# Straight pipe exhaust



## ThoseHubcaps (May 22, 2017)

Hi to all that read this post, just had a quick question about any negative side effects I would experience (other than potential noise complaints) of running a straight pipe out of the cat on my Cruze. It's the 1.4L Turbo, obviously but some friends of mine say that I might experience back pressure issues, and others say it wont do a thing. I would consider getting an actual exhaust put on in the future, but for the price currently, its my first option. Thanks!


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## EricSmit (Dec 21, 2016)

You'll need a tune to run a straight pipe without a CEL; unless you mean behind the second cat, in which case go for it. It's not really loud either.


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## ThoseHubcaps (May 22, 2017)

Yeah thats what I meant, running it out after the second cat


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## EricSmit (Dec 21, 2016)

You don't need a tune for that.


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## cruzeguy2016 (Apr 24, 2016)

I have to agree with EricSmit. Anything post cat doesn't require a tune or any other modifications to function. You're just getting a little less back pressure at the end of the system and maybe some more sound.


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## Taxman (Aug 10, 2017)

It seems like a way to make the car lighter while letting the engine breathe a bit better, although the factory rear muffler seems adequately large. 

Questions for those who've done this (@EricSmit):

What were the results:
Noise from the driver's seat?
MPG gains?
Response/feel?
Measured performance gain?


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## Blasirl (Mar 31, 2015)

Anything to reduce back pressure for a turbo is good, whereas on a NA vehicle, you need some for scavenging.


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## Robby (Mar 1, 2013)

Kind of hard to describe but I'll try.

The actual exaust outlet, post turbo, is about the diameter of a half dollar coin. 
All exaust components are significantly larger than the outlet and the cats, by virtue of the internal volume, contribute nothing to back pressure.

So, if there is any source of back pressure it is the actual turbo outlet......nothing beyond that point contributes.......and that backpressure point is only at WOT near the engines redline.

So, beyond noise and removing maybe 20lbs. of mass, I see nothing that would change overall operation.

Rob


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## 5banger (Dec 9, 2016)

Robby said:


> Kind of hard to describe but I'll try.
> 
> The actual exaust outlet, post turbo, is about the diameter of a half dollar coin.
> All exaust components are significantly larger than the outlet and the cats, by virtue of the internal volume, contribute nothing to back pressure.
> ...


Have you any data? If the engine can breath more air by ways of freeing up the exhaust flow, the engine can make more power if tuned properly. The tiny turbo produces high egt's and if you remove flow restrictions post tubo, egt's would decrease thus making the turbo more efficient and thus make more power. I don't see why so many people on forums share bad information most likely because of some ridiculous agenda.


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## 5banger (Dec 9, 2016)

So there are zero negative performance side effects to running a straight pipe after the turbo before or after the cats.


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## Robby (Mar 1, 2013)

5banger said:


> Have you any data? If the engine can breath more air by ways of freeing up the exhaust flow, the engine can make more power if tuned properly. The tiny turbo produces high egt's and if you remove flow restrictions post tubo, egt's would decrease thus making the turbo more efficient and thus make more power. I don't see why so many people on forums share bad information most likely because of some ridiculous agenda.


Don't need data to see a small outlet leading to a large outlet.
The outlet size is satisfactory for the maximum amount of exaust flow that occurs at maximum WOT rpm that the factory tuning allows.

No agenda on my part.....just answering a question.
In so many words, I said you can do whatever you want, post outlet, without concern, but not to anticipate any measurable performance change, on your stock configuration.

Rob


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## 5banger (Dec 9, 2016)

Flow is hurt when the tube I.D. increases, has a tight bend, goes into a catalytic converter, another tight bend, then abruptly necks down to 2". This is why bnr makes a catless downpipe from 2.5" mandrel bent tubing. The stock downpipe has terrible flow characteristics even with the cat gutted.


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## Taxman (Aug 10, 2017)

5banger said:


> Have you any data? If the engine can breath more air by ways of freeing up the exhaust flow, the engine can make more power if tuned properly. The tiny turbo produces high egt's and if you remove flow restrictions post tubo, egt's would decrease thus making the turbo more efficient and thus make more power. I don't see why so many people on forums share bad information most likely because of some ridiculous agenda.


Robby is saying that removing a minor flow restriction downstream of a major flow restriction will not make a perceivable difference, which is entirely plausible.

From what I've read elsewhere on CruzeTalk, the computer is programmed to make x amount of torque, and if you manage to improve airflow to the point that the engine makes more torque than it expects, it'll simply detune until it's running 'normally' again. 

If Robby is wrong and the computer controls work as I described them, a free flowing catback system will make a few more HP for an hour or two (and maybe a little more mpg forever).


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## Jascnscott322 (Jan 25, 2016)

5banger said:


> Flow is hurt when the tube I.D. increases, has a tight bend, goes into a catalytic converter, another tight bend, then abruptly necks down to 2". This is why bnr makes a catless downpipe from 2.5" mandrel bent tubing. The stock downpipe has terrible flow characteristics even with the cat gutted.



I just gutted my cat and thenb inning 2.5 all they way babbyy


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