mysterious exhaust drone

[nathansnathan]

I've been having an issue where my exhaust note seems to like switch. It's a 1.7L type 4 in my 72, running dellorto drla 40's with stock exhaust and dansk heat exchangers. As I'm driving, it will go from relatively quiet/normal to like this deep mid bassy rumble.

I've been trying to figure this out for some months now, and I've had no luck finding out what's causing it.

1st I thought it was the seal between the heads and heat exchangers as 2 of the 8 exhaust studs were stripped. I put a set of heads that I'd had rebuilt by Adrian at hfm, thinking this would fix it, but no.

I noticed the flanges of the dansk heat exchangers were leaking between the pipes and the flanges, and braze welded them up with my tig. I believe the seal is decent there now. This seems to have made it quieter, but still has this switching on/off drone.

I noticed that my muffler had a crack in it right at the indent for the oil filler at the back(front), and swapped it for a new stock Ernst unit.... but still the same loud and switching drone....

I'm thinking maybe there is a problem with my carbs.... The bus sounds fine until I am going like 60, then it is like this deep basey "whoooom". What is weird is the sound like comes and goes?

I'm going to make an effort to figure this out as it's been bothering me to no end....

[timvw7476]

are you positive it's not intake drone instead?
34 icts can get "wheezy" when I run them without filters,
dual carbs can make weird sounds in that echo chamber back there.

[nathansnathan]

The sound is more like a big dog barking, like "whuuumbbbb". .. trying my best at describing it, it is deep. It's fine until 60mph, no weird deep noise, then at 60 or so it will start and stop sporadically. Like on for 3-5 seconds then off and on again, not all the time, but consistently. It seems to goe away when I bring it past like 68 or so, or else it gets obscured by the louder sound at that speed. I'll see if I can record it somehow on my drive to work.

[LivinInnaVWBus]

I have been chasing that same noise for months. It sounds almost like a harmonic sound, right? Whooooommmm.... whooooooom.... whooooooommm.

I thought my crank was out of round, nope.
Clutch slipping, possibly, it was weak.
Trans noise, maybe.

I pulled down my engine last week and found the exhaust had leaks, a valve spring broken, weak pressure plate and a failing cam bearing.
I couldn't say any one of these issues was causing the noise, but they have all been replaced with new parts so I'll let you know if it's coming from the engine or trans once the rest of my parts come in and I'm up and running.
I will be changing the gear oil before I re-install the engine though, so that might clear up the issue as well but it may give insight as to the condition of the trans guts once the oil is inspected.

[nathansnathan]

Yeah I imagine it being like air resonating the whole chamber that is the engine compartment, like some harmonic coupling of how much air is being drawn in the carbs to to volume of air in the engine compartment? ...could be my imagination though.

This engine has never been split I think. I got it out of my parts 914, engine code EB, so it's a low compression CA motor, equivalent to a bus motor. I believe it was a dealership replacement judging by the stampings and that it has both the dipstick off the breather tower on top as well as having the lower boss drilled for a bus fill tube/dipstick.

I wasn't able to record it today, I'll give it a go tomorrow though.

[LivinInnaVWBus]

I tried to record it as well but there was too much other noise to pick it up. I have it between 55-65mph, much more so if going up a hill.
Whatever the issue is, you find it in a T4 and I find it in a T1 upright.

[airschooled]

[LivinInnaVWBus]

[airschooled]

What about Nathan?
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[nathansnathan]

no, it goes away in neutral.
It seems to be more likely to happen under load, though it is pretty random, so I'm not 100% on that.

[BIGSERG]

Did you ever find out what the exhaust drone was . Going through the same issues I have a 2.0 with vintage speed equal length headers and vintage speed muffler . It’s there quietest exhaust but at 55-60 I hear the drone .

[SGKent]

sounds like a loose exhaust vibrating. A drone sound can also be the exhaust note reflecting off nearby objects and arriving back to the driver out of phase. Since we aren't twin engine cars we can't sync the engines ........ .
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[raygreenwood]

The original problem....from back in 2015.....is one of the characteristics of a stock muffler that has a crack in the paired together pipe especially on the left side.

Yes.....it can switch off and on depending on how hot it gets. I have had two mufflers do this to me.....a Dansk and an Ernst. I finally cut the Ernst open to disect it and see how they die.

Both....if you look carefully into the 3 and 4 exhaust inlet holes into tje muffler.....they are paired in a stamped sheet metal pipe that takes a hard left to go to the right end of the muffler. This "F" pipe is made of two stamped pieces with a welded seam on many units.

From what I could tell....a combination of rust and high temp gas erosion splits the rilled welded seam. It stays generally sealed until it gets hot, expands and vibrates a bit and then it just seems to turn on an exhaust leak drone.
Ray

[BIGSERG]

I’ll check all the support mounts ,How about an exhaust leak ? What’s the best way to check for leaks . I thought about the vibration or maybe the way the exhaust tail pipe angle so I aimed it down a bit .

Could it be coming from the carbs . When I’m going 60-65 and I let go of the gas it gets louder before dying out . It’s a loud hum low tone woooohhhh wohhhh wohhhh

[aeromech]

I have no idea what your muffler looks like. I can tell you this though. Several years ago I installed a high performance engine into a bus with a very expensive muffler. When I drove it I got a very bad sound resonance and that lead me to find the cause. It was the direction of the outlet. It was pointed straight down which caused the sound to bounce back up. I simply had the outlet cut and welded to point aft which took the sound away.
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[airschooled]

[BIGSERG]

so the drone changes when i let go of the gas . almost matching the deceleration of the motor. thats whats making me think it's the muffler.


im afraid to take it out of drive when im going 65 . its an automatic.

does not change drone with time.

maybe the carbs are not 100 percent synced so its creating some weird interference?