Turn down got installed on my truck today, I was tired of polishing the soot off the back bumper. An additional, and unintended, side effect was that the truck is now much quieter inside the cab. Had the drone before, but it didn't really bother me, put the wife and kids to sleep on road trips, can't be all bad right? Any way, pretty well killed it off except at exactly 70 mph or above. Seems almost more pronounced now, because it's not masked by the exhaust noise? Anyone ever heard of such a thing? The stock tailpipe got shortened by about 6" or 8" (there was some "or so" that went into this) and the turn down welded on. It dumps straight down, still exits the rear of the truck at approx. 45 deg. from the rear of the truck. Has anybody else noticed this?

 

My stacks were 6" above the cab, now they are 2' above and the noise went down considerabily.

Cheers, Kevin

 

I never got the dreaded drone, maybe because of the turndown.

 

Try removing the bracket from the tranny for the exhaust,,,, it helps,,,,,,

 

From everything I have read it pertains to the length of the exhaust. Obviously the baffled muffler takes care of the canceled sound waves but for a straight piped exhaust it seems to relate to the length of the pipe. I have also heard about the turn down quieting the exhaust as it is directed elsewhere. I have also heard of some welding or strapping angle iron on the pipe which I assume affects it natural resonance. I think another factor which will play is the wall thickness of the tubing. I have played around with hanging weights in different areas but the one thing that has helped the most is removing the rear seat. A lot of sound comes from behind the seat. I took a ride to home depot without the rear seat and it was if all the windows were removed. I bought a can of the "Great Stuff" and filled all of the cavities I could find. I looked into dynamat but that stuff is expensive so I took a moving blanket and held it in place with tape until I got the seat installed. You can't it there and it is squeezed between the seat and back wall and it eliminated a lot of the vibration. I still have a faint drone at 2000-2100 rpm but I think I hear more of a growl (sounds like it is around the down pipe) than anything else. The resonance occurs as a 125HZ sound wave. The wave length for 125 HZ is around 9'.

I can tell you one other thing. I am running 4" down pipe then going to 5" to the tail pipe. I added a Nelson 9" dia. x 18" resonator to try to tone down the drone. All it did was drop overall sound 1db @6ft but it did reduce 150Hz and 175Hz quite a few decibels @6ft but did nothing for 125Hz.

If you really want to get rid of it but keep the sound, I found a muffler shop selling a 9" dia x 45" 5 in/out non-baffled for $140.

It just hasn't gotten bad enough for me to spend the extra money.

Hope this helps,

Bob

 

Rballard,

Forget the dyno mat. Go to Home depot or Lowes and ask for Fire and Ice shield. It is use for roofing insulation. It is the same as dyno mat. You can get a roll of about 30' for about 30 to 50 bucks. This would cover the whole inside of your rig.

Good Luck,
Bill