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Thread: Let's talk pipe size

  1. #1
    turbo addict
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    Let's talk pipe size

    This came up in conversation with a fellow turbo car guy at work, so I thought I'd pass along the conversation and get some of your input.

    Basically, let's say you have a dead stock turbo 2.... Say the inlet to the turbo is 2 1/4 inches, so let's start there. Is there any reason to increase the size of that pipe from the air filter? I doubt there is a downside to having a 4 inch pipe, but fitment would be an issue. Are there gains to be had with a 2 1/2 or 3 inch pipe? Considering the actual inlet of the turbo compressor is under 2 inches.

    From there, the discharge from the turbo is maybe 1 1/2 inches, maybe 1 3/4 inches? From there, the factory pipes enlarge and reduce until it gets to the intercooler inlet that's maybe 2 1/4 inches. Is there any reason to go larger than the intercooler inlet diameter?

    Assuming the intercooler outlet is the same 2 1/4 inches, and the throttle body is the same size, is there a reason to have a pipe with larger diameter?

    When things are modified, the question gets more complicated, My personal feeling is the factory has price restrictions to work within, but we don't, so is there gains to be had in pipe sizing?

  2. #2
    Hybrid booster
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    Re: Let's talk pipe size

    I can tell you on a VR4. the stock turbo inlet pipes are no smaller than the diamter of the turbo inlet at any point.
    turbo inlet is 1-/34 inch

    I changed from stock turbo inlet pipes to some 2.5 inch exhaust pipes and filters.

    I went from 298 HP to 319.

    https://www.3si.org/forum/f1/diy-pre...owthru-587530/

    no other changes. just pre turbo pipes.

  3. #3
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    Re: Let's talk pipe size

    Quite an increase! How much larger were the new filters? Same dyno too?

  4. #4
    Hybrid booster
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    Re: Let's talk pipe size

    https://www.youtube.com/watch?v=XYaxk50h1DA

    week apart. same dyno. correction factory got it to 319.

    it was a single big filter through a single intake tube. then 2 filters. maybe 4x7 inches. and single pipes for each turbo

  5. #5
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    Re: Let's talk pipe size

    going from the factory T2 airbox, with a T1 smooth hose to the turbo to a 3" mandrel bent pipe that at the time was sucking hot underhood air gained me a consistent 1mph at the track which equates to 10hp. I then built a cold airbox around the filter but didn't get back to the track with it. So yes a bigger pipe makes a difference as the turbo has to work less to get the air. This allows slower shaft speeds and the WG opening sooner and more for less back pressure in the motor. So you gain power on both the intake and exhaust side of things.

    A look at my daughters Caliber SRT4 shows that even with a turbo smaller than our Mitsu's that going bigger on the turbo to intercooler piping helps. As the outlet on one of our mitsu's is tiny the SRT4 funnels out into what would be a 2.5" diameter pipe just looking at it. And some of the other owners run a 4" inlet pipe though I think that's a bit overkill for the turbo as a 3" or so would be more than enough but I don't want to lose the fact that the factory has a cold ram air box.

    Can you go too big? Yes for a point of diminishing returns. But you want the turbo to be able to grab as much air as possible, and then have someplace for it to go. 2.5" ID pipe on a slightly modded T2 isn't overkill between the turbo and throttle body. For over 300hp I'd go 3" with 4" to the turbo.

  6. #6
    Supporting Member II Turbo Mopar Contributor
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    Re: Let's talk pipe size

    I would expect that larger post turbo tubing while leading to more HP due to reduced delta P (pressure drop), will also show increased spool up time on the Turbo due to increased volume
    Regards,
    Miles

    DD '87 Sundance T1, 12PSI, SLH with rear disks
    '87 CSX #432

  7. #7
    Supporting Member Turbo Mopar Contributor A.J.'s Avatar
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    Re: Let's talk pipe size

    The factory air box/air filter is pretty small. Being in AZ I wanted to continue pulling air through the SMEC (when I still used a SMEC) so I went with a Ford V8 enclosed air filter. Last time I went to the dyno with my 2.4 in my van I made 395 horsepower. My dyno guy was curious as to how much of a restriction my air filter was so we took it off. I gain 4 hp. Not enough to risk a turbo so I leave it on. Just so you have an idea how big your air filter needs to be around the 400 hp mark. Start at post 50: http://www.turbo-mopar.com/forums/sh...triction/page3

  8. #8
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    Re: Let's talk pipe size

    This is all good info. So maybe a 3 inch pipe from air filter to turbo inlet with a reducer right at the compressor is beneficial

    How about having charge pipes larger than your intercooler inlet - outlet and throttle body? Say the outside of a 58mm throttle body fits a 2 1/2 inch coupler. Has anyone run a larger pipe and seen gains?

  9. #9
    Supporting Member Turbo Mopar Contributor iTurbo's Avatar
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    Re: Let's talk pipe size

    On my old black/tan GLH Turbo, I had modified it with a very large Isuzu NPR intercooler. It had 2.75" I/O, so I got 2.75" mandrel bent aluminum pipe and used silicone reducers at the throttle body (2.75" - 2.25") and turbo discharge (2.75 - 2.00"). It must have used at least 4 feet of pipe just for the intercooler. For the intake, I used 3" mandrel bent aluminum with a (3.00" - 2.50") silicone transition.

    I never had it to the track, but it spooled up faster than ever with a stock TII turbocharger. I suspect the large pipe slowed the velocity down and probably increased cooling (more time in IC), but it still pressurized the whole thing near instantly even though there was way more volume between the turbo and TB.

    I did have it on the dyno once and it made 198 WHP/270 WTQ with 14psi. The dyno operator (Brent at PFI) was laughing when the boost would hit hard down low.

    Attachment 63788
    87 Shelby Lancer #127
    91 Dodge Spirit R/T
    08 Dodge Caliber SRT-4
    86 Dodge Omni GLH Turbo

  10. #10
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    Re: Let's talk pipe size

    Quote Originally Posted by iTurbo View Post
    On my old black/tan GLH Turbo, I had modified it with a very large Isuzu NPR intercooler. It had 2.75" I/O, so I got 2.75" mandrel bent aluminum pipe and used silicone reducers at the throttle body (2.75" - 2.25") and turbo discharge (2.75 - 2.00"). It must have used at least 4 feet of pipe just for the intercooler. For the intake, I used 3" mandrel bent aluminum with a (3.00" - 2.50") silicone transition.

    I never had it to the track, but it spooled up faster than ever with a stock TII turbocharger. I suspect the large pipe slowed the velocity down and probably increased cooling (more time in IC), but it still pressurized the whole thing near instantly even though there was way more volume between the turbo and TB.

    I did have it on the dyno once and it made 198 WHP/270 WTQ with 14psi. The dyno operator (Brent at PFI) was laughing when the boost would hit hard down low.

    Attachment 63788
    Is than on an otherwise stock t2 engine?

  11. #11
    Supporting Member Turbo Mopar Contributor iTurbo's Avatar
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    Re: Let's talk pipe size

    Quote Originally Posted by shadow88 View Post
    Is than on an otherwise stock t2 engine?
    Actually, it was a junkyard '89 2.5L TI from a Caravan with about 180k miles. I only put new gaskets and seals in it, but didn't touch the rotating assembly. I did have to get a remanufactured cylinder head because that and the cam were wasted. Manifolds and TB untouched (from a TII Maserati TC), but it did have a stock TII turbocharger and full 3" exhaust and grainger valve boost control.
    87 Shelby Lancer #127
    91 Dodge Spirit R/T
    08 Dodge Caliber SRT-4
    86 Dodge Omni GLH Turbo

  12. #12
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    Re: Let's talk pipe size

    Intake, charge pipes, intercooler upgrade and 3 inch exhaust on 14 psi? What size is the maserati throttle body?

  13. #13
    Supporting Member Turbo Mopar Contributor iTurbo's Avatar
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    Re: Let's talk pipe size

    Quote Originally Posted by shadow88 View Post
    Intake, charge pipes, intercooler upgrade and 3 inch exhaust on 14 psi? What size is the maserati throttle body?
    It was a junkyard TII Maserati TC top end. Just a stock 48mm TB. Even the 16v ones only have a 48mm.

    I was very lucky not to hit overboost on the dyno. It still had factory overboost protection (no adjustable zener, which I did later to get to 17 psi).
    87 Shelby Lancer #127
    91 Dodge Spirit R/T
    08 Dodge Caliber SRT-4
    86 Dodge Omni GLH Turbo

  14. #14
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    Re: Let's talk pipe size

    I ran a 09+ ram 1500 hemi intake on my omni. It was 3.5 inches. Worked real well.

  15. #15
    Supporting Member II Turbo Mopar Contributor tryingbe's Avatar
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    Re: Let's talk pipe size

    3 inch filter to 3 inch pipe to 3 inch compressor housing, out 2 inch compressor outlet, to 2 1/4 inch pipe that goes to 2 1/2 inch, then to 2 1/2 inch intercooler, then back to 60mm pipe and hoses all the way to the 52mm throttle body.
    Made 367whp at 26psi on E52.



    Maybe I do need a bigger throttle body.

  16. #16
    Supporting Member II Turbo Mopar Contributor
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    Re: Let's talk pipe size

    Before the turbo you want as little restriction as possible while maintaining smooth flow. Larger pipe helps (to a point as has been mentioned). The smooth, cool flow will yield more power than turbulent hot flow.

    Post turbo the theory from physics is that you should avoid size transitions is because each transition will reduce efficiency. My personal "recipe" is a pipe size that matches the turbo outlet to the intercooler. After the intercooler either the size of the intercooler outlet (usually about the same as the throttle body), or one pipe size larger if it can fit.

  17. #17
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    Re: Let's talk pipe size

    Quote Originally Posted by thedon809 View Post
    I ran a 09+ ram 1500 hemi intake on my omni. It was 3.5 inches. Worked real well.
    What was there before? Any other changes? Any guess on power gains?

  18. #18
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    Re: Let's talk pipe size

    Quote Originally Posted by Reaper1 View Post
    Before the turbo you want as little restriction as possible while maintaining smooth flow. Larger pipe helps (to a point as has been mentioned). The smooth, cool flow will yield more power than turbulent hot flow.

    Post turbo the theory from physics is that you should avoid size transitions is because each transition will reduce efficiency. My personal "recipe" is a pipe size that matches the turbo outlet to the intercooler. After the intercooler either the size of the intercooler outlet (usually about the same as the throttle body), or one pipe size larger if it can fit.
    Let's say you're using an air filter minder type of device. http://filterminder.com/ If it indicates no restriction, could there be gains to a larger pipe? Obviously cold air and smooth transitions are preferred.

    Maybe I'm reading it wrong, but your second paragraph says to me that transitions will reduce efficiency, yet suggest one size larger than throttle body which would require a reducer.

  19. #19
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    Re: Let's talk pipe size

    Quote Originally Posted by iTurbo View Post

    I did have it on the dyno once and it made 198 WHP/270 WTQ with 14psi. The dyno operator (Brent at PFI) was laughing when the boost would hit hard down low.

    Attachment 63788
    very close #'s to my 2.5 that was in my daytona, 2.5 tucked under 87 T2 top end, ported exhaust manny, S60 turbo, 3" straight pipe side exit from the turbine wheel back. 2.5" intercooler tubing through a cummins i/c and a 3" air filter to turbo pipe. 12psi on a grainger valve.

  20. #20
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    Re: Let's talk pipe size

    Quote Originally Posted by shadow88 View Post
    What was there before? Any other changes? Any guess on power gains?
    Just had a cone filter on the turbo before this. No clue on power gains. I switched a lot of things at the same time.

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