# *TECH DISCUSSION* Turbo intake piping size...when does it become a restriction?



## [email protected] (Oct 6, 2009)

Say you have a 60mm turbo with a 4" anti surge inlet...

But you only have space to run a 3" pipe.

How much air can a 3" diameter 24"long or 36" long pipe support?

I'd imagine so long as the pipe can support more CFM than the turbo is rated for you're good...right?:beer:


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## wabbitGTl (Jan 2, 2007)

opcorn:


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## Pat @ Pitt Soundworks (Nov 14, 2008)

I just follow the same as the expected limits for exhausts.

2" 300hp
2.5" 500hp
3" 700hp
3.5" 900hp

But in reality, as big as can possibly fit.

Think about it this way, the anti surge inlet is larger to accommodate the porting around the wheel. I would bet you could drop down to the size of the inducer and be ok for the full flow.

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## [email protected] (Oct 6, 2009)

Pat @ Pitt Soundworks said:


> I just follow the same as the expected limits for exhausts.
> 
> 2" 300hp
> 2.5" 500hp
> ...


so in reality it could be a little more due to the fact exhaust tends to be much longer than 3ft or less...unless you be one dem drraaaaaag cars:laugh:


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## Pat @ Pitt Soundworks (Nov 14, 2008)

Well also hot rod magazine proved even a 500whp supra saw a 50whp gain from going to a 3.5" from a 3" exhaust. And 100whp at the 700ish hp level. But you're right, length makes a difference.

I'd be really interested in seeing the mathematics behind it, though. 

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## [email protected] (Oct 6, 2009)

Pat @ Pitt Soundworks said:


> Well also hot rod magazine proved even a 500whp supra saw a 50whp gain from going to a 3.5" from a 3" exhaust. And 100whp at the 700ish hp level. But you're right, length makes a difference.
> 
> I'd be really interested in seeing the mathematics behind it, though.
> 
> Sent from my SCH-I535 using Tapatalk 2


i remember that issue. 

Yea i'd love to see the math as well and how turbos are affected by smaller intake paths...


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## leebro61 (Nov 7, 2001)

I would say to avoid going smaller than the inducer diameter is a good, practical rule of thumb. Practical because the compressor housings usually have a decent contraction upstream of the rotor inducer and this let's you drop down a pipe size or two for fitment concerns and still have some margin left over. For example, my to4s cover at 4" houses a 2.4" inducer 

The only penalty you will pay in going with a somewhat undersized pipe is that you will have higher pressure loss upstream of the compressor so the compressor will have to work harder to reach your target boost pressure. The downside to this of course is increased heat load on the intercooler, radiator, and additional backpressure on the exhaust (turbine has to work harder to turn the compressor). In terms of length vs. diameter; losses scale linearly with length and squared with diameter, so a longer pipe of a greater diameter is better than short and small diameter.

As far as the mathematics go, the compressor flow function (horizontal axis on compressor map) is dominated by the inducer relative inlet Mach number^2 and rotor passage area. This is why compressor maps are a strong function of what we call "corrected speed" and turbine maps are nearly independent of rotor speed (need to increase a/r to increase flow/drop back pressure, can't just spin it faster ).


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## cifdig (Jun 4, 2005)

Upon researching for my own behalf I came across this and would be a perfect add to this thread..

*.4 Mach is the point at which air becomes turbulent and losses in efficiency start to occur exponentially. The key is to stay under that speed. You want to use the smallest piping possible that still flows enough to meet your needs. Larger than necessary piping increases lag time with no measurable gain

The velocities are in miles per hour and mach, and the flow rates are in cfm. Measurements for the piping are in inches.


2" piping
1.57 x 2 = 3.14 sq in
300 cfm = 156 mph = 0.20 mach
400 cfm = 208 mph = 0.27 mach
500 cfm = 261 mph = 0.34 mach
585 cfm max = 304 mph = 0.40 mach


2.25" piping
3.9740625 sq in = 1.98703125 x 2
300 cfm = 123 mph = 0.16 mach
400 cfm = 164 mph = 0.21 mach
500 cfm = 205 mph = 0.26 mach
600 cfm = 247 mph = 0.32 mach
700 cfm = 288 mph = 0.37 mach
740 cfm max = 304 mph = 0.40 mach


2.5" piping
4.90625 sq in = 2.453125 x 2
300 cfm = 100 mph = 0.13 mach
400 cfm = 133 mph = 0.17 mach
500 cfm = 166 mph = 0.21 mach
600 cfm = 200 mph = 0.26 mach
700 cfm = 233 mph = 0.30 mach
800 cfm = 266 mph = 0.34 mach
900 cfm = 300 mph = 0.39 mach
913 cfm max = 304 mph = 0.40 mach


2.75" piping
5.9365625 sq in = 2.96828125 x 2
300 cfm = 82 mph = 0.10 mach
400 cfm = 110 mph = 0.14 mach
500 cfm = 137 mph = 0.17 mach
600 cfm = 165 mph = 0.21 mach
700 cfm = 192 mph = 0.25 mach
800 cfm = 220 mph = 0.28 mach
900 cfm = 248 mph = 0.32 mach
1000 cfm = 275 mph = 0.36 mach
1100 cfm max = 303 mph = 0.40 mach


3.0" piping
7.065 sq in = 3.5325 x 2
300 cfm = 69 mph = 0.09 mach
400 cfm = 92 mph = 0.12 mach
500 cfm = 115 mph = 0.15 mach
600 cfm = 138 mph = 0.18 mach
700 cfm = 162 mph = 0.21 mach
800 cfm = 185 mph = 0.24 mach
900 cfm = 208 mph = 0.27 mach
1000 cfm = 231 mph = 0.30 mach
1100 cfm = 254 cfm = 0.33 mach
1200 cfm = 277 mph = 0.36 mach
1300 cfm max= 301 mph = 0.39 mach


In order to convert from Lb/Min to CFM for the equation above, you take the flow rate in Lb/Min for your turbo (generally an educated guess based on the pressure ratio and power created) and multiply it by 14.27. That will yield the CFM flow for your setup.

For Example:
T3/T04e 57trim .63ar @ 21psi makes 452 whp
This turbo is known to have a 50lb/min compressor wheel which will make ~500bhp. Since we're using whp above, we can assume this turbo is pretty close to its max of 50lb/min.

Now to convert that to CFM, you take 50lb/min x 14.27 = 713.5 CFM. When you refer to the table above, you can see that we're starting to max 2.25" piping, but we're still in the "good" range for 2.5"

but it also depends on how smooth the piping is inside... and all the bends. this i would say is " perfect piping conditions" and if you would pick a number to upsize your piping at it would be when you hit about the .3 maximum .35 mach region.


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## Pat @ Pitt Soundworks (Nov 14, 2008)

For intakes that appears to work. I wonder how pressure affects this. There's a local evo that makes 500whp on a mustang and uses all 2" piping.

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## leebro61 (Nov 7, 2001)

Pat @ Pitt Soundworks said:


> For intakes that appears to work. I wonder how pressure affects this. There's a local evo that makes 500whp on a mustang and uses all 2" piping.
> 
> Sent from my SCH-I535 using Tapatalk 2


The calculations posted above are based on volume flow (CFM, not lbm/min) so the pressure does not have an impact. If you wanted to do the same calculations downstream of the compressor you would have to account for the fact that the speed of sound is proportional to temperature, so as the air temp goes up so does the speed of sound. You would also need a new conversion factor to get from CFM to lbm/min since the air density would also be different (this is where pressure comes in). Obviously post compressor where densities are higher you can flow more through a given pipe size, so 2" boost piping is less of a restriction than 2" intake piping (for similar length pipes).

One more thing... the "Mach 0.4" guideline doesn't really have much to do with turbulence. It's true that losses do increase proportional to Mach (or velocity) squared, but whether you draw the line at Mach 0.3, 0.4, 0.5 it's really only a judgment call. Keep in mind that the calculations above are using an "average" Mach number to compute CFM but in pipe flow the maximum air speeds are really ~twice the average (in a laminar flow, slightly less for turbulent) so even at the computed Mach 0.4 the centerline pipe velocity is significantly higher.


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## [email protected] (Oct 6, 2009)

good reading in here:beer:


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## skydaman (Dec 16, 2005)

Race cars dont have intakes or filters on their turbo's... let it eat! I dont have room for an intake because I kept functioning A/C plus there is no real need for one. I don't notice any downsides in spool time, horsepower level, or performance compared to those who run 3", 4" piping to a filter or even those who run a filter right on the turbo. I'm currently running a 59mm, no anti surge, 24-25lbs.


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## cifdig (Jun 4, 2005)

Not sure about intake but as far as charge pipe I think most tuners tend to over do it with size not realizing there losing velocity which equates To slower spool wether it be 700-300rpm. I chose 2 1/4 to aid a bit with reaction and to combat a bit of lag. Not so much in our scene I see it, but I've seen so many cars with 3" charge pipes pushing 250-350hp.. Over kill


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## 05JettaGLXVR6 (Jan 25, 2006)

skydaman said:


> Race cars dont have intakes or filters on their turbo's... let it eat! I dont have room for an intake because I kept functioning A/C plus there is no real need for one. I don't notice any downsides in spool time, horsepower level, or performance compared to those who run 3", 4" piping to a filter or even those who run a filter right on the turbo. I'm currently running a 59mm, no anti surge, 24-25lbs.


My car loses a few MPH intake (out the headlight) Vs. No intake


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## [email protected] (Oct 6, 2009)

05JettaGLXVR6 said:


> My car loses a few MPH intake (out the headlight) Vs. No intake


 maybe because removal of the headlight is causing alot of turbulence throughout the engine bay when moving? Or you have one dem fancy fully sealed deals with the bellmouth and all?

Pics?


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## 35i 2000 (Jan 27, 2005)

Pat @ Pitt Soundworks said:


> Well also *hot rod magazine proved even a 500whp supra saw a 50whp gain from going to a 3.5" from a 3" exhaust*. And 100whp at the 700ish hp level. But you're right, length makes a difference.
> 
> I'd be really interested in seeing the mathematics behind it, though.
> 
> Sent from my SCH-I535 using Tapatalk 2


that's really neat to learn! i feel like you always hear bigger is better


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## V-dubbulyuh (Nov 13, 2005)

35i 2000 said:


> that's really neat to learn! i feel like you always *hear *bigger is better


Isn't that what he's saying though?

To: 3.5"
From 3.0"
Net = performance gain


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