91 Eunos RHD, no serious mods yet, automatic
Budget is around $2500, but the more I spend the longer it takes to get to FI

So I'd like to supercharge (maybe turbo, but different torque delivery characteristics call for different trans/drivetrain setups) my Eunos at some point, shooting for 1:10 power/weight so not looking for anything over 230rwhp/230tq, but first I need to lay down the framework because it's a daily and needs to stay that way. I also want to stay slowmatic, so the trans and drive train need to be well developed, and are first on my list.
I'm thinking a cooler, full syn fluid, valve body mods, and maybe a better torque converter for the trans. The cooler and fluid are self explanatory and since heat is the #1 killer of transmissions should be fairly effective on their own, but I don't know who does trans upgrades besides level 10. I wouldn't mind picking up one of their torque converters, but I'd like to do the valve body myself if possible. Are there any references or people who have worked on this kind of thing? I haven't been able to find write ups or much discussion about autos at all.

Second is the rear end which is apparently made of glass on the 1.6. Are there any differences between the JDM/EDM/USDM versions as far as the rear end goes? I'm assuming not. What would be the most cost effective for those power goals? I'm thinking a 1.8 LSD might cut it and it's a fairly minor swap process if I've read correctly. If that won't work there's the 7.5 rear end that the V8 guys use, and a couple people who make subframes for it. Are those the only real options without massive fab work? I can weld, but not well enough for that big of a project most likely.

Thanks in advance for not flaming the new guy.

 

Any 1.8 rear is more than strong enough for your purposes.
​​​​​​At the power levels you are looking for, your auto should be OK.
Somewhere or another I and someone else posted some info about increasing the rpm range before shifting.
Can't recall where at the moment, but if I remember later, I will look it up.

 

That's good news.
Am I looking for a specific gear ratio depending on supercharger or turbo? I assume that since the power band is higher in the RPM for a turbo I would want a lower gear ratio, keeping the RPM higher for the same highway speeds? and the inverse for a supercharger?

is OK good forever with maintenance and not too crazy driving, or is OK 10k more miles before I need a new one?

What would increasing the shift RPM do? I feel like it would make sure you spent enough time in each gear and not be in 3rd before the boost even starts, but I also feel like the shock of having to go higher in the RPM then dropping farther at the shift would be more detrimental. Am I not understanding that right?

Thank you

 

I'm not sure about the gear ratio bit with that specific auto trans, but I would try to keep close to the ratio you have in your case.
As far as how long "OK" will last, I can't give you a definitive answer, as most of us avoid autos.
Yes, increasing the rpm range will help you stay in boost, and will definitely give you a much sportier driving experience.
I'm going to link a thread after this post that I recommend you read through.

 

The auto trans talk starts somewhere in the middle.
https://www.miataturbo.net/build-thr...t-build-87547/

 

Ok, I think stock is 4.1, right?
Ok, that makes sense on the RPM range
Oh, cool, I started reading that one but only got a few pages in. Thank you.

Edit: I bookmarked the hell out of that. That was by far the best thing I've ever read about auto transmissions in a miata. I haven't been able to find anything even close to that before now. All the ones specifically about auto trans turn into flame wars or swap to manual bs. I guess you have to prove yourself by having some crazy setup before anybody takes the idea seriously. I really appreciate that link