Exhaust Drone

[engineman]

Following a lengthy 3 year layup of the car due to a house move and major renovation I am now driving the Stag again
and determined to fix an ongoing exhaust drone problem I have has since fitting a Rimmer stainless steel manifold onto the car. The
problem is a loud exhaust drone at approximately 30 mph and 1500rpm when the car is in top gear, I have a Type-35 automatic
gearbox. If I drop down to second gear the drone is less noisy at 30mph / 2000rpm, if I let the rpm in second gear drop
to 1500 rpm the drone comes back loudly, although the manifold is new the rest of the stainless steel exhaust system
is probably 20 years old. After surfing the net I come to the conclusion that there is a new resonance in the system caused by
the new stainless steel exhaust manifold. I have thoroughly checked the exhaust underneath the car to make sure that the
pipes are not touching the bodywork and the manifold is not touching the front sub frame and steering, all the mounting rubbers are hanging
the exhaust pipes like they should. The new exhaust manifold has been heat wrapped also the two down pipes along to the silencers.
my decision is what to next? I could go back to my old cast iron manifold but that seems to be defeatist attitude or find
a solution without just slashing money out on say new silencers. Any new ideas would be greatly appreciated.

[StagnJag]

Hiya,

Two things spring to mind, firstly the tubular manifolds pass very close to the steering column shaft, could it be touching htat and vibrating? secondly, not directly related to the exhaust, but is the viscous coupling on the fan on the way out? That can cause all sorts of horrible noises throughout the car.

regards

Dave

[rac]

Following a lengthy 3 year layup of the car due to a house move and major renovation I am now driving the Stag again
and determined to fix an ongoing exhaust drone problem I have has since fitting a Rimmer stainless steel manifold onto the car. The
problem is a loud exhaust drone at approximately 30 mph and 1500rpm when the car is in top gear, I have a Type-35 automatic
gearbox. If I drop down to second gear the drone is less noisy at 30mph / 2000rpm, if I let the rpm in second gear drop
to 1500 rpm the drone comes back loudly, although the manifold is new the rest of the stainless steel exhaust system
is probably 20 years old. After surfing the net I come to the conclusion that there is a new resonance in the system caused by
the new stainless steel exhaust manifold. I have thoroughly checked the exhaust underneath the car to make sure that the
pipes are not touching the bodywork and the manifold is not touching the front sub frame and steering, all the mounting rubbers are hanging
the exhaust pipes like they should. The new exhaust manifold has been heat wrapped also the two down pipes along to the silencers.
my decision is what to next? I could go back to my old cast iron manifold but that seems to be defeatist attitude or find
a solution without just slashing money out on say new silencers. Any new ideas would be greatly appreciated.

you say the exhaust pipes are as they should be but are they clear of the rear subframe where they pass thro-can cause terrible droning if they are not clear-how do I know ????

[engineman]

Dave thanks for the reply, the steering column is clear of the manifolds, as far as I can tell the viscous coupling looks OK there is no apparent looseness and it doesn't make any sort of noise at 1500-2000 rpm when the car is stationary

[engineman]

I will jack the car up and make a double check on the clearance in the sub frame.

[Andrew S]

Just another thing you could try as a test is to remove the gearbox to exhaust Z shaped bracket and see if it makes any difference

[mike@thenook]

Also,
Is the pipe that crosses over behind the diff well clear of the boot floor? If it is too close it will produce resonance which gets emphasised by the boot acting as a sound box.

[Seaking]

also it might be worth checking exhaust clamps close to manifolds mine looked clear until I got her up on a ramp with engine running soon found it just touching. I also needed to help it around the steering shaft.

Cheers Glenn

[Stagsongas]

Engineman,

To go in the complete and opposite direction, and assuming that no part of the exhaust is touching the frame ( still the most likely cause), did you run the engine after fitting the manifolds, but before wrapping?

Everything has a natural resonance, but that can be affected favourably or adversely by a change in an object's shape, mass, density, temperature etc. etc. Although you might not want to consider it, the wrapping just could be the cause of the unwanted, noisy resonance. If you ran the engine before wrapping, can you recall the sound being present then?

Good Luck

Steve

[rac]

Engineman,

To go in the complete and opposite direction, and assuming that no part of the exhaust is touching the frame ( still the most likely cause), did you run the engine after fitting the manifolds, but before wrapping?

Everything has a natural resonance, but that can be affected favourably or adversely by a change in an object's shape, mass, density, temperature etc. etc. Although you might not want to consider it, the wrapping just could be the cause of the unwanted, noisy resonance. If you ran the engine before wrapping, can you recall the sound being present then?

Good Luck

Steve

not wanting to be contradictory but have wrapped my tubular manifolds to try and lower the under bonnet temp and cant say I find any difference or resonance-my problem was the clearance thro the rear subframes

[engineman]

The tubular manifold was heat wrapped before I installed it on the car so I do not have a comparison with and without wrapping, I am going to have another physical look at the exhaust again to make sure its not touching the bodywork. The R/H manifold is very close to the front framework but not touching I suppose with the car moving it could possibly touch, I may have to remove the manifold and put a flat in the side to give it as much clearance as possible.

Peter

[Stagsongas]

not wanting to be contradictory but have wrapped my tubular manifolds to try and lower the under bonnet temp and cant say I find any difference or resonance-my problem was the clearance thro the rear subframes

Fully understood, but we can't assume both manifolds are the same make/type/material/batch. It's just a shot in the dark if all else fails.

Regards

Steve

[Stagsongas]

The tubular manifold was heat wrapped before I installed it on the car so I do not have a comparison with and without wrapping, I am going to have another physical look at the exhaust again to make sure its not touching the bodywork. The R/H manifold is very close to the front framework but not touching I suppose with the car moving it could possibly touch, I may have to remove the manifold and put a flat in the side to give it as much clearance as possible.

Peter

Well good luck Peter. I understand the lack of comparision and there's little chance it is that, but worth a mention all the same. I can't remember if anyone else suggested worn engine mountings allowing the engine to lower and turn with torque, and then touch the frame with the manifold, but if not, it's a good bet. If you can get it to make the sound when stationary then sometimes you can see if it disappears by raising the engine a little (technique is your choice). In the Dim and distant, I got lucky by slackening the engine mounting bolts top and bottom both sides, jacking the engine up using a wooden sump protector and then doing all the bolts back up. Clearance was then restored. For a while.....

Regards

Steve

[engineman]

I have reformed the R/H exhaust manifold so that there is adequate clearance between it and the steering shaft and front chassis member, The exhaust has been removed and repositioned so that there is maximum clearance in the rear subframe holes , there is also 3-4 cm clearance between the exhaust pipe and the boot floor, I have driven the car with and without the exhaust bolted to the gearbox brackets. The exhaust drone has not improved one bit, the only good news is that I did not get sunburnt this week working under the car. The two stainless steel silencers were fitted on the car when I purchased it back in 1990, the engine was burning lots of oil before it was reconditioned and the new manifold was fitted, could the silencers be so gummed up with oil residue that its restricting the exhaust flow? should the car be fitted with maybe sports silencers to match the manifold? any thoughts would be appreciated.

[TR5convalescent]

This is going to sound mad, but years ago I had a new exhaust fitted to a Golf GTi. I had a similar boom at certain speeds obviously caused by harmonics. The exhaust man simply fitted a large jubilee clip over the pipe and the noise went.