Exhaust pipe resonance!
Discussion
Does anyone have any ideas what causes exhaust systems to resonate?
I made my own system on my car (TVR Vixen, rwd x/flow) with a 4-2-1 manifold supported by a rigid mount at the gearbox. I then have a canvas stap type mount located halfway along and finally a rubber mount at the tailpipe. All interconnections are rigid push-fit types with U-type exhaust clamps.
Whenever the car is accelerating/decelerating everything is fine but when holding a constant speed at anything over 2000 rpm I get a horrible metallic rattling from underneath?
I recently made some adjustments to the exhaust to centralise the exit of the tailpipe, which involved putting the sytem under quite a bit of tension on the rubbers and the noise dissappeared for a few miles, but now it's back!
Does anybody know which way I need to go with this? More slack on the system or more rigid??? I'm thinking maybe I need to put in flexible joint somewhere??
Any advice welcome.
Darren.
stevieturbo said:
A rigid system with no flexible joints anywhere, is going to vibrate !!! thats why OE systems have flexi joints and flexible or rubber hangers everywhere !!
I was kinda thinking that might be the case (this was my first attempt at an exhaust system), thanks for the advice.
Not sure that any TVR has flexi joints in its exhaust system that I know of 
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So long at the mounting points are isolated from the chassis (rubber) and its not accidentally touching the chassis anywhere else, it shouldn't be the problem. I'm no expert but I would look at the actual design, such as what gauge of steel have you used. Cheap SS systems can have a horrible ring at certain revs. It may even be worth just getting a relatively cheap thick gauge mild steel system made up as it can kill the ringing (resonating) more effectively, that's not to say if you use a decent gauge SS it wouldn't also cure it 
.
. So long at the mounting points are isolated from the chassis (rubber) and its not accidentally touching the chassis anywhere else, it shouldn't be the problem. I'm no expert but I would look at the actual design, such as what gauge of steel have you used
. Cheap SS systems can have a horrible ring at certain revs. It may even be worth just getting a relatively cheap thick gauge mild steel system made up as it can kill the ringing (resonating) more effectively, that's not to say if you use a decent gauge SS it wouldn't also cure it . None of the front engine rear wheel drive cars I've had used any sort of flexible coupling in the exhaust and none resonated - well not until they got old and started to fall apart. 
Would agree with HarryW that it's worth making sure the exhaust isn't touching anywhere. Also worth checking that all the clamps are properly tight. Might even be worth trying without the mount by the gearbox if the other mounts will take the weight OK.
Would agree with HarryW that it's worth making sure the exhaust isn't touching anywhere. Also worth checking that all the clamps are properly tight. Might even be worth trying without the mount by the gearbox if the other mounts will take the weight OK.
I made the exhaust from 18 gauge (1.2mm ish) mild steel 2" diameter tubing for the main pipe after the manifold. As mentioned, I didn't think these cars had a fexi joint but will give it a try if all else fails.
Although it's a tight sqeeeze, I'm pretty sure she's not touching the chassis but I'll double check (along with the tightness of the clamps).
I'll try getting rid of the gearbox mount as suggested and see if that helps.
Many thanks for the advice.
Even if the exhaust is fully isolated from the body the fact that its a tubular piece of steel bolted to a hefty block of cast iron with hundreds of fast moving reciprocating parts means that at a certain frequency (engine rpm) its going to turn into a tuning fork and resonate. Ive seen various jap cars with huge heavy blocks of steel as standard bolted onto the exhaust to stop resonance. It may be harder to stop than you think but the suggestion of putting a flexible link towards the front of the system is a good one as that should help to absorb any resonance. I know that it may not have had one as standard fit but then your not fitting a standard exhaust as a replacement.
Know anyone who wants a 68 vixen, a bit shabby but would walk an MOT, zorst is a bit loud but doesn't resonate, it's just sat on my mates drive.
I know that he had a total nightmare welding his 4-2-1 SS manifold which cracked. I can't believe it is the system as we mounted his as normal and it's fine. I think mini metalastic bushes fetured quite heavily.
I know that he had a total nightmare welding his 4-2-1 SS manifold which cracked. I can't believe it is the system as we mounted his as normal and it's fine. I think mini metalastic bushes fetured quite heavily.
I can't help thinking that when the engine is moved the right amount on its mounts under load the exhaust is rattling against something even though you'd think it was impossible for that to happen looking under the car at rest. Drive somewhere dusty then drive on normal roads for a bit then look for suspiciously clean bits adjacent to the exhaust along the length of its run.
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