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Fatboy 18
10-21-2021, 03:24 PM
OK, Gen II stock headers, cat delete and Belanger Cat back system. Loud! 112db tested at Goodwood circuit UK. If I fit Hi flows will that bring the db limit down or do I need to re-install the oem cats?

its A db limit of 105

GTS Dean
10-21-2021, 04:48 PM
Aevus has a good exhaust thread going elsewhere on the site. He's using HF cats and large resonators IIRC. We need him to update us on results.

Fatboy 18
10-21-2021, 05:51 PM
Aevus has a good exhaust thread going elsewhere on the site. He's using HF cats and large resonators IIRC. We need him to update us on results.

I'm going to try to get my new longtime girlfriend 27yrs / now wife to attempt to experience the car on track and join the fun, so i need to get the db down to track level

RedTanRT/10
10-22-2021, 01:46 AM
That’s pretty loud. So your saying you have stock exhaust manifolds, no cats and a Belanger cat back set up?

If so you should be below 105 db

I have Belanger headers no cats and a high flow muffler and I usually around 103+/-

Was it a cloudy or overcast day? If so the weather is impacting you as it’s amplifying the exhaust

Fatboy 18
10-22-2021, 04:26 AM
A yard stick was placed at the rear exhaust tips with a db meter, i was told to hold the car at 4000 rpm and the reading was 112db

Stock engine, factory headers and turnout, cat delete (3" steel tube in the side rockers), belanger cat back system (so just the rear resonators by the exhaust tips).

They also measured a stock Gen V TA on 1 exhaust pipe and that read 105db

Old School
10-22-2021, 06:48 AM
I wouldn't install a cat as a muffler, I would install an additional short muffler(s) in place of the cat.

Aevus
10-22-2021, 07:42 AM
OK, Gen II stock headers, cat delete and Belanger Cat back system. Loud! 112db tested at Goodwood circuit UK. If I fit Hi flows will that bring the db limit down or do I need to re-install the oem cats?

its A db limit of 105

7db (8-9db to be safe) reduction is a lot. You'd be lucky to get that with high flows only but not impossible. Oem cats are a safer bet

Aevus
10-22-2021, 08:06 AM
I wouldn't install a cat as a muffler, I would install an additional short muffler(s) in place of the cat.

My Vibrant 7'' (3.5 in/out) straight through race muffler did basically.... almost nothing.

They advertise 4db reduction but I doubt that. Probably 4db in a very limited frequencies band, and not necessarily at the peak, the mic on the racetrack might get that peak at 350hz while the 4db reduction is at 600hz (not real numbers here) so it's kind of tricky to guess without actual measurements and numbers (db/hz)



The word “ideally” is used because under actual conditions, there are different and varying frequencies of sound coming through the muffler and only some will be canceled.

Automotive muffler designers use some very simple sound physics concepts to reduce the amount of noise emanating from your vehicle. Engine noise is comprised of many frequencies and these frequencies vary as the speed of the engine varies. Because of the many and varying frequencies, muffler designers cannot eliminate all of these sounds, but attempt to eliminate the higher frequency sounds, which are more noticeable to the human ear.

https://www.thermaxxjackets.com/sound-waves-muffler-designs/


So, the problem here is: we're not fighting noises that will be heard by the human ear but a mic. The mic will most likely pick up (be more sensitive to) lower frequencies.

But then again, at WOT you need to measure the frequency (Hz) where the peak is (decibel)

Aevus
10-22-2021, 08:25 AM
That’s pretty loud. So your saying you have stock exhaust manifolds, no cats and a Belanger cat back set up?

If so you should be below 105 db

I have Belanger headers no cats and a high flow muffler and I usually around 103+/-

Was it a cloudy or overcast day? If so the weather is impacting you as it’s amplifying the exhaust


I have a 92db limit here on my local racetrack and it doesnt mean it's more restricted than his 105db limit, because it all depends of the distance where the mic is (and peak frequency). If measured with a yard stick placed directly at the rear exhaust tip, obviously it's gonna be much louder than a fixed mic installed 30ft away on the side of the racetrack.

But if that track limitation rule is @ 4,000rpm, then find the peak frequency at that rpm and work on a solution that will target that frequency band. As a general rule, lower are the frequencies, bigger/longer the muffler/resonator needs to be. That's why short race muffler works on smaller engines but not much with a huge one as ours

RedTanRT/10
10-22-2021, 11:54 AM
A yard stick was placed at the rear exhaust tips with a db meter, i was told to hold the car at 4000 rpm and the reading was 112db

Stock engine, factory headers and turnout, cat delete (3" steel tube in the side rockers), belanger cat back system (so just the rear resonators by the exhaust tips).

They also measured a stock Gen V TA on 1 exhaust pipe and that read 105db


Wow, the track measures db's with a yard stick after the exhaust????

My comments were based on how the tracks in California monitor sound.
- They use a sound meter that's just off the side of the track, in most instances it's 10 feet off the edge of the track.
- That's why I mentioned weather as: Days with cloudy or foggy atmospheric conditions can make compliance with limits even harder, as sound bounces off the cloud layer.

That's really going to hard to get down to 105 db for you!!! In the case of the G5 TA you referenced, they were only picking up sound from 5 cylinders, not 10 like you.

Aevus
10-22-2021, 12:03 PM
49562


49563



Ok so I did a sound test using my calibrated pro RTA and mic.

1 meter distance pretty close to the ground (as pictured)

Cold start then idle after engine warm up, 2000, 3000, 4000, 5000, 6000 rpms.

Stock 8.3 engine + M&M headers + high flow cats + custom 28'' resonator 3.5 in/out.

Of course, the results will vary with a load. Most likely will lower the frequencies, especially above 4krpm. But it gives a good idea. My resonator's efficiency is probably around 200-400hz or so, along with some lower frequencies.

Note that at idle we get, with our Viper's engines, peak as low as 30hz and even less... We are talking home theater subwoofer frequencies here! Of course that moves up quickly in the frequency band up to roughly 700hz @ 6,000rpm. Which is still pretty low, all things considered.

Aevus
10-22-2021, 12:05 PM
Also interesting to note that I got a SPL reading of 115.2 db at cold start (the first second when I started the engine), then quickly dropped to 113db then 107db once warmed up. That is a big different, no wonder we can disturb our neighbors!

Aevus
10-22-2021, 12:25 PM
To sum it up:

If it's a 4,000rpm test with a yard stick (we don't have that in Canada) your best bet is to use a resonator that is very efficient in the 250-500hz band. Basically the car will be loud as hell at higher rpm, but if that's the way they control noise... that's their problem! lol
Educated-guess here: no muffler but the longest resonator that fits... and ''tuned/designed'' for the target frequencies.

But if it's a fixed mic installed near the racetrack, that measures the peak SPL at ANY frequency... it's another story. You better work on a solution that is efficient when you're at WOT, which means a muffler AND a resonator that is efficient at roughly 300-700hz. Educated-guess here: a short muffler combined with a medium-sized resonator. Use all the space you have, move the cat near the headers (downpipe) if not done already, to get more space for the muffler/resonator, so you can have a fighting chance.

About cats, I have no idea how many dBs it would reduce nor the frequency. But it's sure not designed for that purpose anyway.

Also sidepipes and rear pipes have different impacts in regards of SPL readings and space available for the muffler/resonator. I would sure be kicked out of my local racetrack with that exhaust system, as it is. That is why my (temporary) tips are about 1.5'' too long, so I can clamp a firing-up secondary tip which will help with any fixed mic.

Aevus
10-22-2021, 12:45 PM
That's really going to hard to get down to 105 db for you!!! In the case of the G5 TA you referenced, they were only picking up sound from 5 cylinders, not 10 like you.

Not exactly.

Lower frequencies are definitively picked up by the mic, from the other side. It's sure adding to the total SPL reading. If you could silent the side measured, you would absolutely hear the other side.

The lower the frequencies, the less they need to be directionnal (on-axis) to be heard. And the bad news is: the huge Viper's engine is producing tons of low frequencies. Human ear is very tolerant to those, microphones are not.

Aevus
10-22-2021, 12:52 PM
- That's why I mentioned weather as: Days with cloudy or foggy atmospheric conditions can make compliance with limits even harder, as sound bounces off the cloud layer.

Yes and I wouldnt be surprised to see differences sunny/cloudy of 2-3 decibels. I've been allowed and kicked out of the same racetrack with the same car but different weather. That is why you better have an exhaust set-up that is ''too efficient'' than otherwise.

The other option is to bribe the yard stick guy :dude3:

Fatboy 18
10-23-2021, 04:03 PM
Thanks for the detailed breakdown, I was thinking of bashing a few holes in some baked bean cans and stuffing them up the rear exhaust tips to secure them with a screw.