In a quest to witness and explain the magic that lies inside the air-squeezing contraption called a turbocharger, I headed to my local junkyard to grab those slimy snails and tear them apart. Here’s what I found in the first installment of our new video series, David Dissects.
The first step was to actually find a car with a turbo, and seeing as I’m located just outside of Detroit—home of naturally aspirated ‘Murican cars— that was a tall order.
So I bee-lined to the foreign makes, spotting a VW Jetta with the 2.0-liter turbo. Sadly, my search wouldn’t end that quickly, as two guys were already wrenching hard to get that snail out of the back of the engine bay. “Whatever, that’d be impossible to get out,” I thought. So I continued my search, and after being disappointed time and time again by V6 Audis and Two-Point-Slow VWs, I finally stumbled upon a Saab 9-3 convertible.
It ended up being the perfect car for the job, since that front-mounted exhaust meant the turbo was easily accessible, and since it was freezing outside, I wanted to get this over with quickly. So, after about two to three hours of high-quality wrenching, I had the turbo out of the car, and the next day, my coworker Jared and I returned for the intercooler. The result was an entire turbo system on my workbench:
Just looking at the system on my workbench made it very easy to explain how it all worked. We all talk about things like wastegates and bypass valves, but actually seeing the hardware makes it that much easier to really grasp.
This particular turbo was a Garrett water-cooled, journal bearing turbo. Breaking it open, you can see the snail-shaped exhaust housing, into which exhaust enters tangentially to spin up that turbine wheel. That turbine wheel is on the same shaft as the compressor wheel, which is sitting on the table in the photo above. As the exhaust gases spin that turbine wheel, they turn the compressor wheel, sucking air from the airbox, through the center of the cast aluminum compressor housing, and out tangentially toward the intercooler.
Compressing that air increases its temperature, which means the air’s density goes down and efficiency drops. So, the air exits the compressor and enters that big black heat exchanger called an intercooler or charge air cooler. After the intercooler, high density air enters the engine nice and cold, and you wind up with lots and lots of power.
Another part of the system shown in that photo above is the bypass valve. It’s on the post-compressor side of the turbo, and the way it works is simple. Imagine you give your car lots of gas, and your turbocharger is pumping tons of compressed air into your intake manifold. Then you let off the gas, closing the throttle. Now all that high-pressure gas has nowhere to go, and the pressure waves bounce off the throttle plate, back towards the compressor.
This can cause a “surge” condition, stalling your turbo, and yielding terrible turbo lag when you get back on the throttle. So, the bypass valve is the solution; when you close the throttle, the pressure in your intake manifold changes, and a vacuum line from that manifold then actuates the bypass valve, taking those pressure spikes when you let off the gas, and bleeding that air back into the low-pressure side of the turbo (in this case, directly into the clean air duct).
On the center section of the housing, you can see a bunch of holes. Two of them are the oil inlet and drain, and the other two are the coolant inlet and drain.
The oil inlet hole forks off to allow oil to access two bronze journal bearings like the ones shown below. Those bearings, essentially just sleeves slipped over the shaft, include holes that allow oil between the two parts so the shaft can glide on a smooth film of lubricant at tens of thousands of rotations per minute.
This particular turbo is also liquid cooled, meaning it uses engine coolant as well as engine oil to keep the bearings from burning up. The coolant runs in a water jacket in the center housing, and it aids in keeping the oil from breaking down due to high temperatures.
It especially helps when the car is off, and the turbo is undergoing “heat soak.” In other words, even though hot exhaust gases are no longer flowing through the turbo, the metal housing retains the heat for some time due to what’s colloquially referred to as the turbo’s “thermal mass.”
To prevent damage to the turbo at these high temperatures, the coolant provides what’s called a “thermal siphoning” effect—just a fancy term for natural convection. Put simply, when the car is turned off and coolant and oil flow stop, the hot coolant in the turbo has a lower density than the colder coolant in the rest of the cooling system. As a result, hot coolant floats to the top of the cooling system and is replaced by colder coolant.
For this to work, the turbo’s coolant inlet and outlet ports have to be oriented in the proper directions, but as coolant picks up heat from the turbo, and is replaced by colder coolant, the turbo’s temperatures drop, and the bearings and oil within remain at safe temperatures.
Then of course, there’s the wastegate, which controls how much boost the turbo makes— in other words, it regulates your intake air pressure by changing how much the air charge gets squeezed by the compressor wheel.
It does this by strategically diverting exhaust gases through a valve, and around the turbine wheel. By diverting gases around the turbine wheel, it slows down the compressor wheel, which results in less boost.
It’s all fascinating stuff, and I plan to break open more cool junkyard car parts in future installments of David Dissects. This will be fun.