projectmg
projectmg New Reader
5/2/22 12:00 p.m.

I've had a thing for roadsters for a while, especially the old MGBs and Midgets, as well as rotary engines. Recently I've had the idea to put a 12a/13b from an FB/FC RX-7 in one, and I'm wondering if anyone has experience or knows what might be required. 

Lower weight and a bit more power, and even possibly reliability with a rebuilt or well-kept engine all sound good to me, as well as that great rotary sound.

I have done some research - it will fit with some cradle modification. Mounts will need to be made/fit, along with some way of adapting it to the transmission (or adapting the RX7 transmission to the driveshaft), fabricating an exhaust, and finding a way of hooking it up to the fuel tank, radiator, and throttle cable, as well as mounting the ECU and other hardware.

I have lots of mechanical experience in restoration, but very little in modification or fabrication - how feasible would this be without prior experience in swapping, and around what kind of estimated budget might be required? Any advice or suggestions would be greatly appreciated, thank you.

nlevine (Forum Supporter)
nlevine (Forum Supporter) Reader
5/2/22 12:17 p.m.

Not rotary-specific, but there are several ongoing threads in the Builds and Project Cars Forum with MG content that could give you a frame of reference about adapting to engines that didn't live there originally.

Here are a couple:

https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/1971-b-gt-restomod-hopefully/174704/page1/

https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/1970-mgb-gt-supercharged-beast/179010/page1/

and there was the GRM "Ro-Spit" project - rotary-powered Spitfire - https://grassrootsmotorsports.com/project-cars/1974-triumph-spitifre-ro-spit/ - that could provide some inspiration as well.

 

NOHOME
NOHOME MegaDork
5/2/22 1:19 p.m.

All doable and has been done. The tricky bit is fitting stuff around where the frame,cross-member and steering  rack input all come together.  Word on the street is that the rotary exhaust is extra hot and that needs to be managed. The people who I have spoken with like the results, but then they would cause they did the work and are invested.

I would be loath to give up any torque in an MGB chassis. The car is a bit of a plodder and the rotary is like a hyperactive hummingbird.  The MGB's mass would be fighting the engine the whole time. The rotary works better in the MG midget with its light weight and willingness to explode into acceleration with the rotary revs.

My other objection is that I don't think the rotary looks good in the engine bay. Purely subjective.

Keith Tanner
Keith Tanner MegaDork
5/2/22 1:51 p.m.

Some magazine did a rotary Spitfire years back. There might be some things to learn there.

I would think that using the RX7 transmission with a custom driveshaft would be the easiest solution by far. There wouldn't be much benefit in trying to keep the MG trans.

Carl Heideman
Carl Heideman
5/2/22 2:30 p.m.

I've driven some rotary-swapped MGBs and NOHOME's torque comment is spot on.  I also drove the Ro-Spit back in the day and it was a monster, but it was far from a stock rotary.  Nonetheless, I was going to do a rotary swap myself in 1999 but changed my mind and went Miata swap after a test fit of both engines in an MGB engine bay. While I've done more Miata swaps myself, I think that the V6 and V8 swaps are more practical as a first swap as there are a lot of kits and recipes and internet information.

I've written a whole series of stories about the Miata swap for GRM and most of them are available on the website. Most stories are written for general principles about making a smart swap, not just a Miata swap, so they should give you an idea of what you'd be in for.  For example, most people think the engine and trans mounts are a big deal.  For me, that's maybe 4-10 hours. I can spend 4 hours on an accelerator cable and more on making the clutch work well.  The work is in those details.

Anyway, here are some links.  I believe there are more, but I'm too lazy to search further.

  1. Swap Science: 14 Steps Before You Swap That Engine
  2. Swap Science: How To Build Custom Engine Mounts From Scratch
  3. Swap Science: How To Make Transmission Mounts
  4. Swap Science: How To Get Your Clutch Setup Just Right
  5. Swap Science: How To Make A Functional Gas Pedal
  6. Swap Science: How To Hang An Alternator
  7. Swap Science: Engine, Steering, and Exhaust Fitment
  8. Swap Science: Keeping Things Cool by Building a Cooling System

I can't find Keith's original build thread on his LS-powered MGB/GT (still too lazy), but he did an update that came up quickly after a search. Any tips from Keith are great, but his car is pretty radical and probably more than you want to do for your first swap.

Robbie (Forum Supporter)
Robbie (Forum Supporter) MegaDork
5/2/22 2:38 p.m.
  1. Do it!
  2. Many people will suddenly come out of the woodwork as a self proclaimed "first person to <insert engine here> swap an LBC" of some flavor. Even more will tell you about their uncle/cousin/stepbrother/high school buddy who had one that was the fastest car on the road. 
Keith Tanner
Keith Tanner MegaDork
5/2/22 3:01 p.m.

I definitely do not recommend following in my footsteps :) There are much easier ways to do a swap but I knew that going in. There's an abbreviated build thread here and slowcarfast.com has a few hundred posts. Carl's Miata swaps will have a lot more in common with a rotary than my shoehorning did.

Robbie (Forum Supporter) said:

2. Many people will suddenly come out of the woodwork as a self proclaimed "first person to <insert engine here> swap an LBC" of some flavor. Even more will tell you about their uncle/cousin/stepbrother/high school buddy who had one that was the fastest car on the road. 

This is 1000% true.

nocones
nocones PowerDork
5/2/22 3:26 p.m.

In reply to Robbie (Forum Supporter) :

I just cannot believe how lucky I have been to meet the first person to ever swap a rotary into a MG Midget.  I've meet them a dozen times so far. 

 

My rotary Midget is super fun.  The biggest issue with rotary swaps is generally that the engine is much wider down low then a normal engine.  The pan also is deepest at the front of the engine.  I can measure the oil pan of a 12a for your reference tonight.  They are approximately a 20" square and because the crank is in the middle of the engine that is all low in the car.  The Midget had enough width between the unibody rails but the Cross member was shaped around the stock MG oil pan so it had to have some amount of material removed and reinforced to allow for pan clearance.  

The exhaust is hot but I've been lucky with a pair of Stainless Magnaflow mufflers and 16 ga aluminized steel for 4 years of use and about a dozen track days.  I run a 18" straight through resonator (stainless steel packed glass pack) into a normal stainless steel Labyrinth style muffler.  This is quiet enough with a street port 13B for SCCA Nationals (93DB IIRC) but I have a insert I made that is a 1-5/8 exhaust tube with holes in it and a washer in the end that I bolt onto the tip for street use just to keep it quite enough in the neighborhood.  Without it is isn't obnoxious it's just louder then I want.  

The Mazda trans is the way to go it's about 25 lbs lighter then the cast iron MG transmission and has ratios to accomodate the 8k RPM input of the rotary.  Rear trans mount is pretty easy on them as it's just a 2 bolt pad.  The Mazda RX7 uses a Flanged bolt input to the differential and for my MG I was able to buy an off the shelf brass bushing and make an adapter plate to bolt the MG input flange to the plate with the OD of the bushing registering on the MG input Flange and then the ID of the bushing registerd on the Mazda shaft flange with a 2nd set of bolt holes.  This let me just shorten a RX7 driveshaft vs buying a custom driveshaft.

The gear lever on the shifter is pretty far back on the RX7 trans.  It has 4 bolts holding it on. It's possible to shorten the input shaft and move the shifter forward about 4".  You do this by drilling a new hole to hold the mechanism to in the transmission shaft and then reassemble the housing with the front bolts cut off just using the rear housing bolts.   I originally did this but because of the leverage the shifter feel isn't great.   For the current shifter I am using a remote linkage I designed and left the stock shifter alone on the transmission.   This allowed me to bring the shifter much further forward on the transmission and also mount it to the chassis.  I can post pictures if you want some ideas. 

Torque is all about gearing.  If you gear the Final Drive to around for the fact that you have 8500RPM of 115 Ft-lbs your wheel Torque will be the same as a lower RPM piston engine of greater toque.  This is why RX7s have a 4.10-4.44 or higher numerical Final Drive.    My MG weighs 1580lbs so the 4.10 Miata Diff still delivers strong performance, in an 1800-2000lb MGB you may want to try to find a 4.30-4.50 FD with a stock rotary. 

Carl Heideman
Carl Heideman
5/2/22 3:29 p.m.
Robbie (Forum Supporter) said:
  1. Do it!
  2. Many people will suddenly come out of the woodwork as a self proclaimed "first person to <insert engine here> swap an LBC" of some flavor. Even more will tell you about their uncle/cousin/stepbrother/high school buddy who had one that was the fastest car on the road. 

I agree with Keith to agree with Robbie.  My feeling about swaps is that you have some personal goal in mind--you like an engine, you want to stretch yourself, something else.  If your swap meets your goals, you win.  There is no such thing as the best swap, just the best swap for you. If you're doing a swap to brag or impress people or have the "best swap", you'll just spend a bunch of time hearing stories like Robbie mentions.

jgrewe
jgrewe HalfDork
5/2/22 3:50 p.m.

The first rotary powered car I ever rode in was a 12A swapped MGB.  The car was a blast to auto-x. It had a mild street port and a couple mufflers to breath through to quiet it down. It used the Mazda trans with a B2000 tail housing that moved the shifter to pretty close to the MG factory spot.

projectmg
projectmg New Reader
5/2/22 7:15 p.m.
NOHOME said:

All doable and has been done. The tricky bit is fitting stuff around where the frame,cross-member and steering  rack input all come together.  Word on the street is that the rotary exhaust is extra hot and that needs to be managed. The people who I have spoken with like the results, but then they would cause they did the work and are invested.

I would be loath to give up any torque in an MGB chassis. The car is a bit of a plodder and the rotary is like a hyperactive hummingbird.  The MGB's mass would be fighting the engine the whole time. The rotary works better in the MG midget with its light weight and willingness to explode into acceleration with the rotary revs.

My other objection is that I don't think the rotary looks good in the engine bay. Purely subjective.

Ok, my thought with the B vs the Midget is mostly that the B has more underhood space and would need less modification with my limited knowledge. I have seen build threads with a rotary in a Midget, and that would be super fun but it seems like a lot of frame and body work to take on without much experience on my part to fit that in the smaller car.

Weight also would be better in the Midget, but I figure in the B's case even if it isn't quite as fast it would still be a huge upgrade over the original BMC B-series and it is still significantly lighter than the RX-7 it originally came in. If I find the right Midget though, I think I will consider that instead. Will have to see the measurements and more of what is required.

projectmg
projectmg New Reader
5/2/22 7:19 p.m.
Carl Heideman said:

I've driven some rotary-swapped MGBs and NOHOME's torque comment is spot on.  I also drove the Ro-Spit back in the day and it was a monster, but it was far from a stock rotary.  Nonetheless, I was going to do a rotary swap myself in 1999 but changed my mind and went Miata swap after a test fit of both engines in an MGB engine bay. While I've done more Miata swaps myself, I think that the V6 and V8 swaps are more practical as a first swap as there are a lot of kits and recipes and internet information.

I've written a whole series of stories about the Miata swap for GRM and most of them are available on the website. Most stories are written for general principles about making a smart swap, not just a Miata swap, so they should give you an idea of what you'd be in for.  For example, most people think the engine and trans mounts are a big deal.  For me, that's maybe 4-10 hours. I can spend 4 hours on an accelerator cable and more on making the clutch work well.  The work is in those details.

Anyway, here are some links.  I believe there are more, but I'm too lazy to search further.

  1. Swap Science: 14 Steps Before You Swap That Engine
  2. Swap Science: How To Build Custom Engine Mounts From Scratch
  3. Swap Science: How To Make Transmission Mounts
  4. Swap Science: How To Get Your Clutch Setup Just Right
  5. Swap Science: How To Make A Functional Gas Pedal
  6. Swap Science: How To Hang An Alternator
  7. Swap Science: Engine, Steering, and Exhaust Fitment
  8. Swap Science: Keeping Things Cool by Building a Cooling System

I can't find Keith's original build thread on his LS-powered MGB/GT (still too lazy), but he did an update that came up quickly after a search. Any tips from Keith are great, but his car is pretty radical and probably more than you want to do for your first swap.

Thank you!! I will throughly look into those resources for sure. Even though it might be objectively worse, if it is feasible for a beginner I would like to go for a rotary, unless I can find another engine that I find really interesting that would work better. I really like the high revs and especially the sound of the rotary, and I'm not looking for something with a ton of power yet before substantial brake, suspension, and safety upgrades. I'm hoping as I get more experienced with fabricating through the process it'll get easier and better, definitely will do more research on swaps and make sure it's the powertrain I want first though.

projectmg
projectmg New Reader
5/2/22 7:27 p.m.
nocones said:

In reply to Robbie (Forum Supporter) :

I just cannot believe how lucky I have been to meet the first person to ever swap a rotary into a MG Midget.  I've meet them a dozen times so far. 

 

My rotary Midget is super fun.  The biggest issue with rotary swaps is generally that the engine is much wider down low then a normal engine.  The pan also is deepest at the front of the engine.  I can measure the oil pan of a 12a for your reference tonight.  They are approximately a 20" square and because the crank is in the middle of the engine that is all low in the car.  The Midget had enough width between the unibody rails but the Cross member was shaped around the stock MG oil pan so it had to have some amount of material removed and reinforced to allow for pan clearance.  

The exhaust is hot but I've been lucky with a pair of Stainless Magnaflow mufflers and 16 ga aluminized steel for 4 years of use and about a dozen track days.  I run a 18" straight through resonator (stainless steel packed glass pack) into a normal stainless steel Labyrinth style muffler.  This is quiet enough with a street port 13B for SCCA Nationals (93DB IIRC) but I have a insert I made that is a 1-5/8 exhaust tube with holes in it and a washer in the end that I bolt onto the tip for street use just to keep it quite enough in the neighborhood.  Without it is isn't obnoxious it's just louder then I want.  

The Mazda trans is the way to go it's about 25 lbs lighter then the cast iron MG transmission and has ratios to accomodate the 8k RPM input of the rotary.  Rear trans mount is pretty easy on them as it's just a 2 bolt pad.  The Mazda RX7 uses a Flanged bolt input to the differential and for my MG I was able to buy an off the shelf brass bushing and make an adapter plate to bolt the MG input flange to the plate with the OD of the bushing registering on the MG input Flange and then the ID of the bushing registerd on the Mazda shaft flange with a 2nd set of bolt holes.  This let me just shorten a RX7 driveshaft vs buying a custom driveshaft.

The gear lever on the shifter is pretty far back on the RX7 trans.  It has 4 bolts holding it on. It's possible to shorten the input shaft and move the shifter forward about 4".  You do this by drilling a new hole to hold the mechanism to in the transmission shaft and then reassemble the housing with the front bolts cut off just using the rear housing bolts.   I originally did this but because of the leverage the shifter feel isn't great.   For the current shifter I am using a remote linkage I designed and left the stock shifter alone on the transmission.   This allowed me to bring the shifter much further forward on the transmission and also mount it to the chassis.  I can post pictures if you want some ideas. 

Torque is all about gearing.  If you gear the Final Drive to around for the fact that you have 8500RPM of 115 Ft-lbs your wheel Torque will be the same as a lower RPM piston engine of greater toque.  This is why RX7s have a 4.10-4.44 or higher numerical Final Drive.    My MG weighs 1580lbs so the 4.10 Miata Diff still delivers strong performance, in an 1800-2000lb MGB you may want to try to find a 4.30-4.50 FD with a stock rotary. 

The measurements would be great! I'm thinking of going with the B because it has more leeway in terms of fitting the engine in, and might be a bit more comfortable for my purposes. If I can find the right one though, I think I might go for a Midget instead. It would be good to see one in person and get measurements and everything.

Thank you for all the other details, those are very helpful. It is great to know what I will have to do in order to swap it, especially in terms of fabrication like the exhaust and modification like the transmission and driveshaft. Pictures would be very much appreciated. If you happen to have any videos of it those would be great as well.

Would the Midget's hardware, especially the shortened driveshaft or rear axles have issues with the increased HP/torque of the rotary? It is sounding like a pretty good option now.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
5/2/22 7:47 p.m.

Having owned 12A and 13B RX-7s and driven a few MGBs... even a 12A has more low end torque than an MGB.  There's just a little more RPM to play with so you don't notice it much.

The 13B in the car next to me would probably tear apart a MGB rearend if you whacked the throttle open at idle smiley

 

I know the swap has been done, but my mind boggles at the idea.  A Mazda rotary is at once very wide but also very tall in the transmission.  The trans tunnel of an RX-7 is only inches from the hoodline.  Swapping would be difficult because of that.  And the width.

If it were me, I would do a Miata 1.6 swap.  Feels very similar in feel to a rotary without the clearance hassles or expense of trying to find good parts in 2022.

nocones
nocones PowerDork
5/2/22 8:19 p.m.

A Rotary transmission is no taller then a Miata transmission.  In a midget the mods you need to make are a notch in the top of the MG cross member that goes across the bottom of the trans tunnel, notch the corners of the front cross member, and remove the heater tray.  This is the biggest issue with a Midget swap.  You lose the heater.  I've heard it can be done while maintaining the heater but all the ones I've seen just remove it.   

I swapped my 12A in a stock tub and had it running before I ditched the tub and tube frame converted the car.  

I blew up 2 stock rear ends in <150 miles due to differential failures.  Apparently a race build on the rear end can make them survive a moderate power Rotary.  The rear end is the hard part of the swap.  The Midget is very narrow and requires custom work for any OEM rear end to be narrowed before swap (or Sick flares).  

This video is of the current build of my MG.  Being tube framed it's extensively modified and not of a lot of help but is still a rotary in an MG.  It's on Miata a stock width IRS Miata rear with 15*9 wheels in Miata offsets.  I have a friend local with a 13B in a more unmodified chassis that I can get some picture from.  He runs a narrowed RX7 axle out of a GSL-SE 1st gen.  

I think you could junkyard a good rear end out of 2 short sides of a Ranger Axle.

If you ever find yourself in central Illinois let me know and you can come check out the car and maybe take it for a spin.  

NOHOME
NOHOME MegaDork
5/2/22 9:04 p.m.

This is the area where it gets interesting. As mentioned, there is a lot of rotary engine below the crank and the crossmember is in the way. So is the rack input and the steering column. It does not help that the frame rails on the B are not straight like on the Midget, but rather neck down where the cross member attaches in this area.

 

Look at the balancer on the rotary and locate it where the balancer on the MG sits. Note how much rotary stuff wants to live where there is a lot of MG stuff already. All manageable since it has been done, but not a natural fit. A ford 302 fits the space better than the rotary.

When it is all said and done, will you have built a better RX7?

 

Don't get me wrong, I am the president of the bad ideas with cars club, but I go in knowing fully aware of the folly of my ways.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
5/2/22 9:25 p.m.

In reply to NOHOME :

You'd have to set the engine back a lot.

I acquired some stuff from someone who had a 12A and a 13B in, IIRC, a Midget.  The 12A had a stock RX-7 oil pan turned around backwards.  The header curlicued to go under the transmission, which as noted sits a LOT higher in the chassis than a piston engine's transmission.  I also have a driveshaft that is about 2 feet long smiley It appeared that he used a '79-82 RX-7 rearend for his swap.

 

The only rotary Midget that I have seen in person had no passenger area, that part of the interior was boxed over and the header and exhaust went through there.

Carl Heideman
Carl Heideman
5/3/22 8:59 a.m.

I did test fit a rotary to my Miata-powered 1973 MGB/GT before I did the Miata swap and it fit pretty well after I cut the factory motor mounts off of the front subframe.  I suspect a rotary would fit a 1975-80 MGB without cutting the motor mounts as they are less intrusive.  I've also seen several rotary-swapped MGBs, so it's obviously possible. My understanding is that someone actually offered a kit 20+ years ago.

It sounds like projectmg really likes rotaries, so let's enable and encourage him.  His goals of learning some fabrication while enjoying the rotary make a lot of sense. Especially if he goes with a carb'd rotary, I think it will be a fairly easy swap and not bad for a first swap.

Driveshafts are pretty easy.  Go to a driveshaft shop with the front of the Mazda driveshaft and the whole MG driveshaft and have them make you one to the appropriate length. Our shop does that for about $150.

As an aside, after I finished my first Miata swap, a rotary-powered MGB was for sale nearby and I would have bought it but the owner thought it was made of gold and priced accordingly. He wouldn't let me test drive it without him in it and when I asked if I could stand on the throttle, he said, "okay, but don't go past 4000RPM."  He didn't understand what he had...

wspohn
wspohn SuperDork
5/3/22 11:29 a.m.

Have a look at this:  https://www.mgexp.com/forum/mgb-and-gt-forum.1/1972-mgb-gt-rotary-conversion.3281478/

The engines have found their way into all sorts of cars. We used to have a guy with a Lotus 7 that used one for autocross. He had relabeled it as a 'Rotus'.

Cactus
Cactus HalfDork
5/3/22 2:02 p.m.

I had one with a 12a. Side draft mikuni, but no real power mods. Tube shock conversion, Mazda 5 speed, wire wheel axle with some kind of minilite reps. The exhaust manifold goes where the passenger's legs do, so you need to do some sheet metal work and insulate it if you want somebody to ride shotgun without melting.

 

It was a fantastic car, I'd have kept it if I weren't too young and inexperienced to fix the previous owner's "custom" electrical system. https://youtube.com/watch?v=ghxVFNTGSxY

 

Keith Tanner
Keith Tanner MegaDork
5/3/22 3:20 p.m.
wspohn said:

Have a look at this:  https://www.mgexp.com/forum/mgb-and-gt-forum.1/1972-mgb-gt-rotary-conversion.3281478/

The engines have found their way into all sorts of cars. We used to have a guy with a Lotus 7 that used one for autocross. He had relabeled it as a 'Rotus'.

Rotus was actually a manufacturer of Lotus 7 replicas. And yes, some of them were built with rotaries but not all. 

paddygarcia
paddygarcia Reader
5/3/22 6:03 p.m.
Carl Heideman said:

when I asked if I could stand on the throttle, he said, "okay, but don't go past 4000RPM."  He didn't understand what he had...

Amen. My GSL-SE barely needed a tach, just shift at the warning buzzer :-)

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
5/3/22 7:51 p.m.

In reply to paddygarcia :

My '84 is driven best when the buzzer is still active after an upshift smiley

Funny thing. My '80 had a tach that was dead nuts. My '85 and '84 were/are both 10% high.  7700 on the tach is 7000 actual.  Buzzer engaged at about 6000 actual.  This is all within Mazda's accuracy tolerance, they allow -.05% to +10%.  Presumably the higher the tach reads, the lower the warranty claims wink

I have yet to accurize the '81's tach.  The (stock 12A) engine pulls right on up to 8k indicated so I have reason to believe that it reads high.  The buzzer is inoperative though.

wspohn
wspohn SuperDork
5/4/22 12:07 p.m.
Keith Tanner said:
wspohn said:

Have a look at this:  https://www.mgexp.com/forum/mgb-and-gt-forum.1/1972-mgb-gt-rotary-conversion.3281478/

The engines have found their way into all sorts of cars. We used to have a guy with a Lotus 7 that used one for autocross. He had relabeled it as a 'Rotus'.

Rotus was actually a manufacturer of Lotus 7 replicas. And yes, some of them were built with rotaries but not all. 

I expect that was where he got the idea (his wasn't one of the real 'Rotus' cars).

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