mdshaw
Reader
1/25/21 9:11 p.m.
I'm helping my son to transplant a turbo K24 in his 318is. He, like me loves Honda engines & cars but also rwd's & e30's. We had a really nice white 318is years ago, a 540i & a 318ti.
I would like to have an idea what it would take to keep from spewing a diff & axle parts. I know we need a medium case diff & can swap the output stub axles. Do we need larger axles?
He sold his K20 swapped 92 hatch to have the $ for this.
We have a red 318is with a frozen motor, a JDM K24A, a G260 transmission & a driveshaft. Waiting on the TF-Works parts.
The K24A will be getting a 50 deg vtc gear, K20 oil pump but stock internals.
Will be lots of work but will be a fun car.
The 318 most likely has a small case diff, get a medium case from a 325is e30.
325is have a 3.73 final ratio. If the tag has an "s" then its an lsd. You can also use one from a z3 6 cyl. 318s usually have 4.10 FR.
The axles will also need to be changed to the medium case ones.
Here is some more info:
e30 differential wiki
Small case on left, medium on right:
Can a large case differential out of a 5 or 7 series be made to fit into an e30? They're quite strong.
In reply to mdshaw :
Swap in the Jaguar IRS. The DANA it uses easily handles a lot more than that.
It comes in a whole variety of widths starting at 50 inches.
I thought people were going Ford 8.8 to handle powerz in an e30.
Maybe I'm making that up....
When there's a way to do it with bolt in parts that retains the existing suspension, it doesn't make sense to rip it all out for something that has to be adapted from a different make of car.
JBinMD
New Reader
1/26/21 1:35 a.m.
Streetwiseguy said:
Skinny hard tires.
... and/or a very weak, slipping clutch.
Plenty of people seem to do it on a medium case diff, but I think the key is that you can't fit much tire under the back of an E30. I'm not sure what the high power racers do, but I don't remember diff or axle failures really being a concern even with a lot of power. Not like a 944 or something. The medium case diff was used with the S54 in the last of the Z3 M cars if memory serves; 320ish horsepower and a lot more tire stock than the E30. I'm not sure if they had different axles.
You can also swap the Z3 M diff covers onto any medium case diff - they have much larger fins on them and can help keep that diff cool. They are as good as you get without external cooling.
If you're in the midwest, I have a 318is and might help you rehome the frozen engine and the stock G240 transmission.... 
Check with the stage rally guys. I think some of them have moved to Supra diffs or at least Supra internals in BMW diffs.
If you want to stick with BMW, the large case diff that came on the E28 M5 and euro 535 are known for handling power. They also have a better 3.91 ratio than most of the medium case diffs. Those diffs are pretty expensive though and you won't find any in a junkyard.
mdshaw
Reader
1/26/21 7:23 a.m.
In reply to gearheadE30 :
We're in the Florida Panhandle. The motor, transmission & all of the other bits will need a new home for sure.
Tom Suddard
Director of Marketing & Digital Assets
1/26/21 7:30 a.m.
On a medium case diff... lots of spares! I've never owned a swapped E30, but have watched a friend constantly shredding parts in his S52 autocross car. I'm not sure what the solution is but I don't think it's stock parts.
Check out Zach33 on youtube. He has an LS1 swaped E30 with over 20k track miles on it. Runs slicks. I believe he's running 275 hoosiers all around on stock 4 lug hubs and stock axles/diff.
A lot will depend on the amount of power you plan to make and how much of that you will try and put down, and how quickly. For a mild setup on a street tire I think one of the larger/stronger BMW diffs will work. Trying to put more than 500whp down on slicks/road race tires will be a different story.
ojannen said:
If you want to stick with BMW, the large case diff that came on the E28 M5 and euro 535 are known for handling power. They also have a better 3.91 ratio than most of the medium case diffs. Those diffs are pretty expensive though and you won't find any in a junkyard.
They were also used in e32 7 series cars, so not terribly hard to find (although those may not have the most ideal gear ratio.)
400hp is no big deal with a medium case.
How much torque is probably the better question ...
I still maintain that a medium case is the best route.
In reply to gearheadE30 :
One of the Columbus autocross guys loved E30s because you could fit a lot of tire under them for the car's size/weight. Like to say he was running a square 275 setup.
With the amount of boost he was running, by memory headgaskets were an issue, but not the drivetrain. This was 10+ years ago so memory is hazy.
I'd start with a factory style medium case diff and a more solid mount. Either an aftermarket secondary diff mount or swap an e36 dual ear cover on.
On a bit of a tangent, does adding horsepower to an E30 result in the same kind of unibody destruction around the diff as in an E36 or 46?
In reply to Streetwiseguy :
No, completely different subframe arrangement compared to e36/e46.
+1 for get a medium case diff, since there should be stuff to bolt in and thats the same as the e36 m3.
Large case diffs are in all the 7 series I think, I know I had one in my 750 challenge car and I swapped to an LSD unit from a junkyard as well. But doubtful you'll be able to bolt up axles and hubs etc.
cmcgregor (Forum Supporter) said:
Just copy whatever this guy did https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/my-garage-imaginarium/75671/page5/
Edited because I got curious and went digging. He's using a large case with an e46 rear cover and a ton of reinforcement, but is somewhere north of 1000hp. Changed the link to page 5 where there's some discussion.
