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Building the "Ultimate" L6 Turbo Exhaust


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I'm going to ask about the ultimate exhaust system, even though the site rules clearly state that there isn't one. ;) I hope I can sneak this thread past the moderators. :D

 

One of the things that came out of my recent L6 turbo engine build, is that I really don't like the sound of my exhaust. This was once a track only car, and now it's going to get more street duty, so a change needs to be made to quiet it down. At the same time, I am planning on going full stainless and building the last exhaust this car will ever need.

 

Right now the car sounds like a glorified motorboat. (or Subaru) The current exhaust comes out of the turbo into a 2.5" mandrel bent downpipe, transitions to 3" and runs through a 3" round magnaflow (straight through) muffler. I've got about a 6" long tip on the muffler to route gases away from the rear of the car. It's loud, it's raw, it's fine for the car as a race car.

 

Video with current exhaust sound:

 

 

As a street car, I want to achieve something a little more exotic sounding, as well as quieter. For example:

 

 

I believe in order to achieve the sound I want, I am going to need to run dual functional tail pipes. I think running a different length to each side will create the harmonized sound I am looking for.

 

In my mind the system plays out like this:

 

  • 2.5" mandrel bent downpipe off of the turbo
  • Custom SS "Y" from 2.5" to dual 2.25"
  • Dual 2.25" each into a 12" long resonator box in the tranny tunnel
    • One 2.25" pipe exits at the outside face of the box
    • One 2.25" pipe exits 8" into the box, making this path a different length than the other

    [*]2.25" pipe runs to the back of the car, one exit on each side, each through a muffler.

    [*]Dual tip exits from each muffler.

 

mtk_e46_m3_oncar_585.jpg

 

  • My rationale is that the staggered exit from the resonator box will allow the same pulse to split and reach the tailpipes at different times, therefore smoothing out the exhaust note to a degree.
     
  • By running 2 larger mufflers (than the current 1 smallish muffler) I should be able to control the volume a bit better.
     
  • The dual tips are more for show, but if I can stagger the paths between the muffler and each of it's tips, more smoothness should ensue.
     
  • Dual 2.25" is effectively larger than the 3" single I have now, and probably easier to package under the car as well.

 

I might be way off base with these assumptions, but I welcome any input those with experience on exhaust tuning or exhaust design may be willing to share.

Edited by Drax240z
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I would avoid the expense of food grade stainless. It is more difficult to weld properly (tends to have a higher carbon content) and is generally thicker then what you need. .049" wall is plenty for what you're planning. You can have the exhaust system polished after its built which will give as nice a look as the already polished food grade material.

 

Be careful with the Y pipe design because a poorly done one will create a high pressure spot in the exhaust. A 2.5" inlet with a narrow merge (7 or 14 degree) out to 2.25" outlets will probably work best.

 

Bends on the exhaust tend to quiet things down a bit by they also create high presusre areas. Use V-band clamps for all your junctions. Use a backpurge or a flux on all the TIG welds.

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Food grade is super easy to work with because the bends are so darn good, but point taken on wall thickness. I will order material to make this system anyway, so i can spec whatever. Food grade is avaialble in the 0.049" area though.

 

TIG welding and backpurging are a given. John, if the food grade stuff is 304, does it really have a significantly different carbon content than other 304SS?

 

The system will use V-Band connections to make removal/installation a snap.

 

I will look around a little for collectors, but I think i can probably make what I need better and cheaper than the alternatives, while keeping the angles to a minimum.

 

Something like this looks like it would work well from the 2.5" downpipe to the resonator box.

ypipe_bmw1.gif

Edited by Drax240z
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There is 304L tubing which I've used on the past for thin wall stainless exhaust (.035"). The extra carbon reduction helped. You don't need it for what you're doing and the merge collector example above is what I was describing (minus the cone transition from 3" to 2.5" in the pictured item).

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There is 304L tubing which I've used on the past for thin wall stainless exhaust (.035"). The extra carbon reduction helped. You don't need it for what you're doing and the merge collector example above is what I was describing (minus the cone transition from 3" to 2.5" in the pictured item).

 

I think I could accomplish the transition from 2.5" to dual 2.25" similar to above but without the cone if I cut the merge portion back far enough and ovalled the tube on the 2.25" end slightly.

 

Does anyone have information on the direct comparison of sound reduction between a 3" straight through muffler with a small diameter case, and the same 2.25" straight through muffler with the same case?

 

Switch to Magnaflow Oval and Notice the Camaro type rumble as opposed to the Trumpet type Howl the round exhaust makes.

 

If it sounds like a camaro after all this, I'm setting the car on fire. :P Seriously, that isn't at all what I am trying to accomplish here, see the videos in the first post...

Edited by Drax240z
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After thinking about it a bit, I think the exhaust sound you're looking for has more to do with the manifold/header than the post turbo exhaust.

 

Going on your subaru reference earlier, here is a video that compares equal length headers to stock manifolds on an sti.

 

 

Also, if you compare the lengths of runners in a stock turbo manifold, you'll see that some runners are nearly 10" longer than others. Those exhaust pulses take longer to get out and don't quite maintain the even spacing of pulses that gives the smooth sound you're looking for.

 

Compare something with a header to something with a stock turbo manifold.

 

header

 

manifold

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After thinking about it a bit, I think the exhaust sound you're looking for has more to do with the manifold/header than the post turbo exhaust.

 

Also, if you compare the lengths of runners in a stock turbo manifold, you'll see that some runners are nearly 10" longer than others. Those exhaust pulses take longer to get out and don't quite maintain the even spacing of pulses that gives the smooth sound you're looking for.

 

Fair comments, I guess I need to find some more video/sound clips of L-series turbo headers... anyone have some to share?

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