A Technical Discussion of Intakes and Turbocharging

I see a good amount of intake discussions popping up on forums and wanted to do a small write-up about what to look for when choosing an intake and some turbocharger and airflow theory.

The rule of thumb with intakes is that the larger the diameter, the shorter the intake and the fewer the turns the better in terms of what the turbo “likes” (less losses = more power).  Let’s elaborate on this a little bit.

Turbocharger compressors (the silver cold side of the turbo) operate at what is called a pressure ratio. What this means is that the compressor increases the pressure of the air from the Inlet (from intake) to the Outlet (to intercooler) of the compressor when they are spun up by the exhaust gases (spooled up). This gives us BOOST!

Boost gives the engine more airflow than it can draw through itself (essentially increasing the volumetric efficiency) allowing it to make more power. The amount of airflow needed is defined by the size and volumetric efficiency of the engine and the power requirements. We’ll go into more detail on this in another write-up.

Turbocharger compressor performance is illustrated by a compressor map. A sample compressor map is shown below with the features highlighted. You can read more about compressor maps in this excellent series by Garrett/Honeywell:

Now, the size of the compressor and turbo really depends on application. Generally, we want very little lag and decent top end so the turbo we choose (and the stock one in most cars especially) is driven very hard. In such a case, the lower this compressor pressure ratio, the more efficiently the turbo works and the less “work” the exhaust has to put into the turbine to make it spin.

This reduces pumping losses (increases volumetric efficiency), reduces heat, and allows denser air to reach the motor while increasing the knock threshold. All good things and this applies to ANY turbo unless it is grossly oversized for the engine in which case there are bigger problems to address.

How is this related to intake and boost we measure at the manifold?

You can imagine air follows this path into the motor:

Air Filter → Intake Tubing/MAF Sensor → Compressor Inlet → Compressor Outlet → Hotside Tubing → Intercooler → Cold Side Tubine → Throttle Body → Intake Manifold → Intake Valves/Head → Exhaust Valves/Head → Turbo Turbine → Downpipe → Rest of exhaust.

The image below shows this using an illustration.

The compressor map shows you how much a turbo flows at a given pressure ratio (pressure difference between compressor inlet and outlet) at a standard temperature.

The airflow is on the horizontal (x axis) and the pressure ratio is on the vertical (y axis). As the RPMs increase the motor requires more air to produce more power. For simplicity, let’s say we are running 20 psi of boost at the manifold. You can plot out where the motor operates on the compressor map based on how much air it consumes at that 20 psi with that particular turbo (this case a K03). This looks something like this when the engine is doing a pull in a single gear.

Notice how in this case the pressure ratio that the turbo is operating at is around 2.8 on the vertical axis. At sea level this translates into 26 psi of boost pressure the compressor must generate. But I just said that the car is operating at 20 psi of boost at the manifold right? How can this be?

When you draw air from outside, you are drawing air in at atmospheric pressure. However when it gets pulled through the filter and air tubing, the pressure actually drops BELOW atmospheric. This is due to something called the Bernoulli Principle.

The turbo then pushes air through to the motor but across all the other intake tubing and intercooler you lose more pressure due to the same principle until it reaches the manifold.

When you measure boost at the intake manifold you are measuring it against the atmospheric pressure (called Gauge Pressure) but your compressor is actually operating at a higher pressure ratio/boost level.

Let’s use an example:
– Say the manifold pressure is 20 psi as measured by the ECU or a boost gauge.
– The pressure drop from the intercooler and the piping AFTER the turbo is 4 psi (for example).
– The pressure drop from the intake and filter BEFORE the turbo is another 2 psi (again an example).

That means that the compressor is actually pressurizing air to 26 psi to get you 20 psi of manifold pressure. The more you reduce this pressure drop in front of and after the turbo, the slower you need to spin the turbo, the more efficiency you’ll gain, and in the case of undersized turbos like the OEM, you will gain airflow all while keeping your 20 psi at the manifold.

All this means to you is that your car will be faster! The diagram below shows the same compressor map with the same 20 psi of boost at the manifold (and 26 psi at the turbo in red). Now, let’s assume we run a more efficient intercooler and bigger and more efficient intake and these only drop pressure 1 psi each. This changes the pressure ratio at the turbo to 22 psi to get the same 20 psi at the intake manifold! This is shown in green.

What can we conclude about running a turbo along the green line? Thankfully all good things about both power and reliability.

– Turbo spends more time in the higher efficiency islands.
– The green line extends further meaning that the turbo will flow more air without over-spinning.
– The turbo spins slower, reduces exhaust back-pressure, increases overall engine efficiency
– Turbo runs cooler at lower pressures.
– Engine is less likely to knock.
– Turbo life is extended.

How is this related to the intake?

Larger diameter intakes that are shorter, have less turns, and a better flowing filter will really help here. Believe it or not even the filter makes a measurable difference when you’re working with a maxed out turbo like what is installed on most OEM cars and most medium sized turbos. Of course the intake must also fit in the engine bay and also importantly shield the incoming air from heat without being restrictive.

OEM components are sized for the power requirements of the stock car and noise constraints but often do a great job shielding from heat. Compare the sizes, locations, and design of OEM and aftermarket intakes to see this. Some aftermarket intakes are definitely better designed than others.

The intercooler and piping after the turbo have a similar effect and should follow the same low restriction, high efficiency principle. There are a few more details to charge air cooling that will be covered in a different article.

These are some of the things to think about when choosing an intake for the MazdaSpeed, Focus ST, or any other turbocharged car!

Happy Turbocharging!
Alex@Stratified

Image Credits:
http://www.cap-ny153.org/forcesthrust.htm
http://blog.perrinperformance.com/alta-billet-56-turbo-upgrade/
http://http://www.audizine.com

The implications of a blown boost pressure line

This is a scenario I have seen a few times and I think it is worth going through and the implications of blowing the pressure signal hose off your wastegate or EBCS.

First of all, this is a very dangerous scenario. In most cases a single event like this will either damage or blow the OEM motor. The most dangerous part of this is that it is UNCONTROLLED and pressures rise much too quickly. This is why we have created our Guardian Angel. It is great insurance for this scenario.

Below is the event shown graphically. This is a car running a GTX3071 with a 3 port COBB solenoid and the hose blew off the solenoid barb. The barbs are really small on these solenoids – this is why we went with 1/4″ barbs on our 3 Port. Better to struggle pushing the hose on than have it blow off or leak.. The same thing would happen if the hose blew off the wastegate port.

You will see boost rising in the left hand graph until it pegs the 3 Bar MAP sensor and it continues rising past this. The boost cut in this car is set at 25psi and you can see that it took over 1000 RPM of being above 25psi in 4th gear for the ECU to cut.

During this time, load (airflow) continued increasing and you can see this in the left hand graph exceeding 3.0 load by 4100 RPM and exceeding 100% injector duty by 4400 RPM. At the point where 100% injector duty was reached the limits of the Autotech internals (or perhaps in-tank pump) are also reached and fuel pressure starts to dip before the ECU initiates the cut.

This is a harsh way to see just how much airflow the GTX3071 can generate (a lot!) as well as where the limits of the fuel system lie in the low end. Thankfully, he was on E85 and the engine stayed together this time but it’s not certain if anything was tweaked internally. I estimate boost exceeded 35psi before the ECU cut based on extrapolating past where the sensor was no longer able to read.

What does this look like on Vdyno next to a controlled 4th gear pull? Yes, there probably was some wheel spin but you get the idea.

To avoid this there are several important steps:
1. Use proper hose clamps and not zap straps (zip-ties) around hoses. Zap straps will either not apply enough pressure or melt and beak in high heat areas.
2. If the barbs are mismatched in size, either get an adapter or different barbs, or a solenoid with properly sized barbs.
3. If you are altering the hoses or boost control system in any way from OEM, the Guardian Angel is highly recommended. It is very cheap compared to a blown motor due to a single mishap. It protects you from mechanical as well as tuning errors.

A BNR S3 turbo walkthrough

A few things have changed about the BNR as it has matured as one of the most common medium turbo upgrades for the MazdaSpeed looking for 320-400whp.New compressor housing surrounding the 71mm compress wheel and two port wastegate actuator. This along with our Boost Dial and the factory 2 port solenoid gives you excellent boost control.

2871 journal bearing CHRA remains. No more smokey K04. Journal bearing means it can be rebuilt and although it spools a little slower than a ball bearing centre section, it keeps the costs down.

The exhaust housing is still a machined K04 unit making it easy to bolt on. The bolts holding the exhaust housing to the CHRA now have special washers so they don’t back out; a common issues with older BNRs.

2871 turbo wheel in machined K04 exhaust housing. The exhaust housing allows you to brace the turbo from underneath meaning tubular exhaust manifolds are less likely to crack.

Happy turbo-ing!

How to set logged parameter list using COBB AP V3

The COBB AccessPort V3 is out and we’ve had a chance to work with this very nice new piece of hardware.

When datalogging as part of the tuning process, it’s important to log the parameters you need and no more than this to avoid clutter. Different tuners have different preferences. We have a parameter logging list that we recommend and we often get asked how to set these logged parameters. The short video below describes this using the new AP V3.0. This procedure works for all the vehicle supported by the COBB AP V3.0 (MazdaSpeed, Mitsubishi Evo, Subaru STI, WRX, Nissan GT-R, BMW, Focus ST)

Happy Logging!