Rat Rod - 1972 MK4 Triumph Spitfire project

Oooo...  Nicey nice. 

It does fit pretty nicely in the stock location and the axles line up well (but since they will be CV axles, really does not matter). The bottom of the front mount keeps the location for the exhaust on each side and I have access to the drain and fill plugs.

What ratio R160 did you get?

It is a 3.54 ratio, out of a Legacy, with a viscous LSD. The 6 speed from the miata was originally using a 3.9 rear, so should be just a little more relaxed on the highway. With a 195/45/15 tire, should let me do 60 in 2nd gear at redline. 

Exactly the ratio I've been looking for.  Unfortunately, proving harder to find than anticipated. 

You can find them, but pricing on ebay is crazy.... https://www.ebay.com/itm/Subaru-Impreza-WRX-STI-2016-R180-3-54-Rear-differential-Torsen-2006-2016-JDM/122727277906?hash=item1c931da552:g:YuwAAOSwNytZy5P1&vxp=mtr

I would love a Torsen, but not for $999 plus $99 shipping!

I got lucky and bought a package of parts collected by someone that was trying to do the swap a while back. Cost me $500 for the diff, rear axle casting, MGF axles, bearings and hubs,  and a complete disassembled Rotoflex plus a spare Rotoflex set and various other parts. sold off the spare Rotoflex suspension for $400, so my cost was $100, but the machining for the installation of the MGF bearings and hubs into the GT6 Rotoflex hubs and machining of the rear axle plate was $900 (ouch) and new rear bearings was $100, so my cost to do the conversion so far is $1,100. Looks like I can get Dutchman to cut the MGF axles to the proper length and splined for the Subaru inside CV joints so with boots and the axles, my cost is $1,300 for the diff conversion (not including all the other stuff to restore the rest of the rear pieces, etc.... But I am pretty confident that this will be worth the cost as it gives me a pretty stout rear diff and suspension to handle my engine and transmission package. This should a lot of fun both on the street and on autocross courses.

Mocking up drive shaft angle and spring mount locations

Damn... makes me regret procrastinating on a couple of R160 3.545 diffs I saw on eBay awhile ago.  Next time I see some I'll have to bite the bullet.

I'm not sure I want a R180 diff as it appears to be a fair bit larger and therefore some amount heavier, than a R160.  It is highly unlikely a Spitfire or even a GT6 engine will ever put out enough power to stress the R160.

I already have a pair of rotoflex CV conversions, so my hope is to somehow get the R160 stub-axles welded to the u-joint mounting plates from a Triumph diff.

Since the R160 (or R160) have mounting holes on the rear plate, I've wondered why no one has simply made adapters to go from those to the Spit frame in lieu of casting and machining an entirely new plate.

Yes, the R160 is the preferred differential and can support up to 250HP so more than adequate for our Spitfires unless you are running a big engine swap.

I did not measure it as I already had the plate, but my understanding is that the Subaru plate is too deep, so a bracket that bolts to the cover and then through the stock mounting location would place the axle too far forward. But with CV axles, that may not be an issue. But if it was easily done, someone would likely have done it before. So far my research has shown that the successful installs are either using the NZ casting or welding up a steel equivalent (like Larry Porter did) as below: But he is a professional welder, so this was easier for him than most of us to accomplish.

Also, you should be able to make or buy an adapter plate to go from the Stub axles to your CV joint plates, should not be that hard to fabricate something that would work.

dherr said:

Also, you should be able to make or buy an adapter plate to go from the Stub axles to your CV joint plates, should not be that hard to fabricate something that would work.

That is my hope, but thus far I haven't found R160 stub axles that aren't directly part of the inner CV joint. I've seen a handful of pictures of them, so I know they exist, but it seems they're rare.

FWIW, while digging I found this site: R160 in a GT6 Convertible Project He did a welded version and commented about having to skim the plate after welding due to heat distortion.

Dave, looking sweet and what a score on those parts!

As mentioned on the other forum, propshaft angle can be a concern, but looking at your images it looks like you nailed it.  I wrote a bit about it here, about half-way down the page....http://stevew10.wixsite.com/spit16/general-fitment-and-functional-concerns

I've been running a GT6 3.27 diff with my 1.6 Miata motor and 5-speed (factory diff was 4.1:1).  It's been really good, however when I get to doing the new rear suspension/diff, like you guys 3.5-ish is exactly what I'm looking for.

I think the R160 is ideal, but are even more scarce in this neck of the woods, especially with the ratio we like.

Yes, I am really happy with the prop angle, since the axle pivots off the back mount, it should be relatively easy to adjust via the front mount to get it really close and since the rear is bolted to the frame, it does not move like an live axle does.

Built the spring mount today. 90% complete, just need to add a piece across the top edges to hold the aluminum spring spacer in place and then bolt the sides to the differential and tack weld it together. Then I'll brace it on the corners, do the finish welding, clean it up and paint it. I measured where the spring mounts on the stock diff, to be sure the spring was located in the same location as stock as this locates the top hub and upright. I can make up a tracing of the three pieces to scale if anyone is interested before I weld it all up together later this week.

It always is more satisfying to build these pieces yourself, I enjoy the challenge and am always amazed what you can do with just a drill press, chop saw and angle grinder. I made two mirrored pieces to bolt to each side, drilled the holes in both pieces and it aligned perfectly with my top piece. I used 1/4 inch mild steel for the whole thing, probably overkill, but the spring really needs to be securely mounted and this will be much stronger than the folded steel design that is more commonly used. I used Larry Porter's design, so can't take any credit, but it is pretty easy to duplicate.

Looks great!

I would weld nuts to the underside of the top plate for studs in lieu of bolts. Otherwise it'll be pretty much impossible to remove the spring without lifting the body or lowering the differential.

Like the build.  The use of the Subaru diff is certainly better than the old swing axle that we used to use straps, chains, etc to try to fix years ago.

I see you are located near me.  Did you happen to go to school in Thurmont?

Yes, the stock Miata 1.8 will make a lot more power and torque than the old Spitfire 1.3, so I have been planning to swap out the Differential as part of the engine swap project. The combination of all the pieces (Rotoflex, CV joints and the Subaru LSD diff) should be a huge improvement and still fit under the stock bodywork.

No, I went to school in Waynesboro, moved down to Frederick (Adamstown) since 2008. I'll let you know when the car is getting close to completion as I'll be at the Frederick autocross this spring to sort out the handling and have some fun.

In reply to Ian F :

Thanks

Yes, I was going to weld the bolt heads to the plate to keep them locked in position, but I do see your point. But considering how easy it is to lift a Spitfire body off the frame (and how often I'll ever likely need to remove the spring), it might not really matter. I also considered tapping threads into the top plate but it really is not thick enough for studs unless I weld a nut under each hole. Larry Porter did the bolts in the manner as I have done them on his GT6 convertible. I'll decide later this week as I won't get back to the car until Thursday or Friday.

Spring mount is completed and installed, so next steps will be the hubs and wishbones and then I can measure the axles and send them to Dutchman before Christmas so I can build my CV axles and finish the rear suspension. I can then work on the engine/transmission detailing and install it in the chassis and run brake and fuel lines.

So ran into a major problem with my top spring mount. The passenger side lined up with the upright just fine when I installed the wishbone but on the drivers side it was almost 1/2  inch back. I must have welded the top of the spring mount 1/16 out of alignment, so it translated to 1/2 inch at the end of spring.......

So on Sunday, I had to disassemble the spring mount and spring and cut the top off it . I then bolted back on the side pieces and bolted the bottom of the spring plate back to the spring and then bolted everything back together (with the plate now sitting on top of the side pieces on each side of the differential. With everything bolted together, it was obvious that the entire plate needed to shift forward 1/4 inch and everything lined up perfectly. I tacked it in place, pulled it all apart, braced and welded it (again) and then cleaned and painted it.

Today, it all bolted together perfectly ,no issues and like it was factory pieces. So other than wasting a day, it now works just fine and I can start on measuring for the CV axles tomorrow.

That looks awesome! 

Thanks, I am really happy with how it is progressing so far!

The axles were measured and sent off to Dutchman to be cut and re-splined. I have all the pieces to reassemble my "hybrid" cv axles once the come back from Dutchman and I'll put them together and that will complete the rear conversion. I shared this list on another board of what parts are necessary to duplicate this piece of the build.

Differential and stub axles - Subaru R160 (mine from a Legacy, with a 3.54 ratio and viscous LSD) This is the type with the snap ring retaining the axles into the diff. Get the axles too as you will want to use the inner CV joint and stub axle 
Suspension - Triumph GT6 Rotoflex rear suspension including the fixed spring, uprights, wishbones, radius arms 
Misc parts for Rotoflex upgrade - Wishbone lower frame bracket (Spitbits or Rimmers), Inner radius arm body support bracket (Spitbits or Rimmers), Radius Arm bracket for GT6 Rotoflex (Spitbits or Rimmers) or used 
Hubs and Rear Bearings - MGF/Rover Rear hub and bearing and c-clip (same wheel spacing as Spitfire so wheels can be rotated, much bigger bearing with out the need to shim, heavy duty wheel studs (Rimmers or used on Ebay.uk) 
CV Axles - Rover 100 (Metro) and outer CV joint, apparently the MGF axles are too short, need the entire axle as it will need to cut down and re-splined to match the Subaru inner CV axle. (Used on ebay.uk or ??) 
Subaru and MGF CV boot kit - so you can reassemble your new CV axles - Rimmer bros and ebay 
Spitfire shocks - your choice here, I used Koni as I already had them 
Rear Diff plate and spring and front diff mount- order from the NZ connection or fabricate your own 
Rear brake drum centering rings - small rings that fit on the hub to center the drums on the MGF hubs, you can have them made up easily 
Misc bushings - I used poly for everything 
Custom machining - The GT6 uprights need to be machined for the installation of the MGF bearings and cut for the retaining clip. 

Everything above will give you the much stronger R160 diff and a proper working rear suspension that will work with the later Subaru diff, yet fit under a stock Spitfire or GT6 body. There are many other ways to do this conversion, but all have other compromises such as you can keep your swing spring suspension, but you then need to have the bolt-in axles as a swing spring will pull the newer Subaru axles out of the diff. Other changes that keep the GT6 bearings and use the Cantley axles (or your own version) will work, but the recipe above addresses all the issues. With that said, some of the parts are going to be hard to find.......

Looks like awesome progress so far!

I haven't gotten a chance to touch the SpitBird since the challenge, but I did pickup a parts car for it that should be QUITE helpful... 

My 2 comments:

I would consider using the 5-Speed miata transmission instead of the 6-Speed. IIRC the final drive is close to the same on both which just means you'll be shifting more frequently in the 6-Speed. (I have a 5-Speed sitting in my garage, and I'd probably be willing to trade for the 6-Speed and a ride once it's finished. )

I know you're not onto the electrical portion of the build yet, and I am also not an expert on engine management by any means, but I think a MegaSquirt PNP would work nicely for your situation. If I understand how it works correctly, then you should be able to just plug it into the miata wiring harness without much if any more wiring. Then you don't have to worry so much about any check engine lights depending on your O2 sensor setup, EGR Delete (I would definitely delete), etc. and since you are probably going to add the turbo later on that would have you covered there too. Also, you would be able to hook up a MAP sensor and be able to get rid of the MAF sensor which will de-clutter the engine bay, and may fit better as space can be tight anyway. I am definitely getting rid of the MAF/VAM on mine.

In reply to dherr :

Damn, that sounds like a lot of work and expense... 
Lately I've read the main weakness in the Spit/GT6 diff is the carrier and if replaced with a Quaif LSD the strength is increased substantially.  Still not a cheap option it seems like it would be easier. 

Hmm...  

Let me know when you start playing with the SpitBird as I would love to stop over and see where you are headed.....

In regards to the 5 speed verse 6 speed, my understanding is that with the 5 speed cars had a diff with a 4.3 ratio and that the 6 speed cars had a 3.9. My car has a 3.54 rear, so when combined with the 6 speed, I will have more of an overdrive 6th gear for the highway, but good gearing for acceleration in the lower gears. According to the Flying Miata calculator, I should be able to do 60 mph in 2nd gear, which is what I want for autocross. The 5 speed would be better in other regards, as the shifter location can be changed easily, but I think for now, especially since I have done all the work to make it fit, I'll keep the 6 speed.

The MegaSquirt PNP is a definite consideration, I figured I would start out stock but may just end up using it immediately as it will help to minimize all the issues of going with the later NB1 drivetrain, and help me to maximize the power as well as put me in a good position for the eventual upgrade to a turbo 

In reply to Ian F :

Yes, that is why I am trying to document all of this, as my experience is not typical since someone else had already sourced all the "hard to find" parts for this upgrade. This project is much more than just finding an R160 on Ebay and ordering the mounting kit from NZ. I have seen many comments on the Triumph Experience and don't want someone to just source a few parts as there is much more involved. The beauty is that if you can get the parts, it really is a factory quality installation that retains all the Triumph pieces that keep it a Spitfire/GT6, but improves on all the weak links of the stock drivetrain.  Since my car will start off with 150+ HP and probably end up with 185-200 at the wheels when I add a small turbo, this was really important. 

I completely agree that the Quaif LSD would be a much simpler option and in most cases probably cheaper for most people looking to improve the performance and longitivity of the differential, especially if you are keeping a Triumph Spitfire or GT6 engine in the car. I read the Ebay advertisement for what I had purchased several years ago, and the seller said he was giving it away for "pennies on the dollar", I now realize how much of a good deal that it really was!