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Rotary MX-5 Nordschleife Build Thread

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Old 01-22-18, 06:57 PM
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Arrow Rotary MX-5 Nordschleife Build Thread

Greetings from Germany,
this project has been going for a while and since I fell in love with rotaries, I'd like to document the process of the swap properly. Any advice would be greatly appreciated!




The car is a '95 NA MX-5. I live pretty close to the Nürburgring so the car hits the Nordschleife regularly.
I'd like to focus this thread on the rotary conversion, but for refence, this car has undergone many mods, such as:
  • Engine: 116hp cams, cold air intake, 14° timing, BBK stainless steel catback
  • Suspension: MeisterR CRD+ coilovers, fully restored suspension parts, regreasable polyurethane bushings
  • Brakes: braided lines, discs upgrade to 255 front 250 rear, no fancy pads yet
  • Wheels: 205/50 Nankang AR-1 on Zender MAS 15"
  • Chassis: Stitch welded, thicc rollbar and doorbars, stainless steel framerail reinforcements
  • Interior: Sparco Sprint bucket, OMP Racing 3" harness, Momo Monte Carlo wheel plus extension
  • Exterior: gv lip, kg works spoiler, moss miata turn signal intakes, hardtop, bumper cut
  • Other: Steering depower including welded pinion, some weight savings
Despite all this fancy stuff, as of right now I am actually a lousy apprentice and broke constantly (and since rotaries especially) but follow my dreams every single day! If anything, this is a manifest of what is possible if you try hard enough, put your all into it and fight for what you love.

Keep in mind rotaries are a rarity in this country, there is no solid community for rx-7's, and parts are very hard to find. Since I started all this, the few people I have gotten to meet on my rotary journey happened to become very good friends. Now, a few months ago a '86 RX-7 N/A 6-port popped up for sale, which I ended up buying for 250€. It was said to have pretty damn rusted chassis and an engine with very low compression. My intend was to either restore the car itself, or use the engine as a mockup for building mounts and all that.



The chassis was, as expected, not worthy of saving. Rust had completely removed the sills, gone up the B pillar, big holes in the floor pan, and so we cut it in half and scrapped it. Maybe the parts will be of use to other FC owners. The engine in our hands, teardown began and to our surprise it was in pretty good condition.



We cleaned everything up afterwards and put the rotors into an ultrasonic bath. Good as new! Everything is in spec, however, the housings have slight chrome chipping.



It's a shame, but I guess I will have to resort to buying new housings, which as of right now is heavily delaying the project.
Anyway, so at this point I had no idea how to tackle the drivetrain adaption. My guess was building new driveshafts or something. But when I had both transmissions next to each other, they looked oddly similar.



Turns out I could just bolt the mx5 rear housing and speedo gear onto the rx7 tranny, and the PPF and driveshaft bolt right up. Nice. No custom fab needed other than engine mounts, I guess. Maybe further down the road I will try to swap the mx5 tranny internals over too.

A few days ago a friend of mine then told me that one of his friends was selling a broken 12A. I did some research, and it turns out the 12A irons have BIG primary ports, which I could potentially use for more power. Hoped for the best and picked it up two days later for 80€. Indeed there was an apex seal failure which killed rotors and housings, however all irons were in amazing condition! Lucky!



Now that this was sorted, I ordered a full rebuild kit including all bearings for about 1100€ from Atkins, and it's time to build the engine. Here's where it gets tricky and I could use any advice. So far it's only been theory, now it's turning practical. I asked a knowledgeable and very cool rotary person in this country for his advice, took a 15 hour total trip to his place and back so he could draw ports for me.

My plans so far are:
  • Use 13B rotating assembly
  • Use 12A center iron and port it to close a fair bit later
  • Machine water seal grooves into the 12A center plate
  • Use 13B end irons, combine 5th and 6th port all the way through, and add a half bridge around the aux area
  • Use new T2 housings and port the exhaust area to close a tad later and be more square-ish
  • Port match the LIM, in the future I would like to build an intake with bigger diameter everywhere to suit the ports
  • Add primary injector bungs somewhere, since the 12A center iron doesn't have them
  • Remove the OMP, only do premix (neither engine had an OMP sadly, too expensive to get one. but I'm cool with premix)
  • Increase oil pressure by modifying the regulator
  • Some oil gallery modifications
  • Remove Fuel Pulsation Dampener, for safety
  • Make an RX-8 oil pan work somehow. The REW pan sticks out under the subframe, and I'd love to avoid tilting the drivetrain upwards too much
  • Get the engine to run with stock ECU and build a Megasquirt further down the road

I think that's about it. Please let me know your thoughts and suggestions! I'm new to the rotary game and trying my best to learn as much as possible. Any help would be greatly appreciated!

Last edited by Kraid; 01-23-18 at 08:09 AM.
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Old 01-23-18, 10:09 AM
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Greetings!

First off, as you've discovered, the transmissions are the same family. Instead of putting the tailshaft from the Miata on the RX-7 transmission, what I would do is put the front portion of the RX-7 transmission case on the Miata transmission. The input shaft has to be modified; https://www.mazdatrix.com/faq/miatainputshaft.htm has dimensions and pictures.

The following engine stuff is my opinion - it's easy for me to suggest things to buy that you may not want to.

If you're getting new rotor housings, what I would do is use all of the 12A irons, and get GSL-SE rotor housings. This combination won't need to be machined to have water seals. The 13B end irons for the 6-port engines can have the ports combined into one port like you have in mind, but it is mostly done for a circuit racing class in the United States where those irons are required to be used. The 12A ports runners are smaller, but the 6-port irons with the ports combined are actually too big.

I would go straight to a Weber IDA style manifold with a throttle body and 2 injectors, controlled by a megasquirt. The 12A irons on a 13B leave you without a manifold that will accept the stock EFI manifold, unless you modify it, which is the downside.

If you prefer your idea of machining the 12A center iron for water seals and temporarily using a modified stock EFI manifold, I wouldn't worry about machining the iron for primary injectors. Two of the Injector Dynamics 1000cc / minute injectors should be enough for the power you will be making, and the technology has come far enough where it should idle well too with tuning. I would not use the stock ECU - to me, trying to make a 30 year old ECU and wiring harness work would be frustrating wasted effort.

*edit* Oh, I'm not sure the RX-8 oil pan will work with the front covers you have, but it does seem like a good starting point to make something that would fit.

Good luck with whichever way you build things - to me, a 13B in a Miata seems like an obvious great swap idea. I started to make one for a circuit racing class before I realized I will have better competition building a 1st gen RX-7.
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Old 01-24-18, 05:03 AM
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Hey man, thanks I really appreciate the feedback!

Ah yes, the tranny swap is what I had in mind, however I have had no success in removing the bell housing off the miata tranny yet, despite removing all c-clips I could find. Will try again some time.

Interesting! I have read before that the 6-port Secondaries are too big, and brought it up several times to the expert, however he elaborated that this setup would be a better choice for good power since, as you have mentioned, the port runners are bigger. Then again, he usually deals with turbo setups, in which I can imagine it would be an environment where it would maybe work well? But for N/A? ****, I don't know what to believe anymore. Wish I could try out both setups and see what works best, but then I would need to buy an expensive set of gaskets every time, right?

Originally Posted by Kenku View Post
The 12A irons on a 13B leave you without a manifold that will accept the stock EFI manifold, unless you modify it, which is the downside.
Actually, the stock LIM seems to have just enough material for the ports to be matched up. Here's a crappy pic I made



But yeah, it will probably not be possible to extend the diameter all the way through since the dremel can't reach those places, so further down the road I'd like to craft my own manifold out of steel tubing.

Originally Posted by Kenku View Post
I wouldn't worry about machining the iron for primary injectors
Oh actually I intended to put the injector holes into the manifold, as far up the collector as possible. Appearently that makes for better power because the fuel has more time to evaporate or something.
Wait, are you saying the stock injectors will not be enough for this setup?

Yeah, the RX8 oilpan is so useful because the oil pickup tube thing is shorther. The front part of the pan will not fit, but modifying it won't be a problem.



Thanks man, the feedback is really valuable. I will do my best!
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Old 01-24-18, 05:08 AM
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Hey man, thanks I really appreciate the feedback!

Ah yes, the tranny swap is what I had in mind, however I have had no success in removing the bell housing off the miata tranny yet, despite removing all c-clips I could find. Will try again some time.

Interesting! I have read before that the 6-port Secondaries are too big, and brought it up several times to the expert, however he elaborated that this setup would be a better choice for good power since, as you have mentioned, the port runners are bigger. Then again, he usually deals with turbo setups, in which I can imagine it would be an environment where it would maybe work well? But for N/A? ****, I don't know what to believe anymore. Wish I could try out both setups and see what works best, but then I would need to buy an expensive set of gaskets every time, right?

Originally Posted by Kenku View Post
The 12A irons on a 13B leave you without a manifold that will accept the stock EFI manifold, unless you modify it, which is the downside.
Actually, the stock LIM seems to have just enough material for the ports to be matched up. Here's a crappy pic I made



But yeah, it will probably not be possible to extend the diameter all the way through since the dremel can't reach those places, so further down the road I'd like to craft my own manifold out of steel tubing.

Originally Posted by Kenku View Post
I wouldn't worry about machining the iron for primary injectors
Oh actually I intended to put the injector holes into the manifold, as far up the collector as possible. Apparently that makes for better power because the fuel has more time to evaporate or something.
Wait, are you saying the stock injectors will not be enough for this setup?

Yeah, the RX8 oilpan is so useful because the oil pickup tube thing is shorter. The front part of the pan will not fit, but modifying it won't be a problem.



Thanks man, the help is really valuable. I will do my best!
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Old 01-29-18, 06:54 AM
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Originally Posted by Kenku View Post
Greetings!

First off, as you've discovered, the transmissions are the same family. Instead of putting the tailshaft from the Miata on the RX-7 transmission, what I would do is put the front portion of the RX-7 transmission case on the Miata transmission. The input shaft has to be modified; https://www.mazdatrix.com/faq/miatainputshaft.htm has dimensions and pictures.
Thanks for the link, this was my original intent, however I had no success in removing the bell housing from the mx5 tranny. Will try again some time.

Originally Posted by Kenku View Post
If you're getting new rotor housings, what I would do is use all of the 12A irons, and get GSL-SE rotor housings. This combination won't need to be machined to have water seals. The 13B end irons for the 6-port engines can have the ports combined into one port like you have in mind, but it is mostly done for a circuit racing class in the United States where those irons are required to be used. The 12A ports runners are smaller, but the 6-port irons with the ports combined are actually too big.
Good point on getting GSL-SE rotor housings. Machining won't be a problem , so I'd like to go down the route anyway so I can swap the end irons out if I want to. Regarding those, I believe you are right, I have read this statement many times, and wanted to use the 12A irons initially as well, however my good friend rotary expert insisted that this would be the better option and I do not want to disappoint him, as it is only due to his help that I've gotten this far. So what I think I will do is build it like this first, and swap the irons later on when I get the chance to get a direct comparison. Gaskets won't be too expensive, and some R&D would be interesting. So please keep the info and help coming still, I highly appreciate the valuable input, and it's the reason I made the thread in the first place.

Originally Posted by Kenku View Post
The 12A irons on a 13B leave you without a manifold that will accept the stock EFI manifold, unless you modify it, which is the downside.
So I tried measuring out the 6-Port Manifold and it seems to fit just enough, here's a pic of how I will have to port match.



Originally Posted by Kenku View Post
If you prefer your idea of machining the 12A center iron for water seals and temporarily using a modified stock EFI manifold, I wouldn't worry about machining the iron for primary injectors.
Actually, I intended to put the injectors holes into the manifold. It was told that putting them as close to the collector as possible, as the additional time the fuel has to evaporate will even help power a bit.

But yeah, this is only temporary to get it to run. Hopefully I will be able to switch to ITBs at some point.

Originally Posted by Kenku View Post
Two of the Injector Dynamics 1000cc / minute injectors should be enough for the power you will be making, and the technology has come far enough where it should idle well too with tuning. I would not use the stock ECU - to me, trying to make a 30 year old ECU and wiring harness work would be frustrating wasted effort.
Will the stock injectors not be enough?

Originally Posted by Kenku View Post
*edit* Oh, I'm not sure the RX-8 oil pan will work with the front covers you have, but it does seem like a good starting point to make something that would fit.
Yeah, the pan won't fit as is, but I will weld something nice up. I'm just glad that by using the rx-8 oil pan and collector, the drivetrain won't have to be tilted upwards or anything.

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Old 01-29-18, 08:38 AM
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As I said, I understand if you have reasons for trying things differently than I suggested - having a friend willing to help is a good reason! Even with what is, to me, a less than optimal port layout should be pretty fun and make good power.

I was also trying to be cautious... from talking on forums over the internet, you never know what people's abilities are. If machining the water seal grooves is not an obstacle, then it's certainly interesting to try.

It seems like the intake manifold you have does things differently than the ones we have - over here, the 5th and 6th ports are closed by rotating sleeves and not a butterfly. So on our manifold, the part that you showed that bolts to the engine, has a second part with runners that bolts to a third part with the plenum and throttle body. The part with the intake runners on our manifolds has a pair of injectors pointing down the outer ports. I would just try using those instead of machining things for 4 injectors, simply because it's less work into a temporary setup.

The stock injector size (over here anyway) for the non-turbo engines is 4x 440cc/minute injectors, and the earlier 13B we got in the 1st gen RX-7 were 2x 680cc/minute injectors. The 4x 440cc/minute injectors should be enough if you use all 4. The more modern Bosch EV14 style injectors (like the Injector Dynamics 1000cc/minute ones) open faster so are easier to tune idle and low load performance, and flow enough fuel for a naturally aspirated engine.
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Old 01-31-18, 03:20 AM
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Originally Posted by Kenku View Post
As I said, I understand if you have reasons for trying things differently than I suggested - having a friend willing to help is a good reason! Even with what is, to me, a less than optimal port layout should be pretty fun and make good power.

I was also trying to be cautious... from talking on forums over the internet, you never know what people's abilities are. If machining the water seal grooves is not an obstacle, then it's certainly interesting to try.
No worries, I understand it is likely not optimal, so ideally I would like to try both at some point thanks to your input.

Originally Posted by Kenku View Post
It seems like the intake manifold you have does things differently than the ones we have - over here, the 5th and 6th ports are closed by rotating sleeves and not a butterfly.
Interesting! I was wondering what was up with that. The good thing about this is, I won't have to put putty or anything into that corner as it is already rounded up from factory.

Thanks for the helpful suggestions on injectors. Although this topic is very alien to me so far, I will see what I can do once the engine is properly mounted in the car.
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Old 01-31-18, 06:15 AM
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It is a bad idea to port match the primary ports in the intake manifold. Air does not like to diffuse and turn at the same time.
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Old 01-31-18, 06:53 AM
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Originally Posted by peejay View Post
It is a bad idea to port match the primary ports in the intake manifold. Air does not like to diffuse and turn at the same time.
Can you please elaborate? I'm not sure I understand
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Old 01-31-18, 06:05 PM
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If you port match, you will be greatly increasing the difference in runner length between the short side of the turn and the long side of the turn. This creates turbulence and when it gets really bad, part of the port will actually flow backwards.
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Old 04-03-18, 06:34 AM
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Sorry I've been gone for a while, winter killed the mood, so I only got to do new brakes, harness, and some other bits to be ready for the track this season. But the sun is finally back out and I started doing some work!

Originally Posted by peejay View Post
If you port match, you will be greatly increasing the difference in runner length between the short side of the turn and the long side of the turn. This creates turbulence and when it gets really bad, part of the port will actually flow backwards.
I see, thanks for the advice. Looks like I will be welding a custom manifold that will keep the diameter all the way up to the throttle body

Here's some progress:



First, I needed a way to work on and assemble the engine, so I bought this engine stand and welded some supports on to strengthen it, as well as built a rotary mount.



Next I got the dremel ready and went ahead to combine the ports. It's a start, but needs to be refined. The thing I noticed here is that I think I can't do a full 0-like shape, because there is not enough material. There is more to be gained than this tho.



Meanwhile in the tranny department. I want to keep the mx5 gearbox because of short ratios, so the input shaft has to be shortened to fit the 13b. One way to do this is to fully disassembly the gearbox until the input shaft is loose. Alternatively I went ahead and cut the bell housing off, because the first half of it is required to keep the input shaft tight so it doesnt wobble around, and put the whole gearbox into the lathe. put 4th gear in for 1-to-1 transmission, and off we go.



More to come soon Step by step.
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Old 04-03-18, 08:10 AM
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That is not a way I've ever seen the input shaft work done before, but I like it - I think that's probably a lot less of a pain than separating the input shaft even if you have all the transmission tools built. And believe me I know all about winter making it harder to make progress, I haven't even gotten around to moving the next race car shell to where I wanted to.

Glad to see you're back to making progress though, keep at it!
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Old 04-10-18, 04:33 AM
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Originally Posted by Kenku View Post
That is not a way I've ever seen the input shaft work done before, but I like it - I think that's probably a lot less of a pain than separating the input shaft even if you have all the transmission tools built. And believe me I know all about winter making it harder to make progress, I haven't even gotten around to moving the next race car shell to where I wanted to.

Glad to see you're back to making progress though, keep at it!
Thanks man, appreciate it. I will

Some small updates. "defect" 12A including most parts on the outside popped up near me for pretty much no money. How could I say no? Picked it up and disassembled it the same day. In addition, I found a bunch of individual 12A parts that I seperately bought as well. Not sure what I'm gonna do with it, but I just can't NOT buy them, you know? I mean, these parts are near impossible to find now, and it will only get worse.



It's kind of fun, you know? You never know what its like inside, so the tension builds up the further you tear it down.
Turns out one Apex seal broke and destroyed and broke a Rotor, just like on the other 12A. Both housings were scrap too, but the rest was fine. Got enough parts to build a 12A after the 13B now if I wanted, whats cool is I could grind the 'broken' 13B ones down to 12A size because they are only chipped on the outside. Some other time.



Sent the rotors off to ultrasonic cleaning.

Not much more done in the rotary department, was busy with... other things

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Old 04-10-18, 09:20 AM
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when port matching the secondaries be careful on the inboard side:



the casting gets very thin once you take apart the divider and you can easily poke through so don't make it 100% flush with the gasket or you will be calling your welding friend to fix a hole
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Old 04-26-18, 07:04 PM
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@fidelity: Thanks for the hint!

Some progress.
The new chassis arrived, so we stripped it down completely, put an empty 13b with the tranny in and bolt it up to the PPF and driveshaft to see where it ends ups.





In general, it fits pretty good. there are a bunch of issues, but nothing that we can't solve. The subframe is in the way, my idea is to modify the PPF and use an early-year short diff with a later-year short driveshaft to move the engine even further back. Some parts of the subframe will still have to be modified/removed. The fuel/brake lines are in the exhaust header area, will have to remove them and re-route them somewhere else. Oil filter flange hits the firewall, gotta cut some away and maybe relocate the oil filter? There needs to be more room in the tranny tunnel for the clutch slave cylinder, I think. It might be a good idea to cut the upper part of the front tranny tunnel up and move it further up. regarding front motor mounts, I have seen people use a tube that connects left and right frame rail, but I think I would rather weld mounts onto the subframe. stock intake and steering column have enough room.

I think that's all so far from what I have noticed. Exams soon, so back to studying
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Old 04-26-18, 10:47 PM
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Grassroots Motorsports magazine installed a 13B turbo into a Miata last year, their project page could be a good reference. One of the articles shows how much they cut out of the firewall to make it fit.
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Old 04-27-18, 09:55 AM
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Originally Posted by j_tso View Post
Grassroots Motorsports magazine installed a 13B turbo into a Miata last year, their project page could be a good reference. One of the articles shows how much they cut out of the firewall to make it fit.
Oh, nice! These articles describe almost the exact some issues, only with more on the top due to the turbocharging. Thanks for sharing, this is good reference!
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Old 04-27-18, 09:55 AM
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Wow, this is great !
I've seen a rotary miata once (on internet, of course) and I'm finding out there are plainty of them in the world... Anyway, I'm very excited about your project !

By the way, I'll be hitting de Ring this week-end with my miata, so if you're around, let me know !
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Old 04-27-18, 10:03 AM
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Originally Posted by tomatoto View Post
Wow, this is great !
I've seen a rotary miata once (on internet, of course) and I'm finding out there are plainty of them in the world... Anyway, I'm very excited about your project !

By the way, I'll be hitting de Ring this week-end with my miata, so if you're around, let me know !
Thanks dude!
I didn't plan on going (yet), but maybe you want to join me and many other mx5 enthusiasts at the "SPS Season Opening"? Its only about an hour away from the Ring on Saturday morning.
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Old 05-02-18, 05:44 AM
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So I made some progress and started reviewing my options with the parts I have. Ideally I wanted to move the engine about 30mm back.



As it turns out, a late model MX5 diff combined with the RX7 shaft will give me the length I need without the need of an adaptor flange. From what I could tell, drilling new holes into the PPF should not cause any fitment problems. If it works out, the new engine position should be something like this:



This, combined with modifying the subframe a bit, should give enough volume for the oil pan, while still providing enough clearance for intake and oil filter (once that framerail connector piece is removed).
Anyway, how well it actually works out remains to be seen. I also started on some other work like dremeling the LIM as well as removing all the unnecessary hardware from the throttle body, pictures for this next time.
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Old 05-02-18, 06:28 AM
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That's great !
Can't wait to see the next steps !
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Old 05-15-18, 02:47 AM
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Yeah.
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Old 05-15-18, 04:52 PM
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I've only street ported 12a side irons. Dont know a whole lot about 6ports. But don't u need that bridge between the two ports so your rotor side seals dont fall out? I myself built a 4 port 13b with streetport 12a side irons and s5 rotor, eshaft, flywheel. Gslse housings. Only thing that sucks is intake manifold selection is limited. 1976-78 13b.
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Old 05-15-18, 05:14 PM
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Makes me wish I still had my Miata......and lots of money/time to do this same project
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Old 05-16-18, 04:05 AM
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Originally Posted by Holdfast View Post
I've only street ported 12a side irons. Dont know a whole lot about 6ports. But don't u need that bridge between the two ports so your rotor side seals dont fall out? I myself built a 4 port 13b with streetport 12a side irons and s5 rotor, eshaft, flywheel. Gslse housings. Only thing that sucks is intake manifold selection is limited. 1976-78 13b.
Well, as far as I know they won't fall out, however someone gave me advice on there being rumors that the seals will start to "bend down" because the port is so big and wear down absurdly quick. But it has not been confirmed, and then again I did not port in any other direction, so maybe it will be fine? Unless anyone can say for sure, I'm willing to try! Worst case, the seals would probably wear uneven and the engine would lose compression after a while, in which case I would switch to 12A irons.

That sounds like a great engine man!
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