408 W2 Stroker Build

[jasperjacko]

looking good. My engine build is on pause while I fix a house we bought. It has taken twice as much time and money than we planned.

 

[M_Body_Coupe]

A little "sumthin'...sumthin'"...needless to say, there IS a light at the end of the tunnel, and it's no freight train either!!!

 

[jasperjacko]

Oh, I think thats gonna be a freight train

 

[M_Body_Coupe]

OK, well the end of the season is upon us, heck more like it is HERE, but I digress and probably wish I could deny the truth of the weather that's out there!

I've got a bit of accumulated video materials to start putting together into the next video, but in the meantime I took a quick capture of my ride yesterday as we had an incredibly awesome day here!!!

Ignore the dust on that thing, yup, it badly needs a little "spit and polish", which will happen this upcoming weekend for sure.

 

[jasperjacko]

Sounds Glorious! Love it, can't wait to get mine going.

 

[9secRR]

Sounds great. It’s going to be a fun ride

 

[M_Body_Coupe]

Thanks you guys!

Well, the season is OVER, unless for some crazy ass reason this Saturday does truly turn out to be as warm as they are calling it to be (so about 15C).

Anyways, for now I'm starting to focus on the Winter To-Do List.

First thing on there is the annoying and disturbing rhythmic vibration I am seeing. As best as I can tell I always had it, only recognizable once the speeds get higher though. I suppose given a few 'pedal to da metal' runs I did so far with this new power-train, those speeds just come up a hell of a lot faster now!!! (no complaints there)

The driveshaft is not the issue (that's a custom piece that's been balanced correctly). The pinion angle is not the issue either, I corrected that a few summers ago. This leaves tire balance, which can be re-checked (easy to do) and the more specific housing and axle straightness checks.

I will be checking the axle flange run out next, as well as the overall wheel-to-wheel distance in case the housing itself is bent???

More to come as I might be able to get some of this measured this weekend.

 

[Aspen500]

By rhythmic vibration, do you mean like 1 second on, one second off type of thing?

Only ask as I chased a vibration like that in my car way back when, with no luck. As a sort of last resort thing, I took the poly trans mount off and replaced it with a new stock rubber one. Vibration gone.

Best I can figure is the normal engine/trans vibration frequency matched the harmonic frequency of the chassis. IIRC, it started around 45 mph and diminished above 70 mph. Drove me almost insane! lol

 

[M_Body_Coupe]

By rhythmic vibration, do you mean like 1 second on, one second off type of thing?

Only ask as I chased a vibration like that in my car way back when, with no luck. As a sort of last resort thing, I took the poly trans mount off and replaced it with a new stock rubber one. Vibration gone.

Best I can figure is the normal engine/trans vibration frequency matched the harmonic frequency of the chassis. IIRC, it started around 45 mph and diminished above 70 mph. Drove me almost insane! lol

Yup....that's very close to what I'm seeing.

I can't quite rememeber when it started, but truth be told the POLY mount is what I have been using since the coupe was completely rebuilt and put back into "service", so this may very well be the root cause of what I'm feeling as well!!!

What is really weird is that this only becomes perceptible once I hit 60 mph and above. Of course the kink is that it then it goes away at about 65-70 (lol, which is typically the speed at which I'm doing all my distance cruising now) and picks back up afterwards and becomes VERY pronounced. So certainly vehicle speed produces frequency changes.

I've been "studying" this thing for a while in my attempts to track it down.

FYI - I had removed the trans tailshaft weight way back when, perhaps this is contributing to this as well given that the weight would be used to absorb some such natural frequencies. Probably going to have a tough time finding the rubber bushings that isolate the weight from the tailshaft body...

Anyways, the axle and housing straightness checks are pretty much the last DIY kind of a thing I can do for now.

 

[Aspen500]

Those kind of harmonic vibrations are exactly the reason they put the weight on. It doesn't eliminate the vibration but changes the frequency.

If nothing else, the rubber mounts are inexpensive at a parts store, and would be worth a try. It was close to 10 years ago but the Advance store I went to had the mount in stock(!)

 

[jasperjacko]

Have you checked for slop in the trans output / yoke bushing and or yoke to shaft clearance. Had a very similar issue once. Replaced bearing, bushing, and yoke... smooth as silk.

 

[Sub03]

I've been fighting a RPM-dependable cyclic vibration in my Aspen around 45 mph (~2000 rpm) for the last couple of years.
You cannot really feel it, but it makes an annoying sound in the floor pans. Noticed it after replacing the trans mount with a cheap Rockauto one.
I asked the guys (and gals) in here for their advice on which brand trans mount to buy.
I ordered one of each brand Rockauto had i stock and tried them all. The one made in Korea was the best one (just as Aspen500 suggested in my thread back then), but the sound was still there.

This spring I got an NOS trans mount from eBay. A little bit better, but still not good.

Then I noticed the exhaust vibrating like a guitar string at 1800-2200 rpm. Googled a bit and read about lots of cars equipped with a rubber dynamic damper on the exhaust to take out the resonate frequencies. Ordered two from a Lexus and clamped them on the Y-pipe and it actually helped some. Then I installed a resonator right after the Y-pipe and the vibrating sound moved up in RPM and got damped a lot.

Lexus exhaust dampener


Next I am going to fab up a damper weight to install on the trans tailhousing hoping to get rid of the remaining sound. I found a dynamic damper Hyundai use on the subframe for this.

Sorry for the long post. My point is in my case the exhaust vibration was transferred via the engine and transmission and got into the floor boards through the transmission mount, so maybe you're on to something here.

 

[Aspen500]

It's tough to get a dampener weight with the right mass for a given frequency. There's all sorts of calculations based on the vibration hertz, vehicle harmonic frequency, etc....... I had NVH (noise, vibration, harshness) training but that was more than 30 years ago. Used to be good at harmonics, frequencies, how to narrow down the source (engine, trans, tires, resonance.....) but alot of that data is lost. Got overwritten by other data since then.

 

[AMC Diplomat]

I put in a heavier carpet with mass backing. That made things a lot quieter. But I'm not building a fast car so I'm ok with adding weight

 

[Sub03]

It's tough to get a dampener weight with the right mass for a given frequency. There's all sorts of calculations based on the vibration hertz, vehicle harmonic frequency, etc....... I had NVH (noise, vibration, harshness) training but that was more than 30 years ago. Used to be good at harmonics, frequencies, how to narrow down the source (engine, trans, tires, resonance.....) but alot of that data is lost. Got overwritten by other data since then.

I see that. Just sharing my experience. Bottom line: it helped.

I downloaded an app to measure vibration on my phone. Not so much help since the vibrations was mostly heard, not felt.
Downloaded an spectrumanalyzer app to measure which frequency I was hearing.
I got my exhaust resonance theory confirmed because the frequency was ~130 Hz (a V8 generates 4 exhaust pulses pr revolution: 130Hz X 60 seconds / 4 = 1950 RPM)
Buying a damper is a swing an a miss because i could not find what frequency it was designed for, but I was happy because it worked.