Exhaust Rattle and Mass Damper

[superbad]

I've got a loud rattle that just started coming on, but it only occurs right around 1400rpm (sounds like it's coming from the cat). Poking around trying to find the source, I found two of the rubber mounts on my exhaust mass damper had failed and it was just flopping around. The other two mounts failed when I tried to remove them, so I just pulled the whole thing off. I can pretty much stop the rattle by wedging a screwdriver up against the pipe near where the mass damper was located. So:

1. Is the cat failing internally? I know it's a common problem, but this rattle is very resonance specific. Does a dying cat rattle all the time?
2. Does the mass damper do anything useful (like, say, stop a nasty resonance at 1400rpm)? If the pics on their site are accurate, it looks like Piper leaves it off. Obviously Lotus found it necessary for some reason though.

[rip]

The early Elans suffered from the exhaust clattering against the bottom of the sump or gearbox (I can't remember which). To get around this, Lotus included a mount to keep the down & underpipe rigid. This caused stress on the mid-pipe as ir tried to flex.

Peco & Piper's answer was to include a flexi-section.

Not sure if that answers anything for you??

[GeoffSmith]

Mine's an S2 and there was no sign of a mass damper on it. How about some pictures?

[steve matthews]

Superbad, by mass damper do you mean a silencer prior to back box?
My main cat rattled at low rpm but upon throttle stopped and ship would
blow out the back. Yes it was gone.
Steve

[superbad]

No, this is a 1/4" thick steel plate attached to the mid section of the exhaust by 4 rubber mounts. It doesn't attach to the chassis at all. P/N A100S6012F, Mass Damper, Exhaust. Item 26 on page 45.01A

ex.JPG

damper.JPG

[steve matthews]

I can see no purpose to this mass damper.
My car did not have this and I had the original exhaust I believe.
Car functions well at the moment without this item, 2 1/4 Peco.
Plus I am saving weight, for the lotus purist.
Steve

[Brit-Car-Nut]

My understanding is the exhaust has a point where it will vibrate at a certain harmonic. If left unchecked, it will destroy the honeycomb in the CAT as well as weaken the welds or brittle areas around the welds.

Even GM doesn't waste money by putting things on cars that aren't needed and justified.

I have seen these on other cars. I think TR7 or TR8 used a harmonic damper too.

[superbad]

Just noticed a new rattle in my cat (this will be replacement cat #3 for me), on the same day the car rolled over 100k miles. The first couple cat failures were pretty definitely caused by running rich and overheating the matrix (bad fuel pressure regulator), but that was resolved the day I installed the last cat, and it has run beautifully since. I think the mass damper does, in fact, serve a purpose.

[Peter_W]

No, this is a 1/4" thick steel plate attached to the mid section of the exhaust by 4 rubber mounts. It doesn't attach to the chassis at all. P/N A100S6012F, Mass Damper, Exhaust. Item 26 on page 45.01A

ex.JPG

damper.JPG

The rattle from a failed mass damper and its removal was one of the "common repairs" on one of the historical iterations of LEC - it was the first fix I had to do on mine. Yours lasted an impressively long time.

Edit: Actually it's still there as part of the Broken Downpipe write-up in Common Repairs (mis-labeled as a hanger instead of a damper): www.lotuselancentral.com/repair/downpipe.htm

[Brit-Car-Nut]

Peter: The mass damper is there to reduce harmonics that will destroy the innards of the CAT. If you don't have a CAT, then fine, but don't heave it if you have a Catalytic Converter unless you want to replace it frequently.

[superbad]

The rattle from a failed mass damper and its removal was one of the "common repairs" on one of the historical iterations of LEC - it was the first fix I had to do on mine. Yours lasted an impressively long time.

Nope, my mass damper has been sitting on a shelf since 2011. I'm putting it back on when I replace this cat- just ordered new rubber mounts. Like the man said, GM put it there for a reason.

[Peter_W]

The rattle from a failed mass damper and its removal was one of the "common repairs" on one of the historical iterations of LEC - it was the first fix I had to do on mine. Yours lasted an impressively long time.

Nope, my mass damper has been sitting on a shelf since 2011...

Ah - I missed that this was a thread resurrection.

[Peter_W]

Peter: The mass damper is there to reduce harmonics that will destroy the innards of the CAT. If you don't have a CAT, then fine, but don't heave it if you have a Catalytic Converter unless you want to replace it frequently.

Thanks John - good info as always. In my case I've installed a custom exhaust.