Don Terrill’s Speed-Talk

Here’s a quick summary of what worked for us for low rpm sound suppression and high rpm flow capacity in a twin-turbo V8 intended for street use.

Make the wastegate pipe large enough diameter and then set the length such that with the wastegate closed the wastegate pipe functions as a closed side branch resonator suppressing certain frequencies.

Use a slim resonator on each side to trap any supersonic blast waves after the wastegate dump pipe merge. This really mostly helps at high loads with high rpms when the wastegate is open. In my case, the resonators can be swapped for catalytic converters.

Combine both sides into a single pipe, which then combines the pulses into a more even rhythm, cancelling some sound frequencies and equalizing pressure. (Equalized pressure allows for smaller pipes downstream with additional losses.)

Divorce the single pipe again into two smaller pipes. Add the largest case volume straight thru packed muffler that fits on each side. The smaller pipes (dual 3” in my case) with large case mufflers allow for good noise suppression.

Run unequal pipe lengths from those muffler into another merge on one side of the car. Unequal pipes lengths again cancel some frequencies.

Add a single large straight thru packed muffler to the end. This will get rid of any flow induced noise etc.

This is not a cheap exhaust if made from stainless steel and with inconel bellows in multiple spots to allow for compliance and prevent cranking, but it works in my car.

ptuomov wrote: ↑Thu Sep 05, 2019 7:52 am That auger muffler style has by my understanding turned out to be inefficient in terms of back pressure, volume, and sound suppression.

That's kinda why I was thinking of the second one.

The exhaust is split into 2 half pipes twisting down a resonator core.

emsvitil wrote: ↑Thu Sep 05, 2019 8:22 am

ptuomov wrote: ↑Thu Sep 05, 2019 7:52 am That auger muffler style has by my understanding turned out to be inefficient in terms of back pressure, volume, and sound suppression.

That's kinda why I was thinking of the second one.

The exhaust is split into 2 half pipes twisting down a resonator core.

The twisting path does two things. First, it increases path length. Second, it increases flow losses, both because of longer path and also because of change of direction. Increasing the path length doesn't do one much, unless the flow is split in two paths of unequal lengths. The auger muffler in the first photo has a straight pipe, too, not just the twisty flow path. Some sound goes thru the straight pipe and some sound goes thru the longer "auger" path, offsetting the sound waves and causing some cancellation. I don't think that just twisting the whole flow path like in the second picture would suppress noise efficiently.

There is this unit as well.

https://www.carchemistry.com/silencer-insert/

I'm wondering if a shared case muffler with 2 in/2 out with a Y at each end would work in a single exhaust. Hmmmm

https://www.hottexhaust.com/Magnaflow_12599

dfarr67 wrote: ↑Fri Sep 06, 2019 5:13 pmI'm wondering if a shared case muffler with 2 in/2 out with a Y at each end would work in a single exhaust. Hmmmm

https://www.hottexhaust.com/Magnaflow_12599

I'm not sure there would be a benefit compared to Borla multi-core muffler from just splitting the flow into two pipes, if the case volume is the same.

However, if you can make one of the flow paths meaningfully longer than the other, either before it enters the muffler or after, then by my logic you'll get a benefit. It'll act like a trombone resonator: http://www.enoisecontrol.com/wp-content ... r_wall.pdf

Let me say again: That sounds lovely.

What fascinating information - - Tuomo, is a formulae used to calculate the split flow path's unequal tube lengths, or do you settle for a convenient difference between say 4 and 8"?

Leftcoaster wrote: ↑Sat Sep 07, 2019 1:10 pm What fascinating information - - Tuomo, is a formulae used to calculate the split flow path's unequal tube lengths, or do you settle for a convenient difference between say 4 and 8"?

That presentation has some formulas for frequency and length, but, for practical purposes and space constraints, the longest difference in path lengths that fits is in my opinion the best.

I already posted this sound video in another thread, but here is it again, with the muffler made during this thread’s active life. I like the sound, it’s quite with a slight throttle angle and angry with an open throttle:

emsvitil wrote: ↑Thu Sep 05, 2019 2:13 am I remember reading (a long time ago) in a book about exhaust systems where the straight section of the muffler core was a continous spiral path.


Something like this, but constant.



https://www.summitracing.com/parts/pte- ... lhEALw_wcB



Maybe vary the twist so there's less backpressure.

Use something like this:

Then use a static paint mixer.

https://en.wikipedia.org/wiki/Static_mixer

The one that is split every turn and reverses but like the one above;...

pdq67

We’re now looking at 2.5 psig back pressure in the down pipe at 850 rwhp. While it’s not ideal, it’s a good compromise. And recall that this is with five mufflers in the system that now delivers a very nice exhaust note at the appropriate sound pressure level.