Exhaust size

Discussion in 'Generation 4 - General Discusson' started by Wsho, Dec 6, 2019.

  1. Wsho

    Wsho SHO Member Supporting Member

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    I know they say bigger is better but my question is what's the best pipe size for our exhaust system I will be making my own exhaust system at my job looking at polished stainless but what's the best size 2 1/4, 2 1/2, 3 in 3 1/4 Etc
     
  2. Johnbigdog

    Johnbigdog SHO Member

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    Quite a few posts that aftermarket exhausts (cat back) have minimal gains in power, so duplicate the same size and you should be good 2 go.
     
  3. Wsho

    Wsho SHO Member Supporting Member

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    Okay cool thanks
     
  4. Zpak

    Zpak Es Aich Oh!

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    I’ve been trying to find information and read up on that. Still don’t know or quite understand all the specific science behind it but as of now my understanding is especially with our engines you would actually lose exhaust scavenging/flow with over 2.5”. I’m also convinced an X pipe in place of the third cat would be the ideal setup for our application as to both equalize back pressure and increase scavenging. it’s actually the perfect spot for it as well.
     
  5. 802SHO

    802SHO FULL SEND

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    I tend to lean toward bigger is better. Jordan does well with no exhaust so it makes me think 3" would be totally fine. I think larger than 3" would give you fitment issues?

    I kept everything 2.5" to match my catless downpipes and I have double x pipes. I actually need to get this touched up. My friend kept 2" of the 3rd cat (2-1/4") to weld the first 2.5" x pipe to.... Photo144.jpg
     
  6. SM105K

    SM105K Streetlight Grand Prix Champ/ IG @egocheckersho

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    I see an X pipe in my future as well.
     
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  7. Zpak

    Zpak Es Aich Oh!

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    My plan post downpipes is flange to a X where the third cat now resides then separate straight through resonators on each pipe instead of two pipes into one and flange to the factory axle back.

    I like how yours, SM105K, was made with the ability to bolt the factory set up right back in.

    Then, cross my fingers as I fire it up and hope that it doesn’t sound like trash.
     
  8. SM105K

    SM105K Streetlight Grand Prix Champ/ IG @egocheckersho

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    Mine turned out really good, and having the ability to put the factory equipment back is place was a bonus.
     
  9. yaycandy

    yaycandy Cleetus Mac

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    I dont know, does mine have a Xpipe or are they just pinched together? Lol
    But i was able to fit my foot in the end pipe, i think its 4.5 inches on mine
    Obx catback cheap on ebay. Pipes still look new and fitment was great. livernois catless downpipes, not so cheap. I yelled alot and threw some tools when installing
    And yes they are “catless” i dont think the cat was included, it just appeared

    D1AF7752-44DE-44FC-9B56-0A19D5A8A93C.jpeg DD1A071A-3354-4348-8E5B-A63796457F9C.jpeg E4E92963-8336-4C27-BFCE-1F4F784B6DFB.jpeg
     
  10. Wsho

    Wsho SHO Member Supporting Member

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    Only if somebody lived near me that had a exhaust kit and downpipes I would bring it to my job and have it duplicated. Flowmaster doesn't make exhaust kits for our cars. They don't have a program specs for the bending of the pipes.
     
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  11. SilverSH0

    SilverSH0 SHO Member

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    What have you read that leads you to conclude larger pipe would reduce scavenging on our engines?

    Scavenging is when the low pressure pulse arrives at the collector and "pulls" the exhaust from another header tube. Too big of pipe causes the exhaust speed to slow and you reduce that effect. All works for NA motors. But when talking exhaust after a turbo, I don't see how it applies. The turbo adds lots of back pressure before the turbo and creates a fairly uniform exhaust flow after the turbo. I'm not seeing how a larger post turbo exhaust impacts scavenging on our engines.
     
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  12. Johnbigdog

    Johnbigdog SHO Member

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    It all depends on the turbo efficiency from my understanding. Scavenging can be a thing.
     
  13. SilverSH0

    SilverSH0 SHO Member

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    Right, but the comment was about specific to our engine. I was hoping to read what he has read on our specific engine
     
  14. Jordan_R

    Jordan_R Stock Block Ready To Knock...er..Rock

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    Aftermarket downpipes are 2.5" and my exhaust is full 2.5". Stock exhaust is 2.25" and stock downpipes are 1.875". Just what I use. I also take off the exhaust at the strip and run open 2.5" downpipes when try harding.
     
  15. Zpak

    Zpak Es Aich Oh!

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    I’m looking but can’t find the specific article I was reading at the moment. It was a long night and a deep rabbit hole bouncing from page to page. However, they were not testing a ecoboost, but it was a boosted V6. I want to say it was a EVO motor but I’m not sure, it was twin turbo nonetheless.

    Anyway, they concluded that obviously open downpipes made the most but once you put a full length exhaust system on it the 2.5” made more power then the 3”. Based on the principle of velocity. Like a hose when you choke it down it sprays out faster. But only to an extent because with a 2” it was the worst of all. The example was like with a absorption muffler flowing more than a straight pipe. While the pulses got hung up on the walls of the larger diameter pipe the increased velocity helped move it quicker with a 2.5”.

    But as I stated, I don’t fully understand all this and never claimed to. All I’m saying is that’s my current opinion based on the little research I’ve done. The problem is, as you eluded to, and are correct, all the science changes from NA engines to Boosted as you are murdered with back pressure.
     
  16. Johnbigdog

    Johnbigdog SHO Member

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    I'm sorry Silversho, I was stuck on the back pressure thing, and the exhaust not helping that on a turbo motor. I read your comment wrong.

    There is math that can be done to figure out exhaust stuff on N.A. engines, I dont know how it would afect forced inductions.
     
  17. Zpak

    Zpak Es Aich Oh!

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    I should have been more clear. “Specific to our engine” was referring to forced induction applications as opposed to naturally aspirated.

    I have learned that in NA cars the 500hp range is about the crossover between 2.5” and 3”. Unfortunately, most of the technical data with exhaust does not crossover between the two. Knowing that though and seeing the results of the other test is what led me to my assumption for appropriate exhaust diameter. It’s also possible that the outcome in that test was entirely engine specific. I don’t, know.
     
  18. yaycandy

    yaycandy Cleetus Mac

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    You dont want the turbos restricted at all right? So like turbo diesels and even turbo added cars. Bigger better for exhaust. It dosnt really need to scavange i guess as its just being pushed out to get more in as quickly as it can. But i dont know, just my thinking.
     
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  19. Johnbigdog

    Johnbigdog SHO Member

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    All depends on the set up. That comp came video has some good points.
     
  20. yaycandy

    yaycandy Cleetus Mac

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    So a cam that thumps is from the overlap. How is it effected with a ghost cam tune on our engines? Is there actual overlap with a cam tune from say...ajp turbo or ortiz? With it causing brown or black burn color in the intake manifold like the comp cam video for turbo with cam overlap?
     

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