In this video I try to defeat exhaust resonance with exhaust wrap.
Background – Exhaust Resonance
The project 1.8T has received a 76mm turbo back exhaust which uses a large absorption type muffler. This has made the exhaust very quiet but there is one slight problem.
Absorption type mufflers don’t work well for sounds below 200hz.
The exhaust was fitted with a Helmholtz Resonator which did a good job of quieting down the 95-140hz frequency range but I wanted to do more.
The car still had a slight all encompassing resonance in the car from 3-3500RPM and up. The sort of resonance where you are not entirely sure where the sound is coming from.
Large Exhaust Muffler
The new muffler was very large, something like 55cm x 24cm x 14cm.
I had the thought that maybe the body of the muffler was finding a resonance frequency and this is what was causing a lot of the hum/resonance sound.
It’s hard to explain the sound but it doesn’t 100% sound like an exhaust, it is more of a loudish hum.
Because the muffler is so large I thought maybe there was a chance that the shell of the muffler was resonating and if it was, I thought wrapping the muffler in exhaust wrap would help dampen the movements of the thin sheet metal.
I’ve done an experiment before wrapping a muffler with exhaust wrap and it showed some effect in lowering db, perhaps it could do something with sheet metal resonance as well.
Result – Defeating Exhaust Resonance
I have to confess, I didn’t do a very good job of making this a fair test.
I recorded the frequency of the hum and the loudness of the hum before wrapping the exhaust muffler.
And I compared that to the frequency of the hum and it’s loudness after wrapping.
What I didn’t do was to record what load was on the engine before and after. This was a big mistake.
On the face of it, it seems the difference between a wrapped and unwrapped muffler is very small.
But I don’t know if I am having to put more load into the engine with the wrapped muffler to get it to make the same db.
When I was recording the results with the wrapped muffler I was seeing if it could make a similar volume of hum, and I could get it very close.
What I don’t know is if the engine was under more load to make the same volume.
So the “results” of the test are far from conclusive.
Before, with the unwrapped muffler, I would rarely go above 120km/h because of the exhaust resonance.
It’s been a few days since I wrapped the muffler now and I find myself driving at 130-135km/h (where the speed limit allows) and I’m noticing very little resonance in the car.
This tells me maybe the exhaust resonance is noticeably less now with the same engine load and same RPM.
I’ll report back in a few weeks to see if the way I drive the car has changed consistently.