Hello All,
Greetings from the land down under.

Could someone point me at a site (or photo) where i can find out how the USA made vehicles that use the LS1 5.7 litre (gen III) engine attach and stabilize the first section of the exhaust system from the exhaust manifolds to the catalytic converters.

We seem to have a problem in Australia in that some vehicles have serious vibration problems which seems to come from this part of the exhaust system. These sections of the exhaust system in australia run from the manifold through the cats to the main exhaust system. The front of the cats is supported by a bracket that goes on the transmission side of the rear mount of the transmission. This is basically a T shaped bracket that supports the cats on each side of the top bar of the T.

There is a significant problem with this design in that the bracket is very light gauge metal and vibrates, I guess its got to be light so that it does not conduct heat from the cats into the transmission and its rubber mount. This however compromised the rigidity it should have to lock both sides of the front section of the exhaust firmly to the motor and transmission so that it is not affected by the attachment of the remainder of the exhaust system.

Does anyone have some underbody photos or drawings of the exhaust systems of GM USA installations of the LS1 engine ??

Here are two photos of the setup I've got trouble with on an Australian Holden Calais:
<img src="http://www.users.bigpond.net.au/furse-group/vibrations/dscf0012R.jpg" alt=" - " /> <img src="http://www.users.bigpond.net.au/furse-group/vibrations/dscf0013R.jpg" alt=" - " />

<small>[ December 16, 2002, 08:00 PM: Message edited by: TonyF ]</small>

 

That's a pretty sweet set-up for a factory exhaust system, "H" pipe and all!!! Too bad GM cheap'd it out here in the USA. I don't have any pic's but the bracketry looks pretty good on your car IMHO. The reason it is attached to the tranny is that the engine and trans will torque as an assembly so this is a good place to attach it.

 

Thanks for the comments John.

The problem seems to be that the cats and the front section of the exhaust are not anchored together firmly enough so that they all torque with the engine and transmission and are not influenced by the weight and inertia of the rear section of the exhaust system.

The rear parts of the exhaust system that bolt on after the cats can make the front section wriggle around because of what seems to me as two problems:

(1) the cats are supported on the side and badly from mechanical stability standpoint (the mounting point is at a mechanical disadvantage), given that the cats also support the front of the rear sections of the exhaust.

(2) The bracket holding the cats to the transmission is very flimsy and offers little support against the weight of the rear section of the exhaust system. Its so flimsy in fact that if you bang on a cat with your hand the whole front section vibrates and the support bracket rings. just look how thin the metal of the bracket is, it has almost no strength in a forward and backward direction.

I was hoping that USA vehicles using the LS1 would have solved this rigidity problem without causing heat from the cats to be conducted back to the rear shock-mount on the transmission. This is why I was seeking photos or drawings of the USA exhaust systems.

<small>[ December 16, 2002, 11:12 PM: Message edited by: TonyF ]</small>

 

looks like you could weld a piece of barstock or angle iron across that support to strengthen it up.
im not really sure i understand what the exhaust is doing.... is it flopping around and rattling against things, or?
jeremy

 

Thanks Jeremy,

It vibrates and makes a lot of noise in the cabin. The natural frequency seems to be at about 750 RPM it really is bad here, it also vibrates in the cabin at harmonics of 750RPM (ie 1500 rpm, 2250rpm and 3000 rpm. Each harmonic is not as bad for vibratiion as the previous one. But the one at 1500 RPM is really annoying because the motor is often at that speed (about 80KPH). As you come up to about 1500 rpm you can feel the vibration start and as you go over by a few hundred RPM it stops.

When the engine is cold and you are just driving off from a standstill there is also a very low frequency vibration which feels like something under the car wagging. This sounds and behaves like a beat frequency between two things that have almost, but not quite, the same frequency of vibration. The difference in frequencies would need to be about 3 cycles/second and below for this to happen.

<small>[ December 17, 2002, 10:48 PM: Message edited by: TonyF ]</small>

 

Have you tried removing the bolts that hold the bracket to the converters to see if the noise changes? I did some development work on a Catera with different engines, and found the trans mount would transfer the vibration into the vehicle. I built a bracket that mounted between the trans mount and the trans and was held with existing bolts. it attached to the converters in the original location. It eliminated the "noise" transfer, and the heat was no longer a issue because it was mounted direct to the trans.

 

Thanks for your experiences.

The Maker (GM Holden) changed the mounting bracket to an experemental one of a different design, This was installed by my dealer and this changed the way the vibration sounded and introduced the beat note problem. I think the beat note is generated because the bracket bar that attaches to the cats is not symetrically welded to the upper part of the bracket, this in effect makes each side of the the bracket arm a slightly different length from the weld to the cat.

The bracket mounts as you suggest, between the trans and the rubber shock-mount, you can just make out the side of the shock-mount on the photo from below. I however, would have thought one would still have to be careful about transfering heat to the rubber shock-mount and even though the transmission body makes a good heat-sink to stop this, could it get rid of the heat from a much thicker bracket without adverse effects to its own operating temperature.

<small>[ December 18, 2002, 05:31 PM: Message edited by: TonyF ]</small>