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I seen a very good thread on hot side piping sizes. Im glad I read it. My question is now what about the cold side. Does size matter My? My old set up was 4" piping from pt 91.5 to intercooler. 4" from intercooler to throttle body. Now im going twin turbo with the same intercooler. It has 4" inlet and outlet. The piping off turbo will merge into the 4" inlet of intercooler. Im assuming 4" out to throttlebodyvwould be fine?
Same rules apply. Depends what you read and who's opinions you respect and trust the most! lol.
From what I read and believe... you size the cold side to the motor's "NA" needs. You don't need larger piping for boost. You are slowing velocity through the IC and adding volume to the system. It will contribute to the overall laziness of the setup if you go too large. Corky Bell had some good theory's. Or at least they were backed up with mathematical facts as to why he chose a specific diameter pipe, and not band wagon BS.
He suggested not to allow the air speed to go much over .5 mach or 450fps. Saying that at speeds above that you run into drag and disruptions in airflow.
Basically if you know your motor makes 400hp, thats Approx. 600cfm. Then you calculate the velocity in "X" diameter pipe at "Y" cfm. 600cfm flows around .at 294 FPS or .27 mach. So one can assume 2.5" piping can be run without unreasonable drag in a 400HP motor.
So IMO 4" is likely overkill... and then some. Depending on your motor's NA output anyway.
CFM is a poor measurement to use with intercooler piping because it has no correlation with pressure.
600 CFM N/A is good for ~400 HP.
600 CFM pressured at 30 PSI is enough for ~1200 HP.
You could make a million HP on 2" intercooler piping if its at enough pressure.
Boost pressure doesn't increase CFM in any linear manner, but it does increase CFM from increased velocity due to higher pressure differentials.
Also, the piping should be sized around your engines N/A needs. Boost takes care of the rest.
Well thanks guys I really appreciate it. Since I sm running e85 I may even ditch the intercooler in whole and just use meth. Alot less work for me anyway.
The way I understood it (which may be wrong!) is that the engine is just a pump. It displaces the same amount of mass na or boosted. Since Air is a compressible, the density of the charge is all that's really changed. You can't calculate velocity without CFM... Since excessive velocity is what will cause drag, seems like its needed?
The relationship between flow, velocity, and pressure can be explained using Bernoulli’s principle. Bernoulli’s equation states mathematically that if a fluid is flowing through a tube and the tube diameter decreases, then the velocity of the fluid increases, the pressure decreases, and the mass flow (and therefore volumetric flow) remains constant so long as the air density is constant. This principle holds true for gases so long as the gas is moving far below the speed of sound and does not vary in temperature (as this would cause small changes in volume).
It's what happens when the temp/density changes that's fuzzy to me.
Last edited by Forcefed86; Nov 23, 2020 at 04:20 PM.
evo guys claim faster spool when going to all 3" cold side vs 2 or 2.5 or whatever stock is.
ive always made the cold side as big as i can or is convenient. same as what i do post turbo.
Wonder what the demographic is on most Evo owners...
You don't want to go too small, but going to big does the opposite and slows "spool". Most 2 liters don't need 3" piping. I don't know of many that would just bump up to 3" piping and leave the factory IC. I'm betting they bump up to a massive IC and that most defiantly will make power improvements over the factory IC. Same deal though, basic physics. If there's more space to fill... it will take the turbo longer to fill it. With a manual car it would be more noticeable. Once the charge pipe/IC is "full" with an auto... Its not much of an issue in terms of response. But Id think IC efficiency would drop w/ larger piping and lower air speeds.
I don't see how an intercooler is any different than a fluid heat exchanger. The faster you can get the air through it (without hitting those overspeed mach numbers) the better I would think. Going to larger pipe slows the airflow at like cfm, mass flow, power.... whatever you want to call it.
ic piping with a lot of bends and length would have a lot of friction, so increasing the OD would help.
one thing about evo guys is they are nuts about spooling response, so i dont really question those trends. even a lot of the 5-600hp guys refuse to go away from the MHI turbine housing cause it spools 2-300rpm faster than the fp/tial one.
I won't pretend to understand all of the math involved. But i stand by my statement! Bends and piping length accounted for, most of those guys still do not need 3" piping. I had a long run with DSM toys as well, nothing against them. Just stating the majority of the owners aren't seasoned turbo folk or setting up their combos according to the math. Easily see by the chatter on DSM talk!
These would be great "Holdner Tests"! No changes other than piping diameter. Hot and cold side... Issue there is they can't measure response on an engine dyno.
im with you on not saying i know which is the right answer, just sharing some anecdotal data points for discussion sake. it does seem counter intuitive that larger cold side spools faster. maybe its because we are measuring boost at the intake manifold after all that piping/TB and not at the turbo before the piping?
Surprisingly, larger piping does spool faster up to a point, and makes more power.
I don't think you realize how negligible the piping diameter is in reality.
Lets say you have 2 sets of intercooler piping, 2.5" and 3", each about 6 feet in length.
The area of the 2.5" is 4.90 sq inch, the 3" is 7 sq inch.
The volume of the 2.5" at 6 feet would be 353.43 cubic inches.
The volume of the 3" at 6 feet would be 508.94 cubic inches.
A difference of 155.51 cubic inches, or a WHOPPING 0.0899 cubic feet.
Lets say your engine consumes a measly 600 CFM.
That means it would suck up the difference between that 2.5" and 3" piping in just 9 milliseconds.
This means the biggest difference in spool, is restriction/friction.
A 2.5" with a lot of bends is going to cause more resistance to flow.
The engine will be trying to consume more air than is available, basically trying to breathe through a smaller bendy straw.
Unless the engine gets the air it needs, its not going to spool up the turbo.
Let the engine get what it needs, without building that slight vacuum inside of the intake/piping at wide open.
i can tell you I have roughly 8ft of 3" tubing from compressor to intercooler with bends. And I see all of 2kpa pressure drop at 7500rpm and a 392" engine
The apect of a larger tubing helping especially near the throttle body is to slow down the air.... So there isn't as much velocity held entering the plenum, which can drastically affect distribution
The point is to give it what it needs to work efficiently. Not restrict or not go overboard.
I don't think its that easy to calculate either as 600CFM through a 2.5" pipe will move considerably faster than a 3" pipe. (Not like many LS guys are using 2.5" pipe either. More relevant would be 3" to 3.5" or 4")
And don't you have to calc surface area as well?
Using something like liters puts volume into a better perspective IMO
6' of 2.5 = 5.8L
6' of 3" = 8.3L
So a 2.5 liter difference volume wise. (then tack on some of those huge 15 +liter intercooler cores if we re talking DSM stuff)
Then you factor in the velocity.
600CFM through 2.5" piping 293 FPS
600CFM through 3" piping 203 FPS
Then more LS related... Guys thinking then need 4" (or more) piping for their boosted LS making 400hp NA.
600CFM through 3" piping 203 FPS
600CFM through 4" piping 114 FPS
600CFM through 4.5" piping 90 FPS.
4" @ 14.8L VS 3" @ 8.3L
6.5L difference.
Again I don't' claim to know any answers. Just don't think there is a super simple answer.
Last edited by Forcefed86; Nov 24, 2020 at 03:32 PM.
The point is to give it what it needs to work efficiently. Not restrict or not go overboard.
I don't think its that easy to calculate either as 600CFM through a 2.5" pipe will move considerably faster than a 3" pipe. (Not like many LS guys are using 2.5" pipe either. More relevant would be 3" to 3.5" or 4")
And don't you have to calc surface area as well?
Using something like liters puts volume into a better perspective IMO
6' of 2.5 = 5.8L
6' of 3" = 8.3L
So a 2.5 liter difference volume wise. (then tack on some of those huge 15 +liter intercooler cores if we re talking DSM stuff)
Then you factor in the velocity.
600CFM through 2.5" piping 293 FPS
600CFM through 3" piping 203 FPS
Then more LS related... Guys thinking then need 4" (or more) piping for their boosted LS making 400hp NA.
600CFM through 3" piping 203 FPS
600CFM through 4" piping 114 FPS
600CFM through 4.5" piping 90 FPS.
4" @ 14.8L VS 3" @ 8.3L
6.5L difference.
Again I don't' claim to know any answers. Just don't think there is a super simple answer.
The point is to give it what it needs to work efficiently. Not restrict or not go overboard.
I don't think its that easy to calculate either as 600CFM through a 2.5" pipe will move considerably faster than a 3" pipe. (Not like many LS guys are using 2.5" pipe either. More relevant would be 3" to 3.5" or 4")
And don't you have to calc surface area as well?
Using something like liters puts volume into a better perspective IMO
6' of 2.5 = 5.8L
6' of 3" = 8.3L
So a 2.5 liter difference volume wise. (then tack on some of those huge 15 +liter intercooler cores if we re talking DSM stuff)
Then you factor in the velocity.
600CFM through 2.5" piping 293 FPS
600CFM through 3" piping 203 FPS
Then more LS related... Guys thinking then need 4" (or more) piping for their boosted LS making 400hp NA.
600CFM through 3" piping 203 FPS
600CFM through 4" piping 114 FPS
600CFM through 4.5" piping 90 FPS.
4" @ 14.8L VS 3" @ 8.3L
6.5L difference.
Again I don't' claim to know any answers. Just don't think there is a super simple answer.
Calculating surface area only favors the 3" piping, as the ratio of free-flowing to friction-flowing air is increased.
The smaller piping might have less surface area, but it has significantly more surface area in comparison to its flow area, meaning more of the mass of air inside will be dragging the walls.
Velocity also creates resistance, as the amount of force required to move X amount of air through Y Diameter tube at Z velocity also increases as Y decreases, or X or Z increase.
You get into both pumping losses and frictional loss.
I also noticed almost all of this here is in relation to max power situations.
We're talking spool. For maximum spool, you want the least amount of resistance on both the intake and exhaust. Restricting the intake isn't going to make the turbo spin faster.
The theory that larger volume intercooler piping will slow spool because it will take longer to fill is also a huge misconception. We're talking milliseconds difference, as I've already done the math for.
I don't think your millisecond math is sound, as I've personally gone form 2" (what ever stock is, 1.75" maybe?) to 3" with no other changes in my old DSM days and felt the difference in "spool". The engine was defiantly less responsive, it was very noticeable.
How much of a friction/pumping loss is the question... How do you calculate it? Using say 600CFM. According to Corky Bell if .4 mach and below aren't showing any noticeable friction/pumping losses, what is the point of adding volume to the system if it isn't necessary? Again, its not adding restriction, its sizing the piping to the motors needs without going overboard.
If V8's with poor high airspeed manifold distribution perform better with lower air speeds due to distribution... maybe bigger is the way to go in our situation. Also since the charge air isn't being recirculated, maybe the IC is more efficient at pulling heat with lower airspeeds?
I don't think your millisecond math is sound, as I've personally gone form 2" (what ever stock is, 1.75" maybe?) to 3" with no other changes in my old DSM days and felt the difference in "spool". The engine was defiantly less responsive, it was very noticeable.
How much of a friction/pumping loss is the question... How do you calculate it? Using say 600CFM. According to Corky Bell if .4 mach and below aren't showing any noticeable friction/pumping losses, what is the point of adding volume to the system if it isn't necessary? Again, its not adding restriction, its sizing the piping to the motors needs without going overboard.
If V8's with poor high airspeed manifold distribution perform better with lower air speeds due to distribution... maybe bigger is the way to go in our situation. Also since the charge air isn't being recirculated, maybe the IC is more efficient at pulling heat with lower airspeeds?
You're more than welcome to check the math for yourself (please do).
Calculate the difference in cubic inches of the 2 different diameter piping setups, convert it to cubic feet, and use a standard number like 600 cubic feet/minute to calculate the amount of time it would take for the engine to consume that difference.
Friction and pumping losses don't need to be calculated. Its obvious that smaller piping has more of them. Their relevance would need to be calculated.
The difference in friction between the 2 would be a ratio that correlates with surface area and cross-sectional area.
Friction wouldn't have much effect on spool. Max power yes, but not spool.
Pumping losses, or their effect, could be measured with a map sensor in the piping that maps out curves in relation to both time and rpm to compare the two.
The velocity of air in the intercooler is a tricky one to deal with, The inlet/outlet size would determine entry velocity, but the core itself will likely slow down the air as it would have a much larger flow area.
Keep the radius of the bends 1.5xs the diameter of the tubing (for example 3” tune needs at least 4.5” radius) That’s fact....
I took off the bullshit silicone 90 at the throttle body and picked up 2psi all else being equal. Data logs to prove it! Went from this
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Coldside piping and velocities 
