Riddle me this ... New alternator eliminates drone

Upier · Upier's Garage

Can anyone explain this? I have an 86 Targa with B&B Headers w/heat and a B&B 2in/2 out muffler. I had drone so I added CC incert in one of the outputs. It removed a bunch of the drone put still had noticeable drone from 2200-2800. I was hearing some occasional grinding noises so I looked at the alternator. There was some play in the fan so I pulled it out. The rear dust cap come out when I pulled it out. There was a bunch of play in the rear bearing so I got a Bosch rebuild. I finished putting it in and it started and ran well with no noise. I drove it to work the next day and noticed that the drone I was hearing before was gone! Big surprise. I can't figure out how a new alternator solved an exhaust drone. Any ideas?

At first I thought is was because I hadn't reinstalled the plastic heater duct (removed to get better access to the spark plug wires on the distributor). I reinstalled it and took another drive but the drone was still gone.

911pcars · 02-11-2015, 06:09 PM

My guess. Something shifted inside the muffler to change exhaust flow. Two independent actions occurring at the same time is sometimes mistaken for a relationship. Or perhaps the alternator is now louder than the muffler.

Sherwood

Upier · Upier's Garage

Quote:

Originally Posted by 911pcars

My guess. Something shifted inside the muffler to change exhaust flow. Two independent actions occurring at the same time is sometimes mistaken for a relationship. Or perhaps the alternator is now louder than the muffler.

Sherwood

The only thing I can think of is that when I replaced the heater duct I changed the length of the sound path from the header to the fan so it no longer acting like a pipe organ amplify the sound. It's weird.

rick-l · 02-11-2015, 08:21 PM

maybe those bearings were making more noise than you suspected?

jjeffries · jjeffries's Garage

I'm rebuilding an 82 SC that came with the engine out and apart, so am cleaning up parts in anticipation of reassembly. Alternator was near new Bosch reman, "Oh Good!" I thought. But held itin my hand and spun shaft and received rotational clicking sound. Took it apart and found two things: 1. The bloke who'd reassembled it had used screws approx 3mm too long to retain bearing cap on the back of the unit, and 2. Two of the magnet sections had nasty scars on them from something like a cold chisel. Lord knows who put them there or why, but they had shiney marks leading me to believe they were rubbing on the stator housing (hopefully correct terminology). I dressed them down with a file. Also chunks of cardboard stuck to magnet assembly where it had been painted and set on said cardboard to dry. Point being, the unit was carelessly re-assembled. This was the first alternator I'd taken apart, and only did so after a kick in the a$$ from one of gurus here ("it's so easy!", he said and was right). Once apart, the problems were obvious. Also shows how "Bosch remanufactured" no longer means quality, as well documented elsewhere here on PP. John in CT.

gtc · gtc's Garage

Bosch reman alternators have been crap for a while. You're generally better off having them rebuilt locally or, if that is not an option, buying a new one.

Ayles · 02-12-2015, 08:31 AM

Do you have any sound clips of this combo on your car?

Upier · Upier's Garage

Quote:

Originally Posted by Ayles

Do you have any sound clips of this combo on your car?

I don't but I will make some and post here or on YouTube with a link.

Upier · Upier's Garage

I'm feeling real good about my new alternator now.

911pcars · 02-12-2015, 09:19 AM

Perhaps Bosch should go into the aftermarket muffler business too.

Tidybuoy · 02-12-2015, 09:26 AM

Quote:

Originally Posted by 911pcars

Two independent actions occurring at the same time is sometimes mistaken for a relationship.

That's hilarious. Many times I just wanted to leave but she thought I wanted more.

911pcars · 02-12-2015, 10:19 AM

Quote:

Originally Posted by Tidybuoy

That's hilarious. Many times I just wanted to leave but she thought I wanted more.

Hey Vern. Did I just coin a Confucius-nism? I'd like to copyright that.

JohnJL · 02-12-2015, 04:02 PM

Harmonics?

Elombard · 02-12-2015, 04:08 PM

I never thought about that those heater ducts are like a pipe organ for the headers. I wonder if I took those hoses off and plugged it in the summer when I dont need heat if it would make a difference?

sc_rufctr · 02-12-2015, 04:38 PM

Quote:

Originally Posted by JohnJL

Harmonics?

I suspect this as well.

A long time ago I owned an original Fiat 500. (air cooled, rear mounted two cylinder engine)
It was always a bit noisy and unrefined but I figured they were all like that. At one point I decided to replace the rear shocks because they were worn so I used standard OEM ones.
Guess what... After replacing the rear shocks the car suddenly became very quiet and smooth even when idling. To this day I still don't fully understand how this happened.

911pcars · 02-12-2015, 11:12 PM

Quote:

Originally Posted by Elombard

I never thought about that those heater ducts are like a pipe organ for the headers. I wonder if I took those hoses off and plugged it in the summer when I dont need heat if it would make a difference?

Yes it would. Engine will overheat. Think about the airflow as it goes into the heat exchanger. When cabin heat not needed, the heater valve directs heated air out of the HE to atmosphere. Plugged, air has no where to go and becomes superheated by the exhaust. IOW, don't do it.

As a reminder, there's no physical connection between combustion gases in the exhaust and the heat exchanger chamber around the pipes. If HE air has a bearing on the exhaust loudness, perhaps someone can play with the heater controls to vary the air flow and thus the timbre of the exhaust. Please report the results to this forum thread.

Sherwood

Upier · Upier's Garage

Quote:

Originally Posted by 911pcars

Yes it would. Engine will overheat. Think about the airflow as it goes into the heat exchanger. When cabin heat not needed, the heater valve directs heated air out of the HE to atmosphere. Plugged, air has no where to go and becomes superheated by the exhaust. IOW, don't do it.

As a reminder, there's no physical connection between combustion gases in the exhaust and the heat exchanger chamber around the pipes. If HE air has a bearing on the exhaust loudness, perhaps someone can play with the heater controls to vary the air flow and thus the timbre of the exhaust. Please report the results to this forum thread.

Sherwood

Another approach would be to put some type of limited baffle in the heater duct. Just enough to break up the sound waves.

Elombard · 02-13-2015, 08:17 AM

Good idea Sherwood. I would think you would be able to hear the exhaust sound we are talking about when you open the heater valves if this was the case. I have never noticed any difference in sound when they are opened.

911pcars · 02-13-2015, 09:57 AM

Watch and learn how Alexis does it:

Not sure what beverage provides a 6 cyl. equivalent.

By the way, when you do this, use a tall Red Bull or equiv. can, otherwise you might be stopped for DUI.

For those of you in the real world, another option is to install a remote controlled variable exhaust cutout in a second, presumably louder, tailpipe:
Electronic Exhaust Cutout vs Aftermarket Exhaust | Electric Cutout Comparison

Sherwood