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Just looking at the pictures of that article it reminds me of the back of the merge collector from racing headers.
It's possible that a design like this is beneficial for a jet engine due to the nature of the design of the turbine as it spins the exhaust air.
In a naturally aspirated engine the gasses barely spin, what I've seen has been longer and shorter merge collectors, megaphone style collectors but never one like a turbine.
Lastly this effect might be lost in a vehicle with a full exhaust
@SonicVQ do you know of any SAE papers or other sciency papers about exhaust sound?
I'm wondering how to apply stuff like the scalloped engine cowlings that newer jets use to reduce noise internally in exhaust flow... that sort of thing. I'm expecting there will be math.
I don't know of any papers/books on exhaust sound. All the stuff I have are on flow and wave propagation and automotive focued.
TL;DR: Spent the weekend at the track with ungodly loud engines, and I recall most tracks for amateur racing at least have noise DB limits.
The scalloped engine cowlings are a different principle than the scalloped core/fan air mixers but still kinda relevant.
Exhaust from a jet engine doesn't spin much - the last few stages of rotor/stator in the engine core are almost at 90 to airflow so it cancels out the spin imparted by the large front fan and earlier compression stages. Want to test it? stand under and just behind (like 2') a running jet that isn't at idle and toss something into the airflow. Wait - maybe don't do that. I'm actually pretty sure *we* weren't supposed to do that either come to think of it.
Anyway...
A huge focus on jet design for both efficiency and noise has focused on better design of the exhaust (and intake) to remove turbulence-inducing elements, and a straight coaxial exhaust exit was something that caused a ton of turbulence with the hot exhaust hitting the cold (-50F at altitude) ambient air.
I'm not saying that cowl scallops would work to any useful percentage for noise reduction on our cars, f'rinstance on the muffler tips, but it's an area I think more attention needs to be paid to. Anytime the airflow path expands and contracts you have pumping losses - IE resonators and mufflers decrease the exhaust's efficiency. Not as much as they did in the 80s true, but for sure more than a straight pipe.
I wonder if injecting a mass of cold air into the exhaust stream after the cats would change anything noise-wise without hurting flow. Or if doing the same - mixing in cold air in the mufflers - would do anything.
As much as I like my snorty FI exhaust, good power that is relatively quiet wouldn't be a bad thing.
It would definitely increase volume. You'd have to insure that the pressure of the incoming cold air was at least as high as the pressure of the exhaust stream at that point to ensure flow was one way only - like a Miller Cycle engine.
If you can't tell, it's the idle time of the month at my work and I've got excess brain capacity at the moment LOL
Edit: think about a siphon. High-speed air running over a calibrated orifice creates suction on whatever is in the pipe with the orifice. See pressure washers, sandblasters, etc. Now treat the exhaust pipe as the orifice/feed pipe. Might need too much cold air to be practical in that design, but if anything would pull a mild vacuum on the exhaust upstream and help with cylinder scavenging.
Now that I'm talking through it, you'd probably need a 3' fan to move that much air, like a Porsche air-cooled V12.
It would definitely increase volume. You'd have to insure that the pressure of the incoming cold air was at least as high as the pressure of the exhaust stream at that point to ensure flow was one way only - like a Miller Cycle engine.
If you can't tell, it's the idle time of the month at my work and I've got excess brain capacity at the moment LOL
Edit: think about a siphon. High-speed air running over a calibrated orifice creates suction on whatever is in the pipe with the orifice. See pressure washers, sandblasters, etc. Now treat the exhaust pipe as the orifice/feed pipe. Might need too much cold air to be practical in that design, but if anything would pull a mild vacuum on the exhaust upstream and help with cylinder scavenging.
Now that I'm talking through it, you'd probably need a 3' fan to move that much air, like a Porsche air-cooled V12.
Venturi oriented type scallops would be the only way. I saw this design on the tail pipes of most diesels, for diesels this is a type of flying under the emissions' radar.
For diesels these entries help bring fresh air to mix it with the thick concentrated coal fumes coming out the exhaust to make them 'appear' or "perform" better and not seem so dirty.
This same tech can be used for what you're describing @rotarymike just farther up into the exhaust stream.
I recently helped install a catback z1 touring exhaust on a friends g37x. It comes with 18 in resonators and paired it with berk resonated test pipes. Ended up being quite raspy sounding at around 3k rpm and loud drone at lower rpms. After some pondering he called me up and we decided to add 12 in resonators on the Y pipe and then later 2 more on the straight section before the mufflers. The exhaust note is very consistent throughout the rpms but still pretty aggressive. Not sure if this was the correct way to do this but it seemed to work. He didn't see any power loss in the 0-60 times after all the resonators. I believe a lot of the rasp comes from the Y pipe as I installed FI catback with resonated test pipes and it wasn't nearly as raspy/droney as the Z1. I believe the X pipe design is what makes it much cleaner sounding as someone stated on this forum earlier. Hope this helps someone in making a decision on an exhaust or eliminating rasp. Here are some pics. The car now has a total of 8 resonators and 2 mufflers to create a more mature but loud exhaust. Seems a bit excessive but oh well, it worked LOL. let me know what you think and if you have any ideas.
I've got resonated FI HFCs and the 18" resonated crosspipe with my FI cat-back which also has resonated tips... total 6 resonators and 2 mufflers. Sounds based. Not annoying or drone-y.
I've got resonated FI HFCs and the 18" resonated crosspipe with my FI cat-back which also has resonated tips... total 6 resonators and 2 mufflers. Sounds based. Not annoying or drone-y.
Exactly the same here. Fast Intentions resonated cats with a pair of 18" resonators and resonated tips.
Everybody has a different interpretation of what rasp and drone is, and I'm of the opinion there's zero rasp with this setup. There's some highway drone, but that's to be expected with an aftermarket exhaust, and it's not annoying at all (to me).
Same and love love love it, well I have AAM RHFCs but basically the same. There are times I wish the volume was a tiny bit less with the FI exhaust, but overall it's pretty much perfect.
Exactly the same here. Fast Intentions resonated cats with a pair of 18" resonators and resonated tips.
Everybody has a different interpretation of what rasp and drone is, and I'm of the opinion there's zero rasp with this setup. There's some highway drone, but that's to be expected with an aftermarket exhaust, and it's not annoying at all (to me).
I don't agree with the drone comment at all. Certain companies have mufflers designed which prevent any drone, even on extreme setup with 3" piping and no cats. Corsa Performance on GM cars is a prime example of that. It can be an extremely loud exhaust at WOT with no drone when cruising on the highway.
Drone = the exhaust hitting a resonant frequency for the chassis at some point in the driving envelope. I'd hope the major players tune those freqs out...
I don't agree with the drone comment at all. Certain companies have mufflers designed which prevent any drone, even on extreme setup with 3" piping and no cats. Corsa Performance on GM cars is a prime example of that. It can be an extremely loud exhaust at WOT with no drone when cruising on the highway.
That does appear to be one of Corsa's main claims, and countless videos back that up. Sure is a nice sound for a V8.
I've always thought the same about Borla. Has anyone ever fitted a Corsa muffler to a G?
05-05-2022, 04:08 PM
