Your guide to turbochargers: how they work and why you should care
If you get your hands on any diesel-powered vehicle these days, chances are they will be helped along by a turbocharger hanging off the side of the engine. Here’s what you need to know.
For a lot of the older folks that were used to a big capacity, naturally aspirated diesel (think the old 2H donk from the 40 Series ‘Cruisers), a turbo was a bit of a gimmick, that sure, gave you a bit more get-up-and-go, but in their early days, were prone to cause pistons to melt and blow up engines. You could tow a 30-foot van up Mooney-Mooney hill with the right foot planted in fifth-gear and never have to worry in a 2H (and only down change cause she was down to 50 Km/h). Then there was the turbocharged 12H-T in the 60 Series, that if you did the same thing, exhaust temps would skyrocket, and if it was even slightly over-fuelling, you’d melt or crack a piston, and be up for a complete rebuild. Those days are long gone, with common-rail systems being the norm. More sensors than you can poke a stick at, and usually a pretty conservative tune from the factory, to stop folks blowing them up.
Despite the change in fuel systems, turbos had stayed relatively unchanged for a long time. Just the single turbo setup, with a wastegate keeping your boost pressures in check and that, was that. Now, with a better understanding of how boost will affect a diesel engine and progresses in technology, things have changed quite a bit.
There are different types of turbos, different ways of them to stop over-boosting, different modifications you can make to them, and different ways to mount more than one turbo. In this guide, we go through some of the different setups, how they work, and what options you have on the new (and some old) turbo diesel 4X4s.
The Basics: How a turbo works
The long and the short of it is a turbo works by generating pressure (boost) that gets forced into the engine. The essential components of a turbo, are a pair of impellers with housings (turbine & compressor) joined by a shaft, and a wastegate that regulates the amount of boost generated.
Exhaust gas drives the turbine which in turn spins the compressor, forcing compressed air into the engine, which then allows the engine to burn more air and fuel each revolution. This increase is obviously exponential; as the engine begins to rev harder, it will generate more boost and so forth (to a point). Essentially, it’s an exhaust-driven air pump.
The issue there is the strain and load that the boost puts on the engine – which is where the wastegate comes in. Once the engine generates a certain amount of boost (generally measured in PSI or Bar), the wastegate will open, allowing exhaust gas to pass by the turbine wheel, thus not generating any more boost.
The way the wastegate works, is by way of an actuator plumbed to the intake manifold. When the boost in the intake manifold reaches a certain point – say 8-PSI for an old 12H-T – the pressure forces the actuator to open, which in turn opens the wastegate, causing any extra exhaust gas to bypass the turbine.
The amount of pressure is set by the manufacturer, to what they believe to be “safe” levels, that the engine will handle over its lifetime. Back when 12H-T’s were a ‘new’ engine, 8-10PSI was about it. These days, however, in common-rail engines, pressures upwards of 20-30PSI are not unheard of as standard, and these engines can handle that amount of boost. They are waste-gated to that level or have a variable-geometry turbocharger.
Multiple turbo designs generally work in one of two ways: in parallel or in series (also known as sequential). Say for a V8 engine, with a twin-turbo set up you’ll have one turbo looking after each bank of four cylinders, plus they will generally be smaller turbos than if it was just one being driven off all eight pots – which reduces lag. The other possibility is to have a smaller turbo feeding into a bigger turbo, again, reducing the lag the larger turbo would inherently have by itself; known as a sequential setup.
Random fact: Your average turbo, when used in normal daily use, will spin upwards of 175,000 RPM. Fingers and compressor wheels don’t mix!
VGT or non-VGT waste-gated: What’s the difference?
We explained above how a waste-gate turbo works – by allowing excess exhaust gas to bypass the turbine once it’s reached the set level of boost. VGT or Variable-Geometry Turbochargers work quite a bit differently. The fundamentals are still the same – exhaust pushes the turbine, which turns the compressor, forcing more air into the engine. How it generates boost throughout the entire rev range and then offloads extra boost is how it’s quite a different animal.
First, we need to know about AR or Air Ratio. The Air Ratio is the ratio of the area of the exhaust turbine inlet, to the radius of the turbine impeller. With a lower ratio (and usually smaller turbo), the inlet is smaller, so the exhaust gas flowing through the turbo will be faster, but with less volume. The opposite is true for a higher ratio; more gas can pass, but at a lower velocity. This changes the engine speed at which the turbo will build boost, and how much gas it can flow. Smaller AR will generate boost earlier in the rev range but will begin to run out of puff up high. Larger AR will as you may have guessed, builds boost later on, but is able to flow more gas and air – thus creating what is quite un-affectionately known as ‘lag’ while it’s building boost.
With a standard non-VGT waste-gated turbo your AR is set, based on your rear turbine housing and impeller. This ratio is set toward the mid-rev range, so you get a compromise of being able to build boost quickly, but still have enough puff when you’re off and moving, but it’s not perfect. Wouldn’t it be great if you could have a turbo that could modify the AR while in motion, and also vent any excess exhaust gas so you don’t over-boost your engine? Enter the VGT.
In the rear (turbine) housing of a Variable-Geometry Turbo, there are a bunch of vanes around the inside of the housing. These are controlled by an actuator, which when they open and close, increase and decrease the aspect ratio of the turbine. When the vehicle is at low RPM, they close down, to increase the speed of the gas flow, and then open up when the RPM begins to get to the mid-high rev range; thus allowing more gas through. Where this does away from a standard waste-gate turbo, is that the boost is limited by the vanes – if the gas can go around the turbine blades, it will – so increasing and decreasing the AR will also impact the amount of boost generated.
Turbo modifications and tuning
We got to sit down with Tony and John from Motovated Turbo and Mechanical in Toowoomba, Queensland the other day and got the ins and outs of turbo modifications and what and what not to do.
Why is it worth high-flowing a turbo?
High-flowing a turbo enables more air to flow into the engine and can improve turbo efficiency. If you have a larger compressor wheel this allows more of a volume of air to flow into the engine under the same max boost pressure. As the compressor wheel is making an increased volume of air easier, the air intake temperature can be reduced and this helps to improve power. You can also increase the size of the exhaust wheel as well. This helps to reduce the amount of exhaust manifold backpressure.
This allows the engine to work easier and improves exhaust flow and which in turn, will increase engine power and torque. On the Nissan TD42T and TI engines, this was the start of the performance gains. From the factory, the TD42T/I had 42psi of back pressure on the standard turbo for 8- 9 psi of boost. With a high-flow turbo, you can reduce this down to 16psi of backpressure and achieve significant power gains. The trade-off in going to the larger turbine is you will induce lag and reduce down low performance. Small high flow turbos can see good increases in power without any noticeable change in down low response. A high flow turbo can be better than fitting a bigger turbo in some situations as all of the original pipe works and fittings all still fit exactly the same as factory. Whereas a new bigger turbo changes this and it loses the factory appearance.
Will high-flowing the factory turbo void warranty?
In short yes it can. It is no different to many other modifications which once fitted will affect the factory warranty.
Is there a VGT upgrade, is it any different and what does it entail?
VGT upgrades are the same as a standard high-flow turbo. You fit larger compressor wheels and larger turbine wheels into the turbocharger. This will improve the airflow into the engine and or exhaust gas out of the turbo and improving performance.
Some engines like the Nissan ZD30 respond well to a larger turbine wheel as it helps to reduce the backpressure in the exhaust manifold and can help with blowing gaskets and also improve performance.
On some vacuum-operated turbos you can change the actuator on them to a pressure type actuator which can help to improve boost control in some situations. Turbo modifications can be custom-tailored to suit your vehicle and driving requirements. It is best to discuss this with an expert in the field of turbo modification and tuning to ensure you get the best turbo and or modifications for what you are trying to achieve.