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Discussion Starter · #1 ·
I have been looking at several intake options and it just doesn't add up. Under full throttle, an intake manifold will have a constant 15psi of pressure from the turbocharger. Switching to a more free flowing intake setup I would imagine does not increase target boost, nor does it change intake temps by much as it is usually in the same location as the stock air box. How is there a consistent increase in power across the rev range (as advertised and shown on dyno sheets) if the turbocharger just compresses the air to a set amount of pressure regardless of intake resistance? Unless the stock intake is extremely restrictive I don't see how a different intake setup would have any benefit after the turbo is spooled up. I'm not doubting it I just don't think I understand.

241 Posts
I made a post about intakes and the fluid mechanic principles behind why they work on a Veloster Forum some time ago in a discussion about airflow.
I have pasted it below for your reference.

While my post below is still relatively simple, it still doesn't dive into VOLUMETRIC flow. This is where a majority of the power comes from.
For example, 15psi on a small turbo like ours and 15psi on a bigger Garrett turbo flow a different amount of air.
Pipes and filters we use allow more air in while the ECU maintains boost pressure.

Other factors include the reduction of the pressure ratio pre-turbo. The Y-axis of a turbo's compressor map is the intake system pretty much.


The ultimate goal is to keep the turbo in it's peak efficiency through all operating conditions. We must base efficiency off the turbo's Compressor Map. Does the Super Stock turbo have an established Compressor Map?
I have found the stock turbo's map for reference.

The Y-axis is the one we deal with when talking about the intake system (from air filter to turbo).
I think we should only discuss operation at WOT for the following discussion to keep things constant, and that is what were are ultimately concerned with anyway

The is the formula for Pressure Ratio below:
(source for the following info - https://turbobygarrett.com/turbobyga...pressure_ratio)

P2c / P1c

P2c = Compressor Discharge Pressure
P1c = Compressor Inlet Pressure

- P2c - Compressor Discharge pressure is your boost pressure you are running, measuring in psia (absolute psi measured). I would guess you will be doing north of 20 psig (gauge psi measured) which is approx 34.7 psia (20+14.7).
- P1c - Compressor Inlet pressure is just that, psia at the inlet of the turbo. Without using gauges to measure actual psia at the inlet, it's hard to tell what value to use here. This value is obviously based on the characteristics of the intake system. We all know the more restrictions you have, the more you have pressure drop, and that can affect the pressure ratio you are operating at. No intake is the best intake for turbo. This way we know what inlet pressure is for the most part, it's normal atmospheric pressure at your altitude.

Another member said this "regardless of intake (air) or exhaust, the desired principles of Laminar flow are to be applied."

I'd like to know how you came to this conclusion. Please cite links or quotes.

Are you saying that laminar air flow is the basis for making more power vs turbulent flow? Because I think that statement is a little too general and not quite the basis for making power, or what you ultimately want and that is the turbo being its most efficient.

I'm going to lay out how what you are saying is may be true, but this characteristic of laminar flow shouldn't be what you are chasing.

Same member said this: "Much reading on the topics thus far has yielded thew following general concepts in terms of accentuating positive laminar flow"

what do you mean by positive? a direction?

Laminar flow is a characteristic determined by a DIMENSIONLESS number called the Reynold's number. This number is calculated from the following equation:
(source: Reynolds Number)

Re = (p times u times L) / v


p = fluid density (in our case, air)
u = velocity based on pipe cross-sectional area
L = length of pipe
v = kinematic viscosity of the fluid (in our case, air)

Reynolds number has a range that determines Laminar flow, transient flow, or turbulent flow

Laminar - when Re < 2300
Transient - when 2300 < Re < 4000
Turbulent - when Re > 4000

If we want to control turbulence, we only have a couple options.

- The fluid density of air (p) is we can attempt to control with Cold Air intakes, or heat wrap insulation. Or we just accept it as it is if we are running the turbo with no intake lol
- We can control velocity (u) by adjusting pipe size
- We can control the length (L) obviously
- We can't control viscosity - that is pretty much a constant

Looking at the equation we can see what causes the Re to go turbulent that we are in direct control of:

- If we increase length L, Re goes up
- If we increase velocity u, Re goes up

We all know the rule of thumb don't make the intake system too long or have too much pipe. However we might sacrifice here for temperature decreases from a Cold Air intake.
A balance between temperature achieved by the cold air intake and pressure drop from the piping used must be achieved. Or, we have a really good intercooler to decrease temps and the intake side we don't care about :p

My argument here is we are optimizing pressure drop, but I'll get there in a minute

Well that brings us to the Darcy–Weisbach equation. This is the basis for head loss (or pressure loss) in pipes.

deltaP = pressure drop
lambda - friction factor (we all use nice smooth pipes, we can treat this as a constant) (This is also why the stock intake tube is so bad, it has all those ribs!!!)
L = Length of Pipe
D = Diameter of pipe
p = density (we will treat this as constant)
w = velocity

Analyzing the equation, we can confirm that:

- if we increase L, pressure loss also increases all other things constant.
- if we increase pipe diameter D, pressure loss decreases. If we apply the principles of a limit to D, as D approaches infinity > L/D = 0. By infinite D, I am saying a pipe of infinite diameter, or no pipe at all
- If L/D = 0, then deltaP = 0 and there is no pressure drop because there is no intake pipe.

Which brings me back to the beginning of my post.

Since we have 14.7 psia in the world around us and that is the pressure as it hits the air filter, then we'd like the pressure at the turbo to be as close to that as we can.

I don't have the time right now, but look at a couple different small turbo's compressor maps (that would be suitable for our engine) and:

- calculate the pressure ratio using our stock boost level at WOT at peak torque - which I can estimate is about 14-15 psig (or 28-29 psia)
- visualize a line across the map at that pressure ratio, I'm willing to bet that line is on the highest efficiency "islands" of the map for most of the graph


TL;DR is the best intake is no intake. That's dangerous though in our daily driver because we don't want FOD in the turbo :)

Again, this is very basic fluids stuff. I'd love to see some thoughts from a seasoned HVAC PE or OEM drivetrain engineer.
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