Shifting issues
 

I am new here so please go easy on me.

I bought a turbo NA Miata this summer and have been working on getting the car sorted out for reliable track days.

The car is now running on e85 producing around 300hp and 240ft/lb of torque @4500 rom. It has happy meal FlyMiata clutch, 6-spead gear box and IRP shifter. The clutch was installed prior to me buying the car. I fixed the shifter installation.

I also changed transmission field and replaced both master and slave clutch cylinders. Bled the system and adjusted clutch per FlyMiata instructions.

The car shifted fine before I tuned it with E85 and increased the power from the standard FlyMiata turbo kit.

The car shifts fine under light driving conditions. However, when I get on boost, it does not want to go in to fifth or sixth gears. It goes into natural and I am just not able to slide it into the next gear after I shift up from 4th. Feels like something is sticking in the clutch system. There is no grinding noise so it does not sound like syncros.

Someone suggested replacing engine and dif mounts. I believe they are currently stock but I can not verify since I caught the car that way. It there anything else I should change while I am doing this as a preemptive measure?

I should also mention that I have OC Giken dif.
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Thanks everyone

Dump the garbage eBay short shifter for stock and put a shifter rebuild kit in for good measure. That's what I would try first before spending a ton of money on other stuff.

I modify cars for a living, and have had SO many issues with IRP shifters. They're hot piles of over-engineered garbage. Start by replacing it with stock or preferably a Miata Roadster short shifter. Check out the motor/trans mounts, that will also help. If they are indeed stock, which would be shocking given the rest of the modification to the car, install some mazdacomp motor mounts and poly diff mounts. Those two changes should fix all your issues. That being said, I have had customers who cannot comprehend that the transmission moves during shifting, some people try too hard and keep hitting the gate, or worse 3rd gear. But try those other things and then we can help with the driver mods.

Yeah, I’d imagine running that much power with stock mounts would be like trying to play whack-a-mole with grabbing the shifter.

Definitely would confirm on what motor mounts are in the car, and would get rid of the IRP shifter if these two smart guys are recommending it ^^

I also had trouble with 4th-5th shifts when I went to a 6 speed, and fixed it via shifting technique. With my 5 speed, I could pop out of fourth, push the shifter as far right as it would go, then forward and it would slide right in every time. Doing this with the 6 speed would result in getting stuck in neutral when driving it at full throttle all the way to redline. Couldn’t get it into 5th even if I pushed really hard on it. Flat-handing and pushing the shifter in a diagonal path as opposed to right then forward fixed my issue. I went from thinking my trans was broken to not even noticing my shift strategy now.

This was on a car with Poly motor and diff mounts and a MR short shifter, though. Don’t think it’ll solve your issue but might be something to play around with while you’re waiting for whatever parts you need to come in.

Thanks guys,

I was wondering about IRP and I appreciate the confirmation. I will do the motor mounts too just because I think it will become an issue. Any recommendation on which motor mounts too get (Flymiata or super Miata)?

Also, should I change dif mounts since I know they are stock?

I'd definitely swap out to poly diff mounts. The motor mounts will make the bigger difference in drivetrain movement, but diff mounts still noticeably reduce slop. Or at least that's what I noted on my car.

I've got the Supermiata 70a poly mounts on my car and have no qualms with them. Maybe others on here that have experience with multiple types of poly mounts can elaborate on different companies' pros/cons, if there are any.

I remember regretting my poly diff bushing choice on my stock diff when I faced the music, I mean the NVH, afterwards.
YMMV

So I might have figured out the issue. Thanks to all of you guys plus Bill from Miata Roadster.

​​​​​​THe original owner of the car put 6 speed transmission and the second owner did not realize that it requires NB1 6-speed shifter. He caught IRPSSMXNA-B shifter which is part number for the 5 speed shifter.

Bill sent me pictures and dimensions for both 5 and 6 speed shifter and of course they are different... Which explains why the previous owner installed the shifter without nylon bushing (only way they could make it work).

New Miata Roadster shifter is coming on Friday so I will let everyone know if the issue is fixed.

I think it's the guy that owned the car before me installed an incorrect shifter. I believe he bought NA 5speed shifter not realizing that 5speed has longer shifter shaft. That would definitely screw up geometry of shifting. We will see once I get new Mazda roadster shifter (unfortunately the car did not come with stock shifter).

Could you tell me what your issue was? I do not have stock lcd.

The noise and vibration was unpleasant.
I specifically went with rubber bushings for my BMW diff project on my Miata for that very reason.

I put Mazda competition mounts in the Spec Miata engine and diff, and did not notice a noise and vibration problem, but ... race car.
However I put the SuperMiata/Mazda comp pairing they sell for the engine in the street car, and found the increased noise and vibration really noticeable, and not very pleasant for street driving in a my turbo street car, NB2.
I would go with the Mazda competition mounts in a street car.
(My street car will become a race car, so the increased vibration is not a problem, and the stabilized mount is a real advantage.)

Need help with transmission
 

So after all the digging we pulled the transmission down and found out that the longer shifter absolutely destroyed the transmission ( lesson learned, always check all the comments when buying someone's race car)

I found a JDM 6-speed transmission with under 50k.
Are the gears on the JDM transmission the same? Any other differences that I need to be aware of?

Thanks,
L.g.

I do not have the answer to your question but believe the internals are the same. I’ll leave that for the smarter/more experienced guys on here. Have had both a USDM and JDM 6 speed in my car and noted no real differences between them, although that’s not saying much…

Can you elaborate on how the shifter destroyed the transmission? What failed and how did the shifter factor into it?

It's a 90 Miata that originally came with 5 speed. The first owner upgraded the car to 6-speed. The second owner replaced the shifter with a 5 speed shifter. The 5 speed trainy has shifter that are 10mm longer from pivot to transmission. Therefore, the shifter pushed too far into gears and something broke off, which caused the parts floating around and destroying 1st, 3rd, 5th and 6th gears . Plus some other parts.

Btw, I am the third owner who figured out the issue and now trying to fix it.

Hey man, respectfully (I’m just trying to help), based on what you’re describing I don’t think the shifter killed your transmission. Yes, the 5 speed shifter has a longer lower rod than a 6 speed. However, with that being said, as long as the fulcrum (ball) is spaced out correctly, all the lower rod does is change the throw of the shifter. That’s how short throw shifters get their short throw.

It’s possible to run a 5 speed shifter in an early 6 speed transmission if you space it out correctly. How to vid here:


I did the same thing on my early 6 speed transmission and was able to adapt a standard eBay 5 speed shifter onto the transmission. Regardless of the length of the throw, the shifter is gonna stop where it’s gonna stop unless you attempt to jam it past a gear too hard.

Again, just trying to help, but the cause of the failure is likely elsewhere.

I agree with your statement. However no one spaced it out correctly so the lower point of the rod went 10mm under where it was suppose to go. That probably could have pushed the connection rod down.

Honestly, there could have been a bad transmission and that is the reason the owner #2 replaced the shifter. This is the reason I am replacing the transmission. We actually took the transmission apart and found out that it's completely destroyed.

Yes, the JDM and North American 6 speed trans are the same; same ratios, same mounting.

Is there anything different internally?

Ahh, I seem to have misread your original post then. Understood!