Home » Public Forums » GMCnet » [GMCnet] Harmonics in my exhaust system
| [GMCnet] Harmonics in my exhaust system [message #319037] |
Wed, 14 June 2017 21:14  |
BobDunahugh
Messages: 2465
Registered: October 2010
Location: Cedar Rapids, IA
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The harmonics happens at about 2100 . Then stops. Then returns at about 3900 to 4100 RPMs. But not as strong. Both at moderate throttle. It's a 403 with a stock exhaust system. I thought it was the double wrapped mufflers. So I put on two Flow master series 40 units. No change. But these are real solid/welded mufflers. I have an idea. Any thoughts? Bob Dunahugh 78 Royale 403
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| Re: [GMCnet] Harmonics in my exhaust system [message #319041 is a reply to message #319037] |
Thu, 15 June 2017 00:42  |
Bullitthead
Messages: 1411
Registered: November 2013
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Make sure you don't have any solid exhaust mounts, accidental or otherwise, and do a compression check on all eight cylinders. Then if there are no large differences in compression readings (20 psi or more), do a power balance test under light load and note differences in power output, and finally, do the power balance test at the rpm where you hear/feel the resonance. Possibilities of weak plugs/wires/vacuum leak on certain cylinder(s), and valves sealing at rpm.
Terry Kelpien
ASE Master Technician
73 Glacier 260
Smithfield, Va.
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| Re: [GMCnet] Harmonics in my exhaust system [message #319043 is a reply to message #319037] |
Thu, 15 June 2017 08:13 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
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I am not sure what you mean by double wrapped mufflers. But Flowmaster 40s are minimal muffling units to substitute with as a test for quiteness. Very loud.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: [GMCnet] Harmonics in my exhaust system [message #319060 is a reply to message #319037] |
Thu, 15 June 2017 13:03 |
BobDunahugh
Messages: 2465
Registered: October 2010
Location: Cedar Rapids, IA
Karma: 11
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Senior Member |
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Larry. You have an excellent point. Next. I should have said double outside wall mufflers. The exhaust system is pure stock ( except the mufflers now ) down to the proper hangers, and GM muffler bearings for the tail pipe. Sound out the tail pipe is up a little. What helps there is that the tail pipe points straight out the back. As to the 403. Has about 7000 miles on it now. Bored .030 over, towing cam, standard bearings, stock oil pump, roller cam chain, 3 angle valve job, new plugs/wires and was balance. Compression test at 1884 miles were 144 to 149 psi. Hope to get the EFI with spark control on soon. A bright side is that I'm at 2100 rpms for very short times. The 4000 rpms can get stretched out on long hills. Most driving is in the 2800 to 3000 rpm range. Bob Dunahugh
________________________________
From: Bob Dunahugh
Sent: Wednesday, June 14, 2017 9:14 PM
To: gmclist@list.gmcnet.org
Subject: Harmonics in my exhaust system
The harmonics happens at about 2100 . Then stops. Then returns at about 3900 to 4100 RPMs. But not as strong. Both at moderate throttle. It's a 403 with a stock exhaust system. I thought it was the double wrapped mufflers. So I put on two Flow master series 40 units. No change. But these are real solid/welded mufflers. I have an idea. Any thoughts? Bob Dunahugh 78 Royale 403
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| Re: [GMCnet] Harmonics in my exhaust system [message #319079 is a reply to message #319066] |
Thu, 15 June 2017 18:44 |
Bill Van Vlack
Messages: 419
Registered: September 2015
Location: Guemes Island, Washington
Karma: 14
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Mechanical or acoustic? A 'drone' is generally acoustic.
Bill Van Vlack
'76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath, Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o mid November 2015.
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| Re: [GMCnet] Harmonics in my exhaust system [message #319086 is a reply to message #319060] |
Thu, 15 June 2017 19:58 |
BobDunahugh
Messages: 2465
Registered: October 2010
Location: Cedar Rapids, IA
Karma: 11
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Johnny. I don't think the tail pipe is vibrating due to the two muffler bearings being as tight as they are. The harmonic sound seems to be coming from the muffler area. I don't lug engines of any type. When I'm going up long climbs. I down shift, and run the rpm's up to the 4000 rpm range with moderate throttle. I like the water, and trans fluid pumps moving fluids faster. That's why I get to those rpm ranges. The thing here is that I never had this problem with this exhaust system in the burned 78 GMC. Bob Dunahugh 78 Royale
________________________________
From: Bob Dunahugh
Sent: Thursday, June 15, 2017 1:03 PM
To: gmclist@list.gmcnet.org
Subject: RE: Harmonics in my exhaust system
Larry. You have an excellent point. Next. I should have said double outside wall mufflers. The exhaust system is pure stock ( except the mufflers now ) down to the proper hangers, and GM muffler bearings for the tail pipe. Sound out the tail pipe is up a little. What helps there is that the tail pipe points straight out the back. As to the 403. Has about 7000 miles on it now. Bored .030 over, towing cam, standard bearings, stock oil pump, roller cam chain, 3 angle valve job, new plugs/wires and was balance. Compression test at 1884 miles were 144 to 149 psi. Hope to get the EFI with spark control on soon. A bright side is that I'm at 2100 rpms for very short times. The 4000 rpms can get stretched out on long hills. Most driving is in the 2800 to 3000 rpm range. Bob Dunahugh
________________________________
From: Bob Dunahugh
Sent: Wednesday, June 14, 2017 9:14 PM
To: gmclist@list.gmcnet.org
Subject: Harmonics in my exhaust system
The harmonics happens at about 2100 . Then stops. Then returns at about 3900 to 4100 RPMs. But not as strong. Both at moderate throttle. It's a 403 with a stock exhaust system. I thought it was the double wrapped mufflers. So I put on two Flow master series 40 units. No change. But these are real solid/welded mufflers. I have an idea. Any thoughts? Bob Dunahugh 78 Royale 403
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| Re: [GMCnet] Harmonics in my exhaust system [message #319097 is a reply to message #319066] |
Thu, 15 June 2017 23:42 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Take a LARGE speaker magnet and attach it at various places along the exhaust system. Do not rely on the magnet staying in place because it is flat and you have a round surface to attach to. Use a large diameter hose type clamp to hold it in place. As you change the magnet locations you should be able find the RPM resonate point if this really is a vibration issue.
I have a resonate muffler at 45 mph on a Blazer. This is how I found it. A large speaker magnet on the muffler eliminated it. I left it installed. It was cheaper and quicker than replacing the muffler.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] Harmonics in my exhaust system [message #319102 is a reply to message #319037] |
Fri, 16 June 2017 07:55 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Oh OK  4100 in third would be stepping out even with a 3.7 final ratio.
Anyhow, as someone noted, do you have a sound or a mechanical problem here? To check for a sound resonance, try opening and closing different windows in the coach looking for a change in intensity or RPM at which it occurs. (The Mousetoad will resonate above 30 mph with a rear window open but not a front) For something mechanical, there are two ways I know of. If the camera doesn't see anything, change the mass of everything one piece at a time. Take some lead sheet and clamp on the muffler, then sections of exhaust pipe, accessories that are belt driven, etc. When you change the mass of the culprit, the resonance will disappear or the RPM at which it occurs will change. We used to wrap scoot carburetors in solder to preclude foaming and power loss from resonant shaking.
--johnny
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: [GMCnet] Harmonics in my exhaust system [message #319127 is a reply to message #319037] |
Fri, 16 June 2017 17:22 |
Bill Van Vlack
Messages: 419
Registered: September 2015
Location: Guemes Island, Washington
Karma: 14
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For acoustic resonance, look up helmholtz or 1/4 wave exhaust drone cancellation. Results for big V8s on the interweb indicate that a 1/4 wave tube at that RPM will need to be a couple feet long, tee'd off the exhaust pipe and capped. The reflected 1/4 wave cancels the offending one. Consider making it out of two pipes, one over the other so that they can slide and be tuned then clamped with a muffler clamp. I understand that a helmholtz resonator can be shorter.
Bill Van Vlack
'76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath, Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o mid November 2015.
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| Re: [GMCnet] Harmonics in my exhaust system [message #319143 is a reply to message #319086] |
Fri, 16 June 2017 23:33 |
BobDunahugh
Messages: 2465
Registered: October 2010
Location: Cedar Rapids, IA
Karma: 11
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Senior Member |
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This is my thought on the source. ( MAYBE ) The entire exhaust system was on the 78 Royale that burned. Plus the front, and rear suspension. Then trans, and 3:70 FD. There are 122 GMC VIN's between the two 78 Royales. The burned one was built in Dec of 77. The Mouse House in Jan of 78. The only item that wasn't from the burned GMC drive line is the 403. And the only difference between the two 403's is the camshaft. The 403 from the burned 78 had compression readings from 130 to 135 psi at 152,000 miles. The Mouse house compression readings were 140 to 145 psi at 1884 miles. Thus the only things different is a more aggressive towing cam. And a little higher compression readings. Bob Dunahugh
________________________________
From: Bob Dunahugh
Sent: Thursday, June 15, 2017 7:58 PM
To: gmclist@list.gmcnet.org
Subject: RE:Harmonics in my exhaust system
Johnny. I don't think the tail pipe is vibrating due to the two muffler bearings being as tight as they are. The harmonic sound seems to be coming from the muffler area. I don't lug engines of any type. When I'm going up long climbs. I down shift, and run the rpm's up to the 4000 rpm range with moderate throttle. I like the water, and trans fluid pumps moving fluids faster. That's why I get to those rpm ranges. The thing here is that I never had this problem with this exhaust system in the burned 78 GMC. Bob Dunahugh 78 Royale
________________________________
From: Bob Dunahugh
Sent: Thursday, June 15, 2017 1:03 PM
To: gmclist@list.gmcnet.org
Subject: RE: Harmonics in my exhaust system
Larry. You have an excellent point. Next. I should have said double outside wall mufflers. The exhaust system is pure stock ( except the mufflers now ) down to the proper hangers, and GM muffler bearings for the tail pipe. Sound out the tail pipe is up a little. What helps there is that the tail pipe points straight out the back. As to the 403. Has about 7000 miles on it now. Bored .030 over, towing cam, standard bearings, stock oil pump, roller cam chain, 3 angle valve job, new plugs/wires and was balance. Compression test at 1884 miles were 144 to 149 psi. Hope to get the EFI with spark control on soon. A bright side is that I'm at 2100 rpms for very short times. The 4000 rpms can get stretched out on long hills. Most driving is in the 2800 to 3000 rpm range. Bob Dunahugh
________________________________
From: Bob Dunahugh
Sent: Wednesday, June 14, 2017 9:14 PM
To: gmclist@list.gmcnet.org
Subject: Harmonics in my exhaust system
The harmonics happens at about 2100 . Then stops. Then returns at about 3900 to 4100 RPMs. But not as strong. Both at moderate throttle. It's a 403 with a stock exhaust system. I thought it was the double wrapped mufflers. So I put on two Flow master series 40 units. No change. But these are real solid/welded mufflers. I have an idea. Any thoughts? Bob Dunahugh 78 Royale 403
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| Re: [GMCnet] Harmonics in my exhaust system [message #319149 is a reply to message #319037] |
Sat, 17 June 2017 08:01 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Given that there is/was a difference, you're still looking at something resonant, no? 'Divide and conquer'. Modify one thing at a time by adding mass until something changes the resonance. You will have then identified the culprit or one of the culprits. Depending what it is / they are will determine a possible cure. Don't overlook the engine driven accessories either.
--johnny
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: [GMCnet] Harmonics in my exhaust system [message #319156 is a reply to message #319037] |
Sat, 17 June 2017 09:44 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Back a lonnnggg time ago, a friend and I did a Halloween prank on the
school bus that we rode. We wired some lead wheel weights to the drive
shaft. When the bus reached a certain speed, it would resonate throughout
the bus. One of my first experiences with harmonics. Really got a serious
butt chewing for that stunt. Probably go to jail for that, today.
I really studied exhaust systems as they relate to 2 cycle engines on
motorcycles and competition gocarts. You can make (or lose) a whole bunch
of power by using chambered components there. Same with 4 strokes, but not
as pronounced. Even mis-shaped pipes will resonate. One easy way out is the
addition of another smaller muffler (called a resonator, oddly enough)
downstream near the end of the pipe. Super-trapp makes 'em for race cars
that are tuneable. They use a stack of crimped discs that can be added or
subtracted to achieve the desired effect. They are often used where local
noise ordinances prevent open exhausts. Usually old age hearing loss
minimizes objection exhaust noise, Bob. (Grin)
Jim Hupy
Salem, Or
78 GMC ROYALE 403
On Jun 17, 2017 6:02 AM, "Johnny Bridges via Gmclist" <
gmclist@list.gmcnet.org> wrote:
Given that there is/was a difference, you're still looking at something
resonant, no? 'Divide and conquer'. Modify one thing at a time by adding
mass until something changes the resonance. You will have then identified
the culprit or one of the culprits. Depending what it is / they are will
determine a possible cure. Don't overlook the engine driven accessories
either.
--johnny
--
76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
"I forgive them all, save those who hurt the dogs. They must answer to me
in hell" - ol Andy, paraphrased
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| Re: [GMCnet] Harmonics in my exhaust system [message #319170 is a reply to message #319143] |
Sat, 17 June 2017 13:54 |
BobDunahugh
Messages: 2465
Registered: October 2010
Location: Cedar Rapids, IA
Karma: 11
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James, James, James. What's this old stuff about? So. The average age of owners of GMC's is near death. Mentally I'm 35. I use that series 40 Flow master on my track Yenko's. There mounted directly on the headers with no tail pipes. The sound to me is a symphony in all it's glory. And at 7 to 8000RPMs full throttle. WOW. Gear heads understand that one. As to the GMC. I have a microphone on my rear view camera. Yes the sound level is elevated some. Inside/up front. Again. Slightly elevated. Then the harmonics. That's not lowed, or last long. I'm just stuck on why. After all. It did break up a new $80 sheet metal muffler. As to a resonator at the back. ONE. I don't have room back there due to a floor drain. Two. As to the sound at the back. I turned the mic on the camera off. Bob Dunahugh
________________________________
From: Bob Dunahugh
Sent: Friday, June 16, 2017 11:33 PM
To: gmclist@list.gmcnet.org
Subject: RE:Harmonics in my exhaust system
This is my thought on the source. ( MAYBE ) The entire exhaust system was on the 78 Royale that burned. Plus the front, and rear suspension. Then trans, and 3:70 FD. There are 122 GMC VIN's between the two 78 Royales. The burned one was built in Dec of 77. The Mouse House in Jan of 78. The only item that wasn't from the burned GMC drive line is the 403. And the only difference between the two 403's is the camshaft. The 403 from the burned 78 had compression readings from 130 to 135 psi at 152,000 miles. The Mouse house compression readings were 140 to 145 psi at 1884 miles. Thus the only things different is a more aggressive towing cam. And a little higher compression readings. Bob Dunahugh
________________________________
From: Bob Dunahugh
Sent: Thursday, June 15, 2017 7:58 PM
To: gmclist@list.gmcnet.org
Subject: RE:Harmonics in my exhaust system
Johnny. I don't think the tail pipe is vibrating due to the two muffler bearings being as tight as they are. The harmonic sound seems to be coming from the muffler area. I don't lug engines of any type. When I'm going up long climbs. I down shift, and run the rpm's up to the 4000 rpm range with moderate throttle. I like the water, and trans fluid pumps moving fluids faster. That's why I get to those rpm ranges. The thing here is that I never had this problem with this exhaust system in the burned 78 GMC. Bob Dunahugh 78 Royale
________________________________
From: Bob Dunahugh
Sent: Thursday, June 15, 2017 1:03 PM
To: gmclist@list.gmcnet.org
Subject: RE: Harmonics in my exhaust system
Larry. You have an excellent point. Next. I should have said double outside wall mufflers. The exhaust system is pure stock ( except the mufflers now ) down to the proper hangers, and GM muffler bearings for the tail pipe. Sound out the tail pipe is up a little. What helps there is that the tail pipe points straight out the back. As to the 403. Has about 7000 miles on it now. Bored .030 over, towing cam, standard bearings, stock oil pump, roller cam chain, 3 angle valve job, new plugs/wires and was balance. Compression test at 1884 miles were 144 to 149 psi. Hope to get the EFI with spark control on soon. A bright side is that I'm at 2100 rpms for very short times. The 4000 rpms can get stretched out on long hills. Most driving is in the 2800 to 3000 rpm range. Bob Dunahugh
________________________________
From: Bob Dunahugh
Sent: Wednesday, June 14, 2017 9:14 PM
To: gmclist@list.gmcnet.org
Subject: Harmonics in my exhaust system
The harmonics happens at about 2100 . Then stops. Then returns at about 3900 to 4100 RPMs. But not as strong. Both at moderate throttle. It's a 403 with a stock exhaust system. I thought it was the double wrapped mufflers. So I put on two Flow master series 40 units. No change. But these are real solid/welded mufflers. I have an idea. Any thoughts? Bob Dunahugh 78 Royale 403
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| Re: [GMCnet] Harmonics in my exhaust system [message #319172 is a reply to message #319037] |
Sat, 17 June 2017 14:41 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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When the Honda 250cc Elsinore series first was introduced, they were more
powerful, faster, lighter, handled better, than the European offerings
from Montessa, Ossa, CZ, Bultaco, Maico, and others. But, shortly, frame
cracking, and exhaust system breaks and cracks reared their ugly heads in
the Hondas. High frequency vibrations along with poor or no heat treatment
on the frames was blamed. Cure, strip the bike down to bare frame and
inspect all welds. Fix the broken ones, and mag check the rest, then heat
treat it and repaint. Trackmaster and Redline both marketed replacement
frames for Elsinores. Racers bought a lot of them. Lots of aftermarket
exhaust systems were introduced about the same time. Better stuff, held up
quite well for the most part. Exhaust system sounds set up vibrations that
crack and break very substantial components. No one cure for everything.
Very situational.
Jim Hupy
Salem, Or
78 GMC ROYALE 403
On Jun 17, 2017 11:55 AM, "Bob Dunahugh" wrote:
James, James, James. What's this old stuff about? So. The average age of
owners of GMC's is near death. Mentally I'm 35. I use that series 40 Flow
master on my track Yenko's. There mounted directly on the headers with no
tail pipes. The sound to me is a symphony in all it's glory. And at 7 to
8000RPMs full throttle. WOW. Gear heads understand that one. As to the GMC.
I have a microphone on my rear view camera. Yes the sound level is elevated
some. Inside/up front. Again. Slightly elevated. Then the harmonics. That's
not lowed, or last long. I'm just stuck on why. After all. It did break up
a new $80 sheet metal muffler. As to a resonator at the back. ONE. I don't
have room back there due to a floor drain. Two. As to the sound at the
back. I turned the mic on the camera off. Bob Dunahugh
________________________________
From: Bob Dunahugh
Sent: Friday, June 16, 2017 11:33 PM
To: gmclist@list.gmcnet.org
Subject: RE:Harmonics in my exhaust system
This is my thought on the source. ( MAYBE ) The entire exhaust system was
on the 78 Royale that burned. Plus the front, and rear suspension. Then
trans, and 3:70 FD. There are 122 GMC VIN's between the two 78 Royales.
The burned one was built in Dec of 77. The Mouse House in Jan of 78. The
only item that wasn't from the burned GMC drive line is the 403. And the
only difference between the two 403's is the camshaft. The 403 from the
burned 78 had compression readings from 130 to 135 psi at 152,000 miles.
The Mouse house compression readings were 140 to 145 psi at 1884 miles.
Thus the only things different is a more aggressive towing cam. And a
little higher compression readings. Bob Dunahugh
________________________________
From: Bob Dunahugh
Sent: Thursday, June 15, 2017 7:58 PM
To: gmclist@list.gmcnet.org
Subject: RE:Harmonics in my exhaust system
Johnny. I don't think the tail pipe is vibrating due to the two muffler
bearings being as tight as they are. The harmonic sound seems to be coming
from the muffler area. I don't lug engines of any type. When I'm going up
long climbs. I down shift, and run the rpm's up to the 4000 rpm range with
moderate throttle. I like the water, and trans fluid pumps moving fluids
faster. That's why I get to those rpm ranges. The thing here is that I
never had this problem with this exhaust system in the burned 78 GMC.
Bob Dunahugh 78 Royale
________________________________
From: Bob Dunahugh
Sent: Thursday, June 15, 2017 1:03 PM
To: gmclist@list.gmcnet.org
Subject: RE: Harmonics in my exhaust system
Larry. You have an excellent point. Next. I should have said double outside
wall mufflers. The exhaust system is pure stock ( except the mufflers now )
down to the proper hangers, and GM muffler bearings for the tail pipe.
Sound out the tail pipe is up a little. What helps there is that the tail
pipe points straight out the back. As to the 403. Has about 7000 miles on
it now. Bored .030 over, towing cam, standard bearings, stock oil pump,
roller cam chain, 3 angle valve job, new plugs/wires and was balance.
Compression test at 1884 miles were 144 to 149 psi. Hope to get the EFI
with spark control on soon. A bright side is that I'm at 2100 rpms for
very short times. The 4000 rpms can get stretched out on long hills. Most
driving is in the 2800 to 3000 rpm range. Bob Dunahugh
________________________________
From: Bob Dunahugh
Sent: Wednesday, June 14, 2017 9:14 PM
To: gmclist@list.gmcnet.org
Subject: Harmonics in my exhaust system
The harmonics happens at about 2100 . Then stops. Then returns at about
3900 to 4100 RPMs. But not as strong. Both at moderate throttle. It's a 403
with a stock exhaust system. I thought it was the double wrapped mufflers.
So I put on two Flow master series 40 units. No change. But these are
real solid/welded mufflers. I have an idea. Any thoughts? Bob Dunahugh 78
Royale 403
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| Re: [GMCnet] Harmonics in my exhaust system [message #319179 is a reply to message #319037] |
Sat, 17 June 2017 17:04 |
mrgmc3
Messages: 210
Registered: September 2013
Location: W Washington
Karma: 2
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BobDunahugh wrote on Wed, 14 June 2017 21:14The harmonics happens at about 2100 . Then stops. Then returns at about 3900 to 4100 RPMs. But not as strong. Both at moderate throttle. It's a 403 with a stock exhaust system. Any thoughts?
_______________________________________________
This sounds like a classic case of exciting the natural frequency of the exhaust system. It excites at 2100 rpm then again at 2x. You'll probably find another node at 6200 or so if it could run that fast.
We hung a tuned mass damper on the 3800 v6 F-car (Camaro / Firebird) exhaust system to fix just such a problem. If exciting the natural frequency is the cause you really only have two choices, add a mass damper (a weight that changes the natural frequency) or stiffen the exhaust system (increases the natural frequency). You may luck out and find that a clamp or hanger is loose which might lower the stiffness of the system causing the issue, and explaining why the same system behaves differently than in the previous GMC.
Chris Geils - Twin Cities / W Wa
1978 26' Kingsley w/ very few mods; PD9040, aux trans cooler, one repaint in stock colors, R134a, Al rad, Alcoas, 54k mi
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| Re: [GMCnet] Harmonics in my exhaust system [message #319183 is a reply to message #319179] |
Sat, 17 June 2017 17:10 |
GMC.LES
Messages: 505
Registered: April 2014
Karma: -2
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Senior Member |
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My F250 diesel has such a weight hanging off the DPF/CAT. I've read of several who installed straight delete pipes and experienced a drone in the exhaust pipe afterwards.
Les Burt
Montreal
'75 Eleganza 26'
> On Jun 17, 2017, at 6:04 PM, Chris Geils via Gmclist wrote:
>
> BobDunahugh wrote on Wed, 14 June 2017 21:14
>> The harmonics happens at about 2100 . Then stops. Then returns at about 3900 to 4100 RPMs. But not as strong. Both at moderate throttle. It's a
>> 403 with a stock exhaust system. Any thoughts?
>> _______________________________________________
>
>
> This sounds like a classic case of exciting the natural frequency of the exhaust system. It excites at 2100 rpm then again at 2x. You'll probably
> find another node at 6200 or so if it could run that fast.
> We hung a tuned mass damper on the 3800 v6 F-car (Camaro / Firebird) exhaust system to fix just such a problem. If exciting the natural frequency is
> the cause you really only have two choices, add a mass damper (a weight that changes the natural frequency) or stiffen the exhaust system (increases
> the natural frequency). You may luck out and find that a clamp or hanger is loose which might lower the stiffness of the system causing the issue,
> and explaining why the same system behaves differently than in the previous GMC.
> --
> Chris Geils - Twin Cities
> 1978 26' Kingsley w/ very few mods; Headers, Progressive Dynamics 9040, aux trans cooler, one repaint in stock colors, R134a, Al rad, 49k mi
>
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| Re: [GMCnet] Harmonics in my exhaust system [message #319185 is a reply to message #319183] |
Sat, 17 June 2017 17:38 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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My grandson had his '04 Mustang on my service rack last weekend.
Protruding out the right side of the top rear of the muffler was a 3/8" rod
with an iron weight hanging on the end of it, probably 1/2-1 lb. I assumed
that was a harmonic balancer.
Similar to my folks' 59 Cadillac which had leaf springs off the front of
each frame side rail behind the front bumper. The chunk of iron on each
leaf probably weighed 5 lbs.
Ken H.
On Sat, Jun 17, 2017 at 6:10 PM, Les Burt wrote:
> My F250 diesel has such a weight hanging off the DPF/CAT. I've read of
> several who installed straight delete pipes and experienced a drone in the
> exhaust pipe afterwards.
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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