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dollraves
dollraves Reader
3/21/13 9:46 a.m.

No, really this time!

The Rotary Miata project got waylaid by my own recovery from surgery in $2011, then I was completely consumed with a new job in $2012. I quit that job in October and determined I'd take at least three months off, if not an entire year. After three months off and a quick/fun two month contract, I'm finally in the garage and dusting off the ol' RX5.

I'll write this build thread starting with the absolute basic decisions. I want everyone who considers this project to see the entire thought process in the hopes that it helps them, too. If you're considering this swap, keep in mind that I have access to two entire cars, so I may have more options than if you only have an RX7 engine. (Also, I have only a vague idea of what I'm doing, so this gives folks time a chance to give feedback...)

History: I paid $500 to drag the silver '91 Miata out of a field in Boulder Creek (damp spot in the mountains southwest of Silicon Valley), and another $700 for a totaled '90 RX7 from a local kid (body was TRASHED, but it cranked when I asked to see it run, no oil leak I could see, so here's hoping it's good). Starting budget: $1200. I sold the Miata engine for $350 and I've sold enough of the RX7 interior to more than recoup the $700. Effective budget: $194.

Getting there...Prepping for surgery...

Decision 1: Engine placement

In poking around the intarwebz, people who've done this conversion before have gone one of two ways:

Option 1: Cut the firewall and transmission tunnel

Option 2: Cut and weld the subframe

At first I thought this would be my primary decision, but this looks like it depends somewhat on which route I go for the transmission. Which leads to…

Decision #1A: Transmission

I've been told that RX7 racers will pull just the gearbox out of the Miata and swap it into the RX7 because the gearing is better. In theory, I should be able to swap either the gearbox and tail, or just the tail, of the Miata transmission to the RX7.

Option 1: Swap gearbox and tail from Miata transmission to the RX7 bellhousing

The gearbox on the Miata is iffy - it couldn't stay in fifth gear the few months I drove it (nothing like cruising along at 70 and suddenly popping into neutral!). On the one hand, how likely am I to get to fifth gear in the Challenge? On the other, it's a problem, and ignored problems tend to just get worse. Pro here is that I'd get the "better" gearbox and the mounting point for the transmission.

Option 2: Swap just the Miata tranny tail to the RX7 bellhousing and gearbox

In setting the RX7 tranny next to one I have lying around, it looks totally doable. Pro here is that I'd get the hopefully working gearbox out of the RX7 and the mounting point from the Miata transmission.

Option 3: Use the entire RX7 transmission

Given the apparent interchangeability of the transmission pieces, I don't see any real reason to go this route, since we lose the Miata transmission mount and then have to figure out how to mount the transmission. Not seeing any real advantages here.

A quick pic comparing the two trannies (the clean shiny stunt Miata tranny, bottom, is not the actual tranny to be used) below. The RX7 tranny is ever so slightly longer, I think; will get measurements up soon. RX5 Initial Fitting: Trannies

Decision #1B: Engine Placement, revisited

Since we'll most likely be using the Miata transmission tail, we're thinking just push the engine forward and into the subframe.

Option 1: Splice the two subframes together

My buddy Jesse is confident enough in his meausuring and welding skills to cut and splice the Miata and RX7 subframes together in such a way that we open up the spacing for the engine and we're using the stock RX7 motor mounts. We'll have to pull the steering control arm (is that the right term?) to get it the engine in, but shouldn't have to move it or anything too janky.

Option 2: Build a tubular subframe from scratch.

J's leery of this; I feel good enough about my math skills that I think could figure this one out, but I'm inclined to let Jay do the splicing since he feels good about that, and if we don't like the results, go tubular. Minor downside: since the axles for the front wheels are attached to the crossmember, we may end up having to redo the suspension.

Here are a couple of pics on the initial fitting. More pics on Flickr. RX5 Initial Fitting: From the CockpitRX5 Initial Fitting: Bottom Front

The goal this week is to get the engine and tranny into the car. I'll let you know which way we go in a few days.

(Edited for pic placement and HTML)

dollraves
dollraves Reader
3/21/13 10:02 a.m.

Decision #2: Fuel Management

Jesse was concerned that the Miata fuel pump couldn't feed the RX7 engine and suggested a carb conversion + fuel cell (Jay builds real RAZKARZ, so I was willing bow to his superior knowledge). I hadn't read of anyone going this route, so assumed the Miata fuel pump could handle it; SVRex confirmed this by pointing out that the Miata fuel pump runs at 35-38 psi @ idle, 48-50 psi @ atmospheric and the RX7 pump is capable of 49-71 psi @ the pump outlet, but it is regulated to 37 psi, so fuel feed shouldn't be an issue.

But, in the interest of fairness, I weigh it in.

Option 1: Carb conversion + fuel cell

RotaryShack sells a carb conversion kit that is spendy - $1k of my budget. Ouch! But, I pinged Robert at RotaryShack to see if he had any second runs or returns he'd be willing to let go cheaper. :D Pure cost alone will most likely rule this out, but I do have room in the budget if I want to go that route. While in theory, it would simplify the tuning down to a screwdriver, I know nothing about carburetors and tuning would be a bit of a learning curve.

Option 2: Stick with RX7 fuel injection and go MegaSquirt

Depending on which way I go, a slightly or significantly cheaper option than the carb conversion, and I have a friend or two (o hai, Johnny Pruitt!) that could help me with the tuning. :D

Option 3: Stick with the RX7 fuel injection and ECU

Curmudgeon suggested swapping out the S5 ECU for the S4 ECU, but either way this is the cheapest option. Limits the tuning, but less work all around.

Edit: Will worry about engine management after engine is in car, obviously, but figured I'd throw this out there for feedback.

jere
jere Reader
3/21/13 10:28 a.m.

Great project idea!

Why didn't mazda put the rotary in the Miatas to begin with?

ransom
ransom GRM+ Memberand UltraDork
3/21/13 11:13 a.m.

Regarding engine/trans placement, how far fore/aft is the difference between the two rough placement options?

The rotary lump is compact, but not particularly light. If using the Miata trans mount shoves the whole engine/trans assembly forward several inches, I'd be thinking very seriously about fabricating another trans mount option and carrying that stuff as far back as possible...

DaveEstey
DaveEstey SuperDork
3/21/13 11:25 a.m.

I have a RacingBeat carb setup (manifold and Webber DCOE side draft carb) you can try for the cost of shipping. If you don't like it you can just send it back.

I don't have any near-term plans on using it for my RX7, so it's hanging out on my work bench. It came off an IT7 that was running a 13B six-port.

dollraves
dollraves Reader
3/21/13 11:36 a.m.
ransom wrote: Regarding engine/trans placement, how far fore/aft is the difference between the two rough placement options? The rotary lump is compact, but not particularly light. If using the Miata trans mount shoves the whole engine/trans assembly forward several inches, I'd be thinking very seriously about fabricating another trans mount option and carrying that stuff as far back as possible...

Not sure how far forward until I bolt the tranny on and try refitting, but it would be mostly be downward into the current subframe compared to the stock engine. In theory, the whole rotary assembly is lighter than the stock engine, but I don't have a way of weighing them, especially since I've sold the stock Miata engine.

The shifting forward is in comparison to where people who cut the firewall put it; I'm not convinced it's enough of a difference from stock to justify cutting into the firewall. I'd rather have the firewall intact. :D

I'm also not finding any corner weights reported for either version.

dollraves
dollraves Reader
3/21/13 11:41 a.m.
DaveEstey wrote: I have a RacingBeat carb setup (manifold and Webber DCOE side draft carb) you can try for the cost of shipping. If you don't like it you can just send it back. I don't have any near-term plans on using it for my RX7, so it's hanging out on my work bench. It came off an IT7 that was running a 13B six-port.

If it looks like I can do it reasonably in the budget, I'll PM you before I buy. :D

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
3/21/13 11:47 a.m.

About the fuel pump: no problem there. The Miata pump will happily do the pressure you need and it'll provide enough fuel flow for 250 rwhp or so. Unless there's a hairdryer on the side of that box of spinning triangles, I don't forsee a problem.

dollraves
dollraves Reader
3/21/13 11:51 a.m.
Keith Tanner wrote: About the fuel pump: no problem there. The Miata pump will happily do the pressure you need and it'll provide enough fuel flow for 250 rwhp or so. Unless there's a hairdryer on the side of that box of spinning triangles, I don't forsee a problem.

Thanks for the confirmation, Keith!

Gearheadotaku
Gearheadotaku GRM+ Memberand UltraDork
3/21/13 4:11 p.m.

Good to see you again Doll! Wish you success at the challenge!

SVreX
SVreX MegaDork
3/21/13 7:19 p.m.

(Barely audible chant, increasing to a violent senseless pounding in your skull)

megasquirt, megasquirt, megasquirt, Megasquirt, Megasquirt, Megasquirt, MEGASQUIRT, MEGASQUIRT, MEGASQUIRT, MEGASQUIRT, MEGASQUIRT, MEGASQUIRT,

Luv ya, babe!

MrJoshua
MrJoshua PowerDork
3/21/13 7:24 p.m.

When you mention cutting the firewall-are you talking about the "ears" in the engine bay, or do you mean actually cutting the firewall. When you cut the firewall what parts are you making room for?

SVreX
SVreX MegaDork
3/21/13 7:27 p.m.

Start with the stock RX7 ECU and fuel injection. Then switch to megasquirt.

The carb conversion is more expensive then you realize. Don't forget the pump, filters, lines, etc.

Let Jesse do the subframe splice. It's down and dirty, but fully functional- that's the way I did the SVreX. Easy swap later, if you want tubular. No need for the added work now.

I'm with Ransom on the weight. Don't move it too far forward.

dollraves
dollraves Reader
3/21/13 10:55 p.m.
MrJoshua wrote: When you mention cutting the firewall-are you talking about the "ears" in the engine bay, or do you mean actually cutting the firewall. When you cut the firewall what parts are you making room for?

When I first read about rotary Miata conversions ages ago, several of the conversions actually cut into the firewall to make room for the bell housing, then split the transmission tunnel (the bell housing is wider on the RX7 than the Miata) and pushed the whole thing back into the car. I could be wrong, but seems to have been done in the earlier conversions; most of what I read now is cutting into the subframe, making a tubular subframe, or spending $750-$1k to buy a tubular subframe from Flyin' Miata or another aftermarket parts place.

dollraves
dollraves Reader
3/21/13 10:57 p.m.
SVreX wrote: Start with the stock RX7 ECU and fuel injection. Then switch to megasquirt. The carb conversion is more expensive then you realize. Don't forget the pump, filters, lines, etc.

I'm really not into the carb conversion; it's an expense that isn't reasonable for this project (and, it's a layer of complication for me, who's never handled a single car with a carb). But, I did want to list it as it is a viable alternative for folks doing this conversion that may stumble upon this thread. :D

SVreX wrote: Let Jesse do the subframe splice. It's down and dirty, but fully functional- that's the way I did the SVreX. Easy swap later, if you want tubular. No need for the added work now. I'm with Ransom on the weight. Don't move it too far forward.

Jesse & I were talking about it over lunch; going to pull the subframes out to do the splicing. Going to give it a try tomorrow or over the weekend before he heads home to Oregan. Yay! My first welding lesson!

dollraves
dollraves Reader
3/21/13 11:32 p.m.
jere wrote: Great project idea! Why didn't mazda put the rotary in the Miatas to begin with?

To give us enthusiasts something crazy to do. :D

Sky_Render
Sky_Render Dork
3/22/13 8:02 a.m.

Oh, this is an awesome idea.

wvumtnbkr
wvumtnbkr GRM+ Memberand HalfDork
3/22/13 2:28 p.m.

Stick with the stock ecu and engine controls. If you want to tune it, get an Rtek 2.X. They are not spendy, you can get one used, and they are easy to tune with.

It is simply a way to modify the stock controls (like a Hondata).

DO NOT MIX AND MATCH S4 and S5 electronics. There are several things that are different enough that you can run into real issues. For example:

The S5 will want to see the electric OMP connected.
The injectors have different impedance(s).
The S5 has controls for the S5 intake. (The S4 will not).

I am sure there are a few other items I am forgetting! Really, taking the entire control system out of the RX7 shouldn't be that difficult. If you have any RX specific questions, there are a number of us on this board that can help!

As far as the megasquirt is concerned... Is it easy to get them to run a rotary? Can they do the timing of the leading and trailing spark plugs? That seems to be the major stumbling block to me.

Rob R.

fidelity101
fidelity101 HalfDork
3/22/13 2:41 p.m.

Screw the OMP go premix, your engine will thank you, however the envinroment is already mad so don't worry about that.

dollraves
dollraves Reader
3/22/13 6:39 p.m.
wvumtnbkr wrote: Stick with the stock ecu and engine controls. If you want to tune it, get an Rtek 2.X. They are not spendy, you can get one used, and they are easy to tune with. It is simply a way to modify the stock controls (like a Hondata). DO NOT MIX AND MATCH S4 and S5 electronics. There are several things that are different enough that you can run into real issues. For example: The S5 will want to see the electric OMP connected. The injectors have different impedance(s). The S5 has controls for the S5 intake. (The S4 will not). I am sure there are a few other items I am forgetting! Really, taking the entire control system out of the RX7 shouldn't be that difficult. If you have any RX specific questions, there are a number of us on this board that can help! As far as the megasquirt is concerned... Is it easy to get them to run a rotary? Can they do the timing of the leading and trailing spark plugs? That seems to be the major stumbling block to me. Rob R.

Great info, Rob, and really understandable. Thanks!

Although, I have no idea what OMP stands for...remember, I'm pretty much a rank amateur wannabe here trying to get her hands dirty! :D

dollraves
dollraves Reader
3/22/13 7:30 p.m.

Okay, have some pics of the RX7 and Miata trannies with their tails off.

First pic: RX7 on left, Miata on right 90 RX7 vs 91 Miata tranny

Second pic, opposite side: Miata on left, RX7 on right

NA Miata vs S5 RX7 tranny

The gear boxes appear to be the same - the RX7 has an extra milled spot or two. The shaft appears to be the same (rough measuring). The RX7 tail fits to the Miata gearbox without a problem, but the Miata tail bumps into the counterweight (?). Anyone think I would I berkeley anything up by pulling the counterweight off the RX7 shaft to put the Miata tail on there?

-doll

SVreX
SVreX MegaDork
3/22/13 9:05 p.m.

OMP= Oil Metering Pump.

Brett_Murphy
Brett_Murphy GRM+ Memberand SuperDork
3/23/13 12:28 a.m.

Pre-mixing is a good thing.

Write up here: http://www.rx7club.com/2nd-gen-archive-72/omp-pre-mix-mod-write-up-436722/

The '90 rotary will have the better intake, so you did well there. Even if you're not rebuilding the engine, make sure your aux ports open up, it is very possible they are gunked up and you're leaving horsepower on the table.

Write up: http://www.rx7club.com/2nd-gen-archive-72/how-cleaning-5th-6th-port-sleeves-51210/

If your 5th and 6th ports are working, there are nifty little sleeve inserts you can use, a-la the RX-8 setup. They cost like $40. Check Pineapple Racing for those.

I'd take the intake off and check your oil metering lines as well. They crack and break. You don't know it until you see the oil leaking or your engine dies. You can rebuild those with $20 of material from McMaster-Carr

Write up: http://www.rx7club.com/2nd-generation-specific-1986-1992-17/rebuilding-omp-oil-injection-lines-614754/

Following with interest

dollraves
dollraves Reader
3/23/13 9:06 a.m.

In reply to Brett_Murphy:

Woo! Thanks for the info...those links are great tutorials. I have more homework after I figure out the transmission. :D

wbjones
wbjones UltimaDork
3/23/13 10:22 a.m.

welcome back to the forum ... we've been missing you

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