My 1974 REPU Project

[chickenwafer]

Dude a Twin turbo REW on that truck is going to be so much fun!!! Can't wait to see it finished.

Haha yeah thanks Eduardo!

My plan is to swap in the REW with the stock sequential twins and keep them sequential. Do a simplified sequential setup and remove all emissions and run a PowerFC as the PFC is 1) Cheap, 2) I know how to tune it and it's easy to tune, and 3) I still believe it's the best aftermarket ECU if you're keeping the stock seq twins as it's setup outta the box to control them perfectly.

So I'll have to a front mount intercooler, obviously, and then I'll probably add some water/meth and upgrade the secondary injectors. Maybe even switch to E85 but with the stock twins I don't think it's necessary, and I'd actually like to have somewhat decent fuel economy as I do plan to drive it to work every now and then.

My plan is 320rwhp on seq twins on pump gas and water/meth. I think 320rwhp sequentially will make this truck ridiculously fast and fun! Given the low weight of the truck (2855lbs empty) I should be able to click off a low 12 second 1/4 mile pass. My goal would be to eventually get it into the 11's, which given the trucks weight and our altitude would be close to 390-400rwhp, which would be an amazing feat sequentially. The stock twins modified to be non-seq have done over 430rwhp.

Hopefully soon I'll have some spare time and some help to get the stock engine out and I'll start selling off stock REPU engine parts. Get the chassis all cleaned up and I'll hopefully have a JDM REW engine here by Xmas. I'm hoping to be have this swap up and running by spring/summer 2017.

[VRx8]

2855 lbs. that truck is that heavy ? My FD wights 2950 with me in it this is with the lighter 8.8 diff setup and no PPF. The FB race car weights 2480 with me in it. Figure it would be way lighter that's crazy.

[speedjunkie]

Eduardo! Back from the grave lol.

I'd like to be the first to say...it has begun. The slippery slope to a full on crazy build. HAHA

[chickenwafer]

2855 lbs. that truck is that heavy ? My FD wights 2950 with me in it this is with the lighter 8.8 diff setup and no PPF. The FB race car weights 2480 with me in it. Figure it would be way lighter that's crazy.

Yeah, the truck isn't as light as I would think. It weights about as much as an FD. This is according the Road & Track's magazine article test, though. I've never actually weighted it. I know Matt has weighted his that has a GSL-SE swap with supercharger, but I don't remember how much it weighted.

Eduardo! Back from the grave lol.

I'd like to be the first to say...it has begun. The slippery slope to a full on crazy build. HAHA

Oh yeah, the slope is REAL slippery.

Plans are always subject to change, but as of now the plan is to swap in a stock JDM REW motor with seq twins. Do a vacuum hose job, replace all check valves, delete all emissions and do simplified sequential with resistors (aka ghetto mod) to trick the ECU. The only hurdle will be the factory OMP since I'll be running the REPU front cover, the FD OMP won't bolt up and I don't think it would be even remotely easy or worth it to modify the REPU front cover to fit it. But the OE ECU doesn't see the OMP plugged in, it goes into limp mode. So I'm thinking I'll just move the OMP wires out of the harness to under the dash where the ECU will be and have the OMP lay in the glovebox LOL.

The factory engine/ECU will have a pretty much straight exhaust with a single muffler (not cat, obviously), free-flowing custom intakes, and front mounted intercooler. With two manual boost controllers I'll regulate to 12 or 13psi for a 12-10-12 boost pattern on the factory ECU, which should be safe.

After it's running good I'll add a PFC, Full Function Engineering Step-Up secondary rail upgrade kit with 1680cc secondaries, keep the stock primaries and delete the FPD, and shoot for around 350rwhp sequentially, maybe more. I REALLY, REALLY want to keep the engine sequential twin turbo. Honestly, looking back at my FD, it was the most fun when it was sequential. It's a very unique and fun power delivery, with the super fast spool of the primary, a small lull or dip in power, and then BAM!! You get kicked in the gut and the secondary it lit and you're gone! It's wild, it's unpredictable, and it's totally a unique experience to the sequential twin turbo REW motor. Even tho my single turbo FD was making more power, it didn't has the same "feel" to it. So I'm pretty excited to try seq twins in the REPU because I think it will be a hoot. Hell, I'm hoping to even snag a set of BNR Stage3 twins and run them over 400rwhp sequentially. Now that would be FUN!

[speedjunkie]

You can delete the OMP with the PFC, right? Why not just get a PFC right away?

Talk to Jason Kaiser, Dwduc. He's in love with the stock twins haha. He was talking about ditching the solenoids and building an electrical box I believe to control everything. I think the later FDs came with something similar from the factory too, but don't quote me on that. Or maybe I'm thinking of something similar that some company released, I don't remember.

[Dwduc]

Well now I wouldn't say I love the "stock" twins. I think the sequential idea is fascinating. I am not a fan of how it is done on the FD controlled by an 8 bit computer through vacuum solenoids on an awful cast manifold for inefficient 25 year old Hitachi hairdriers with an RPM based switchover. I have been working on some coding for controlling twins with a 32bit microcontroller using PWM to fast servo operated valves similar to motorcycle EXUP valves. But until I have a workshop again its dead in the water.

[chickenwafer]

You can delete the OMP with the PFC, right? Why not just get a PFC right away?

Talk to Jason Kaiser, Dwduc. He's in love with the stock twins haha. He was talking about ditching the solenoids and building an electrical box I believe to control everything. I think the later FDs came with something similar from the factory too, but don't quote me on that. Or maybe I'm thinking of something similar that some company released, I don't remember.

You're right, you can delete the OMP with the PFC easy. I'm going to skip the PFC off the bat just to eliminate variables. Electrical is going to be the biggest challenge in this swap, IMO, so I don't want to risk throwing in a PFC and having issues only to have it trace back to the PFC. On the stock ECU, it should work no problem. But added a PFC will be right up there on the To-Do List as just by swapping the REW into the REPU I'll be over the coveted "3 mod rule" with a basically straight thru exhaust no cat (which is actually all 3 mods right there in one- downpipe, cat delete/midpipe, and catback for a stock FD), custom intake, and front mounted intercooler.

Boost spikes will be difficult to manage even with two manual controllers. The stock ECU hates boost spikes and will aggressively fuel cut the rear rotor- and the rear rotor only- in overboost situations, which is BEYOND stupid by Mazda. No wonder why so many FD engines blew up when being modded.

I'm also not sure about a used PFC given my history LOL- with the FD I went through TWO bad PFC's used! So I might have to suck it up and buy a new one, which will also push back the time frame.

Well now I wouldn't say I love the "stock" twins. I think the sequential idea is fascinating. I am not a fan of how it is done on the FD controlled by an 8 bit computer through vacuum solenoids on an awful cast manifold for inefficient 25 year old Hitachi hairdriers with an RPM based switchover. I have been working on some coding for controlling twins with a 32bit microcontroller using PWM to fast servo operated valves similar to motorcycle EXUP valves. But until I have a workshop again its dead in the water.

I'm with you 100%! The stock twins are fascinating and an amazing technological feat given their design and implementation was in the late 80's/early 90's when conceived. Mazda debuted them on the Comso in as 1992 model year in 1991, which means they were being hatched in the late 80's. Given the material tech of the time, computer modeling, and limitations of car ECU's due to cost/size, they actually created a lot with a little. It's amazing how well they work given the time period they were born in!

That being- as you pointed out- they obviously have plenty of drawbacks. Not only design, time period constraints, and budget concerns Mazda couldn't really overcome such as the cast iron manifold that traps heat against aluminum rotor housings and acts as a heat sinks, and an 8-bit computer controlling the whole sequential mess but also 2 stages of fueling and 3 ignition circuits. Talk about an overworked little computer!

But Mazda clearly made some poor decisions that I think were influenced by not only mass production constraints, but also budget. Like putting the plastic solenoids ON TOP of the rotor housings AND under the upper intake manifold! So they bake! And their vacuum hoses and check valves bake! Not the best idea (along with that plastic air separator tank...seriously Mazda?). So there are obviously improvements 25 years later to be made.

I like the idea of new, stronger, metal-valve and bodied solenoids that should theoretically last forever or over 300K miles, and to have them relocated from under the UIM (although if they're metal and more durable probably no need other than serviceability). I think a microcontroller is a good idea- would that be to piggyback on the stock ECU? I'm pretty sure the PowerFC is far more capable, CPU wise, than the factory 8-bit ECU. The PFC does a pretty solid job of controlling the stock twins.

I have to say the most impressive thing with stock twins is they can make over 400rwhp sequentially. I believe the highest dyno I've seen the 432rwhp/399tq to the tires at 19psi or so sequentially. That's amazing considering the car put down 220rwhp or so stock.

[Dwduc]

I like the idea of new, stronger, metal-valve and bodied solenoids that should theoretically last forever or over 300K miles, and to have them relocated from under the UIM (although if they're metal and more durable probably no need other than serviceability). I think a microcontroller is a good idea- would that be to piggyback on the stock ECU? I'm pretty sure the PowerFC is far more capable, CPU wise, than the factory 8-bit ECU. The PFC does a pretty solid job of controlling the stock twins.

My thought was to initially operate it like a standalone boost controller. It would run a simple speed density based map and control the switchover at a point relevant to compressor efficiency instead of just RPM based. And with servo operated valves the prespool and sequence can be done with a single valve that will open gradually as the primary turbo begins to leave its efficiency range. I'm sure it will take alot of tinkering but that is just a good excuse for trackdays. And the custom mani will be good for TIG practice. For now its all just in my head (and on my computer) until I get my house done and can start such a project. I think the Power FC is a great upgrade to the stock ECU feature wise but performance wise its still a very dated 8 bit module running everything.

[chickenwafer]

My thought was to initially operate it like a standalone boost controller. It would run a simple speed density based map and control the switchover at a point relevant to compressor efficiency instead of just RPM based. And with servo operated valves the prespool and sequence can be done with a single valve that will open gradually as the primary turbo begins to leave its efficiency range. I'm sure it will take alot of tinkering but that is just a good excuse for trackdays. And the custom mani will be good for TIG practice. For now its all just in my head (and on my computer) until I get my house done and can start such a project. I think the Power FC is a great upgrade to the stock ECU feature wise but performance wise its still a very dated 8 bit module running everything.

Servos are a great idea, much more controllable than a simple solenoid! That would be cool.


Does anyone know of an aftermarket gauge package that can display wirelessly to a tablet? Like this:



Only things I can find are for OBD-2 cars, which the REPU obviously isn't and never will be. PLX Devices also has their aftermarket gauge sensors and a Bluetooth module for an app so it's close, but limited to only a few sensors. No speedo, tach, or fuel level sensor mainly, since I'd like to use this tablet to replace my entire instrument cluster instead of running a ton of analog gauges.

[chickenwafer]

I looked more into the tablet gauge idea, but only found really expensive options. So my new plan will be to see if I can get the stock speedometer and tach to work, and add a WB02 and boost gauge. I'll use the PFC Commander for water temp. Maybe I'll add an oil pressure gauge down the road. But trying to avoid the jet fighter cockpit look.

Also debating on the front cover situation. Originally, i was planning on keepimg the REPU fromt cover and oil pan because the REPU chassis engine mounts are attached to the front cover. So, theoretically, swapping to the REPU front cover makes it a nearly bolt in affair.

I'm having some second thoughts now because a little more happens on the front cover. First, the primary turbos oil drain is on the front cover. So I would have to find another spot to drain the primary turbo imto (maybe oil pan?) or drill and tap a fitting on the REPU front cover and make a custom hose since the stock drain is a flange fitting.

Second is the crank angle sensors that are used for ignition and fuel timing. These need to be a precise location, just a millimeter off amd the ignition timing and fueling is thrown off. These bolt to the fromt cover and pickup on the pulley. I'm unsure if they will be able to be mounted to the REPU cover so that's another wrench.

Last is the distributor hole, since I'm not running it anymore it will be removed. I can simply cover this, but unsure if the flange will contact anything like the throttle body elbow.

Using the FD fromt cover solves all the issues above, but predictably causes other issues. The only big issue is I loose the front cover located engine mounts which allow me to easily bolt in the engine. So I'd either need to weld something to the front cover, or use the FD engine mount locations. The ladder would be really diificult, given the FD mounts sit in a subframe and would be a terrible amount of work to get right.

At this point, leaning to using the FD cover. This also means using the FD oil pan, which I'm unsure will fit above the REPU crossmember. Ug.

[VRx8]

FC CAS ?

[chickenwafer]

FC CAS ?

Yeah, if I run the REPU front cover then that's what I'd have to do. The FC CAS has far less resolution than the stock FD CAS and more "float" or room for error. Additionally, no way to run the factory FD ECU it with. I plan on running the OE ECU not only for simplicity and to eliminate variables, but to keep cost low initially. Eventually, it will get replaced but I'm not sure how long it will be in there. Plus, I want to run the twin sequentially.

After considering all options, I decided I need to mount the REW motor the same way it gets mounted in a stock FD- with the proper FD subframe. That means aquring an FD subframe and welding it in to the REPU. Luckily, I've got a mint subframe today so that's ready to go

Going to be like this:



That's the FD subframe cut in half for just the rear motor mount section and welded in. I picked up the subframe today so I can work on getting it in the REPU. I plan on welding it in and then drilling through the frame and dropping a long grade8 stainless bolt with giant washers in that will do through the frame and into the subframe piece for additional support in-case of a weld failure.

Overall this is the "proper" way to mount the engine as,

-it's stronger than the front mount since there is no "twist" across the engine as with the front mounts
-Allows me to retain the FD front cover with the primary turbo oil drain, FD CAS, no dizzy hole, etc.
-I can also shift the engine slightly forward to move the FD trans shifter forward an inch or two to get it closer to the stock shifter hole

Lastly, I was also incorrect about the weight of the REPU. MotorTrend did an article that said the curb weight was 3050lbs, so that's what I thought. Well, I don't know what they tested but the average REPU seems to weight around 2750lbs. With mine, I removed the heavy ass rear bumper, spare tire and ratchet chain for the spare, and some weight reduction through advanced rust composite LOL.

My plan is to get the truck down to 2500lbs or so. 2500lbs with 340-360rwhp sequentially (eventually) should be HELLA fun.

[Shadowden]

When do you think you will be welding the subframe in? Would like to see that. You going to mig, tig, or stick it?

[chickenwafer]

When do you think you will be welding the subframe in? Would like to see that. You going to mig, tig, or stick it?

Most likely MIG. I'm no expert welder (hardly even an average one). My plan is to just get some sold beads on it and then drill a hole through the frame and the subframe and put a long bolt and nut through it. This will be a backup in case the welds fail. But you're welcome to come watch haha.

Today was a good day. I have to give a MASSIVE thanks for Eric and Matt because today wouldn't have been possible without Eric's muscle and Matt's engine hoist. Eric drove all the way up from the Springs to help me pull the engine and transmission out of the REPU. I had the engine pretty stripped with the exhaust, intake mani/carb, radiator, etc all removed. Only thing we had to do was drain oil, remove bellhousing bolts and motor mount bolts, and lift her out. And surprisingly, it went really smoothly. One bellhousing bolt was a little rounded but luckily I was able to get my Visegrips on it and it came right out.



We also pulled the 3spd auto gearbox with the engine hoist but chaining it to the hoist and lifting it out. Again, not too bad save for a stubborn blown trans mount bushing. And we managed to catch most of the trans fluid in the drain pan! Amazing!

And, I WAS going to save this info until it actually happened, but due to the mishaps today, might as well let the cat outta the bag. I bought a 13B-REW enigne off eBay! It's a complete swap with engine, harness, ECU, trans, etc. I actually bought it about 2 weeks ago. And today was the day I had scheduled with UPS freight for it to be delivered, which is why Eric was coming up.

So right on time, UPS comes with the engine. SWEET. It's on a pallet and wrapped in black shrink wrap so I can't easily see anything about it. As the UPS guy is lowering it off the liftgate Eric notices it looks like a TurboII trans mount on the back. Weird. So the pallet is dropped off and I cut the shrink wrap off. That's a T2 tranny...and it's a T2 engine. WTF!! I check the packing slip and it's has all the right info on it.

So this is what I ordered:





I got THIS:





Clearly a T2 engine/trans and not what I ordered.

I call up the number for the engine import company fully expecting no one to answer or to get a machine and leave a message. Surprisingly, someone answers and I tell them what's going on, he takes all my info and says he'll call me in an hour or so. 30 minutes later I get a call, he asks me to email him pictures of the freight label and engine. I do and he calls me right back and says they sold the engine I bought and the one I received the same day and his shipping guy got them switched somehow (great). Luckily the engine I ordered is yet to be delivered and still sitting in a warehouse, so they are rerouting it and having UPS freight come pickup the wrong engine tomorrow.

I don't have the right engine yet with all the pieces, but so far they have handled this pretty good, even though it's 100% their mistake. I always say everyone makes mistakes, it's how we correct them.

While this was all going on, Eric got his rotary compression tester and we ran a comp test on the T2 engine just to see what it's got. It's showing 81psi front rotor and 0 (as in ZERO) psi rear rotor. We were having a hard time believing the tester so I hook up an old analog piston comp tester to the rear rotor and sure enough, during cranking, the needle never moves. The rear rotor is completely toast!

So, it will be another week until I have the right engine, which I'm hoping to maybe find the time to weld the subframe in so once I get this new engine I can work on dropping it in. I will for sure compression test it as this T2 engine now has me worried.

So there's the updates. Plan is to have the RIGHT engine by next week and hopefully that one has compression and I can drop it right in once I get that subframe welded in.

Again, BIG THANKS to Eric and Matt!

[RX-7 Chris]

Those engines look slightly different, lol. Glad they are working it out.