EFI Tune for Quietest Possible Exhaust Noise

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Cal Tonsley

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Bit of a different query but here goes!
Last piece of my restoration journey for registration is to pass our Department of Transport Roadworthy inspection. One of the tests in this inspection is for maximum exhaust noise, which for my vehicle requires no greater than 96db be emitted at 3300 RPM. I’ve had a preliminary test performed and my car is currently at 96db at about 2800 RPM.
My 410 stroker is running a 3” stainless twin system with dual mufflers, an x pipe and resonators. It’s also running an Edelbrock ProFlo4 EFI - timing specs are 20 degrees initial with advance starting at 1300 RPM and 33 degrees all in at 2800 RPM. Air Fuel Specs are currently Idle 13.4, Cruise 13.9 & ACC 12.8.
I don’t necessarily wish to modify my exhaust system just yet, so, given my ability to digitally alter the “engine tune” I’m interested to hear people’s suggestions for (temporarily) altering the tune of the motor to achieve the quietest possible exhaust. My thought was to maybe advance timing from the fairly conservative 33 total especially given the motor will be under no load during the test. Is this or any other possibilities worth exploring?
Appreciate your advice, thanks.
 
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Pull the plug on your oxygen sensor, then your engine monitoring computer will go into "Closed Loop" altering the sound of the exhaust note.

After you pass the test, plug it back in.
 
Make an insert for the exhaust that chokes down the inside diameter of the pipe to say 2” and make it removable with say a simple nut and bolt. 3” is huge and emits a lot of noise. There is a technical term for that - I just cannot pronounce it let alone spell it. lol!

I seen this before: a washer looking item with a long steel stick tack welded.
It drops down the tail pipe(s) and bolts to the side with like a 6 or 8mm nut and bolts.
 
Can you drop a few cylinders at a time? Or are they monitoring emissions too?
 
I dunno how you can incorporate any of the following, maybe it's just food for thought;
I have a carbureted, factory centrifugal-advance type Distributor, 11/1, 367 Mopar LA: a nice spicy 400hp engine.
I have a dash-mounted, adjustable Timing module, with a range of 15 degrees. I use it in several ways including;
> up to Plus 6* for cruise-timing
> Plus 2>4 Degrees for Power-Timing
> up to Minus 9* for a lower idle rpm, for parading, and
> the full Minus 9* for tip-toeing past cops. And, I have turn-downs on the pipe-ends .

Late ignition timing reduces my cylinder pressure out the tailpipes, which reduces the intensity of every pop, as the expanding gasses chase after the piston on the PowerStroke. But my Second cam had longer powerstroke, to suck the energy out of the expanding gasses and stuff it into the clutch. By the time she gets to the exhaust cycle, there's not a lot of energy left in the exhaust gasses.
The large, thick-walled, 3" exhaust pipes, and large 3-pass mufflers, suck the heat out of the exhaust, reducing it's volume and speed out the tail pipes. Plus then there's the turndowns on the end.
All in all, this system removes the hi-pitched, higher intensity, annoying sounds, leaving the lower bass tones .
You could maybe quiet it a lil more by fattening up the fuel curve as well.
At 3300rpm, you can take a lot of timing out.
Conversely, you could add timing which will increase the exhaust pressure, but the gasses will be cooler.
I think, a lil experimentation might be required.
Good luck
 
I was going to say retard the timing and find and figure a way to add "muffle", either put a flange in the rear pipes so you can replace a section with some sort of resonators, or inserts into the pipes

I'm not absolutely sure, but I think maybe for "quietest" you want a resonator very close to the rear, to avoid a long piece of "echo" tail pipe
 
are you running headers? I made a 2 inch db killer like a motorcycle, put it instead of the header gasket/primary pipe, kills a ton of db's.
 
Can you drop a few cylinders at a time? Or are they monitoring emissions too?
No, they’re not monitoring emissions, this mightn’t be a bad idea as the injection has a soft touch rev limiter that I can set to work from 3000 RPM upwards
 
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I dunno how you can incorporate any of the following, maybe it's just food for thought;
I have a carbureted, factory centrifugal-advance type Distributor, 11/1, 367 Mopar LA: a nice spicy 400hp engine.
I have a dash-mounted, adjustable Timing module, with a range of 15 degrees. I use it in several ways including;
> up to Plus 6* for cruise-timing
> Plus 2>4 Degrees for Power-Timing
> up to Minus 9* for a lower idle rpm, for parading, and
> the full Minus 9* for tip-toeing past cops. And, I have turn-downs on the pipe-ends .

Late ignition timing reduces my cylinder pressure out the tailpipes, which reduces the intensity of every pop, as the expanding gasses chase after the piston on the PowerStroke. But my Second cam had longer powerstroke, to suck the energy out of the expanding gasses and stuff it into the clutch. By the time she gets to the exhaust cycle, there's not a lot of energy left in the exhaust gasses.
The large, thick-walled, 3" exhaust pipes, and large 3-pass mufflers, suck the heat out of the exhaust, reducing it's volume and speed out the tail pipes. Plus then there's the turndowns on the end.
All in all, this system removes the hi-pitched, higher intensity, annoying sounds, leaving the lower bass tones .
You could maybe quiet it a lil more by fattening up the fuel curve as well.
At 3300rpm, you can take a lot of timing out.
Conversely, you could add timing which will increase the exhaust pressure, but the gasses will be cooler.
I think, a lil experimentation might be required.
Good luck
Thanks, appreciate your input, I’ll do some experimenting.
 
I was going to say retard the timing and find and figure a way to add "muffle", either put a flange in the rear pipes so you can replace a section with some sort of resonators, or inserts into the pipes

I'm not absolutely sure, but I think maybe for "quietest" you want a resonator very close to the rear, to avoid a long piece of "echo" tail pipe
Thanks, I’ve decided to do some tuning experiments and will likely still add some type of db killing baffle to the tailpipes if I need to!
 
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Retarding timing at idle area will bring HC ppm massively down, thats the reason for the vacum idle retard in the 80s.
 
I dunno how you can incorporate any of the following, maybe it's just food for thought;
I have a carbureted, factory centrifugal-advance type Distributor, 11/1, 367 Mopar LA: a nice spicy 400hp engine.
I have a dash-mounted, adjustable Timing module, with a range of 15 degrees. I use it in several ways including;
> up to Plus 6* for cruise-timing
> Plus 2>4 Degrees for Power-Timing
> up to Minus 9* for a lower idle rpm, for parading, and
> the full Minus 9* for tip-toeing past cops. And, I have turn-downs on the pipe-ends .

Late ignition timing reduces my cylinder pressure out the tailpipes, which reduces the intensity of every pop, as the expanding gasses chase after the piston on the PowerStroke. But my Second cam had longer powerstroke, to suck the energy out of the expanding gasses and stuff it into the clutch. By the time she gets to the exhaust cycle, there's not a lot of energy left in the exhaust gasses.
The large, thick-walled, 3" exhaust pipes, and large 3-pass mufflers, suck the heat out of the exhaust, reducing it's volume and speed out the tail pipes. Plus then there's the turndowns on the end.
All in all, this system removes the hi-pitched, higher intensity, annoying sounds, leaving the lower bass tones .
You could maybe quiet it a lil more by fattening up the fuel curve as well.
At 3300rpm, you can take a lot of timing out.
Conversely, you could add timing which will increase the exhaust pressure, but the gasses will be cooler.
I think, a lil experimentation might be required.
Good luck

You know, this got me thinking... It's probably getting way too far into the weeds on the engineering side, but the other thing you could do would be to do a spectral analysis of the noise and see if there are any particular frequencies that have large peaks. Sounds complicated, but I think there are plenty of apps you can download on your phone nowadays that can show you a live view of sound frequencies. Maybe not laboratory grade accuracy, but if there are some particular resonant peaks you might be able to work on just addressing those if you can find what is causing them. You would probably expect to see peaks that are some fraction/multiple of your engine rpm, but there's always a chance that some of the noise could be coming from resonance of other parts. Mostly just me being nerdy, but exhaust tone can really change the perception of noise, and depending on if they are using standard weighting (A weighting is pretty typical since it reflects how humans perceive sound levels), if you can shift the tone around you might not have to reduce the level as much (by tuning exhaust length or something like that).
 
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