You have to look at the turbo, intercooler, piping, and throttle body as a system. One part affects the efficiency of the others…you have to look at the CFM flowed by the turbo, the capacity and pressure drop of the IC, and the diameter of the IC piping and TB.
Piping size has a huge impact on flow...the CFM flowed through the pipes is what gives you the power out of a turbo. It has two big components - velocity & volume (the CFM). You can look at boost psi as "rate"...it will increase flow, but only to the point where velocity becomes critical...that is 300 ft/sec. Flow also becomes more turbulent above this velocity and hinders flow…the measurement for this is the Reynolds Number...the higher the value, the more turbulent the flow. In a perfect world, you want air flow to overcome inertia and produce laminar flow...in our IC piping, that is very difficult (if not impossible) to do. The best we can do is keep the Reynolds Number to a minimum.
To get the max efficiency, velocity needs to be close to this “magic number”. Above it, you will get a compounding increase (it's not linear) in pressure drop and will reach a point pretty quickly where you're achieving no gain. The same effect occurs with the Reynolds Number…air flow will become much more turbulent. The combination of both creates a sort of “back pressure” in the system negating any further gains. Below critical velocity, the turbo will take more time to fill the volume of the piping and you will get lag. How much lag is going to depend on the volume produced by the turbo.
The volume flowed is determined by the most restrictive pipe diameter in the system. Below critical velocity, increases in the CFM produced through a set pipe diameter by the turbo will show as large gains (both felt and dyno). Above critical velocity the motor will “run out of breath”, especially at high rpm. The most restrictive diameter in the system will cause this…it’s a pressure drop combined with turbulent flow. Mike Mallory experienced this due to using a CarTech IC with 2” inlet/outlets with IC 2.5” piping and a stock TB:
http://www.supramania.com/forums/showthread.php?t=31165
To see what I’m talking about, here’s a neat little calculator for determining mean gas flow inside a pipe:
http://not2fast.com/gasflow/velocity.shtml
We need to make a couple of assumptions:
Turbo Flow – 930 CFM (Boss 67mm compressor @ 75 lbs/min)
OAT – 75 deg
IC flow is sized to turbo flow
Compare 2.5” dia to 3.0” dia pipe.
For the 2.5” pipe…Mean Velocity: 454.6 ft/sec…Pressure Drop: 1.71 psig…Reynolds Number: 588857
Clearly less than optimum for this turbo.
For the 3.0” pipe…Mean Velocity: 315.7 ft/sec…Pressure Drop: 0.82 psig…Reynolds Number: 490714
Much better…note the pressure drop is cut to less than half.
Also of interest is the effect a stock TB has using this turbo…its diameter is 2.36”:
Mean Velocity: 510.1 ft/sec…Pressure Drop: 2.15 psig…Reynolds Number: 623979
Note the pressure drop is 2 1/2 times that of the 3” pipe. I point this out to illustrate the restrictive effect a sub-sized TB can have…it’s significant. If you were to substitute a 3.5” TB the Mean Velocity drops to 231.9 ft/sec, the Pressure Drop is 0.44 psig, and the Reynolds Number is 420612. Since the TB is so short, using a larger diameter TB will have little or no effect on lag. This is what I meant when I said using the larger throttle body is a better choice above.
Now, I realize this appears like I’m making a case for using larger IC piping…for this 67mm turbo I am. Keep in mind this is one of the larger turbos available for the 7M…lets run the numbers for a turbo that flows 600 CFM:
For the 2.5” pipe…Mean Velocity: 293.3 ft/sec…Pressure Drop: 0.71 psig…Reynolds Number: 379908
Kinda looks the same as the above turbo flowing 930 CFM on 3.0” piping
For the 3.0” pipe…Mean Velocity: 203.7 ft/sec…Pressure Drop: 0.34 psig…Reynolds Number: 316590
Hmmm…nearly a 1/3 drop in velocity. Think that might cause a little lag getting the boost to the motor?
One other thing about all this: The IC itself can cause a restriction. In Mike’s case above, it was the small inlet/outlet…the flow capacity of the IC is also a big factor. Too small an IC for the CFM produced by the turbo, same restrictive effect. Too large and you will get lag.
Also I want to point out theory can be quite different than reality. This little analysis does not take into account bends in the piping, the interior surface RA, or little leaks in the system. All these (and more) can impact performance. It does give you a very good place to start though and illustrates that bigger piping is not always better. However, in the case of the throttle body, it is better to err on a larger size.
