How to fit a BMW IRS into a GT - Part 1

[jtb]

How to fit an IRS unit from an 87 BMW 325 into an Opel GT.



The good news: - You don't have to narrow the axles.
- The bolt pattern is 4 X 100mm, same as stock Opel.
- Many different gear ratios are available, and it’s easy to swap the pumpkin (diffy.). I have a 2:93 and a 4:10 Limited slip.
- There are only three mounting points: two on the moustache bar, and one on the pumpkin.
- IRS, Rear disc brakes, and limited slip.



The not-so-good news:
- There is some fabrication required. The BMW unit is too wide, so the mounting points need to be moved inward. (Alternately, I suppose you could do some kind of voodoo on the rear of the Opel rocker panel)
- The stock BMW springs are too long.
- You’ll have to run a FWD offset rim to compensate for the wider track of the Bimmer if you don’t want fender flares.
- If you want a lower-than-stock ride height, some additional fabrication is required.
- With the lower ride height, you don’t have a heck-of-a-lot of suspension travel. As my project is more of a racecar than a street car, a cushy ride is not a priority for me.


Step one - Get the BMW unit. When I pulled the BMW rear end in the junkyard, I made sure to keep the parking brake cables intact, and even pulled the BMW parking brake handle. First thing to do is drilling these large ugly holes in the moustache bar to move the attachment points inward to line up with the GT's box section (~ 36" Ctr to Ctr). See picture in previous post: http://www.opelgt.com/forums/performance-suspension-braking-upgrades/4892-shortening-half-shafts-irs-conversion.html

Step two, I took a 3” long piece of 1” dia. schedule 80 pipe. (Why this material? It’s what I had on hand, I’m sure you could use some nice DOM tube or something else) and bored it out so that I could snugly fit a piece of ¾” heater hose inside it. I referenced off the original BMW mounting point to position the 3” piece of pipe before welding (just some flat bar and clamps) Then I hacked off the original mounts.

Step three – Cut off the original spring perches, they’re going to have to move inward slightly (I think it was about 1 ¼” or 1 3/8”) to line up with the Opel. I made new perches out of some 12 gauge plate and 1” schedule 80 pipe (hey, this stuff comes in handy!). Then I capped off the cut ends of the moustache bar with some 12 ga plate. [NOTE – If you want to run coil-overs, I have some thoughts on this in step eight.]



Step Four – On to the Opel! I supported the car on a hoist (you could just as easily use jack stands), and dropped some plumb lines to the floor to and made reference marks of the location of the stock rear end. Then I pulled out the stock rear end and stuck the BMW unit underneath (supported on jack stands). I tried to line up the BMW unit as close as I could to the reference marks from the Opel (while compensating for the different wheel offset). This took a bloody long time. It should be close, but the final alignment will come later, so it doesn’t have to be perfect.
Then I scribed a line on the Opel floor up through the new BMW mounts. Drill a big hole in the Opel floor from underneath. This will be the home of the mount built in step five.
In picture four A, you can see three things:
1 – Cut off the Opel’s spring perch. I made a new one from (yup!) 1” schedule 80 pipe and a 5/8” Bolt (The bolt caps off the pipe and screws into the hole in the Opel body to hold it there for welding)
2 – There’s another piece of…you guessed it, 1” schedule 80 pipe hanging down through the new hole in the Opel’s floor. This hole goes into the box section of the Opel. More info on this mount is in step five.
3 – Oops! I wanted to tuck the moustache bar as high up into the Opel body as possible, so I had to ‘relieve’ the inner rocker panel here.

[jtb]

Step five – The body mount.
There are several things going on here. I wanted to isolate the moustache bar mount in rubber (thus the heater hose mentioned in step two) to avoid hard mounting and cracking something. I also wanted some fine adjustability in the location of the rear end, because those holes I drilled in the Opel were unlikely to be perfectly located. Also, I wanted to accommodate a crossbar, if needed, to really beef up the mounts by tying them together. This is the design my brother came up with (picture five A):

The ½” bolt is moveable within the pipe. When the alignment is perfect, the ½” bolt gets tightened up, then the two small holes in the upper plate are used to transfer punch to the main plate, which gets drilled and tapped, so that the upper plate can be bolted down to secure the location. I say weld it, but my brother likes bolts. Due to the slope of the Opel’s box section, I had to shim one end up by ¾”. Note the reinforcing plate underneath the car. Driver’s side shown. The yellow is the Opel box section (picture five B)

Picture five C is the passenger side as viewed from inside the car (Again, the crossbar is optional)


Step six - The pumpkin mount.
The location of the front of the pumpkin mount lines up almost exactly with the Opel’s beefy body section where the shocks mount. I took out the gas tank and drilled a few holes in the Opel body to facilitate plug welding the pumpkin mount. Underneath the car, some sheet metal covering the shock mount had to be cut away. While I was cutting that, I cut away the old panhard rod mount as well. I used cardboard templates to figure out the mounting plates, with the moustache bar bolted into the body mounts and the pumpkin sitting pretty much level. I then transferred the cardboard to 12 gauge plate. I bolted the two plates to the pumpkin, then tack welded them in place. Then I removed the BMW unit, stuck a spacer and the bolt back through the pumpkin mount, and welded away. In this pic you can see the rear plate and the holes ready for plug welding the front plate:


You can see that I also put some side plates in to reinforce the mount back to the Opel’s body. You can see the heat marks from the plug welds.

[jtb]

Step seven – The stock BMW springs are too long (unless you like the jacked-up rear end look!) I used some 5” long (free length) springs that I had lying around, eventually I’ll get some Eibachs. The problem with the lower ride height is that you get too much negative camber, as shown in the pic:


The solution to the negative camber problem was to move the inner mounting point of the trailing arms up by 1 inch. There was no modification to the Opel body required to do this. You can also see the triangular gusset I added after raising the mounting point. To keep everything aligned, I bolted one flat bar to the rotor, and another to the two holes on the moustache bar (probably used at the factory for a jig), and measured the angle between the two flat bars. I made sure the angle stayed the same when welding in the piece with the new hole 1” above the old hole. The brown things in the picture are the lower shock mounts.

Step Eight – Shock mounts. Once you get the ride height figured out, some shocks might be nice. I used a U-bracket shown in a previous picture for the lower shock mount, bolted into the original BMW location. The stock Opel upper mounts might work for a taller ride height, but I had to move them about 4” outward on each side. After cutting a square hole in the body, I just used some angle iron (normally I shy away from this material, but it was ideal for this!), and filled in the triangular gaps with sheet metal later. The angle iron sits at a compound angle; I used a shock bolted to the lower mount to align the upper mount (remove the shock after tack welding!!). I had to relieve some of the sheet metal under the car (nothing structural) to allow the shock to clear. Also, the stock Opel shocks are too long, so I’ll have to buy some shorter ones (anyone want to buy some 30 year old Koni’s?)


[An alternate method to the whole spring perch and shock mount thing would be to cut out the shaded area shown in the last picture, and build a structural tower to house a coil-over unit. You could attach to the beefy Opel body sections on two of the sides for strength. Your lower mounting point would be near the BMW sway bar mount on the trailing arm.]

Step nine is wheels, I’m planning on running a 205/50-15” tire on a 40mm offset rim (not 100% sure on that offset yet…). I’ve made a spacer for the front wheel so that I can run the same rim front and back. According to the tire-store guy, spacers should be bolted to the rotor, and then the wheel bolted to the spacer, to be street legal.

Well, that it! I’m done for now. I suppose Step ten is the driveshaft, but I don’t believe that requires too much explanation. Oh, and I still have to do a final alignment. Hope you found this info interesting.


PS, the swaybar that came out of the Opel (I think it is stock, but I don’t know) will be going back in. It fits neatly over the BMW differential, and the width is almost identical to the BMW swaybar (and it’s thicker, too!).

[RallyBob]

Hope you found this info interesting.

Absolutely! Nice presentation! I like the 'magazine style' step-by-step procedure. Makes it clear for those intending to do the work themselves. Keep us up to speed on how well the IRS works for you.

Bob

[N61WP]

Did you think to weigh the bimmer unit before installing? I'd be interested to see how much weight you save.
Jc

[jtb]

Hmmm... weighing it may have been a good idea. I know they're both pretty heavy but I'm sure the BMW is lighter, especially the sprung weight (or is that unsprung weight?), as the pumpkin is mounted to the body.

Thanks, RallyBob. I've just yanked the engine for the winter; it gets to live inside, but the rest of the car must live outside for a few months, so it'll be awhile before I can find out how well it works.

Anybody need a stock Opel rearend?

Cheers,

James

[jeff denton]

I like what you're doing. I like what it will do to your unsprung weight factor. In fact, I like anything about cutting and grinding and welding on a GT. Too bad Speedway GT is finished. I wanna do more!

[mrtzmd]

Nice work, i recently installed the Isuzu Impulse Rear end with disc brakes on my Gt and have had pretty good results but i am worried about the future since parts for the Isuzu unit are very hard to obtain. I have seen plenty of 80's BMW 3 series cars at the junkyard with rear disc brakes ( i believe the later 80's models) so this upgrade is definitely valuable especially due to their availability and capacity to handle more HP.
Mauri

[jtb]

Thanks for the reply, Mauri.

The local yard had six 3-series BMW's, and they all still had the rear-ends. Apparently the pumpkins from the 325's are the same ones used up into the 7 series, so they should handle some serious hp. Which also means in a 325 they hardly ever blow up, so you can get them cheap!

I considered the Isuzu, but availability was an issue for me being in Canada. Plus the IRS challenge was interesting...

James

[v6transplant]

Hope you found this info interesting.

I agree also. You've got a lot of great information here! And a lot of parts availability possiblities...

Looking forward to hear how it does...

GREAT JOB!!!

[jtb]

Hi,

After reading some other threads on rear engine conversion plans, I'm kind of glad I didn't run coil-overs in the back. According to these other threads, the body shell gets very weak when you mess with the rear floorpan (I guess you lose a lot of torsional rigidity). Makes sense.
SO... If I were to go with the coil-overs, not only would I stick them in the corner to attach to the box sections, I would also suggest running a cross-bar at the top to link the two towers together. Just to be sure.

Cheers,

James

[simplemind7]

well it looks like spring is here, have you had it out for a run yet? i'm quite interested in doing this since my friend is buying a 325 soon and i think we could split the cost of a parts car for him so i can grab the rear end out of it and whatever else if i feel like getting creative. let us know about any bugs that show up, looks like you did a great job and an excellent write up of it

[jtb]

NO! Haven't had it out yet, Damn working for a living gets in the way of hobbies!!! The only work I've done on it lately is to shorten the shifter remote on the mazda tranny to fit the opel.
I should have more time to spare in the summer, hope to drive it then.

It SHOULD handle well; the BMW's I've driven all did!

Cheers,

jtb

[- GT Style -]

Originally Posted by jtb I should have more time to spare in the summer, hope to drive it then.

Care to give a preview to a local

I'd kind of like to look over a local's car so I can get an idea of what to look for on any future potential cars I find.

[jtb]

Step 8 changed (again!)
OK, well, I never even got it rolling with the chopped BMW springs, nor did I buy any other springs… I decided to go with coil-overs. In fact, I decided to use the same coils and shocks I have on the front, AFCO 1553 (that’s right, no transverse leaf on this car!). The engine is quite far back, and so with the fuel tank and battery in back, I hope to have weight distribution fairly even front to rear.
My original coil-over scheme of making a box section would probably have worked, but they would be quite far inboard. So, I decided to go behind the drive axle. There is just enough room. I used the top shock mount I made in step 8, with the addition of a bracket to change from the stud- mount type shock to a clevis mount shock. (see picture of top bracket). I made the mounts wide so that the shock can swivel, and location can be adjusted by use of spacers (you can see one aluminium spacer in the photo).

For the bottom of the shock, I had to make a bracket which ties into the swing arm via welding, and into the stock BMW shock mount location via a bolt (you can see it in the green circle in the photo). This is a simple bracket, the only oddities are that the stock bmw shock mount is at 15 degrees from the hub, and the lower part of the bracket needs to be coped to the swing arm.

I will keep the bump stops made in step 3, and use the rubber bumper off the BMW.

Tires are 205/45/16’s on an alloy rim, which were removed from a Honda. Not my original plan, but the whole set came up cheap, and they fit, so what the heck.

Sway bar is stock bmw (for now ).

jtb

[mrtzmd]

Originally Posted by jtb Step 8 changed (again!) OK, well, I never even got it rolling with the chopped BMW springs, nor did I buy any other springs… I decided to go with coil-overs. In fact, I decided to use the same coils and shocks I have on the front, AFCO 1553 (that’s right, no transverse leaf on this car!). The engine is quite far back, and so with the fuel tank and battery in back, I hope to have weight distribution fairly even front to rear. My original coil-over scheme of making a box section would probably have worked, but they would be quite far inboard. So, I decided to go behind the drive axle. There is just enough room. I used the top shock mount I made in step 8, with the addition of a bracket to change from the stud- mount type shock to a clevis mount shock. (see picture of top bracket). I made the mounts wide so that the shock can swivel, and location can be adjusted by use of spacers (you can see one aluminium spacer in the photo). For the bottom of the shock, I had to make a bracket which ties into the swing arm via welding, and into the stock BMW shock mount location via a bolt (you can see it in the green circle in the photo). This is a simple bracket, the only oddities are that the stock bmw shock mount is at 15 degrees from the hub, and the lower part of the bracket needs to be coped to the swing arm. I will keep the bump stops made in step 3, and use the rubber bumper off the BMW. Tires are 205/45/16’s on an alloy rim, which were removed from a Honda. Not my original plan, but the whole set came up cheap, and they fit, so what the heck. Sway bar is stock bmw (for now ). jtb

Looks cool, any road tests? (I can hardly wait!!)
Mauri

[jtb]

No road test yet. Still finishing the drivetrain, should be able to drive it come Spring

[jtb]

OK, So I've gotten to the point where I need the driveshaft to go from the Mazda transmission to the BMW rear end. I didn't think this would be a problem; I've done other swaps with American stuff where it was a piece of cake. Not so in this instance-
Went to a driveline shop, they wouldn't touch the BMW driveshaft, and wanted to build me a whole new (shorter) Mazda one (at great expense) The reason? BMW and Mazda both use u-joints which are non-replaceable (peened in). What happens when the u-joint goes bad? You are supposed to buy a whole new driveshaft!
So... once again I find myself custom fabricating. I've cut the diffy end yoke off the Mazda shaft, which I will modify to fit the BMW diff. Then I'll shorten the tube and weld 'er back together. Hopefully I can get it balanced somewhere, after I true it up best I can in a lathe. Thanks to the rx7 owner's club forum, I found that you can buy replacement u-joints for the non-replaceable u-joints! www.rockforddriveline.com

Well, if nothing else, this project is an education.

jtb

[GoinManta]

I am really interested in how well it works. I think it might even be easeir in the Manta. I was thinking of the BMW or Impulse IRS (THe 88-89 Impulse was IRS). But the argument you have for the BMW (Ease of parts, etc..et). Makes it definately appealing.

Especially if i get into the KA24DE motor and do the turbo and other stuff told it easily can get oer the 200hp/torque range.

All I know is I have a donor Manta coming. GOt offered a 71 Manta Rallye in Norfolk that is a total basket case so I dont have to feel bad about butchering it up.

Charles

[GoinManta]

OK Basket Case was a TOTAL basket case..

But luckily found a non-basket case, but one not good enough I cant modify without feeling tinges of guilt.

So again looking at rear options for the Mantissan.

Looks like a 1987 325i it is..

But obviously I guess I will have to document my changes since a Mnata and a GT mount a bit differently.

Great help though and should be a wonderful guide to helping my project..

Now the BIG question.. ever get a road test done?

Charles

[jtb]

Originally Posted by GoinManta Now the BIG question.. ever get a road test done? Charles

Nope, sorry Charles, I'm still working on the rest of the car, but it is getting closer all the time. The drivetrain is almost done, but it still isn't driveable yet. This custom stuff just takes a long time! plus progress is slowed by work, kids, etc.

jtb

[GoinManta]

Understand how that all goes..

I am going to cut the rustmobile up and flip its bottom up side down so I can work on it from the top. Might make for a bit easier inital fitment.

Charles

[GoinManta]

BTW.. what was the hub to hub dimension originally on the BMW ?

Charles

[jtb]

BMW hub face to hub face is 56", I believe.
I used aftermarket mags from an Acura, not actually sure on the offset, as I bought them from a guy at work, so I got to try them on the car before I bought them.

HTH,

jtb

[simplemind7]

if you don't mind me asking, how much did the rear end cost when you pulled it?

i'm torn between this and the toyota rear end for the manta, and now i'm starting to lean toward this. charles, if you do start the swap, then my mind is made up and i'll try to start it at the same time and maybe we'll be able to help eachother out. usually when you're stuck with a problem, you just need someone else to look at it from a different angle... then make fun of you when they realize its an easy fix