Tuning Turbodiesels

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n many respects, modest turbodiesel tuning is similar to modest tuning of a petrol engine. The tuning of TDs does not appear to be so straightforward, but in reality, it is. There are a few ways to tweak turbodiesels. The methods vary according to the engine and the technology involved. For indirect injection diesels, you can tweak the fuel pump, whereas for newer direct injection diesels, you can either reprogram the ECU or fool the ECU.

How Do Turbodiesels Work, Then?

Before we think about modifying our turbodiesel, we should consider how they work. Now I don’t intend this website to be a comprehensive guide into the technical details of a myriad variety of turbodiesel engines because I’ve other articles to write, but instead as an overview.

The first thing we should consider is the turbocharger.

The Turbocharger

The turbocharger is a logical piece of engine design. It’s an air pump. Engines already have fuel, oil and water pumps, why not an air pump? A turbocharger, then, pumps more air into the engine. With more air being fed into the engine, we may burn more fuel and so produce more power.

Turbochargers rely on the exhaust flow from the engine to drive a turbine, which is in turn used to drive a compressor in front of the engine air intake. The turbocharger uses some of the energy that would otherwise be lost through the exhaust; we are recycling energy back into the engine.

Most turbochargers as fitted to car diesel engines can start to produce some boost by as low as 1,500 rpm, but the majority need a few hundred revs more to produce meaningful amounts of power.

Depending on the design, the turbine spins at speeds of up to 150,000 rpm, which is one reason why a turbocharged engine needs good quality lubrication.

There are a number of disadvantages associated with turbocharging. One is that a certain amount of power is needed to drive the turbine. Having this turbine in the exhaust flow increases the restriction in the exhaust, thus the engine has to push against a higher back-pressure. This subtracts a little bit of power from the cylinders that are firing at the same time. Conversely, under certain conditions the power produced by the turbocharger more than compensates for the exhaust restriction and as such, one may benefit from energy that would otherwise be wasted down the exhaust pipe.

There is a further efficiency to be had from using a turbocharger to compress the air entering the engine without burning more fuel to produce more power.

The other disadvantage is that the turbocharger loses boost when one lifts off the accelerator. Thus, when one punches the accelerator down the engine delivers materially less than maximum power and torque until the turbocharger is operating at maximum boost pressure. Turbodiesels tend to soot during the time period between the driver pushing the accelerator pedal down and the turbocharger spooling up to produce power, although modern electronic control systems have done much to reduce this.

The final disadvantage is that many turbocharged engines have a relatively low compression ratio, which means that when running off boost the engine is not especially efficient. This is more important for petrol cars than diesels.

Intercooling

All turbocharged cars benefit from an intercooler, and turbodiesels are no different. An intercooler cools air going from the turbocharger and into the engine. Cooler air is denser, so if you can stuff more air into the engine, which means more oxygen and this in turn means more power. It also has important thermodynamic benefits - the cooler the air, the more boost pressure you can run. Almost all modern turbodiesels have an intercooler, but in most cars, the positioning, size and efficiency leaves much to be desired. The location of an intercooler varies from above the engine immediately under the bonnet or at the front, close to the radiator.

Intercooling is more critical in petrol applications, since petrol engine exhaust gasses are significantly hotter than turbodiesels. This said, intercooling helps produce higher specific output figures and perhaps more importantly, maintains these figures. In heavy traffic the intercooler can warm up in the higher temperatures and when you do finally move off, the car breathes in a lung full of warm air. Intercooler heat soak is a particular problem if the intercooler is mounted above the engine.

Fitting a larger, or more efficient (ideally both) air-to-air intercooler will make a difference to performance right across the rev band. Most intercoolers are rather small affairs, but do make a substantial improvement nevertheless (Ford’s 1·8 litre engine gains approximately 17% power and torque when fitted with an intercooler, the General Motors 2·0 DTi gains over 20%). Furthermore, this improvement is through increasing the efficiency of the engine - you usually reduce fuel consumption.

Chargecoolers

Here, a chargecooler is an air-to-water intercooler, which is just another name for a water-to-air intercooler. Chargecoolers require rather more plumbing under the bonnet than an intercooler, and are more expensive as a result, but achieve greater gains through reducing the temperature of the air going into the engine even more. I do not know of a light car or van with a charge cooler, but I do know that you can have them fitted.

It is probably also possible to use water injection systems for a turbodiesel, in the same way you can for a high performance petrol turbo, but again I do not know of anybody with this modification carried out.

The traditional tuning techniques applied to petrol engines also apply to turbodiesel engines - improving the exhaust and induction is reputed to make a difference. This will depend on how good the manufacturers systems are: in come cases, fitting an aftermarket induction kit can cause a loss of power (the most notable is the Peugeot 205 GTI). In some cases, turbodiesels use restrictive induction systems to help keep the noise down, so fitting an improved system will improve things. The older, indirect injection turbodiesels may also benefit from this as it will “lean out” the engine, reducing black smoke (this is usually the case where the air filter is clogged up).

It is also possible to use a different camshaft and make cylinder head modifications in order to improve a TD’s performance - I have yet to meet anybody who has done any such changes. Any such changes to the diesel’s head are reputed to shorten the engine’s lifespan.

So how do you get lots of power from a 2·0 litre turbodiesel?

At the time of writing, Alpina’s 2∙0 diesel in the D3 produces two hundred brake horsepower. Other manufacturers’ change the maximum power output of the engine to suit tax reasons rather than engine longevity or performance concerns.

A turbodiesel produces power just like a petrol engine. Power and torque are linked as I discuss here, the more torque at a given engine speed, the more power. If your engine produces less torque but is turning over at a higher engine speed, it’ll produce the same power. To produce more torque we need to stuff more air into the engine so as to be able to burn more fuel. To stuff more air into the engine, we may redesign certain aspects (the head, the induction system and the turbocharger) or make it bigger.

The use of ultra high pressure fuel pump has helped increase power figures. It’s not a case of the higher the fuel pressure, the faster you can squirt the fuel in but more that the higher the pressure, the finer the fuel mist and the more complete the burn in smaller period of time, thus, the more torque the engine can produce at higher revs. More torque at higher engine speeds means lots more power.