exhaust drone?
 

My truck has a 250 inline 6 in it. The exhaust I built exists under the running board in front of LR Fender. the muffler and tailpipe are 3" which is overkill for a stock 250 but I had the muffler and some u bends on hand.
I like the look of the exhaust poking out under the running board, but may give that up to make the truck quieter.

There is an annoying drone at at 2500-2700 RPM I'd like to eliminate. A sound spectrum analyzer on my phone shows the spike in DB with the drone is 173hz
Some options I am considering

1) extend tailpipe out the back of truck. This is the high effort solution as I have a c10 trailing arm rear suspension to snake it around. but the longer pipe will probably make it quieter overall which is not a bad thing.
2) make a silencer to stuff into the tailpipe. This is the low effort solution
3) make a 1/4 wave resonator J pipe 20" long
3.5) extend option 3 to be a muffler bypass with a butterfly valve at the 20" point so I can make obnoxious noises on demand

Anyone have any experience or suggestions to share?
Here is what the exhaust looks like now and where the J Pipe might be added

Attachment 2293778

Re: exhaust drone?
 

Curious what kind of exhaust hangers you are using too? With our old trucks I have found that with any exhaust system, quality hangars designed to help insulate and allow the system to flex helps a lot (not solid mount with exhaust grommets alone). Terminating any exhaust out at the rear bumper helps reduce noise and to keep fumes out of vehicle. I run a stock 292 in a K10 with stock style exhaust system this way and it's very quiet on the interior, so much so that I hear gear whine more than anything. My Thread below with pics:
http://67-72chevytrucks.com/vboard/s...d.php?t=836212

Re: exhaust drone?
 

Attachment 2293801

Walker 35119

Re: exhaust drone?
 

Those exhaust hangers are great, and running exhaust out the back is going to help but may only reduce drone a little. I would say the muffler is contributing, not certain if 3" pipe is also a factor. Hopefully others will chime in with suggestions.

Re: exhaust drone?
 

I had a very hard time with muffler drone. Both pipes dumped in front of rear axle near end of running boards and was the stuff of migraine headaches. Talked to several shops and all pretty much agreed running any exhaust as far out the back on ADs is way to go. I did and its great

Re: exhaust drone?
 

I drove my 48 for over 100K with the tailpipe exiting in front of the rear tire with a pretty well blown out glasspack to look like I had a muffler. That was with 2 inch exhaust and yes it had a terrible drone. So did my 71 GMC with a 350 and glass packs and the pipe(s) exiting in front of the tires. It helps a bunch to get the exhaust to exit behind the rear wheel either angled out towards the corner or straight out the back. Since most likely you will never have a camper shell on it with people riding in the back straight out the back works fine. A good exhaut guy will cuss you a bit but usually can get around the Trailing arms.

The 3 inch might be a challenge but you could split that behind the muffler and run dual 2 inch pipes on out the back

Re: exhaust drone?
 

you probably already know this but a larger pipe can actually not flow as well as a slightly smaller pipe. it is a science to figure out the pulses and the associated slight vacuum created behind each one. if the pipe is smaller the flow in the pipe is faster so the vacuum pulses created help to scavenge the exhaust from the cylinders as well as help pull the next pulse along the pipe.
anyway, a longer muffler would help, a chanmbered muffler would help, a resonator would help, running the pipes out the rear would help. I have driven the family van long enough (years ago) to know I am not a loud exhaust guy. I am using a 2004 GMC Envoy frame and Atlas inline 4.2L inline 6 cyl engine. I will be keeping the stock exhaust because, firstly, it's already in place and quiet, secondly, I'm cheap. lol.

https://www.motortrend.com/how-to/wh...pes-work-best/

Re: exhaust drone?
 

The exhaust drone on mine was so bad it really did trigger headaches in driver and passengers alike. Even stopped driving it for a while when wife/friends declined to go. Was so concerned i had the exhaust pipes run out as far as possible almost beyond rear bumper. This worked and since my puter took a dump can be seen on post #137 on build link. IMHO they look cool and work well with a harmonious pleasing rumble/roar.

Re: exhaust drone?
 

Following up,

I agree with everyone that extending to the back is the best solution. Having too many projects on the go this summer I decided to go with the quickest, option 2 above, making a silencer to stuff in the tail pipe.

I'm quite pleased with how much difference it made, the brain pounding drone is gone and the truck is quieter overall. Any immediate need to spend a weekend fitting chunks of U bend up over and around rear axle, trailing arms and sway bar is gone! No apparent loss of power and the engine still has some rumble

Here it is, not really finished yet as it is just tacked together so I could drive to work with it bailing wired in and see if it was worth more effort. I'll finish weld it, trim it to be hidden in tailpipe and bolt it in.

Attachment 2299037

Re: exhaust drone?
 

I have a version of that in the exhaust of my road bike. it is 2 head pipes into one larger rear pipe that has a slip in section similar to that which necks down at the tip. the difference is that at the front section there is what looks like the end of of a tin can welded on with a section of the inner pipe protruding through. this ensures that all the flow goes through the inside of the pipe with all the holes in it. the outlet end of the pipe has the same thing. that donut shaped blockers are formed so they have a flange that fits snugly inside the outer pipe and the rearward donut has a couple of screw holes so the whole thing can be retained inside the outer pipe but also can be removed for service. the area between the 2 donuts of metal is then wrapped in what looks like dirt bike exhaust silenecer material. this material burns out in a summer or 2 of highway riding. the slip in part can be pulled out and repacked. I have used stauinless steel wool wrapped around the smaller pipe and then a length of galvanized sheet metal from home ducting that I had laying around to keep it all together. then I keep it all in place with a wrap or 2 of stainless steel wire. if you find the tip seems to do the job you could easily make a larger pipe for the tip of the exhaust system that would allow you to have a larger diameter slip in part such as the motorcycle has. just a thought. my bike is still quite loud and has the sound of a bike bagger harley but without the slip in you wouldn't want to ride it far. it would be less loud on a truck like yours because you actually have a muffler. this is the muffler on my road bike.

Re: exhaust drone?
 

my bike exhaust was too loud for me (yeah, I know, too loud? huh?) so I modified it. there are 2 sections to the slip in now. the exhaust is directed to the outer part of the forward cell, then through the holes in the pipe back into the inner section of the rear cell. the rear cell is wrapped in silencer material. the forward section has a bell shaped collector at the most forward end. here is a pic or 2. it is quite a bit quieter with this set up but definately still has the roar effect of a big road bike. it may be a thought for you to try, even without the 2 cells, if you have time to fab something up. I remember supertrapp used to make a weld on or clamp on style end tube that had a stack of washer shaped baffles that could be added or subtracted to the tip for different sounds or backpressure as required to tune the exhaust for performance.

Re: exhaust drone?
 

here is the supertrapp version
https://www.pegasusautoracing.com/pr...asp?RecID=4305

Re: exhaust drone?
 

on the pics of my pipe the rearward section is wrapped with duct tape. it burns off over time but the steel wool and other wrap material is also wrapped with stainless steel wire so it stays put. the tyape is just to help with sliding it into the outer pipe. this works really well actually. no power loss that I have noticed.

Re: exhaust drone?
 

we deal with a lot of drone on 4 and 6 cyl motors. this is the exhaust for the 13-18 ford focus ecoboost. left pipe is connection to exhaust, right pipe is just empty capped of resonator. of course it would involve welding and testing to get the right length for your application.

Re: exhaust drone?
 

what I did was not as fancy as what dsraven shows.

the tailpipe is 3"
the silencer is 2" x about 16", the end towards engine is capped, the fat end of the silencer is a piece of 3" slit and rewelded so it is 3" OD and slides into the tail pipe, a circle of plate fills in between the 2" and 3" part of the silencer. So all the exhaust has to go through the small holes and then exit out the end of the silencer. There is no steel wool or other baffle material.

end towards engine:
Attachment 2299408

view inside from tailpipe end
Attachment 2299409

it is trimmed flush with the tailpipe and held in with 2 sheetmetal screws

Re: exhaust drone?
 

Heimholtz Resonator. look it up. pretty slick how science takes care of pesky drone!