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I'm thinking of doing a round to rectangular exhaust and bringing it underneath the tunnel then back out to round to a muffler/resonator assembly.
I want to do this so I can make a TVR Sagaris style exhaust out the side of the rear bumper area.

Does anyone have a link where someone did something similar? I know I've seen someone do this on this site (or at least something similar) but I can't find the thread.

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My build -> viewtopic.php?t=15585

ccrunner did a rectangular exhaust on his Berkeley build

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Chuck.

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I had to Google Image TVR Sagaris, but I found some interesting images.

This looks like the "factory" style exhaust


This looks like an aftermarket version.


This is someone's project car.


I'm wondering why you think you need to make a rectangular exhaust. I assume it's for space requirements.

The project car illustrates something interesting. You can actually make a resonator using a end capped tube of the same size off of the exhaust. I'm not an exhaust expert, but I've always been interested in making mufflers/resonators for my own build, so I could control the space allocation for the system. I want my exhaust to exit at the rear and I don't want a big muffler (or even a hot exhaust pipe) running along the entry/exit point that the driver and passenger have to dodge.

If you look at the project Sagaris above, he has essentially build a resonator into the exhaust system. His system looks way overbuilt to me. You do not have to tie back to the exhaust flow to make one work. However, he did for some reason. It's not clear to me what his intent was.

Would a simple capped tube as a resonator work for your build?

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

You're right Lonnie, just looking at doing it for space requirements. I'm planning on running it around the side of the transmission fairly low then changing to rectangular under the transmission and to get underneath the pinion yoke of the differential. I'm planning on factory style Sagaris exhaust since my car will be pretty similar to a donkervoort dimension wise and body wise as well. I figure it's look pretty decent. If I have the room I'm planning on running just a single resonator as well as a valve to either be straight piped after the resonator or go directly into a muffler then out.

I think my Maine problem now is going to be packaging a catalytic converter somewhere in there. Going to have to do some major heat shield fabbing I think.

As far as the resonators you are thinking of Helmholtz resonators. They work really well and I've actually got a spreadsheet made up to figure out sizing. Only problem is that they should really be only used to get rid of specific frequencies such as if you have droning exhaust. Regular resonators work over a much broader range.

Also yeah Rx7locost, that's the build I was looking for, thanks!

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My build -> viewtopic.php?t=15585

I needed more clearance on a Lotus Elan exhaust many years ago. I crushed round tubing into an oval in my shop press using 2x4's and slowly working down the tube. Flat surfaces in an exhaust system make me nervous. I know ccrunner used 1x3 (?) in his exhaust system and it seems to be working for him.

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Flat sides on exhaust pipes or mufflers causes drumming, which in a Locost probably doesn't matter compared to all the other noise, but is a no-no in "real" cars.

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@KB58

Nice to know, Kurt.

I wonder if this style would solve the problem. It's just a style I like and I saved the photo for reference in case I wanted to do something similar.


Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

Do you mean drumming as in an actual drum sound?

If so I may have to just see if I can fit 2.5" tubing in the tunnel, or at the very least ask ccrunner if he dealt with that at all.

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My build -> viewtopic.php?t=15585

If you do as kartracer has describe, to form an oval, I would increase the diameter of the tubing at a min of one size locally, at the pinch point. That will allow for a better match in cross section area with the rest of the system.

The details of the design are important. Round is a more ideal shape for even loading of the exhaust pulses than rectangular but wall thickness can offset any excess oscillation of the wall. Rectangular is a definite no-no for the bean counters but not a deal breaker. A thicker wall will also pass less of the noise.

The purpose of rectangular on the berk was for ground clearance without losing pipe area.

The discussion(s):

viewtopic.php?f=27&t=20927

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The frictional losses are definitely something I'll need to consider. I don't think I should have much of an issue with corves because the section I'm planning to be rectangular will only be under the tunnel. So basically right out of the turbo into a small downpipe elbow with oxygen sensor, catalytic converter next, then into another elbow to align with the front of the tunnel alongside the transmission and once it's under the transmission then it'll go to rectangular all the way back until right after the rear bulkhead then it'll transition back to round.

I'm going to do some finessing of fitting the catalytic converter in the engine bay but aside from that my only issue will be fabbing a transmission mount to go around the exhaust and figuring out how to fit a flex pipe in there somewhere.

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My build -> viewtopic.php?t=15585

Meaning that the internal exhaust pulses can physically flex the flat sections of the exhaust or muffler, effectively forming a speaker and transmitting some exhaust noise through the walls. As was noted, oval or slightly thicker wall rectangular section tubing can reduce the effect. I'm just bringing this up for awareness, because since Seven-type cars have so many sources of louder noises, but the above is a contributor.

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Midlana book: Build this mid-engine Locost!, http://midlana.com/stuff/book/
Kimini book: Designing mid-engine cars using FWD drivetrains
Both available from https://www.lulu.com/

Gotcha. That makes sense. I think if I end up doing it I will probably use fairly thick stainless such as 16ga to keep that from happening. I don't mind a little heavier car since I'm already going to be using an SR20DET and even have a ford 8.8 IRS out back.

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My build -> viewtopic.php?t=15585

That's because it's not a resonator at all, it's an 'active' exhaust. Two flow paths, with a pair of (vacuum operated) valves in them. Seeing as how the valves are placed to close off the muffled path, rather than the unmuffled path, the they were apparently going for loud and louder since there sill always be at least some exhaust (sound) bypassing the muffler.

https://youtu.be/ZAblkolIFRs?t=102

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-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie

I did this on a Miata I had about 4 years ago. Surprisingly it droned less on the highway at constant throttle application than with the muffler. I think I just had a small resonator in the exhaust and it could for sure used a Helmholtz resonators in addition. Personally I like setups that can be switched on and off like this either completely automatically or by a switch. Makes it very nice in case you are coming home after a decent dinner and don't want to disturb the neighbors.

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My build -> viewtopic.php?t=15585