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Street fighters on a budget

TDI-tuning I.

26/03/2013 00:32 | Comments: 

Contributing editor

Ahead of most of us in the race of „who bought the most cars in the last two years”, Göbi is the real techno-pro of the mother magazine, He has an engineering job at a motorcycle brand, so he makes up for time the lost time with Torx screws in his garage, which is as clean as a pharmaceutical factory. A nice VW Bug came up in the ads with Porsche suspension? He buys it and tunes it to 146 PS. A green, bog-standard Bug is on sale in the neighborhood? He collects that too, restores it. What about a nice Fiat 127? An as-new Merc W124…? And his technical writings are just as precise as his passion for cars – they steamroller the readers.

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Does the idea of souping up your 1.4-litre car instead of simply buying the 1.6-litre version sound silly and irrational to you? Well then, there is no reason for you to continue reading, please move on, nothing to see here.

Car tuning is, as the name implies, all about fine-tuning your vehicle. In other words, it is about optimising something available but less than satisfactory. You want to keep what you have (your car, in this instance) but would prefer it to be better and, more importantly, different from what the guy next door could ever have. Say car tuning and most people will think about upgrading your engine power. Others think more in terms of visual enhancement.

What we are about to explore here, with the mindset of an engineer, is one of the most popular and simplest ways to get more oomph out of your engine – namely the infamous software upgrade of modern diesel engines. Also referred to as chip tuning, it is a bland and highly unimaginative umbrella term for software-based modifications.

What you do is meddle with the engine control software. You modify a few parameters which will yield certain results. These final results may or may not coincide with your original objective, which is invariably an increase of engine power. For the time being we are not going to talk about hardware-based modifications which require structural changes such as an oversized cooler or the installation of different carburettors. Later, we will get to them as well, but for now, let us stick with software upgrades.

For ”chip tuning” you will definitely need to know the serial number of the engine control unit (ECU). Sometimes otherwise identical car models have different ECU's

Why is software upgrade aimed towards diesel users? Well, modern diesel engines have electronic control as a requisite , and a turbocharger as well, which makes your job easier. You ultimate aim is to let more air into the combustion chamber, and to inject a correspondingly increased amount of fuel, which in turn will up your engine power. Since the whole process is governed by electronics, all you need to do is modify the control software. Modify, yes, but how and to what degree? That is the knowledge that sets wizards and charlatans apart.

As for now, the answer is simple: you just need to modify a few parameters in the EPROM of the ECU. If your EPROM is read-only, you need to remove it, build a socket, extract the program, modify it, save it to a new chip and insert that into the socket. If your ECU is a modern one and you have the right tools, you can use a diagnostic cable to upload the modified program without having to remove the processor.

All of this sounds more complicated than actually it is: a TV repairman of average skills can do it blindfolded. This is not the tricky part. You can also attach a self-contained external unit, commonly known as a tuning box, to the same end. It can modify parameters in the same vein as described above, resulting in more fuel being injected into the engine, and thereby, more power. If you are lucky, that is. If not, there will be no gains. If you are extremely unlucky, there is a chance you'll blow the engine. But that is jumping too far ahead.

But here is the catch: you need to know exactly which program to upload, which programming lines to modify and where each modification will take you. To keep it simple, chip tuners make use of the fact that manufacturers must, by law, comply with various emission regulations. This, in fact, means OEM software programs run diesel engines with a vast surplus of excess air. You don't even need to raise turbo pressure: all you have to do is slightly increase the amount of fuel injected, and the engine promptly and tangibly becomes more responsive.

The gain is most obvious in the lower rev range, giving you significantly more low-down torque but flat-out power is also going to increase. Is there going to be more smoke? You bet. You will not necessarily be able to see it, but yes, measured values will be worse. Will there be a higher mechanical stress on the engine? Naturally. . The more power you extract out of a machinery which has alternating weights moving in it, the more physical load it will be subjected to, and the higher mechanical wear you'll need to put up with.

Will your engine fail prematurely? Not necessarily. If you make use of the extra power on a regular basis, the answer is a big fat yes. If you just use it once in a while, there should be no loss of life expectancy for the engine. This is the rule of thumb, but in order to truly understand the implications of meddling with the software, you need to look at each component separately, just like you need to deal with different engines and different injection systems in their own right.

The inside of an ECU. This is the control unit of a 1997 Mercedes-Benz C-class. A VW TDI ECU will look mighty similar to this one

There is one thing regular TDI units, newer PD (Pumpe Düse, or unit injector) engines and common-rail powertrains have in common: a turbocharger. Whether a variable geometry turbo or a simple wastegate unit, certain things will be identical for each type.

When engineers attach a turbocharger to an engine, they first examine the air intake of that engine and match the smallest possible turbocharger to that measured value. Smaller turbos cost less, are simpler to manufacture and, more importantly, rev faster and more freely, resulting in a reduced turbo lag. If you modify the electronic control of the turbocharger to achieve higher pressure, you will end up degrading its efficiency optimised for the specific size and air flow conditions.

Choke on the exhaust side will be higher and in the end you may end up not helping the cause at all. This is why 90 and 110 PS versions of the same engine have different turbo scrolls installed. It would in fact be more cost efficient to manufacture just a single component. For that reason tuners must be really careful when selecting the right charge pressure.

In principle, you can make informed decisions based on the performance curve or by measuring the pressure on the exhaust side. With a thoroughly written software this is something that has been checked and double checked. At least it should be. If you operate a turbocharger outside its optimum range you'll end up reducing not only its efficiency, but also its useful life. Sometimes radically so, by a factor of ten. And this is not only true for simple turbochargers, but for all kinds, from variable geometry units to older models.

The indestructible old TDIs

The next thing you do is increase the amount of injected fuel. You do need to look at the different engine types separately, since they have radically different fuel delivery systems. First I will look at original TDI technology featuring electronic control and fuel pumps, then move on to TDI units from VW (1989 and onward), followed by PD (ca. 1999) and finally CR TDI powertrains.

OEM EPROM governing engine control and fuel injection

These archaic TDI engines are highly reliable and simple. For fuel delivery they use a Bosch rotary pump (VE) with added electronics, operating basically the same way previous units of similar structure always had. Electronics are used for controlling the amount of fuel injected. When altering the software to increase the injected amount, you basically modify the signal that this dosage controlling electronic unit puts out, making it to inject more fuel into the chamber. Since no other modifications are carried out, the system will be forcing more fuel through the same injector.

And this is where trouble strikes, since you cannot achieve a pressure high enough to force the increased amount through the injection ports within the same period of time. If you are not careful when rewriting the program the injection phase can last too long, resulting in high thermal load and smoke, not to mention the risk of overdriving and damaging the dosage controller.

This goes to show that caution should be exercised when modifying the software. If you want to do it right, you should also slightly increase turbo pressure, thereby increasing excess air: this allows adding more fuel to increase power without visibly aggravating the smoke issue. You might also want to adjust the pilot injection but that is for the pros. Pre-programmed software are highly complex, their development process is supported by several dyno runs.

OEM EPROMs removed; and a new EPROM with reprogrammed software

You can buy tuning boxes for this system. These are usually wired in serially between the injector and the control unit and serve to modify the signals reaching the injector (i.e. the duty cycle of the square wave) thereby forcing the pump to inject more fuel. There is really nothing else you can do here. There is no increase of boost pressure or any other modification of the rev range. All this does is change 50% to 55%, 70% to 77%; basically shift the curve upwards by 10% in the entire rev range.

Does this make the engine more powerful? Certainly, but results are rather pathetic compared to a well-written software. On the other hand you can install or uninstall the unit in ten minutes with no tools required. However the slight power gain also means a slight increase of load, not as much mechanically but rather thermally. Soot particle emission also increases.

To put some specifics on the table here: a 90 PS, 210 Nm Golf Mk3 TDI from 1997, when equipped with a modified software (chip tuning), produced 118 PS and 298 Nm with basically no smoke, while a tuning box from a respected German company yielded 108 PS (against the rated 115 PS) and 277 Nm; all values dynoed on the same bench. In the first instance, charge pressure was increased from standard 0.7-0.8 bar to about 0.9 bar. The tuning box offered no such option, and we did in fact experience a bit of black smoke under heavy acceleration.

PD system – that means the VW-Audi Group

Operation completed, with the new reprogrammed EPROMs in

Next in turn is the unit injector (PD TDI) type diesel engine, manufactured mainly by the VW Group. By now, these engines are on the verge of extinction. With this kind of technology the camshaft not only controls valves but it also drives the unit injector which, as a result, injects fuel into the combustion chamber at high pressure.

By modifying the software you can change the amount of fuel injected. You can also alter turbo pressure, in a slightly different manner, but to the same end. Another thing in common with the previous method is that you are not expanding the port size of the injector. When you are trying to push 30 percent more fuel through a hole of unchanged diameter, the forces at play will be unfathomable. As a clear indication of the extreme load exerted at the components, the service life of your toothed belt will be reduced from 90k to 60k km.

Also, the extra pressure is exerting so much lateral force on the unit injectors that they come loose within the cylinder head and cause diesel oil to leak into the lubricant. The unit injector (PD) system is more sensitive to tuning but tuning boxes are readily available at favourable prices. Of course these systems are affordable exactly because they are nothing more than simple resistors which cheat the engine into ‘thinking' that it is still cold outside, or the coolant has not reached its operating temperature, and are therefore ordering the injector to inject more fuel into the combustion chamber. It is as primitive a method as it gets, but it works, although don't expect too much from it. Manufacturers' claims of +30 PS are usually unfounded but the low cost of 25-40 Euros is just too attractive to miss... steer clear of these.


The third and final category is that of common-rail (CR) diesel engines. The high pressure pump unit builds up pressure in a shared pressure container (common-rail) with injectors installed. These injectors are governed by electric signals (much like in petrol engines) which cause them to inject fuel into the combustion chamber at high pressure. This is a well regulated and modern system which complies with all emission and noise reduction standards. You'll find this technology in most modern diesel engines.

Of course it does allow for some fine tuning, from modifying injector opening times through the customary increase of turbo pressure to employing tuning boxes. The latter usually increase pressure in the common-rail system, allowing more fuel to pass within a single opening of the injector. This means additional load on what is already a highly sensitive pump, meaning a sure-fire shortcut to having to replace the pump. What they do offer though is relatively lost cost and simple installation.

Adjustable tuning box for a V6 TDI engine. Advertised as digital, this is in fact an analogue system but it really works fine

To sum things up: you can indeed soup up your TDI with purely electronic (software) modifications, no structural changes are required. Effects will be readily tangible and unless you are unscrupulous about rewriting your software you are only looking at a minor decrease of useful service life, depending on how often you make use of the added potential of the engine.

You may rightly ask the question why manufacturers do not offer their vehicles with such upgraded software in the first place. The answer is rather straightforward – because as they stand, these souped-up engines fail to comply with environmental regulations and would be prone to failure under extreme circumstances.

These modified software make use of the mechanical and thermal safety buffer zones manufacturers construct their engines with - these are the buffers that allow you to run your A/C even when driving flat out in the scorching heat of the Death Valley without your engine overheating or the piston crown melting. You do that in a car with a modded engine control software (what we have been referring to as ‘chip tuning') and you'll find yourself constantly looking at the water temp gauge. Also, your diesel particulate filter, if you have one, is not going to last much longer than the warranted minimum.

Certain engines tolerate power upgrades quite well, others, however, take them in an ill way. With a bit of mechanical modification anything can be repaired but we'll discuss that next time, maybe.

Russian Roulette

Maximum combustion pressure, upstream temperature and maximum turbine rpm – these are the factors that a manufacturer measures and uses to calibrate the nominal values of injection and charge pressure. Limits are set so that all components can tolerate the arising stress, even manufacturing tolerances are taken into consideration. Some components could take a lot more; others will disintegrate at anything that is even slightly higher than factory values.

Tuning professionals never measure these threshold values. First, it would take a highly expensive diagnostic toolset to do so. Second, their modifications are based on the assumption that factory components tolerate higher loads than specified by the manufacturer. This is usually true, but certainly not in every case. If worse comes to worst all you can do is cry and whine over the ruins of your assembly. Software upgrades are nothing less than playing Russian roulette with factory units.

Gulyás Gusztáv

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