So... This will be a long write-up with a lot of detail.


This is becoming a bit of a mess now. Before I go any further I should explain that I did replace the engine on my car alone on my own. The part where I say alone might have a critical role here. After the replacement I remember, that I thought that I noticed a difference in the shaking and flexibility, but I thought that this was just a placebo since I had not been driving the car for a while.


I got the car on a lift today to tick of stuff that has been mentioned in the thread. I learned couple of things I think:

PPF indeed might have been a bit loose, especially the bolts on the diff-side were not in spec, opened without any effort. On gearbox side they were also noticeably looser than with proper torque. This is a bit weird, as I have replaced the engine in the car by myself, and I clearly remember the pain of tightening every single PPF bolt while laying in the tight space under the car without any light.

I opened all PPF bolts and engine mount bolts, gave the drivetrain couple of good shakes and then tightened the whole package back with correct torque.

After this I noticed, that the since the car had a rust-hiding treatment done by the previous owner, the spray pattern on the PPF did not match with the bolt-heads (meaning that the bolts were not lining up where they were when I bought the car)

I realized that I can (and propably should) push the gearbox up before tightening the bolts on the PPF to align everything better. So I loosened the bolts again and had my friend push the gearbox up until the bolt-heads aligned with the rust-paint in the PPF, then tightened the PPF bolts. During this operation the car was lifted in the air from the side-sills.

After this I went for a spin, and I swear to god that even opening and closing the doors feels different now. The hard-top does not creak/resonate like it used to while driving over un-even surfaces. I did not have a chance to test it without the hard-top, but I am pretty sure this made a difference just based on quick feeling with my ass. I will update when the weather allows me to open the roof completely again. Only annoying thing is that now my gear-stick is noticeably higher than before this exercise.

I am not sure if the difference was made by the fact that the bolts were actually loose, or the fact that the gearbox and diff are probably a bit closer and better aligned towards each-other than before (probably). To me it makes sense that the alignment of the gearbox and the diff could make a difference, since I was not able to push up the gearbox alone when I was tightening the PPF half a year ago during the engine replacement.


Now this is where things get confusing for me:

Before you read any further, you should know that I have four (4) friends with miatas, and every single one of us has the same problem with "clonking" noise coming from the drivetrain sometimes when the clutch is pressed while driving. One guy has a six-speed and the rest has fivers.

After the drive I still wanted to still check if all the rotating things are spinning without much wobbling. I lifted up the car so that the wheels are free. I put on fifth gear and let the clutch up so that the car was idling on fifth gear. And jesus christ what a horrible noise it makes. ( )

At this moment I got a bit worried that something is very fucked up in my drivetrain. I gave the car some revs and immediately all noises disappeared. I went slowly through the rev-range in fifth gear all the way to 180km/h, and the car made no weird sounds and there was absolutely zero vibrations.

At this point I got very confused and I put the car to first gear and let it idle, magically it now made no noises whatsoever. Then I told my friend to pump the clutch while I was under the car, and now I could hear the exact same noise I heard on idle with fifth gear. ( , )

After this I turned off the car and tried rotating the axle back and forth by hand from the diff-end. Definitely this is the same sound as before. What makes it interesting is that now the car is on neutral. ( )


This is now where I kinda gave up and just said "fuck it" and went home. During driving I did not hear any weird noises and felt no vibrations, except the exact same noise I have always heard from four different miatas sometimes when pressing the clutch...

If anything, definitely after my little play with the PPF, the gear-stick is definitely transmitting a lot less shakes or vibrations to my hand if I rest it on the stick while driving.


TL;DR: PPF might have been loose or misaligned, drive-train makes annoying noises. Maybe nothing that I say makes any sense.

Look for play in all of the driveshaft universal joints and check the little damper collar on the pinion.

You may not realize how much the difference in angle between the front and rear of the driveshaft can make. The output actually speeds up and slows down twice every revolution. Very troublesome and can definitely create a lot of vibration.

Everyone should watch the video below if they aren't already familiar.

Interesting write up. I had a similar noise back when I had my open diff in there. It surely wasn't as bad as yours. But that all went away when I put in my torsen diff. I drive a european model btw.

You're not saying all 5 friends have this same issue right?

Obviously there will be some play in the driveshaft rotation but if it's so bad you can't really tell if the main issue is at a single point or multiple. Try spinning it with your hands when it's disconnected from the diff... Then pray that the problem is gone xD

This I was kinda half-aware of, and this would also explain why I feel a lot less vibrations now on the gearstick since the gearbox is pointing more towards the diff now.

I wish I was not serious but yes, we all have this same stupid noise. And as I said one of them is a six-speed with a mystery diff, one is supercharged with torsen and two are completely bone stock with open diffs. I think that the only common thing we all have is that the clutch has been replaced atleast once. The idea about disconnecting from the diff end sounds logical, I will try that. I have also replaced my diff to a torsen like a year or two ago, but I don't remember anymore if this noise was present then or not. I have been kinda living with it for a while since it does not seem to be a issue that actually would break the car. I have put many km's on the track to the car after noticing the sound.

I also have this drivetrain noise, I was wondering what the cause was especially as a clutch replacement didn't change it in the slightest. I will check my PPF alignment to see if its up to spec.

Are there actually some measurements/specs from factory with which you could make sure that the PPF, transmission and diff are all aligned between eachother? You know like cross-measurements or something?

Ppf adjustments
 

Go to Flyin Miata's website. They have instructions to install their clutch. In that write-up is the measurements and points to measure from. Can't copy and paste link on phone, so you'll have to find it yourself.

Oh, thanks a lot for this! I feel like a idiot now for not realizing to search for this information.

https://cimg7.ibsrv.net/gimg/www.mia...d25ed5ab11.jpg
Bolts were already torqued, and the spec is right on. Could be my driveshaft, there is a decent amount of play & a clicking noise occurs when moving it back and forth.

Also took a video of my driveshaft play for comparison

https://youtu.be/XsIpDQY_jaA

Can't tell if that's normal or not, I think it is.

All of the noises in your videos are completely normal driveline lash. The lashing in 5th gear at idle is because you are spinning the driveline fairly quickly (lots of momentum) and the idle valve is constantly adding or reducing power to maintain that speed, which causes the driveline to accelerate and decelerate. When you spin it up to speed, it takes constant power application to maintain that speed, and the lash goes away. When your friend pumps the clutch, you are alternating between acceleration and deceleration which, again, causes the lash to become audible.

Lash is caused by the required fitment between gears or splines. You have splines between the clutch and the input shaft, two sets of gears inside the transmission (except when you're in direct drive), a set between the output shaft and the driveshaft, a set of gears in the rear end, and a set of splines between the differential and the axles. Add it all up and you'll hear some noises if there's not a constant load being applied.

Your car is fine. If you want to get rid the cowl shake, add frame rails or fender braces.

Thanks a lot for the input, in conclusion I will just forget about the noise then. I think I found some decent looking framerails from uk-based shop "Bofi-racing" though. Is there any experience if such a butterfly-brace between the frame-rails would have a big impact?

https://bofiracing.co.uk/product/ski...t-frame-rails/
https://bofiracing.co.uk/product/ski...chassis-brace/

I have on my car the following. They are listed in order if impact for how much I feel they impacted the stiffness of the car. (Order of instillation could have affected this, I won't know)

Frame rails.
Door bars.
Butterfly brace.
Frog arms.
Roll bar.

my miata does the same thing as this for like 2 years now.. axelwal video