Jon_Valjean

This is going to be a long thread, so to summarise, myself and a friend put independent rear suspension comprised of largely Nissan 240SX/Skyline components in a street FB RX-7, as well as a FD 13B-REW engine and a FC front subframe. Here are some pics:







Beyond this point there’s a lot of technical stuff and a bit of a novel.

The car is an Australian-assembled 1981 FB RX-7 (series 2) with a shade over 200,000km on the clock. I first bought this car as a wreck in 1996, sold it in 1998 then tracked it down and bought it back again in 2004. I’ve driven it for about 100,000km all over New Zealand with a combination of standard and bridgeported 12A engines, and various suspension and wheel combinations. You could say I’m attached to it. Here it is before the work:



It’s been in pieces pretty much since 2004 and I’ve set up various suspension and brake combos on the car in an attempt to modernise it, but nothing really stuck. This is because the stock running gear is prohibitive – the suspension and brakes are capable in stock form with a mild power increase, but if you’re staring down the barrel of a good 500 RWHP the factory steering, brakes and suspension aren’t up to the task of getting the power to the ground in a safe and effective manner. That’s about as diplomatic as I’m going to get about recirculating ball steering, live axles and single piston brakes.

I’m not going to go over the FC front subframe swap in any great detail, mainly because it’s been done before on here and you’ll be able to see by the pictures that I haven’t really done anything different. It’s the same old 25mm rearward relocation, 15:1 rack depowering, FC coilover and sway bar setup that most folks will recognise. The only departure from convention is that I’ve fitted the entire FC steering column instead of messing about creating a hybrid FB/FC setup. But I think that’s probably been done before. I also used FD front calipers, which required a bit of tweaking to get the hard brake lines to clear the struts.



The main motivation for changing the front subframe was the desire to eventually run slicks on the front, which necessitated thicker spindles than factory fitted to any FB, and the elimination of wondersteer by fitting the FC rack and pinion. To be honest, I'm not sure if I ever will run slicks at a track day, but I don't want to be in the position to never be able to.

I did go to the trouble and expense of having Wilwood brakes mated to large vented disks and custom hubs for the FB, but changed my mind and sold them on earlier this year. I just couldn’t stomach the idea of running slicks on the tiny FB spindles, seriously put FB and FC spindles next to each other on a bench and take a look, the difference is astonishing.

The FD engine swap required the creation of custom engine mounts which I’ve welded directly to the FC subframe in order to use the rear mounts on the FD engine. This is a better way of distributing the power of the engine to the mounts, and it also allowed me to move the engine rearward and downward 30mm in each direction for a better centre of gravity. The FC gearbox has had the shifter moved forward 104mm too, this is also a fairly common mod.

The engine has a large extend port, new rotor housings, new seals throughout, a 5kg flywheel and Exedy clutch, it’s doweled and has a heap of internal work done to it. The turbo is a Borg Warner S300SX.





So, why IRS? As mentioned before, I just didn’t want to mess around with a lot of power through a live axle.

Yeah, I’m aware that the live rear axle can be tweaked. There’s 3 link, 4 link, better bushes, adjustable spring platforms and a whole heap of aftermarket parts that can help. I looked at them all. You can spend thousands making a live axle easier to live with, and even pull some impressive lateral G figures on a skidpan, but ultimately you’re stuck with a setup that was originally used to save Mazda money in order to allow them to ship more units at a competitive price rather than a suspension designed purely for the sake of performance and comfort.

It’s not like I didn’t try to stick with the live rear axle either. This car in 1981 had an open diff head out of the factory, but even swapping in an expensive aftermarket setup limits you to about 350HP, or even less if you really lean on it. I wasn’t happy having such a weak link in the drivetrain, so it had to go anyway.
As you can see from this pic, I initially replaced the factory Mazda rear axle with a shortened Borg Warner 78 unit out of an Australian EA Ford Falcon, but I was never comfortable with the characteristics of the trailing arm setup.





It was also a 25 spline axle unit, meaning LSD centres were hard to find and expensive, and the axles weren’t even really that strong to begin with anyway. Not to mention the hassles of routing a 3” exhaust pipe over a large rear axle and still having room to lower the car to the deck.

Yeah, I could swap in a 28 spline Ford unit. Hell, I could go set up a Toyota Hilux rear end for a couple of grand, or even a Ford 9” if reliability was the only problem. But that kind of stuff – beneficial as it is in a straight line – won’t help you much on the track, or on some of our winding roads here in New Zealand. This is not a car designed to go from traffic light to traffic light on a Friday night; I built it to get into and drive challenging roads in comfort and at speed with solid reliability.

In reality, I’d already spent a lot of time and money on a setup that was never going to compete with a half decent IRS.

I looked around this forum and others to see what IRS setups had been done in the FB before. From what I can find it’s really only Miata and FC that have been swapped in, and neither really appealed. The Miata stuff is too weak for the FD engine, and the FC stuff looks archaic with the trailing arms and passive 4WS that people spend good money to delete anyway.

Around mid-2015 it occurred to me that there might be a fairly common OEM IRS setup with a similar width to the FB that I could investigate using. I’ve always been a huge fan of the Nissan R32 GT-R, and early Nissan RWD stuff in general, and upon looking at the hub to hub measurements of the S-chassis and R32 found they were within 5mm of the FB.

Nissan shared the rear suspension design between a lot of cars in the late ‘80s through to the early 2000’s. For example, the S13 Silvia (240SX in the USA), R32 Skyline, C33 Laurel and A31 Cefiro all have (more or less) the same rear subframe. Sure, there are different axles, brakes, different diff options and various other minor changes, but the width is the same. Even the floorpan is similar, other than alternating between having a spare tyre in the rear wheel well or attached to the rear floor.

In order to investigate this further, in late 2015 I bought a stripped rear subframe from a BNR32 GT-R Skyline and took some measurements.



I saw pretty quickly that the four mounting points would physically fit under the FB and there were no weird angles or design decisions that specifically prevented its use.

Most importantly however, the rear subframe houses the diff, axles, hubs, sway bar, lower arms, camber arms and toe arms. It’s all entirely self-contained – you can literally undo four large nuts (plus brake hoses/cables obviously) and drop the entire rear suspension on the ground. From an engineering perspective, this means no need to set up brackets on the body to house suspension arms. In other words, no difficult and time consuming laser measurements to get each link positioned precisely – just make sure the four mounting points are aligned front to rear, left to right, diagonally and vertically and you can guarantee everything else is correct.

Keen to take this further, myself and my friend picked up an A31 Cefiro from a local vehicle dismantler. This Japanese Domestic Market car shares a floorpan with the 240SX and was originally fitted with a RB20DE six-cylinder auto. They’re common as anything in NZ, but probably don’t exist at all in the USA – just imagine a four door 240SX with grandpa-spec interior and you’re about there.

Jon_Valjean

All we needed was the rear of the car. We started by cutting out the entire rear floorpan behind the front seats all the way to the very rear panel. We used cutoff disks, a reciprocating saw and a plasma cutter, and only set fire to it once!







Once the floor section was extracted, we could take a closer look at the subframe mounting pins to determine how to attach them to the RX-7. It would be extraordinarily difficult to attempt to retrofit the four pins that locate and mount the subframe to the existing RX-7 rear floor, there’s simply too much to chop out and change and you’d never get it millimetre-perfect, which was a prerequisite. At that point it pretty much confirmed our suspicion that the only way to do it was to swap the entire rear floor section.

Next step then was to chop out the RX-7 floor and make a hole big enough to insert the A31 floorpan in its place, keeping the RX-7 outer guards with the A31 inner guards.







No going back now! You can see the RX-7 rear floor sitting on top of the A31 floor, and how similar the width is.



A bar was welded between the Nissan strut towers to keep it rigid, but in reality it was more beneficial as a handle to help lug it in and out of the car a few dozen times when we were measuring it all up.

Jon_Valjean

Some may have noted by this point that the A31 open differential and axles aren’t up to the task of surviving 500HP either. Most of the 4x114.3 PCD RWD Nissan gear is based off the R180 platform, that is, a differential with a 180mm crownwheel and 29 spline axles. These axles are bolted to the diff ears with so-called “3x2” bolts, three sets of two bolts grouped on each of the tripod corners. This is common across S13, S14, R32, R33, C33, C34, A31, basically any 1989-2000ish RWD Nissan with the 4x114.3 PCD.

But by going to the larger 5x114.3 PCD, you gain 30 spline axles with “5x1” proper CVs, the R200 diff, bigger brakes and better rim choices. In order to do this, we chose the 4.083:1 viscous LSD out of a 300ZX, R32 Skyline turbo axles and hubs, along with vented disks and calipers from a Laurel.



To change from four lug to five lug, simply bolt in whatever hub you want – in this case, R33 Skyline turbo:



We used BC Racing Gold coilovers all around too – FC in the front, S13 in the rear, and topped it off with a BNR33 GT-R rear strut brace (yet to be fitted in the pictures). Good bendix brake pads and braided lines are used all around too.

In order to get the R200 diff to go into the R180 subframe, we chopped out the back of the (now useless) R32 GT-R subframe and welded it in place of the A31 diff mounting bracket. The GT-R has the so-called HICAS four-wheel-steer system that dynamically changes the toe-in and toe-out characteristics of the Skyline, this is a feature I didn’t want to use so we had to make a couple of changes to the toe-arm mounting brackets to swap things over. We could have sourced a R200 non-HICAS subframe to use but they’re not super common and changing the mounting plate over was a half-hour job.





Mixing and matching is pretty important – you can’t use an R33 or S15 rear subframe for example, they’re too wide. The 300ZX is even wider, as is the Q45 and the J30. But you can use some componentry here and there, which takes a good deal of research along with trial and error. I think I must have spent more than fifty hours researching every component before deciding which would complement each other.



All that said and done, the rear of the car required I-CAR approved joins to connect the chassis rails. This is a road-legal vehicle here in New Zealand, so the standard required to modify this car safely is set high. The amount of time required to connect the floor to the rest of the car is immense, every connection has to be done to the satisfaction of a qualified engineer before this sets one wheel on any public road.

Jon_Valjean

After physically joining both Nissan and Mazda sections, we had assumed we’d be able to use the A31 fuel tank (all set up for EFI) or at the very least, re-use the FB tank (with a swirl tank). Unfortunately neither would be possible – the A31 tank is designed to straddle one rear chassis rail, which was no longer possible. Having removed a good 300mm from the back of the floor, the tank was now so far forward that it was actually within the inner guard on each front corner of the tank.

The FB tank wouldn’t fit either, it’s too wide to go between the Nissan rails and too long to fit between the back of the new subframe and the rear of the car.

The only solution was to make up a custom drop tank, so we decided to make it a large one. It’s a shade over 90 litres in capacity, which means it can double as an additional device to add weight over the rear axle if traction turns out to be a problem.









Also, by chopping out the back of the car to accommodate the tank, we could fabricate new rail ends and rear panels to tidy up the transition between Nissan and Mazda, along with a false floor to hide the top of the main tank, the swirl tank, two fuel pumps and two fuel filters.

Jon_Valjean

All the EFI componentry is contained on the right of the car, meaning the exhaust will exit the car on the left Nissan-style. This is mainly because the rear sway bar has a bend to accommodate the exhaust pipe exiting on the left, and also because how many FB RX-7s have you seen with a single exhaust on the left?

In doing all this, we added a factory rear bumper back on to the car. During the period of time between 1998 and 2004 when other people owned the car, it had been repainted and the rear bumper removed – probably due to a minor rear impact which there is ample evidence of. I never liked this “shaved” look so I was only too happy to cut it all out with a grinder and replace it with a more factory look. I do like however that the side indentations on the car have been filled, it makes it look much newer. I will also add the rear garnish between the lights eventually too.





Yeah, the purple colour is stupid, it was silver originally and I’ll be painting it gloss black eventually. But that’s a job for the future.

So having fabricated new floor sections, and finished welding the car back together, we ground everything fairly flush and covered it all liberally with zinc rustproofing and Sika seam sealer. Eventually the floor will be covered with dynamat before the carpet goes on.

We even managed to reuse the original Nissan hard brake lines, and the 76mm driveshaft has been custom made with new replaceable UJs (a Spicer 2603-25 yoke with Mopar 7260 UJs and a new Nissan flange) - It’s good for the task of transferring 500HP between the FC gearbox and the 300ZX diff. The emergency brake was easy enough to make out of half Nissan and half Mazda bits and pieces too.

At this point however I have to leave New Zealand and travel to Ontario, Canada for the foreseeable to fulfill a work contract. This means the car goes back into storage and I get to think about the next phase in the build – interior and bodywork, most likely. It will be at least two years until I can get back to it.

I’ve run out of time to get the engine running and actually take it for a drive – the car is 95% ready to go, but such is life. If it helps, I’ve towed it down the road a bunch of times and it handles like a lowered skateboard, despite not having a proper wheel alignment and all four wheels pointing in different directions. It’s eternally frustrating that I can’t post a video of it powering through a corner, but that will come in a couple of years. I have to remind myself that it’s been 12 years since I drove it last, two more won’t be the end of the world.

I also figured that showing you all the progress to date would be better than sitting on it for a couple of years and probably forgetting how we did most of it anyway. At the very least, you can certainly see the potential.

In 2018 it’ll get a repaint and I wouldn’t mind making a completely custom interior. I’d like to pick up a Haltech ECU to replace the ageing Microtech LT10S too, and some additional Nissan go-faster bits for the rear suspension. Adjustable camber arms and possibly even a clutch-type NISMO LSD out of a GT-R or something, I’ve never been a huge fan of the viscous units, as strong and cheap as they are. Seriously, the Nissan diff cost $100 and came with free axles, it’s so refreshing not paying the rotary tax for a change.

Here are some more pics, but just a note about the crazy rear camber, the coilovers are set about as low as they can possibly go, and the factory camber arms are set to maximum negative - this is not the final configuration, it was just set this way to check clearances and I haven't had a chance to wind it all back out again. It's actually too low to get a trolley jack under the diff like this, and it turned into a tripod going down my steep twisty driveway.

Anyway, comments & suggestions welcome, and thanks for reading this far. Also, if there’s anyone in Ontario with an FB and the rear half of a 240SX lying about who feels like a challenge, I might have a few weekends free over the summer













Summary:

Vehicle: 1981 FB RX-7

• Engine: FD3S 13B-REW, large extend port, dowelled, lightened rotors, 3mm apex seals, new rotor housings, high capacity oil pump, windowed bearings, 5Kg flywheel, Gilmer belt drive, FC water pump.
• Turbo: Borg Warner S300SX 66mm T4 twin-scroll on custom steampipe manifold w/50mm wastegate.
• Transmission: FC3S 86-88 turbo gearbox with shifter moved forward and short shifter kit. Custom 75mm driveshaft, Exedy solid plate clutch & heavy cover.
• Suspension: BC Racing Gold coilovers all around, FC3S front, Nissan S13 rear.
• Brakes: FC3S vented front rotors, FD3S front calipers, Nissan C35 Laurel vented rear rotors & calipers, bendix pads & braided lines, FC3S master & booster, FB pedal box.
• Differential: Z32 300ZX 4.1:1 R200 viscous LSD with R32 Skyline turbo 5x1 axles, R33 Skyline hubs, A31 Cefiro hub carriers, LCAs, sway bar, camber arms & toe arms, BNR32 GT-R Skyline subframe diff housing, BNR33 GT-R Skyline rear strut brace, HICAS & ABS removed.
• Steering: FC3S 15:1 de-powered rack, welded quill with FC3S turbo column, aftermarket steering wheel.
• Fuel system: 90L alloy tank, Holley Black lifter pump, 100 micron pre-filter, 3L surge tank, Walbro 255 high pressure pump, Ford V8 EFI filter, 3/8" solid & flexible EFI lines.
• Wheels: JDM 16x7" 5x114.3 PCD +45
• Tyres: 205/50/16 asymmetric front, 225/50/16 directional rear.

craaaazzy

iTrader: (9)

That's just too cool but so much work. Jealous of ur fabrication and knowledge. Hope u get to pick it back up in 2 years like u've mentioned.

Qingdao

iTrader: (3)

How did you fill in the line that runs down the side of the car; where the trim goes originally?

Very nice though.

Jon_Valjean

I didn't do that. Bondo, I'm guessing?

Qingdao

iTrader: (3)

Looks kinda slick. More 70's feel.

Jon_Valjean

Yeah I like it. Although I'm not looking forward to repainting it, whoever did the work has cut a lot of corners. This was behind the rear panel when we cut it off:

KansasCityREPU

iTrader: (9)

Very impressive. Even more so doing it on top of a single axle trailer.

ACR_RX-7

iTrader: (1)

Are you concerned at all about how far rearward the tank is? All I can see is a Ford Pinto issue.

Other than that, very impressive and unique for sure.

Jon_Valjean

Yeah to be honest, the tank fitment is really the only thing I have reservations about. It's constructed of 3mm aluminium, and has a rollover valve, but its depth means that the rear bumper had to be stripped and a custom inner bumper made out of steel to support the outer skin and physically prevent the tank from exiting rearward.

The steel inner bumper will protect the tank in a rear impact - coupled with the stronger nature of the tank it's probably just as safe as a factory FB. Probably. I just don't know. So I think in a couple of years I will probably revisit it - if I were to take maybe an inch out of the depth of the tank, it'll still be ~85 litres and I can double up the thickness of the rear support panel to make it that much stronger. That's probably what will happen. It also bothers me that the tank is offset to the right about an inch because the Nissan rails aren't symmetrical, they kind of curve to the right to allow a space for the factory spare wheel position in the left inner guard, and I'd like to change the shape of the tank to make it more aesthetically pleasing.

This is the issue with a build like this, as anyone who has followed the likes of Project Binky on YouTube will attest - you start down the line of making something, then you discover that future modifications or alterations require you to completely re-engineer what you've already done. Through this constant iteration you end up with a great product, as you can't just make a custom setup like this in complete isolation with a bit of tracing paper and some imagination.

It's been a learning curve, for sure!

ACR_RX-7

iTrader: (1)

I'm glad I'm not the only one watching Binky. I was upset when they had to reengineer the whole front suspension early on due to the rack position.

I didn't realize that the tank was that thick though, and that it had a reinforcement behind it. I'm no engineer, but have you considered a fuel cell bladder? I'm unsure if you can get one made to your application, but it may be worth looking into.

Jon_Valjean

Yeah I have no idea how they're planning on keeping it looking stock at all. Amazing project though, wish I knew what those guys know.

Hadn't considered a bladder at all - cheers for the suggestion, I'll take a look around and see what options there are

SpikeDerailed

iTrader: (7)

love this thing already, shame there wont be updates for a few years.

stilettoman

iTrader: (1)

"Through this constant iteration you end up with a great product, as you can't just make a custom setup like this in complete isolation with a bit of tracing paper and some imagination."

My friend Sean, who is a master fabricator and helps me with my various projects, has been quoted as saying that a really good fabricator only has to do everything twice. I don't know if you have seen my projects, but they are shown in some detail on my website: stilettoman.info in the automotive section.

I owned a lot of FBs over the years, just never had any interest in the FC, and I can't fit comfortably into an FD. MY V8 powered FB was fun to drive, but I sold it after 9 years. My current daily driver is the RB20 powered FB. I thought a lot about using the Skyline rear suspension, but with the wheels and tires I have it handles just fine for the kind of driving I do on the street. I am only running about 240 HP, so the stock LSD is just fine. I would have to say I have owned a lot of cars, starting with a 47 Cadillac I had in college, and a lot of sports cars beginning with a TR-3 I had back in the early 60s, but this RB20 powered FB is the nicest driving car I have owned. I liked the RB20 so much I bought another one and installed it in a 48 Studebaker, along with the R32 front suspension, R33 rear suspension, R33 front and rear brakes, and numerous other parts. You might enjoy reading about this conversion which is detailed on my web site.

I hope you get back to your project and get it on the road.

I had a wiring meltdown in the Stiletto, and I have to rewire it. I decided to ditch the Kawasaki engine management and built a custom intake system with an RB20 throttle body. I am putting it together with RB20 ECU, TPS, MAF, Cam position sensor, etc. - in other words a research project. Wish me luck.

In case you are wondering, I am 39 years old, and as of this year I have been 39 for 39 years.

RGHTBrainDesign

I love this build. Truly one of the greats on here.

Jon_Valjean

That's a very cool restomod, nice to see someone else using the Skyline running gear like this. I like all the small touches, the customised bits and pieces that really make a big difference overall. I can also appreciate how many hundreds of hours you've spent on that, very cool indeed

Also a bit strange seeing two of my cars merged into one with your RX-2/RX-7 convertible

RGHTBrainDesign

I'm more interested in how it drives. Shoot some videos of the car on mountain roads!

Jon_Valjean

Just because I'm in Canada and the car is in New Zealand doesn't mean I can't buy some sweet OEM parts.

I had to wait six weeks for this to arrive from Japan, but it should do the trick:




Still about 14 months until I'll be working on the car again. I have a few more OEM parts to buy in the meantime so will update later

Frankenmazda

iTrader: (2)

Always cool to see these projects. for me it's as much doing it as it is anything else. I mean it will be fun to drive but the journey to get to that point it whats important.

Good luck with the rest!

Jon_Valjean

Yeah I agree, the planning is quite a lot of fun Thanks man.



Picked up these two OEM door rubbers, the originals are a bit tatty after 35 years. Surprisingly cheap and available overnight in North America. There were 7 sets left after I purchased these

nonsensebureau

This thing is sick! that cefiro tub fits up so well, really gives me some insight on finishing my rx8 IRS install

Jon_Valjean

Work begins again! I'm back in NZ after a hellish 30 hour trip from Ontario, but first things first - time to get the "stress relief" vehicle legal.



The old girl flew through a safety check after two years in a garage, and will be used for contemplative breaks when things aren't going well.

Time to drag the seven out of the same dusty shed and get it into the workshop:



It hasn't fared too badly in the temperate climate here. Unfortunately my Borg Warner S366 is sporting some new exhaust-side surface rust, but that can be cleaned.



I thought about it for a while, and wasn't sure where to start again. So I just started where I left off - cleaning the sound proofing off the floor. It's not a sexy job and doesn't make for good photos, but it has to be done.



Next steps will involve completely rewiring the car. I'm not messing about with 40 year old wires, it's crazy not to do it now while it's all apart.

Whilst I was in Canada I accumulated all the parts I need to get this thing running, so that should happen soon. I want to get it going before I turn 40 in November, which will be a nice thing to do considering the first SA models are 40 years old now too.

More very soon!