How Hot Does An Exhaust Pipe Get at the bottom bend of the down pipe?
#1
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Hi Guys
Got a Problem with the Exhaust Pipe/Down Pipe on '50 Shades' my Grey XJS V12
This started with an Exhaust Leak at the bottom of the Down Pipe, where the Sealing Ring is Clamped in place by 3 Bolts, which was leaking enough Exhaust Gas to cause a Flat Spot Hesitation on Take Off
Having re-positioned this, it has been ok for a while although unfortunately its started leaking again, so this time I am planning to bed the Ring in place with RTV Silicone, as I'm reluctant to use exhaust paste which can go hard and crack
I've got some Bathroom RTV which I am thinking of using, so any thoughts on whether this would do the Job
Got a Problem with the Exhaust Pipe/Down Pipe on '50 Shades' my Grey XJS V12
This started with an Exhaust Leak at the bottom of the Down Pipe, where the Sealing Ring is Clamped in place by 3 Bolts, which was leaking enough Exhaust Gas to cause a Flat Spot Hesitation on Take Off
Having re-positioned this, it has been ok for a while although unfortunately its started leaking again, so this time I am planning to bed the Ring in place with RTV Silicone, as I'm reluctant to use exhaust paste which can go hard and crack
I've got some Bathroom RTV which I am thinking of using, so any thoughts on whether this would do the Job
#2
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orangeblossom (08-20-2020)
#3
#4
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JB Weld do a GOOD range of RTV now, really good stuff. I saw some at work, back in early 2019, just before I retired, very impressed.
Maybe like Greg, Amazon etc is the go.
I would not use that 2 pack stuff, it will seal it, but, you will never get it apart in the future, without a lot of effort, and foul words.
Maybe like Greg, Amazon etc is the go.
I would not use that 2 pack stuff, it will seal it, but, you will never get it apart in the future, without a lot of effort, and foul words.
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orangeblossom (08-20-2020)
#5
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Alex
The temp inside the combustion chamber is about 2,500 C. according to wikipedia.
Exhaust temp in the manifold is about 1,500 degrees C
I reckon the bottom of the downpipe will be 900 once the car gets going.
I think you have a flange problem, which requires welding, a bodge will not do it. How sure are you that this is the cause of the flatspot? Cn you hear and feel the leak.
Silicone will melt above 350 C. Exhaust paste and bandage will hold it for a bit, and it will come apart easily enough.
The temp inside the combustion chamber is about 2,500 C. according to wikipedia.
Exhaust temp in the manifold is about 1,500 degrees C
I reckon the bottom of the downpipe will be 900 once the car gets going.
I think you have a flange problem, which requires welding, a bodge will not do it. How sure are you that this is the cause of the flatspot? Cn you hear and feel the leak.
Silicone will melt above 350 C. Exhaust paste and bandage will hold it for a bit, and it will come apart easily enough.
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Grant Francis (08-20-2020),
orangeblossom (08-20-2020)
#6
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Alex
The temp inside the combustion chamber is about 2,500 C. according to wikipedia.
Exhaust temp in the manifold is about 1,500 degrees C
I reckon the bottom of the downpipe will be 900 once the car gets going.
I think you have a flange problem, which requires welding, a bodge will not do it. How sure are you that this is the cause of the flatspot? Cn you hear and feel the leak.
Silicone will melt above 350 C. Exhaust paste and bandage will hold it for a bit, and it will come apart easily enough.
The temp inside the combustion chamber is about 2,500 C. according to wikipedia.
Exhaust temp in the manifold is about 1,500 degrees C
I reckon the bottom of the downpipe will be 900 once the car gets going.
I think you have a flange problem, which requires welding, a bodge will not do it. How sure are you that this is the cause of the flatspot? Cn you hear and feel the leak.
Silicone will melt above 350 C. Exhaust paste and bandage will hold it for a bit, and it will come apart easily enough.
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orangeblossom (08-20-2020)
#7
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Hi Greg/Grant/Xjeffs
Thanks for you help and advice
I managed to find some Exhaust paste which can handle Temps of 1,000/c but decided not to use it as from what I've been reading it apparently cures rock hard, which could cause a problem if I ever wanted to replace a part of the Exhaust
So instead I also managed to find some RTV Silicone that can handle Temps up to 180/c which may be just enough to do the job, as its only a very small leak around that donut joining ring, though more than enough to throw the Engine off kilter during take off
It was only £4 for a great big Tube, so I will give that a try and see what happens
Thanks for you help and advice
I managed to find some Exhaust paste which can handle Temps of 1,000/c but decided not to use it as from what I've been reading it apparently cures rock hard, which could cause a problem if I ever wanted to replace a part of the Exhaust
So instead I also managed to find some RTV Silicone that can handle Temps up to 180/c which may be just enough to do the job, as its only a very small leak around that donut joining ring, though more than enough to throw the Engine off kilter during take off
It was only £4 for a great big Tube, so I will give that a try and see what happens
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#8
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orangeblossom (08-20-2020)
#9
#10
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Exhaust leaks don't cause flat spots. Retarded ignition, faulty advance, manifold air leaks, incorrect fuel grade, fuel pressure to name a few but not exhaust.
Back pressure is an urban myth that just won't die. To get proper scavenging of the cylinder you need exhaust velocity at the lowest pressure possible - search for pulse scavenging -
This vid really explains it pretty well Stop Saying Car Exhausts Need Back Pressure
If you have a flat spot you have an issue elsewhere too.
Back pressure is an urban myth that just won't die. To get proper scavenging of the cylinder you need exhaust velocity at the lowest pressure possible - search for pulse scavenging -
This vid really explains it pretty well Stop Saying Car Exhausts Need Back Pressure
If you have a flat spot you have an issue elsewhere too.
#11
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#12
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I had mounted the probe right at the end of the exhaust manifold, so the downpipe will be a bit cooler.
#13
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Exhaust leaks don't cause flat spots. Retarded ignition, faulty advance, manifold air leaks, incorrect fuel grade, fuel pressure to name a few but not exhaust.
Back pressure is an urban myth that just won't die. To get proper scavenging of the cylinder you need exhaust velocity at the lowest pressure possible - search for pulse scavenging -
This vid really explains it pretty well Stop Saying Car Exhausts Need Back Pressure
If you have a flat spot you have an issue elsewhere too.
Back pressure is an urban myth that just won't die. To get proper scavenging of the cylinder you need exhaust velocity at the lowest pressure possible - search for pulse scavenging -
This vid really explains it pretty well Stop Saying Car Exhausts Need Back Pressure
If you have a flat spot you have an issue elsewhere too.
it's a good catch though, and I realize I don't know where he's taking about. At the manifold exit to the first catalyst? My only 3 bolt flange is at the IRS.
Last edited by Xjeffs; 08-20-2020 at 03:21 PM.
#14
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Hi Benkenobi
You could well be right, or maybe I just didn't explain what happens as well as I could have done
She idles perfect at 750 RPM but when you hit the Throttle, the Revs momentarily dip before She picks up but once the Engine gets hot that almost doesn't happen at all and when I went around the Three Bolts that sandwiches the donut ring and tightened them up a bit more, that also made an improvement
So I will refit the donut ring with some RTV and then report back with the findings
You could well be right, or maybe I just didn't explain what happens as well as I could have done
She idles perfect at 750 RPM but when you hit the Throttle, the Revs momentarily dip before She picks up but once the Engine gets hot that almost doesn't happen at all and when I went around the Three Bolts that sandwiches the donut ring and tightened them up a bit more, that also made an improvement
So I will refit the donut ring with some RTV and then report back with the findings
#15
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What you describe sounds like retarded timing to me but it would be interesting to check mixture - is it better when hot or cold ?.
May be worth making sure those balance weights on the dizzy aren't siezed. Of course then theres all the other stuff like making sure the throttle bodies are synchronised - so many things this can be - A blowing exhaust needs fixing either way, I don't know so much about things on the V12's, later cars that I've seen with thesear donut type seals to accommodate movement are spring loaded - or at least aided. I've also seen these type of joints over tightened and once you do that they never seal properly as it distorts the exhaust.
I'm happy enough to have a Jaguar old enough to have no such BS as cats and o2 sensors, exra air and copious complexity all in the name of emmissions - but I do have a big cat with them and am having no end of issues with this 'emmissions' stuff currently - working on a car without is refreshingly simple ...
May be worth making sure those balance weights on the dizzy aren't siezed. Of course then theres all the other stuff like making sure the throttle bodies are synchronised - so many things this can be - A blowing exhaust needs fixing either way, I don't know so much about things on the V12's, later cars that I've seen with thesear donut type seals to accommodate movement are spring loaded - or at least aided. I've also seen these type of joints over tightened and once you do that they never seal properly as it distorts the exhaust.
I'm happy enough to have a Jaguar old enough to have no such BS as cats and o2 sensors, exra air and copious complexity all in the name of emmissions - but I do have a big cat with them and am having no end of issues with this 'emmissions' stuff currently - working on a car without is refreshingly simple ...
#16
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Hi Benkenobi
You could well be right, or maybe I just didn't explain what happens as well as I could have done
She idles perfect at 750 RPM but when you hit the Throttle, the Revs momentarily dip before She picks up but once the Engine gets hot that almost doesn't happen at all and when I went around the Three Bolts that sandwiches the donut ring and tightened them up a bit more, that also made an improvement
So I will refit the donut ring with some RTV and then report back with the findings
You could well be right, or maybe I just didn't explain what happens as well as I could have done
She idles perfect at 750 RPM but when you hit the Throttle, the Revs momentarily dip before She picks up but once the Engine gets hot that almost doesn't happen at all and when I went around the Three Bolts that sandwiches the donut ring and tightened them up a bit more, that also made an improvement
So I will refit the donut ring with some RTV and then report back with the findings
But, if the leak is upstream of the O2 sensor, I think that might have an impact but I would think it would be hot and cold.
Last edited by Xjeffs; 08-20-2020 at 08:38 PM.
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