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Senior Porsche Financier
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Discussion Starter · #1 ·
Our story thus far ... I went in many directions trying to remove the drone from my borla cat-back exhaust with very limited results; the final step was to take it off my car and attempt my own.

This simple / basic exhaust was in the same configuration as virtually all the other aftermarket exhausts out there -- given the limited room and tight layout, this is what (generally) fits. Note well that the piping lengths are generally all the same.

The build thread:
http://www.planet-9.com/987-cayman-...9529-simple-basic-cat-back-exhaust-build.html

... which resulted in this:

Auto part Muffler


It worked very well on the car, sounded great and droned.

Added a quarter-wave resonator and the center section became:
Automotive exhaust Auto part Muffler Exhaust system Pipe


This was partially successful, at certain air and car temps but was a pain to try to tune.

I'd already tried running just the flowmaster HP2s and it sounded great, droned and was too loud to live with. That in mind I decided to keep the two side (flowmaster) sections and build (yet another) center section to try.

It kills virtually all the drone. [Narrated in a deep slow voice to build drama] This is its story.
 

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Discussion Starter · #2 ·
Background and details: the existing center section Y-pipe was mild steel & breathed great but a local guy couldn't get ceramic to stick w/o it rusting thru the coating. I was also never happy with the amount of front-back room it took behind the transaxle -- made any attempts a tight build.

I'd also found mention of a type of muffler I'd never heard of -- which reportedly killed drone. This is the good bit and I'm saving it for the last -- it was also the final thing added to the new center section.

When I found this Y-pipe from Magnaflow I fell off the wagon again and decided to tackle drone One More Time.
Magnaflow #10734:
Product Pipe Automotive exhaust


This gives a very smooth exhaust flow from either side w/o each side's flow pointing head-on into the other's. it is made of 409 stainless 16ga (my estimate), is ~ 13" wide by ~ 8" deep. the inlets and outlet are all 2.5". I'd have preferred a 3" exit but it's the only bottleneck in the build. A very nice piece at a best offer of $75.

Several commented that flex sections could help prevent some vibration from the exhaust pipe itself -- so I ordered a pair of these from eBay:
Auto part Muffler Steel Metal Cylinder


... 2.25" inner diameter, 4" long and many choices to be had. I went with the inner spiral sleeve rather than the braid most others had - hoping it would flow smoothly.

And since these were the second-last thing delivered I clamped together an exhaust w/o flex sections, using a simple piece of 2.5" tube for a tip. Droned less that the previous center section but it still droned.

Moving in the right direction. stay tuned for the good bit.
 

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Discussion Starter · #3 · (Edited)
The Good Bit

I'd been googling 'till my fingers were dead and found mention of an exhaust handbook and in that, the description of an Auger -type muffler to go inside the tail pipe, tip, or farther upstream. It was specifically mentioned to remove drone.

I found a lot of corroboration in the muscle car world but nothing remotely like a CS. And then I found a post to a board about a car with a turbo'd flat-4, aftermarket exhaust and a lot of drone -- a subaru forester.

This guy tried a 3" auger muffler insert and said it eliminated his drone.
This one:
Metal Dumbbell Auger Steel

Really nothing fancy - made to fit inside 3" tube, (somewhat) mild steel, 11" long. $15 + shipping from speedway. A series of spirals to smoothly spin some of the exhaust stream while the rest goes thru the center tube.

No idea why it works.

The plan was simple - trim some from each end to get down to the ~ 9" I had from the inside of the Y-pipe to the exhaust tip. Grind the first auger and part of the 2'nd so they'd fit inside the 2.5"ID of the Y-pipe outlet.

Painted it with rustoleum barbecue black to prevent rusting.

That cutting / grinding was the largest part of the effort. I didn't place the insert entirely into the Y-pipe -- I left some room for flow just downstream of where the left & right sides of the Y-pipe meet at that dimple in the middle.

The rest is contained in a simple slant-cut 3" exhaust tip, 304 stainless, $18 on ebay, shipped and a very nice piece:
Auto part Cylinder Muffler Pipe Automotive exhaust


The Tip inlet was 2.5" ID and so I had to grind about 1/8" so it would fit *over* the 2.5"ID Y-pipe exit (didn't want to lose 1/8th inch of tube ID using a stub of 2.5" to join the 2 -- and didn't have the front - back room anyway.)

I'd already tried the flex sections and that was an improvement but drone persisted.

Adding the auger took care of nearly all the drone -- I won't use the word 'eliminated' because there is a resonance between 2K - 2.4K rpm but it's very mild and I haven't driven it enough under varied conditions.

I'd call it the baseline resonance of the body w/ the stock exhaust, but it's been a long time since I had one on the car.

I'm really happy with the results:
Auto part Automotive exhaust Exhaust system Pipe Muffler
Auto part Automotive exhaust Exhaust system Muffler Vehicle


It seems to breathe *slightly* less well than the prev center section but performance is really good and I just haven't had enough time with it to really tell. could be variations in weather, my last tank of gas or sunspots ...

I'll find something to add to dress up that 3" tip ... at least mask and paint the inside to hide the weld havoc but for now, so. happy!

-PM.
 

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Re: The Good Bit

Seems like a couple of years that you have been pursuing this. Thanks for the update and congratulations. I'm still working with sound deadening material and bass traps. Some progress but the drone is still there under load between 2-3K. Less than it was, but still there.
 

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Discussion Starter · #6 · (Edited)
Re: The Good Bit

Quick update - went for a longer test drive, curves, varied conditions, polished the rust from the rotors ...

This is working. the driving is just simpler ... if I'm lugging it in traffic it's no big deal - I'm no longer rattled and reminded by drone if I find myself in the low 2000s rpm. Every. Damn. Time.

I'm just driving; the floor no longer vibrates. Nice.

There's a bit of a rush to the exhaust above 5K rpm. I may add a dump tube or build my own auger to flow a bit more freely.

mmmv,
-PM.
 

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I made a couple of these a while back out of 1 3/4" inner and rolled sheet metal around into my 3" straight pipe header extensions on my '28 Model A. I only made them roughly 4" long and it has about a 270° full twist. Since then I have had a headache-less power tour cruise every time. Glad you found the answer, wish I had seen the struggle earlier. If I were you I would've gone further upstream to uphold tip look. But that's just me, driveability is most important. Cheers!

Sent from my SAMSUNG-SM-G935A using Tapatalk
 

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Discussion Starter · #8 ·
Quick update ...

I built a 3.5" version (tip and auger dia.) with the goal of it breathing better - which it did.

But the build was loose enough that there were sizable leaks between the outer edge of the auger and the inner surface of the 3.5" tip. More drone until it was thoroughly warmed up.

I've since added more metal to fill the gaps and that fixed the drone. A bit ugly but it's still a work in progress.

Since this wasn't the simple slam dunk I thought it would be -- and so I can tinker -- I'm going to add a collar to clamp different tip assemblies to for test ... rather than welding.

The rpm I want to run is now more driven by traffic & throttle response, and oil pressure concerns. not trying to drive around drone anymore.
 

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Discussion Starter · #9 ·
Re: Quick update ...

One key point re drone, when comparing the results for different stages of this experiment: the drone is worst at cold start.

Even a few minutes into a drive it will be pronounced. I'm guessing that the fuel mix doesn't lean out to actual running trim at all immediately.

The drone continues to lessen until about 10 - 15 minutes into the drive when 9+ quarts of oil reach operating temp. I'd seen enough cases of little drone after a warm restart; much less than on cold start.

mmmv,
-PM.
 
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