Exhaust drone

[mlipmd]

My '90 with about 32,000 miles on it, with Walker (Dynomax) Super Turbo mufflers and exhaust put on by a PO, has a great sound but a loud drone which is starting to annoy me, at around 1800+/- rpm. This exhaust system is about 15 years old and I have read somewhere that with age the packing inside the mufflers can wear out or get degenerated and can cause this. People with this exhaust do not report drone so I wonder if with time this is true. Sound deadening pads in the rear and behind the seats from a forum vendor have very little to no effect. I have read a lot about eliminating the drone with J-pipe 1/4 wave resonators, or extending the exhaust tips beyond the rear bumper (to stop the sound from reverberating underneath the car), or, lastly, replacing the system with new cat-back mufflers. The J-pipes sound like a great way to eliminate this altho I don't know if there's any room underneath to place these. Other than replacing the mufflers, anybody have any other ideas? I really don't feel like spending $2000 for a new exhaust.

[WARP TEN]

Since the first ZR-1 was produced, there has been a great deal written about drone or resonance in ZR-1 exhaust systems--the search function is your friend. Essentially, the LT5 (and many other engines) have natural frequency points around 1,800 and 3,400 rpms that create resonance at those engine speeds under part throttle operation that many describe as drone. It can be very annoying, especially when cruising around 70 mph and accelerating a bit. The stock exhaust system uses Helmholtz resonators to quiet this drone. That is like a closed coffee can with a pipe into it which does not allow exhaust to flow through but allows the sound waves of a given frequency in where they bounce around and cancel each other out. Helmholtz resonators are designed and sized to affect specific frequencies, such as those generated at about 1,800 and 3,400 rpms. The stock system has resonators in the mufflers and in the center resonator designed to tackle one frequency each. Corsa also used Helmholtz resonators but they call them "Quarter Wave Tuners" or something like that. Other systems may do the same, or use some other approach. Larger 3" exhaust systems seem to aggravate the resonance. I experimented with a half a dozen aftermarket exhaust systems in the Nineties and settled on Corsa for the most power, best sound and no drone. Had Corsa's on both the '93 and '95 until on the '95 I decided to go back to a complete stock system with electric cutouts right in front of the rear mufflers. I liked that the best. FYI, on the '95 I had Marc's 510 package with SW headers and no cats. It is possible that someone might be selling a used Corsa or stock system that might fit your needs without too great an expense.--Bob

[mlipmd]

WT- I have done all that research and am well aware of everything you said.
What a J-pipe is is a Helmholtz resonator by a different name. It is a length of closed pipe of a particular length (for the frequency you are looking to cancel) attached perpendicularly to the exhaust pipe.The pipe length is easy to calculate and is 1/4 the wavelength of the sound frequency you are looking to cancel. The sound wave is reflected back into the main exhaust stream 180 degrees out of phase with the original, and the desired frequency is totally cancelled, no longer exists. The same way sound cancelling headphones work.
It is called a J-pipe because it is frequently bent into a J shape to lie alongside the exhaust pipe underneath the car. As long as the pipe comes off the main exhaust at a 90 degree angle, the shape of the pipe, be it straight, curved, whatever, does not matter, only the length is critical for it to work. You accommodate it underneath the car any way that fits.
Mufflers may do this with an incorporated empty area at one end, where the sounds bounces around and again comes out 180 degrees out of phase with what came in - nothing magical about it. I believe the Super Sports also incorporate this design feature, but I am wondering if over time, if packing or some other material inside has degenerated and is now not cancelling the drone. No one who has these mufflers report drone, and with all else the original PO did to this car, if he had drone when the system was first installed, he would have immediately removed it instead of leaving it like that for 15 years.
And Hawaii - all I can see underneath my car because I do not have a lift, is straight 3" SS pipes leading to my mufflers, ending in (4) 2 1/2" tips which terminate about 2" forward of the rear fascia. You can't see squat from the sides but I do not see cats or any other resonators in the line, just pipes to mufflers.
I am thinking of fabricating temporary extensions on my exhaust tips to get the exhaust to exit further out the rear instead of underneath my car, seeing if that will modify some of the noise.
Rear cargo area and behind the seat expensive sound absorbent mats purchased from a forum vendor have absolutely no effect on the drone and noise.
I may start to look for a set of used Corsa axle back or catbacks, but I doubt if I will be able to find some.

[mlipmd]

Do you mean for me to cut my 2 existing exhaust pipes somewhere underneath the car and insert that Dynomax Ultraflo Welded X offset muffler into the system, one pipe in each inlet and outlet, before my Super Turbos? So I have in effect 2 mufflers in each line and that extra one acts as the resonator to get rid of the drone? That sounds like a good idea if it would work, and a whole lot cheaper than buying a new exhaust system. I'll give them a call and discuss it further with them. Thanks for that idea.

[mlipmd]

Hawaii - as far as I can see without picking up the car, like I said, I only have straight pipes leading to the mufflers, no cats or resonators or anything else in the line, just pipes. I spoke to a knowledgeable guy at Dynomax yesterday and he advised me to try a pair of their Ultra flo welded SS round mufflers inserted inline in my pipes before the Super Turbos, thinking that, acting as the resonators, they will quiet down my drone significantly. They are 6" in diameter so I think I may be able to fit them in there, if not side by side, then maybe a little off, in tandem, in the center tunnel. He thinks that will work much better than the X pipe muffler added in before the Super Turbos.
I could cut out a section of pipe to fit them in and use SS band clamps for the install rather than welding them, to give me a chance to see how they work.

[Highegt]

Hello - chased drone ever since replacing stock exhaust with SW headers and 3 inch chambered mufflers which they laughably claim have none - 1800 RPM it starts with vengeance and made driving on highway unbearable. Rather than bore you with all attempts tried after reading multiple posts on subject which included swapping mufflers, found the solution which eliminated drone for my 1990. Once you realize inherent design of Corvette exhaust having dual outlets on each side was cause, that through the years factory mitigation was either dummy port on older or valves in one pipe on newer to remove drone it allowed focus to solution would be to change flow. Picked up a set of Car Chemistry exhaust inserts, removed dual tip, slipped one each wrapped with stainless steel medium grade wool into inner pipe each side, put tip back on and all good - NO Drone. Sound was still mean, power had no noticeable difference as 3 inch pipe through mufflers actually goes down to dual 2.5 inch tip openings as designed. $120 shipped to house. Car is now total pleasure to drive!

[tf95ZR1]

Looks great, Craig!
What kind of rear tires are those?
Looks like monster grip!

[Highegt]

You did what my next step was going to be if the inserts didn?t work - looks great and glad to hear works just as well!

[jss06c6]

Originally Posted by Highegt

Hello - chased drone ever since replacing stock exhaust with SW headers and 3 inch chambered mufflers which they laughably claim have none - 1800 RPM it starts with vengeance and made driving on highway unbearable. Rather than bore you with all attempts tried after reading multiple posts on subject which included swapping mufflers, found the solution which eliminated drone for my 1990. Once you realize inherent design of Corvette exhaust having dual outlets on each side was cause, that through the years factory mitigation was either dummy port on older or valves in one pipe on newer to remove drone it allowed focus to solution would be to change flow. Picked up a set of Car Chemistry exhaust inserts, removed dual tip, slipped one each wrapped with stainless steel medium grade wool into inner pipe each side, put tip back on and all good - NO Drone. Sound was still mean, power had no noticeable difference as 3 inch pipe through mufflers actually goes down to dual 2.5 inch tip openings as designed. $120 shipped to house. Car is now total pleasure to drive!

Which muffler are you using? I've got Stainless Works on mine Headers to muffler tips. 3" exhaust.. SW HiFlo cats installed.

Thanks,

Steve

[GOLDCYLON]

If somebody wants to install a corsa resonator I have a spare in my attic I might be willing to part with