Diff Question

[rb1971]

I'm running an S38 (which I thought was a b38 but now I think may actually be a b36) through a 280 (pretty sure it's a 280) coupled to an LSD 3.45 side loader. The car runs great, but I can't get into 5th anywhere close to something that looks like highway legal speeds in the US.

Anyone done this or a similar swap and have any ideas for a diff? I made the decision a while back to stick with the side loader set-up, at least for now. Seems like a 4.11 would be too high - has anyone tried a 3.90 in a similar set-up?

Any thoughts appreciated.

 

[M5dCS]

diff ratios

The M5 used a 3.93 diff ratio, which when you figure in the overdrive of 5th gear, makes it come out to 3.15. I'm using a non overdrive trans with a 3.07 diff so my 5th gear is fairly close to the M5 gearing. Coupeguy did have a 3.63 diff with limited slip for sale a while back
that I traded to him, that's about the lowest ratio that I know of that is fairly available for our side loader diffs. Metric Mechanic sells a modified diff/trailing arm mount that will allow you to use the rear loader diffs and then a lot more ratios are available to chose from.
Good Luck, Leroy

 

[rb1971]

Thanks. So I take it swapping to a non-OD 5-speed and keeping the 3.45 would be overkill as well.

Since the car is definitely fun to drive as-is, I should probably wait a while to make this upgrade, but I do most of my driving in SF so would love to fix the ratios. Maybe next stop is to call Korman and see if they can swap the gears out for something higher (and up the lockup in the process).

 

[M5dCS]

gears

Somebody, I don't remember who, once told me that some of the later model gears and limited slip units will work in our sideloader cases. I seem to recall that they were talking about 735 diffs. Some of the 735 diffs are the same as 535 diffs, but some are the same as the M5, and by that I mean a larger diameter ring gear. This requires some very precise setup and Korman would be one of the places that could set it up properly. Yes the 3.45
gears with a non-overdrive trans would be a little buzzy at freeway speeds but a 3.25 that came in the later 3.0CS's would probably be just fine, in fact, I will probably change my car out to the 3.25 once my car is finished and I have the S38 B36 engine.

 

[x_atlas0]

Don't forget to change the speedo. Actually, if you are running a 280, how are you driving the speedo?

As for the diff internals, I really doubt the internals from a later front-loader would work in the side loader. Just from looking at the two cases, the dimensions and connection points inside the unit are vastly different. Medium case front loader diffs are very cheap, though, so it would be a cheap experiment to get one and check.

 

[rb1971]

There are a few companies that sell an electric-to-mechanical conversion unit, so there is a box that sits behind the dash and drives the speedo cable. Kind of a neat trick actually, and you can adjust the settings in the box to accommodate different gear ratios, etc.

 

[x_atlas0]

There are a few companies that sell an electric-to-mechanical conversion unit, so there is a box that sits behind the dash and drives the speedo cable. Kind of a neat trick actually, and you can adjust the settings in the box to accommodate different gear ratios, etc.

I've heard of those before, but I didn't know anyone who actually had one on their coupe. Nice to hear it works well, and the ability to adjust for different ratios is very nice.