Engine and Transmission Tech

There's an element of luck.

My TR6 - six-pot - had a twin system that died of rust, replaced it with a stainless twin system from Moss, horrible resonance.
Tried changing the mounts, wrapping it, no good, but that system was sold in numbers and worked fine on most cars
I changed to a big-bore single-pipe system, problem gone.

Our Pug 205 GTI with an aftermarket system used to have a drone at 4,000 which was motorway cruise, a change to a stock system cured that.

My Land Rover with 215 had an awful crackle around 2,000 like a Gatling gun, cured with a larger silencer, a Hooker Aero Chamber. Now it has a resonance at around 1,000 but I can live with that.

Manufacturers have whole departments sorting out this sort of thing, us DIYers need an element of luck.

Ivor

True, but it IS possible to use a more scientific approach.

Jim

I started with the science (lots of calculations), moved on to trial and error (adjusted tube lengths for the resonators), and ended up with luck (the system works unbelievably well).

Phil apparently used the SWAG method first, then went with WAG. :-)

Droning exhausts are a real PITA and can be hard to fix.
Here are a few ideas that have worked for me in the past.
The number one cause of a drone in my experience, is engine harmonics transferred into the exhaust system.
Installing flex couplings into the system as close to the engine as possible will give dramatic results.
Vibrant makes a good one, like the 62606. Make sure that whatever you get has an inside liner. If it's just the corrugated stainless it will whistle.
Tailpipes make a difference. A slash cut pipe drones far less than a straight cut pipe. Position counts as well. Try to position the tailpipes out past the bumper slightly.
Tail pipe diameter matters. If you have a 2 1/2 inch system, for example, try a 3 inch tailpipe. It will lower the sound frequency out of phase with the rest of the system.
While it is more work, a termination box will do wonders for noise and power. It's just a rectangular sheet metal box that the exhaust dumps into. It's mounted where you would normally put the H or X pipe. It's function is to terminate the exhaust pulse on a full length exhaust system. Very simply, the engine thinks its running open headers. The rest of the exhaust doesn't see the heavy pulses and the drone disappears.
Make sure that the box is ribbed or reinforced in some manner or it will rattle. Rule of thumb for box volume is 8 to 10 times the volume of one cylinder.

Hope that helps.
Cheers
Fred



Edited 1 time(s). Last edit at 10/28/2021 07:53PM by DiDueColpi.

The Heimholtz resonator is a dead-ended tube that is length tuned to the drone frequency. Because it is 1/4 of one wave length at that frequency it cancels the sound wave. So if the drone is at a specific rpm it is very effective.

Jim

In my case, since there is no H or X pipe before the single muffler, the Helholtz resonator needed to be after the muffler.
This is due to the frequency you are trying to cancel. Low frequencies required a larger chamber, and I didn't see that I have
the room for that-and especially since it would require two, one for each pipe. So, I still have two resonators, one for each
tailpipe, but they are in the extensions after the dual outlet muffler, and since the frequency is now higher due to the exhaust
pulses being combined in the internal "X" of the muffler, each resonator is smaller.
The conclusion I seem to be coming to is that the resonance is not confined to a specific rpm only, but begins, reaches a maximum,
and then trails off. I quieted the peak, but the rise and fall still exist. This could be cured with a variable volume resonator,
but that is beyond my willingness to try. The biggest thing about the sound, is in nice weather, with the top down, I
don't notice it as much. And, now that winter is nearly here, it won't bother me at all!
Dennis