would be! Before the swap I did numerous starts in 2nd gear so I could get the feel of it. I was really hoping it wouldn't be like starting in 2nd gear every time. The 3.45 is slower off the line than the 4.44 diff, but the car doesn't lug at all. It still feels fairly quick if I push it. I think 1st is short enough in this car that the difference isn't too large.

Crusing: 70 mph does sit right at 2350 in 4th (top gear) Hopefully that's not too low and I'm on the back side of the "efficiency curve" (if that exists). Only a good road trip will tell.

Another plus is the noise level. My old diff always had a little whine, and this new one is silent as can be. The freeway noise of the 318ti has been bothering me lately. Essentially I'm getting old I want this thing quiet, comfortable and efficient.

[thesk8nmidget]

very nice i swapped from a 3.45 to a 2.79 and it was such an improvement for the highway cruising!

[PettitWC]

nice swap and great gas mileage

[paddyhart]

any update on the mpg?

[clex2]

Originally Posted by thesk8nmidget

very nice i swapped from a 3.45 to a 2.79 and it was such an improvement for the highway cruising!

Josh,


Do you still have your 3.45 lying around and if so, do you want to sell it?

[thesk8nmidget]

Originally Posted by clex2

Josh,


Do you still have your 3.45 lying around and if so, do you want to sell it?

yeah i do
but i did the swap on my last ti. so i only have an open diff sitting around.


my club sport still has the stock lsd in it