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Upgrading My 1990 Miata Miata

7898 Views 140 Replies 13 Participants Last post by  ScythianNightingale
This will be the thread about my attempt at upgrading my working (albeit slow and loud) Mazda Miata conversion. Everyone's feedback and comments are welcome. I can take it. :)

First - about the car at present - the body is in great condition for being 30 years old. The top is new, all the lights and accessories work. It has a custom battery I made (mostly because I wanted to learn about the process) which is 21s (77V) and 136 AH. (10kWh). For V1 I used two 72V motors connected with the loudest chain on the planet. I have a Soliton 1 controller (340 V, 1000 amps) waiting to go in. I invite you to check out details on my website, here.

The other thing - I am a electrical engineering college student who likes doing things myself on the cheap - I am doing this project for the process and not as much the result. I didn't build my own battery (and my own spot welder, for that matter) because I thought I could do it better than Elon... So ditching everything and switching to a high voltage AC system is out of the picture.

The current plan:
- Acquire beefy forklift motor. (In progress, if you have had success with this and are in the DC area let me know)
- Remove the transmission and directly drive the differential. Add reversing solenoid for reverse.
- Extend the drive shaft and attach it to the motor. There is a universal joint on both ends of the drive shaft already.
- Possibly (see below) extend the C beam and attach it to the motor.

Here's my question for everyone - it is necessary or advised to balance the torque running through the driveshaft? We know that if my driveshaft is transmitting X amount of torque, an equal and opposite X torque has to somehow be transferred back from the differential to the front. The transmission presently has a C beam (see image below) that connects its output to the differential - the reaction torque is absorbed by this C beam.

One idea I had would be to extend this C beam forward to my motor, so both the drive shaft and beam would be attached to the motor. The motor would not exert torque on whatever is supporting it (aka, my 30 year old frame).

Thoughts on whether this is this necessary?

Here's an image of the output of the transmission. A driveshaft connects it to the differential. The C beam is attached quite heftily to the transmission and the differential housing.

Font Line Auto part Automotive window part Parallel

Other bits:
- I have access to basic tools. Lots of electronics stuff. No welding on site, but I know a guy.
- I plan to upgrade the battery pack from a 21s to a 40s after the forklift motor is good and secure. So if this first edition goes 40 mph, its ok.
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How much power are you wanting out of the forklift motor? I can't imagine it will do anything to the Miata driveshaft. If you are concerned about it, you could get a beefier one made, which you might need to do anyways unless you place the motor where the trans output shaft was. And there's probably no room for that in a Miata.
 

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Discussion Starter · #5 ·
I figure this - the stock miata engine is rated for 100 ft lbs of torque. The transmission can do a 3:1 in first gear - so max rated torque on drive shaft is 300 ft lbs. I am not worried about the drive shaft itself. I am worried about the reactionary force that is usually handled by the C beam (Learned this part is called the PPF, or power plant frame). If I remove the C beam when I take out the transmission then the torque would go through the mounting ears as mentioned.

As per that thread - user "gavin_eakins" found that:
[The PPF] Makes the whole drivetrain one piece, effectively.

However, diff's ears are rubber as are the engine mounts, so driveline can still move relative to MX5 chassis.
At this point I am leaning toward extending the PPF based on the thread and continue to allow the motor to rotate slightly with respect to the diff. Looks like the engine that broke their diff made about 200 ft lbs, so in first gear call it maybe 600 ft lbs that the diff had to resist? Who knows. Also in my experience momentary torques due to bumps and whatnots on drivetrains can be 10x what the regular torque is.
 

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1967 Ranchero (x2) / 1983 Fairmont
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Honestly what I would do (but only because I can fabricate) is swap in a solid rear axle. Junkyard Ford 8.8's are cheap, and easy to narrow if you get two passenger-side axles. There are a lot of benefits to it, and the Fabrication Series has an excellent set of videos on how to do it, and the link explains some of the benefits. It's way overkill for a forklift motor, but you'll never have to worry about it again. I wouldn't bother with an IRS, because half shafts are going to be the next limiting factor if not the ring/pinion itself on the Miata diff
 

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With that being said, an IRS 8.8 would also likely be enough for what you need to do, and probably easier to swap in. I'm a drag racer, so I say solid axle swap everything haha. But a solid axle would be a lot more work, and I don't know if there is much gain to it in your application compared to an IRS. Either way, I would say lose the Miata stuff...
 

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The PowerPlant Frame does not take reaction to propeller shaft torque. It is a beam under bending stress in the vertical plane, not a torque tube. It is a C-channel, and that would be a terrible choice to handle torque.

Torque reaction to motor torque on the shaft is through the final drive (differential) housing mounts (that's why there are two and they are widely spaced), then the rear subframe, then the body structure, then the front subframe, then the engine mounts, engine block, and transmission housing. After conversion everything at the back is the same; the motor mounts need to transfer torque to the body structure, and this still has nothing to do with the PPF.

In the stock Miata the PPF ties the final drive housing and transmission housing into one structure which then only needs mounts at each end (on the engine and final drive) - there's no transmission mount because none is needed.
  • If the same method is used on the conversion, the PPF is retained, the motor just needs two mounts (to take output torque reaction), and there's no change at the back.
  • If the PPF is eliminated, the motor needs mounts front and back (and at the output end it needs mounts left and right), and the final drive needs an added front mount to the subframe (this is commonly done in Miata engine swaps that change the transmission to one that doesn't have provisions for attaching the PPF).

There is a torque reaction function of the PPF: reaction to axle torque. The PPF is a long arm that, like a pry bar or wrench, keeps the final drive from twisting nose-up in reaction to drive torque. That's why if the PPF is eliminated the final drive needs a front mount... and there's a lot of force on that mount.


It would also be possible to replace the PPF with an actual tube which would both perform the current function of the PPF and take the motor torque reaction; this has been done in several production vehicle car designs (the best known are some Alfas, the Porsche 924/944/968 and 928, and C5/C6/C7 Corvettes), but it would not be a trivial project. Old Miatas handle the torque of even radical engine swaps, so this is not necessary.
 

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Depends on goals. I would much rather a well set up solid axle than IRS in a drag car. This isn't what this build is though. But there is a place for live axles lol
 

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I'm thinking a good place for them is hay wagons 😛

Ranchero's close enough to being a hay wagon (lopped off station wagon), yet you are not going solid axle there, are you? 🤨

The beauty of an EV's torque curve is in the twisties as a road car. 10 seconds and put it away? Half assed road handling in trade for 10 seconds every month?
 

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If I can make it work I will. Probably not in the Ranchero we were discussing on the other thread because it doesn't need it, but if I ever make a dedicated racecar then I would like to keep it solid axle
 

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Probably not in the Ranchero we were discussing on the other thread because it doesn't need it, but if I ever make a dedicated racecar then I would like to keep it solid axle
This is if I build a dedicated racecar. I agree, you don't need a solid axle for a while. I do think that there are advantages to it, and it's hard to beat a 4 link setup for the simplicity/tunability/cost. Can we just say there are advantages to both and leave it at that? Lol
 

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Hi
I had similar issues
The diff - a Subaru diff - reacts out the Wheel Torque - so about 4 times the motor torque
With the Subaru diff the front of the diff is a long way forwards which drops the loads

I have the rear of the diff mounted as usual with rubber mounts to the subframe
The front of the diff mounts to four of those rubber mounts usually used for exhausts - two upwards and two downwards
The idea was to always load the rubber mounts in compression

The motor is mounted to the chassis with six of those rubber mounts
Four at the bottom - loaded downwards
Two above them loaded upwards

The front motor mounting bolts onto the front of the motor and downwards onto the chassis rails

The rear mount has the bottom piece with two bottom mounts - and a top piece with two upward facing mounts

The mounts are about a foot apart - so with say 300lbft of torque the mounts on one side are pushed down by 150 lbs and the other side are lifted up by 150 lbs

This has worked rather well while I have been abusing it for the last 9 years

With your car I would be looking at that "C" beam as part of the motor mounting assembly
 

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@Forrest You will want to tie the PPF into the motor somehow. The issue is going to be that your motor is much shorter than the original engine + transmission so where the PPF connects to the trans is a good 16-20 inches or so from where the motor mounts are. The motor mounts are also in the front subframe so they can't really be moved backwards. The rest of the car is just thin sheetmetal so you can't really mount it without a lot of fabrication and building new subframes etc.

You could extend the PPF and mount the motor in the engine bay, but then you also need to extend the driveshaft to make up the difference, and the whole trans tunnel is wasted space.

This is the issue with front engined Miata conversions, the chassis does not accommodate it very well and you end up with a lot of wasted space.
 

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Discussion Starter · #20 ·
Thank you everyone - I am going to leave the rear axle and differential as they are. Too much work to do otherwise. I understand alternatives have their place and appreciate being taught about them and what I currently have.

Thank you Duncan for sharing the pictures of your parts. Really helpful to see what I'll need to build.

The plan as it stands (this week is finals week for me, so I'll start work next Sunday when I get home)
  • Somehow extend the driveshaft (Current plan is to cut it in half and then weld a tube with an inner diameter that equals the diameter of the driveshaft onto both ends)
  • Create a PPF extender so I can mount it to something hard up front (the PPF is necessary for the reactionary wheel torque force that would otherwise send the front of the diff upwards). If the wheels have 300 ft lbs * 4.3 diff ratio so 1200 ft lb of torque, that point where I connect my extender to the PPF needs to be able to handle hundreds of pounds.

You could extend the PPF and mount the motor in the engine bay, but then you also need to extend the driveshaft to make up the difference, and the whole trans tunnel is wasted space.
Looks like this is the current plan. Yeah, the space where the transmission used to be will be a waste of space.
 
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