Turbocharger, VNT, and diesel turbo FAQ-page 3
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Oil supply and turbo timers
The biggest area of concern in the turbo is the oil supply. Insufficient oil (especially journal bearing turbos) or excess oil (especially ball bearing turbos) or dirty oil may wear out the bearings, causing wear and shaft play in the turbo. Because of the high temperatures seen at the turbo, the oil may also break down faster than a comparable non turbo car. Synthetic oil is recommended for turbo cars because it doesn't break down as quickly as conventional oil. Because the best engine oils for a diesel engine are synthetics, this is another reason to use synthetic in turbo and diesel applications if you are not already doing so.
In addition, since the turbo gets hot when running, an engine idling period of 5 seconds before every engine shutdown is enough to let fresh oil circulate to the turbo bearings. If you were driving hard and hot, a 1 minute idling period or a few minutes of sensible driving before shut down should be enough to let the turbo cool down and receive fresh oil. If the turbo is too hot and does not receive cooler oil upon shutdown, the oil could become burnt and "coking" may occur. This is more of an issue with non synthetic oils.
Another issue is letting fresh coolant circulate to the CHRA. After engine shut down, the coolant heats and expands in the cartridge if the CHRA is too hot. This creates a natural circulation to drain away the heat and bring in fresh coolant. The reason it doesn't boil off is the same reason engine coolant doesn't boil off - the engine coolant is a sealed system. Some cars have auxiliary pumps that circulate coolant after engine shut down. There would be no benefit to this on a TDI since the turbos are oil cooled only and not water cooled, and because of the lower temperatures that you should see during engine shut down due to a diesel engine and from good shut down practices. Even on gasoline water cooled turbos, if it didn't come from the factory with an auxiliary pump, I would not add one since the engineers didn't put one there and because there is some natural convection of coolant and oil. I do not recommend rerouting the oil or coolant lines in your turbo unless you are sure they are routed properly. I also recommend never using radiator "stop leak" products because they can gum up and clog the turbo coolant lines.
You should also not install any kind of inline oil prefilter upstream of the turbo oil supply line. Some newer Subaru gas turbo cars suffered destroyed turbos from oil starvation. These were traced to a design change consisting of an inline oil filters added at the factory - these became clogged, causing oil starvation. Here is the reference.
The other concern is mounting angle. If you've seen turbos on engines they're all mounted so that the shaft is parallel to the ground. I haven't seen specs from VW but Garrett says their turbos must be mounted below 15o tilt. Beyond this can cause bearing wear and drain issues.
Use of VW approved engine oils in the TDI is also recommended to ensure proper lubrication to the turbo. The big shift for North American market cars was in 2004 with the introduction of the pumpe duse engine and in 2009 with the common rail engine. These engines see very high pressures in the head and should use VW approved engine oil to keep your warranty intact. See 1000q: pumpe duse engine oil and 1000q: non pumpe duse engine oil to see lists of approved oils and reference links to oil manufacturer's websites. The common rail engine in the 2009 and 2010 VW TDI uses VW/Audi 507.00 spec engine oil only. At least for warranty purposes, stick to the VW spec, especially since this is a new engine and there isn't any aftermarket engine oil analysis out there yet.
Some people install a turbo timer to keep the engine idling so they can walk away from their car during a cool down period. I do not recommend these products for a number of reasons. First, if you have a manual transmission, you should always put it in first or reverse gear when parking in addition to applying the parking brake, so the convenience of walking away with the car idling is not possible. Also, a turbo timer requires spending money on the timer, cutting wires and introducing an unnecessary failure point. Lastly, for diesel applications, coking is not as common of a problem due to the lower rpm and cooler exhaust gas temperatures, and you should be using synthetic oil anyways which is more resistant to coking. If you are truly concerned about turbo care, just make sure that you drive at medium rpms and load when the engine is still warming up and just drive sensibly a few minutes before shutting the engine down.
Another component in a good turbo setup is the intercooler. After intake air passes through the turbo, it heats up partly because of higher pressure. The ideal gas law states that when all other variables are constant, if pressure is increased, so will temperature. An intercooler lowers air temps before passing the air into the engine. (Some other sources of heat are the intake piping soaking heat from a hot engine bay, because the turbo is so close to the exhaust with hot exhaust gasses passing through the exhaust side of the turbo, and mechanical agitation of the air by the turbine wheel.) Without an intercooler, hot air increases the likelihood of uncontrolled detonation and engine damage.
An intercooler is basically a heat sink that takes away the heat of the intake charge. Here is a picture of an intercooler in a Jetta TDI. Cooling ambient air moves through the front bumper, through the intercooler, and through the wheel well in the direction of the arrow. More air moves through the intercooler as the car moves faster.
You don't see intercoolers on non-turbo cars because the intake air is already at ambient temperature. An air intake directly connected to an intercooler or anywhere not after the turbo would actually decrease performance by restricting airflow. Below is a funny picture of an "interfooler", someone who put an intercooler on a non turbo car. It's there because they want to look cool and are ignorant of what its function is. Even worse, the air filter is exposed and low enough to suck up water and damage the engine.
The goal of intercooling is to produce the least pressure drop (so the turbo doesn't have to work as hard) and remove the most heat. Depending on the exact setup, the average well designed intercooler in a car may have .5-2.0 psi pressure drop. There is always some pressure differential between the turbo and the engine to get air moving from one spot to another. An intercooler acts more like a heat sink and less like a radiator when boosting because boosting heats up the intake air. This heat is transferred into the intercooler like a heat sink. Then the intercooler releases the heat into the ambient air or coolant. Most of the heat leaves with the ambient air flow (while the car is moving, air is passing through the air ducts) but a little heat can go back into the intake air once air temps have dropped (heat moves from hot to cold).
A good air-air intercooler can cool the air to within 20 degrees of ambient temperature if it has steady airflow to take away the heat. The advantage of a good air-water intercooler is more consistent intake air temperatures because water is a better heat sink. Water (coolant) is not as quickly affected by rapid changes in ambient air temperatures and car speed. But once water is hot, some heat goes out a radiator and some goes back into the air-water intercooler's intake air. Some cars don't have the routing or space for a good air-air intercooler so they must use an air-water intercooler.
An air-air intercooler is preferred for diesels because they are normally front engine so there's plenty of space for plumbing. An air-air intercooler is also easier to fabricate with less chance for leaks. If there is a major water leak into the intercooler core, it's possible that this could hydrolock the high compression diesel engine. A air-water intercooler is more appropriate on a mid engine car due to difficulty of intercooler packaging or a car with more peaky temperatures.
In a gasoline engine, the engine is operating at vacuum or low boost most of the time. Low boost doesn't heat the intake air as much as hard boosting and as a result, doesn't transfer as much heat to the intercooler. In other words, a larger intercooler is not needed unless you need the extra heat sink capability! Most modified gasoline cars would benefit a little from a larger intercooler due to higher than stock boost levels. However, how much it's needed in only lightly modified cars is debatable due to variations between cars, ambient outside temperatures, intended use (street vs. track), desired safety margin and fuel octane, etc.. For example, a large front mount intercooler will cool better than a small intercooler but it may not fit, may be blocked by the bumper, cause overheating problems due to blocking the radiator, etc.. Also check for leaves or dirt blocking the face of the intercooler.
A diesel engine has a greater need for an effective heat sink vs. a similar gasoline engine because of higher sustained boost levels. Turbos are also smaller for a number of reasons, for example, the smaller rpm range. I think that even lightly modified VW TDI could benefit from more efficient intercooling for maximum peak power. The best way to determine the need is to log pressure and temperature at the turbo and at the intake manifold. Especially for a front engine TDI, an air-air intercooler (which you already have) is the best option. The VW TDI naturally puts an oily mist on the inside of the intercooler from the crank case ventilation (CCV) system but trying to keep the inside clean is like trying to keep the oil dipstick clean. Gasoline cars shouldn't have any oil inside the intercooler.
If you must paint the intercooler to help hide it, use 1-2 light sprays of radiator paint or even better, a heat shedding coating like Swaintech's "BBE heat emitting coating". I don't know how well it works since bare Al is already very good at shedding heat. My guess is that because it sells well and measuring before-after intake air temperatures is so easy (assuming equal ambient test conditions), that it probably works.
Spraying coolant onto the outside of the intercooler is very effective because it can lower the temperature of the intake air below ambient air temps. CO2 (compressed carbon dioxide gas), N2O (nitrous), and just regular water all work very well at increasing intercooler effectiveness but only work until your coolant runs out. If you are preparing a short race, placing bags of ice on an air-air intercooler or chilling the coolant in a water-air intercooler works well too.
1000q: boost leak testing. A common issue with the VW TDI is the sudden loss of power known as limp mode. The VW TDI ECU has pressure and air temp sensors and if the ECU senses a problem, it cuts power to prevent damage to the turbo and engine, preventing damage to the turbo from an overspeed. See 1000q: limp mode diagnosis for more details on a sudden loss of power.
To the right and above is a thumbnail of an aftermarket off-the-shelf intercooler next to the stock intercooler, click for a larger view. It features bar-plate construction instead of the stock tube-fin.
Further flow improvements
Another way to increase the efficiency of your general setup is to improve the pre and post turbo and intercooler piping. This reduces pumping losses and restriction (reduces boost - see the next section for more on this). On most TDI engines, this can be difficult due to the turbo, intercooler, and battery locations. The best piping would be relatively smooth on the inside (mandrel bends), have a relatively straight path or gradual angles and transitions, and be as short as possible. The shortest, smoothest pipe routing on a transverse 4 cylinder engine would be from a turbo in the front, with a 180o loop to a front or side mounted intercooler and then a 180o loop back to the intake manifold. This is not possible on the VW TDI due to the rear mounted turbo location but you can still improve the existing piping. When putting together an aftermarket setup, use piping that has mandrel bends with straight silicone couplers instead of using straight pipes with bent silicone couplers. Silicone couplers tend to collapse at tight spots and can bend, reducing the cross sectional area. Due to varying fitment, they also tend to have more gaps between the piping, disturbing airflow more than necessary. They economical and easier to assemble but the best system is a simple system.
Shortening the intake piping, making the transitions between piping as smooth as possible, and and routing the piping as straight as possible will reduce the amount of required pressure to produce a certain amount of power, increasing reliability and efficiency. A rough rule of thumb is that each 90o bend in pipe adds as much airflow resistance as 25 ft of straight piping. Of course, actual resistance depends highly on diameter, smoothness of bend, etc., but (big surprise here) short straight piping results in the best flow.
Some people think that larger piping or a larger intercooler increases lag. This is true because it takes longer to fill and pressurize the larger piping and intercooler. However, the difference in response is extremely short, especially considering the small, quick spooling turbos on the TDI. In addition, the loss of throttle response is generally not a factor at all since the larger piping increases overall efficiency and the power gain from other mods offsets any additional lag. Exhaust backpressure, chip tuning, and turbo size are far greater factors in throttle and turbo response than larger diameter intake piping, so don't worry about piping being too big. Intake piping makes a difference but on the TDI the priority is lower compared to a turbo, injectors, exhaust, and chip improvements.
The one thing to be wary of with Volkswagens is using high flow air filters. The mass air flow sensors (MAF or MAS) on the Mk4+ body seem to be sensitive to the additional dust and debris that a high flow air filter, especially aftermarket oiled cotton filters let into the intake tract. This can lead to a failed MAF, see 1000q: MAF FAQ for more details. The stock air filter and housing was overbuilt and uses the same air filter as the 240 horsepower Golf R32 so there is little-no gain by switching to a high flow air filter. Lastly, many cold air filters don't use a cold air intake snorkel. This draws in hot underhood air and can actually reduce power.
Common turbo myths dispelled
The biggest myth is that every turbo car can make more power just by turning up the boost. Boost is only a measure of intake pressure. Pressure can only be created when there is resistance from a restriction.
Everything else being equal and within reasonable limits for the setup, more boost makes more power only if the turbo is operating in an efficient range of performance and if the rest of the setup can benefit from it. Most turbocharged cars have a little room to safely increase boost. If you were to increase the boost to the point where the turbo is trying to move too much air, it actually reduces performance. This is because past the point of diminishing returns, a turbo is basically blowing hot air. This hot air creates intake air pressure and more boost because boost = measure of pressure. Again, back to the idea of volumetric efficiency, you want the maximum mass of air for the engine. Unless the air can be cooled sufficiently by the intercooler, the density of the air might actually be less than it would have been at a lower boost level. This psi level of diminishing returns is different for every setup and every car and even varies by ambient conditions. At that point, some modern cars compensate by using their computer and sensors to adjust the timing to prevent detonation. The TDI engine car computer has air temperature and pressure sensors and a program that will prevent increased power if the only change is increased boost. You need a chip or other performance enhancement to increase fueling, see 1000q: basic performance upgrades for the TDI for more details.
Again, it is a common mistake to equate boost, or intake pressure, with denser air. Assuming the other variables are constant, the ideal gas law PV=NrT shows that if you raise pressure, temperature increases. Also keep in mind the above paragraph about operating a turbo outside of its areas of efficiency. It's easy to get so caught up with quick power gains from more boost pressure that one can forget that the ultimate goal of turbocharging is increasing air density, not just pressure. In designing the engine as a whole system, you want to create the same amount of power with the least amount of boost, within a range, to reduce stresses on the engine and turbo and to keep air moving at a reasonable speed throughout the intake tract.
One more time: boost pressure is a measure of intake restriction. You could put a choke in the intake air path and that would also create boost (but reduce power). A turbo moving a lot of air but showing relatively low boost on a boost gauge means there is low air restriction in the intake air path. Remember, the goal in increasing power is to move more air, more efficiently, not just create boost. Changing camshafts to allow more air into the combustion cylinders, changing the combustion cylinders by boring and making the diameter of the cylinders wider, or stroking the engine and making the length of the piston travel longer, can all increase the amount of air moved.
Adding a larger turbo does not mean the engine will make more power. In a modern car, the turbo is regulated by sensors, computer feedback, and solenoids set to control the boost at a certain pressure. The computer measures the pressure with sensors normally at the intake manifold or some spot right before the intake manifold. Everything else being equal (load, rpm, etc), one large turbo and one small turbo will flow identical pressures of air at a given psi but remember that psi is just a measure of pressure - air mass is what matters and is what makes power!
Here is an example: to flow a certain amount of air, where a smaller turbo may have already passed its maximum efficiency and is blowing mostly hot expanded air, a larger turbo will still be operating in its area of maximum efficiency and is moving cooler denser air at the same psi. Again, assuming that one turbo is stressed too much and the other is in its peak efficiency, they are both giving the same psi but not the same density of air. 20 psi is always 20 psi, the difference between an efficient turbo and a turbo blowing hot air is the temperature of the air coming out of the turbo which affects density. 20psi of 50oC air is not the same as 20psi of 14oC air. There are also other factors that effect this such as the size of the turbo housings, backpressure, etc.. You want to select a turbo which balances responsiveness with moving your desired mass of air. Do a lot more research and consult your performance and parts vendor before crunching the numbers and selecting a turbo setup. The same turbo on a 4.0L engine will respond totally different than on a 2.0L engine. Garrett turbo's website has some more info on calculating airflow. Turbo manufacturers often publish graphs which show where the turbo is operating most efficiently.
Also remember that the control systems and sensors for the turbo are normally located after the intercooler in the intake manifold or piping. If there is an air leak in this section of piping, the turbo has to work even harder to make up for the lost and provide the same reading to the sensors. Because it has to work harder, it may even be operating outside of its optimum efficiency range and creating excess heat over a leak free car. See 1000q: boost and vacuum leak testing for a simple way to test for boost leaks.
What does this mean for detonation in a gasoline car? Detonation or engine knock is the explosive ignition of fuel. It often occurs from preignition on hot spots inside the engine cylinder which can be caused by pitting from earlier detonation. If an engine starts to knock at 20 psi, it will always knock at 20 psi, everything else being equal (ambient conditions, same octane and fuel quality, same exact engine). In this example, a more efficient turbo will move more air mass at the knock limit of 20 psi than a less efficient turbo at 20 psi. Therefore, the larger turbo can move more air (and make more power). This goes back to the last section on flow improvements: move more air, more efficiently to make more power.
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