Exhaust Noise Questions

[Gary Lewis]

Yesterday we took a 150 mile trip in Big Blue, and at one point where we were going north with the wind I realized that there is a rumbling noise in the exhaust - pretty much directly under the driver. Janey says there's no noise on the passenger's side.

The noise completely goes away when you are coasting, and it is smoother when you are climbing a steep hill and the AFR goes down around 13:1. But when you are cruising and the AFR is running in the high 14's there's a rumble that I can only liken to softballs in a dryer. It isn't a hard hit, and it isn't steady, but it is there consistently when cruising.

I've checked the gaskets and they are intact and don't appear to be leaking, so I'd say it is in the headers. Or maybe "header" as Janey isn't hearing it on her side.

I've pulled the plugs, as shown in the Spark Plugs Questions thread, and there's obviously no fouled plug. In fact, the plugs look almost new with just a slight tan showing in one area on each plug, so it looks like each cylinder is running well.

Have any of you experienced a noise like this in headers? Thoughts on what might be causing it?

 

[viven44]

AFR meter is definitely nice to have ... definitely on my list..

Just curious.. wouldn't detonation be more likely at lower AFRs i.e. richer mix ?... rumbling sound could be detonation but AFR trend you are observing is the other way around.

[85lebaront2]

I'm thinking it could be the obviously lean mixture possibly causing some rattling inside the headers.

[Gary Lewis]

I'm thinking it could be the obviously lean mixture possibly causing some rattling inside the headers.

Vivek - The limited info I have is that detonation in the cylinder is reduced with a richer mix. That's because the more fuel in the mix cools things, and because the richer AFR tends to slow the ignition process.

Bill - Do you think if I were to change the AFR to 14.0:1 it might reduce the noise in the headers?

I'm guessing that the noise is the unburned fuel from one cylinder being lit off by the hot exhaust from another cylinder - in the header. Does that make sense?

[viven44]

Vivek - The limited info I have is that detonation in the cylinder is reduced with a richer mix. That's because the more fuel in the mix cools things, and because the richer AFR tends to slow the ignition process.

Bill - Do you think if I were to change the AFR to 14.0:1 it might reduce the noise in the headers?

I'm guessing that the noise is the unburned fuel from one cylinder being lit off by the hot exhaust from another cylinder - in the header. Does that make sense?

Got it.. I got my logic mixed up. I thought of EGR and how it works to prevent detonation by making the mixture "less desirable"... had assumed EGR would cause the mixture to lean out as there are more gases... but then the gases are inert so technically AFR is lowered with EGR.

 

[ArdWrknTrk]

Vivek - The limited info I have is that detonation in the cylinder is reduced with a richer mix. That's because the more fuel in the mix cools things, and because the richer AFR tends to slow the ignition process.

Bill - Do you think if I were to change the AFR to 14.0:1 it might reduce the noise in the headers?

I'm guessing that the noise is the unburned fuel from one cylinder being lit off by the hot exhaust from another cylinder - in the header. Does that make sense?

This is the entire reason that 460's from the factory have thermactor ports and the exhaust manifold 'spiders'.

To "cleanly :nabble_laughing-25-x-25_orig:" burn off hydrocarbons in the manifolds -before- they make it out the tailpipe.

Fact is that ALL cylinders are dumping fuel -with a carburetor- because combustion efficiency is pathetic in an iron headed two valve V-8.

I guarantee that a stock 4180 is going lean 16-17:1 under cruise.

I refrained from comment about "headers" because I've never wanted them on my truck.

But I've experienced exhaust leaks at both the #7 & 8 ports and the drivers side manifold flange, multiple times.

You can definitely tell that the leak is near your right foot when driving.

[Gary Lewis]

This is the entire reason that 460's from the factory have thermactor ports and the exhaust manifold 'spiders'.

To "cleanly :nabble_laughing-25-x-25_orig:" burn off hydrocarbons in the manifolds -before- they make it out the tailpipe.

Fact is that ALL cylinders are dumping fuel -with a carburetor- because combustion efficiency is pathetic in an iron headed two valve V-8.

I guarantee that a stock 4180 is going lean 16-17:1 under cruise.

I refrained from comment about "headers" because I've never wanted them on my truck.

But I've experienced exhaust leaks at both the #7 & 8 ports and the drivers side manifold flange, multiple times.

You can definitely tell that the leak is near your right foot when driving.

I don't appear to have any exhaust leaks. I've checked with a scope and a hose and can't hear any leaks.

But the sound isn't ahead of me. It is slightly to my left and below me, which puts it in the tailpipe after the forcing cone. Or maybe right at the forcing cone.

But yes, when I had a carb on Big Blue I saw 16:1 routinely as I added throttle to climb a hill or to accelerate slowly. Then the vacuum would drop enough that the springs would push the metering rods up and the AFR would come down to 14:1 or so. So the EFI system is keeping the AFR much more consistent.

[ArdWrknTrk]

I don't appear to have any exhaust leaks. I've checked with a scope and a hose and can't hear any leaks.

But the sound isn't ahead of me. It is slightly to my left and below me, which puts it in the tailpipe after the forcing cone. Or maybe right at the forcing cone.

But yes, when I had a carb on Big Blue I saw 16:1 routinely as I added throttle to climb a hill or to accelerate slowly. Then the vacuum would drop enough that the springs would push the metering rods up and the AFR would come down to 14:1 or so. So the EFI system is keeping the AFR much more consistent.

To your left?

Like... In the next lane???

I wasn't saying that the L&L's were leaking, but that when I had noise in the exhaust that got louder under acceleration/throttle it was typically the #8 port, because when the manifold flange heated up the expansion happened there.

If you think your Carterbrock got lean, imagine the 4180 that wasn't tuned for anything but to meet federal mandate (and STILL needed forced air to flare off unburned fuel!)

 

[Gary Lewis]

To your left?

Like... In the next lane???

I wasn't saying that the L&L's were leaking, but that when I had noise in the exhaust that got louder under acceleration/throttle it was typically the #8 port, because when the manifold flange heated up the expansion happened there.

If you think your Carterbrock got lean, imagine the 4180 that wasn't tuned for anything but to meet federal mandate (and STILL needed forced air to flare off unburned fuel!)

To the left of the center of my body. Maybe I'm hearing it through the floor, but more likely the door as the floor has sound deadening, insulation, and carpet on it. And we were on 2-lane roads with no one visible for miles.

And yes, my Carterbrock was tuned to match the setup, so it didn't get too lean. I can only imagine what the 4180C did. If Ford had that jetted the way they had the 2100 jetted on my '72 F250 w/a 390 it must have been L.E.A.N. On the 390 you didn't accelerate until the power valve opened. Ease into the throttle and watch the vacuum drop and drop and drop with essentially no acceleration, and then all of a sudden away you went.

[ArdWrknTrk]

To the left of the center of my body. Maybe I'm hearing it through the floor, but more likely the door as the floor has sound deadening, insulation, and carpet on it. And we were on 2-lane roads with no one visible for miles.

And yes, my Carterbrock was tuned to match the setup, so it didn't get too lean. I can only imagine what the 4180C did. If Ford had that jetted the way they had the 2100 jetted on my '72 F250 w/a 390 it must have been L.E.A.N. On the 390 you didn't accelerate until the power valve opened. Ease into the throttle and watch the vacuum drop and drop and drop with essentially no acceleration, and then all of a sudden away you went.

I don't have duals either. 🤷‍♂️

I'd say you're hearing it reflecting off the inside of the frame rail, ahead of the midship..