Designing a QUIET, Straight Through Catback Exhaust!

I just sold my AutoSpeed CBE. Loved the sound quality, but I need this car as quiet as a stock sports car. So going to design a one-off custom exhaust.

 

Project Requirements:

 

• Straight Through Mufflers, no chambers or louvers.
  
• QUIET as possible, granted limited by straight through mufflers.
  
• Mindful of cost
  
• Preferably steel mesh over fiberglass packing, for longevity.
  

I'm looking at a 3" mid-section, splitting into two 2.25" lengths each feeding a 2.25" muffler.

 

Hoping for oval tips, but that's more of an afterthought.

 

Top brands considered:

 

• Borla
  

Secondary brands considered:

 

• Vibrant
  
• Magnaflow (don't love because fiberglass will blow out over time)
  

So far this is what I have worked out as some options:

 

For 3" muffler:

 

Borla XR-1 40944 http://www.summitracing.com/parts/BOR-40944/

Perfect specs, but a bit expensive. $143 shipped

 

For 2.0" mufflers:

 

Borla Turbo 40651

13.125"x9.5"x4"

$112ish shipped

 

Borla Pro XS 40344

14x9.5x4

$103 shipped

 

For 2.25" mufflers:

 

Borla Turbo 40652

13.25x9.5x4

$105ish

 

Borla Pro XS 40357

14x9.5x4

$103 shipped

 

Borla Turbo is supposedly their retail line.

Borla Pro XS is supposedly their wholesale line, like for exhaust shops, internet clearance houses, etc.

Same thing.

 

 

Fitment Considerations:

 

I backed her up on ramps and took measurements... I WANT A LIFT!!! Waaa 1st world problems...

 

Behind two little nubs that protrude out of the well 1" behind the driveshaft center support area and stock heatshield, to in front of the rear diff shield, there is room for a:

16" long x 8" wide (assuming oval style) x 4.5" thick/tall/high muffler / resonator case.

Summary: 16" x 8" x 4.5" muffler case under driveshaft.

Offset in/out considerations: I recommend straight through in and out, as offset in or out may run into side clearance issues.

 

*Note these specs above are assuming a 1/2" buffer inbetween body contact, which I suspect should be fine and plenty achievable IF my fabricator does a good job...

 

In front of the rear bumper, there is room for two mufflers of the following dimensions:

16" long x 12" wide x A LOT" thick/tall/high muffler case. The limit to the height/thickness of the muffler is purely up to the preferences of the end user. You could put a round muffler with a 8" diameter in there, but if it is a center exit then you have 4" of muffler hanging below exhaust tip center, which may look bad.

Offset in/out considerations: I recommend offset for the inlet, offset toward vehicle center. I recommend center exit, as that is dead center with the muffler tip cutouts in the rear wheel well.

 

Muffler tips: The AutoSpeed tips are long. From side view, their outlet visual cutoff slants upwards, like seen here:

http://www.lextreme.com/pul/DSC08455.jpg

On my LGT the tips appear to protrude 1/2" or 1" depending on your angle of incidence, out of the rear bumper. This is a great way to do it IMO, as there is zero sign of bumper burning.

 

My AutoSpeed tips are 10" long on top, 9" long on bottom due to this effect.

This is with the muffler right at the furthest rear position possible in front of the bumper.

So to replicate how far comparable new tips stick out, you would add on your buffer space between bumper cover edge / pinch weld, and muffler. I am going to aim for 1/2" clearance on all muffler positioning.

 

Lastly, that leaves pipe needing to be purchased...

 

My single pipe section between DP to axle will stretch around 66", but could be as long as 70" to be safe... this includes muffler length.

 

My Y section with split pipes will take up a grid 24" long by 40" wide or so, between the Y entrance and front of the rear mufflers. Will make it easier to know how much pipe to buy.

 

 

If you know anything about designing a quiet exhaust, please post!

You need as much volume as possible for the mufflers and the smaller the pipes, the quieter it will be.

 

My BMW goes down to a single 2.5" pipe and a single muffler, but of course it is pretty low on power

You need as much volume as possible for the mufflers and the smaller the pipes, the quieter it will be.

 

My BMW goes down to a single 2.5" pipe and a single muffler, but of course it is pretty low on power

 

I've been tossing around the idea of 2.0" mufflers in the rear instead of 2.25" There is a LOT of pipe length to shed exhaust heat and lose velocity.

 

Now that you mention it, necking down to 2.75" right before the midpipe muffler would probably make a nice difference. Then split the 2.75" into 2x 2.00" sections.

 

The math verifies this, attached. Though I have nothing on heat loss.

 

How am I doing?

Legacy Muffler Diameter Calculations Exhaust.pdf

This is my setup, i have a 2nd muffler installed, its an ebay one and it's very quite now, compared to without. Also had a prospeed racing 100cell 5inch cat installed as well. At idle with my Equal Length headers its quite subdued but open it up WOT, and there is a banshee like scream to it makes the power too.

http://www.smugmug.com/photos/i-MxrrwsZ/0/L/i-MxrrwsZ-L.jpg

BTW this is an interested before and after comparison at idle;

 

first video below is Kando Equal Length Headers, with 3inch Turbo Back with Hi flow cat in dump pipe and 3inch resonator with MRT mufflers

 

[ame=http://www.youtube.com/watch?v=eLfma4NenTM]YouTube - ‪Liberty MRT Mufflers‬‏[/ame]

 

And next video is Kando Equal Length Headers with 5inch 100cell cat in midpipe, second muffler to replace resonator and quad Magnaflows. *notice the idle with the Kelford 264 cams*

Oh don't mind the condensation, it was early in the morning.

 

[ame=http://www.youtube.com/watch?v=5z0zRS_pPW4]YouTube - ‪Liberty Magnaflow‬‏[/ame]

I have been looking for a quiet exhaust. I have a little one on the way and my magnaflow with catless dp is just too loud for me (mostly too loud for the wife). The droning on the highway is annoying.

 

 

adam

What is wrong with the stock cat axle back?

Gambit, we have very similar setups, I too have EL headers and a high flow cat, for what it's worth. The second video sounds much better

 

Going to miss the car actually sounding good since the AutoSpeed is on its way out, but c'est la vie.

 

Your magnaflows are going to sound very similar to my Borlas or whatever cans I get in the rear. The wild card is that eBay can. Trouble with mufflers is they're incredibly simple, and yet some are made to be a lot quieter than others.

 

Your video is the second one I've seen that verifies how quiet these cars can get, this being the other http://legacygt.com/forums/showpost.php?p=2386291&postcount=35

I have been looking for a quiet exhaust. I have a little one on the way and my magnaflow with catless dp is just too loud for me (mostly too loud for the wife). The droning on the highway is annoying.

adam

 

See my link in post #7. He too has MagnaFlow cans, but he welded a borla in the center section and just see how it sounds.

FWIW I use a vibrant streetpower muffler and a vibrant resonator in the mid pipe on my catless single exit set up. Its fairly tame....on par with a quiet version Perrin exhaust.

 

I would say that if you were to use dual mufflers plus a mid muffler or a resonator from either vibrant or Borla, you would have no issues at all obtaining the noise level you are looking for.....especially if you have a cat too...

 

Its pretty tame on the highway also.

What is wrong with the stock cat axle back?

 

 

Too quiet?

Those do sound decent. Kinda what I'm hoping for one day.

See my link in post #7. He too has MagnaFlow cans, but he welded a borla in the center section and just see how it sounds.

 

 

I think i have a highflow cat already, but with a borla resonater it should be perfect. This is good stuff!

 

Thank you sir!

Thanks mate, because it is a daily and I transport a young baby I needed it to be quite. Checks all the boxes re: quite yet makes the power because it's all 3inch apart for the magnaflow 2.25in inlets.

Found one hitch, there is no 2.75" in out muffler, at least not from magnaflow or borla... but there is this often-talked-about big thing http://www.magnaflow.com/02product/shopexd.asp?zone=main&id=9074

What is wrong with the stock cat axle back?

 

By buddy here has a stock catback sitting in his garage... Might see if I can "borrow" it for a while after finishing the tune of my most definitely free-flowing catback.

This is the centre i got off bay, just find something similar:

 

http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&item=290418974464&ssPageName=STRK:MEWAX:IT#ht_814wt_907

I just sold my AutoSpeed CBE. Loved the sound quality, but I need this car as quiet as a stock sports car. So going to design a one-off custom exhaust.

 

Project Requirements:

 

• Straight Through Mufflers, no chambers or louvers.
  
• QUIET as possible, granted limited by straight through mufflers.
  
• Mindful of cost
  
• Preferably steel mesh over fiberglass packing, for longevity.
  

I'm looking at a 3" mid-section, splitting into two 2.25" lengths each feeding a 2.25" muffler.

 

Hoping for oval tips, but that's more of an afterthought.

 

Top brands considered:

 

• Borla
  
• Vibrant
  

Secondary brands considered:

 

• Magnaflow (don't love because fiberglass will blow out over time)
  

So far this is what I have worked out as some options:

 

In the 3" midpipe:

 

• Borla XR-1 Sportsman #40944 Oval 16" x 4 1/2" x 7 7/8" Center/Center $142 Shipped
  
• Borla Turbo #40359 Oval 14" x 4" x 9.5" Center/Offset $109 Shipped
  

For the 2.25" rear mufflers:

 

• Borla Turbo #40359 Oval 14" x 4 1/4" x 7 7/8" Center/Offset $145 Shipped Each (awfully expensive)
  

More to come later with cheaper vibrant mufflers, and perhaps some cheaper Borla options.

 

If you know anything about designing a quiet exhaust, please post!

 

Just wanted to throw in a quick mention for the round Magnaflow XL 3 style. They seem to be straight through but with a small chamber. Seem for work well as a balance between quiet and performance. Check this out, seems to be the MPE-13743:

 

http://my.is/forums/f88/turbo-tube-muffler-quest-no-drone-345428/index3.html

 

Initial perception is one of a little surprise. It is so quiet that it is almost stock sounding inside the car. Now when you push the gas pedal you do hear it unlike stock, and when I floor the car, yeah it is just as loud as a straight thru exhaust I have always had.

 

But overall, it sounds like a stock exhaust with a little extra grunt when I push the gas, then loud as hell when I floor it.

 

http://i19.tinypic.com/8c136sp.jpg

Thank you for the post!

 

I stopped posting in this thread after Rao asked what's wrong with the stock exhaust. The more I read about it, the more I find I don't know.

 

When switching to a straight-through exhaust:

Many people claim an improvement in responsiveness, which I interpret to be similar to switching to many aftermarket intakes. This could be explained by a lowering of exhaust backpressure, which may increase the difference between pre and post-turbo exhaust pressures, which would spool up the turbo quicker and with less input energy.

I saw a post on a modification like this allowing a tuner to add more timing, indicating it would allow for a higher knock threshold, which would allow for more power potential via tuning.

From my limited research, I feel comfortable assuming that the lower end of the value added would be a power increase.

There may not be anything wrong with the stock exhaust, if you're running the stock turbo.

 

The stock setup just doesn't flow well enough to support the higher rpm power of larger turbos, though.

There may not be anything wrong with the stock exhaust, if you're running the stock turbo.

 

The stock setup just doesn't flow well enough to support the higher rpm power of larger turbos, though.

 

Momswagon has a GT35r on his wagon with stock or STi exhaust, IIRC. Makes fine power. It is certainly not going to flow as well as a straight-through, but perhaps the pumping losses are not as severe as internet hype makes it sound. Also, once you have already built target boost, it becomes a function of the turbine map, which none of us know the shape of, for most turbos. That could either be a benefit or detriment, depending on the plot map we don't have any idea about. But holding that constant, there is only a pumping loss, which is probably far more due to ANY turbo than the stock exhaust, or so I speculate. Boost is being made, and thus power can be made. Theoretically...

 

On the other hand, Infamous1 did have my buddy w/ VF39 and FMIC get a straight-through exhaust because he said it was the main thing holding him back. I would like to know more...

I put the stock catback back on my car and immediately felt a difference. I lost something like 40 MAF g/s from the moderate flowing 2.5" HKS catback that my car was tuned with. Spool suffered as well.

 

I had a cutout welded before the cat on my Cobb DP and I dynoed it back to back.

 

http://i12.photobucket.com/albums/a204/TRSCobra/cutoutdyno.jpg

 

Although not exactly an apples-to-apples comparison to a free flowing exhaust, the open cutout freed up 46whp and 58wtq. Also, notice the decrease in spool time with the cutout open. I'd assume a catless, 3" straight through system would be resonably close to the performance as the open cutout.

I put the stock catback back on my car and immediately felt a difference. I lost something like 40 MAF g/s from the moderate flowing 2.5" HKS catback that my car was tuned with. Spool suffered as well.

 

I had a cutout welded before the cat on my Cobb DP and I dynoed it back to back.

 

http://i12.photobucket.com/albums/a204/TRSCobra/cutoutdyno.jpg

 

Although not exactly an apples-to-apples comparison to a free flowing exhaust, the open cutout freed up 46whp and 58wtq. Also, notice the decrease in spool time with the cutout open. I'd assume a catless, 3" straight through system would be resonably close to the performance as the open cutout.

 

Not doubting your results, but IMHO it's highly likely that the car made (much) more boost with the cutout open, if no other changes were made, accounting for the additional power.

 

I personally think catbacks make a minor but very noticeable difference in power and response in most cars.

No changes (tuning or otherwise) were made between runs. Boost target was 20 psi; the turbo hit 21 with the cutout open and struggled to even hit 19 with it closed.

 

Most of have heard the addage that an engine is an air pump. Flow more air, make more power.

No changes (tuning or otherwise) were made between runs. Boost target was 20 psi; the turbo hit 21 with the cutout open and struggled to even hit 19 with it closed.

 

Most of have heard the addage that an engine is an air pump. Flow more air, make more power.

 

Well that makes sense. Even an active boost control system has a limited range it can be effective in without adjustment. If you massively reduce back pressure without changing restrictor pills or enlarging the wastegate, it's pretty typical to see extra (over) boost.

 

Watch out for boost creep at cooler temps with the cutout.

 

I would be curious of the results at similar MAF levels or similar boost levels. That would give a clearer indication of actual power made from the reduced restriction.

TRS Cobra that is very interesting and thank you for having done the test and posting it! I realize it's foolish to ask what difference 19psi vs 21 psi gets you with the same setup back to back. No one want to waste time playing with WG tables all day to get a number... but do you have an educated estimate based on your tuning? If you're making 400+ on pump, your turbo should be just as capable of making 15psi as 30 psi. And timing would taper, further complicating things...