Help me fight this frieken frequency (resonance)
 

I have a '92NA with the longnose 1.6. I diy turbo'd it at the beginning of the summer and have been running it for 2 months at 6psi with absolutely no problems.

Last two days, I pulled off the wastegate, dp, and manifold and welded up a longer divorced gas tube for my wastegate and dp. Put everything back together. Smooth at idle, but I pull out of my driveway and the first time it hits 4000rpm i get the harshest vibration coming from motor/entire exhaust area. As soon as the motor goes above or below 4000rpm it's smooth as butter.

I was wondering if I could throw out my suggestions to what the cause for this is and get some advice from anyone with a suggestion.

1.) Problem: New wastegate pipe has a nasty resonance at 4000rpm. Solution: figure out how to break up the harmonic in the tube at 4000rpm(whatever that entails).

2.) Problem: With dissassembly and reassembly involved in making my new wastegate tube, i have some part of the exhaust rubbing the body and it resonates with the car body at 4000rpm. Solution: get the exhaust off the body.

3.) Problem: Harmonic balancer is shot. I have been noticing signs of the inner and out pieces of the balancer slipping in between times i set base timing. Solution: throw a new harmonic balancer on.
Question: Anyone ever have a harmonic blancer go out and only get nasty vibration at a single specific rpm?

4.)Problem: After filling remote mount oil filter and starting the car upon reassembly of new w.g. piping. I reved the car before i got adequete oil pressure and spun a main or rod bearing.
Solution: Rebuild bottom end
Question: Anyone ever spin a main or rod bearing and notice vibration or knocking (not predetonation) at a certain single specific rpm.

5.)Problem: You insert your hypothesis here....
Solution: Give it your best shot....I need help...s.o.s


Anthony


p.s. PACE miata club meets at columbia burger king 8/19 at 11am

south central pa miatas....represent!!!!

Did you ever figure out the cause of your 4000 RPM engine vibration? I'm dealing with the same problem in my '03, it sounds like. Strong vibration, mostly noticeable in shifter, appearing near 3,500-4,000 RPM and nowhere else.

This started a couple thousand miles ago, not too long after I installed a blower (supercharger in my case).

my guess is number 2.

Probably not in my case, though -- this doesn't feel anything like an exhaust issue, purely something rotational. I'm hoping the original poster is still around and can tell me what the problem turned out to be.

number 2 motor has some movement and rubs

surely that's exhaust hitting the bellhousing

+1 on body contact. When my DP touched my trans tunnel, it rattled around 3500-4000, horrible buzzing to be exact. Drove me nuts.

Sigh, i have this exact thing at 4-4.3k.

Took entire exhaust off. Still there. Put a piece of wood between bell housing and downpipe to lock it all in... still there.

Hurm... I wonder if wood would have carried enough vibration to make it annoying also...

Time to dent the downpipe at the only close spot.... With my turn buckle and Mazdaspeed motorsport mounts... i get almost 0 movement though.

Hurm.. found this on everyones favorite site...

" Question: Several folks on the list have commented on a mysterious dash rattle that shows up between 2000 -2400 RPM. After having the same really annoying rattle for the past year, and trying all the usual cures ( banging on the dash, stuffing foam rubber between all the electrical connections, etc.) I set aside three days and decided to completely disassemble the dash to find and cure the rattle. Actually, I found three surprising sources.

* A1:
The most annoying rattle was a slight hum (or vibration) that occurred consistently at 2100 RPM or when the car was revved through the 1800-2500 RPM range. After completely tearing apart and taking out the dash, I found that this rattle was caused by the almost complete disintegration of the rubber grommet that surrounds the outermost hot water pipe that runs to the heater through the firewall. The grommet was destroyed on the inside, therefore giving the appearance when you are under the hood that it is still functional. However, revving the car to 2100 rpm revealed that the water pipe vibrated happily at that frequency and set up quite a din on the firewall. This grommet is only $2.65 at the local Mazda dealer and is easily replaced from within the engine compartment (I.e. you don't need to tear apart the dash)."

It could be a resonance in the DP/ wastegate tube. There used to be an SRT4 here in Huntsville that had some sort of muffler shop 3" dp on it, and at WFO, that sucker sounded like there was a duck stuck in the exhaust. It was aweful. Dude bought a decent aftermarket piece, and it went away instantly. Whatever was resonating (a close crush bend, I imagine) wasn't in the new pipe, and the sound went away. That said, its much more likely that the new pipes are hitting the floor pan/ transmission.

Yah, my downpipe is full mandrel bent 3". There is one spot where it closes in on the transmission... I stuffed a wood shim in there and the vibration was still there. Of course... that might not have dampened it at all... the metal of the DP at the close spot doesn't look like its rubbed/bumped at all though.

Maybe i'll just pull it, dent it so I can assure myself that isn't the issue.

The floorpan isn't being hit, I can assure of that. With the exhaust off... and open DP... the vibration/noise is still there. And the Downpipe doesn't hit the floor or even come close. That and my turnbuckle stiffener and engine mounts prevent almost all movement.

I have a few things to try tonight... video/audio will hopefully come eventually.

Try pulling off the engine stiffener.

That was added after the vibration tracing started. It was an attempt to isolate it as a engine movement noise... or just something else vibrating.

My exhaust pipe was hitting my butterfly brace in the front and making a horrible vibration in the 3500 to 4000 range as well.. So I drilled two holes in the brace and clamped the pipe to the brace. Vibration gone.

...But brace very hot.....the propper solution is to clearance the brace or the exhaust to clear. By clamping the exhaust to the brace you're limiting it's movement with the engine which will lead to broken exhaust bits sooner and broken floor pan spot welds later.

I think some folks are confusing what I'm talking about with "noise" or "rattle" symptoms. That is not what is happening, and it is not due (in my case) to the exhaust touching the bell housing. That would create some actual rattling/banging/buzzing noise, not just severe vibration in the shift handle... unless I'm just completely off base.

My brothers car does the same thing. It is the new exaust tubing moving to rub when the motor tweeks under acceleration. Yours may be different. I would put it on stands and have someone work the pedal. And if it comes really close anywhere. I would try to make more space.

My 99 had this issue and it was the heat shield for the CAT hitting the heat shield. But seeing my CAT is empty I just removed the shield.

My 2000 has a funny resonance at about 4k rpm also the exhaust note changes noticeably and there seems to be a bit more vibration, after that it's gone. I find it odd, but it hasn't caused any major issues :shrugs:

Digging this up....since I fitting my new IM I noticed this as well. I feel it in my foot if I'm part throttle around 4K. if I go WOT I don't feel it.

Here's the weird thing. If I keep my foot in the same position and hit 4K, the car wants to pull to 5K so incredibly fast after that. I feel I'm hitting some or frequency that's allowing the motor to pull the damn butterfly open.

I'll try to record a video, but its nearly impossible to hold the revs at idle at 4k because once I get to this point the revs jump and i feel the tension on the throttle lessen.

pretty intense.