Basic questions about "tuning" CR engines

Today, I was offered a Fiat Punto. It was manufactured in late 2004 and first registered in early 2005. It has a 1.3 JTD (common-rail turbo diesel) engine with 70 standard horsepower. The car has a mileage of 55,000 km and is intended to be purchased as a second car for my partner. The price and features are right, and the service history is complete.

The car has a "chip tuning" device installed, of unknown origin, and of course, it's not approved for use according to German road traffic regulations (StVZO). The (private) seller is trying to sell it to me as an "ultimate selling point." To find out how long the device has been installed, I asked for a receipt for the box – "for warranty purposes," of course. He showed me a receipt that was issued to him 5 weeks after the Punto was first registered. I assume that the vehicle may have had approximately 5,000 kilometers (or miles) on it at that time.

The box is connected to the rail pressure sensor and will likely interfere with its readings. The intention is probably to increase the rail pressure, obviously! My questions for understanding: If the rail pressure is increased, the fuel pump must work harder. The injection timing of the injectors will not be changed by continuing to use the original control unit. Where does the increase in performance come from? Is it from over-pressurizing the injectors? What impact does this have on the lifespan of the injectors and the fuel pump? What other changes result from the (deliberately) increased fuel injection amount?

I do not intend to leave that box in place after a potential purchase, simply because I don't want to let my partner drive around in an "illegal car." Can I still buy the car, or should I expect pre-damaged components or a damaged engine or turbocharger?

I understand that this community is not intended to provide support for poorly executed modifications. That is perfectly fine, and it's my opinion as well. My concern is about the fundamental principles I mentioned earlier. Therefore, I hope for some interest...


[OT:] For all the curious people... Of course, I drove the car! Both with the box (transmission) and without, because sometimes even shoddy work can be quite entertaining! When starting the cold engine, it clearly rattled and coughed out a small puff of black smoke. The soot stopped after a few engine revolutions. The rather harsh combustion noise subsided after about 3 km. However, engine noises also penetrate directly into the cabin, probably due to very stiff engine mounts. The engine is hardly movable by hand in the mounts.

We drove about 15 kilometers on the Berlin city motorway, then exited and entered an industrial area. The temperature gauge indicated that the engine was at operating temperature.

The initial dyno test was quite impressive. The engine (with the box) revved up nicely and pulled consistently. Full power was achieved (due to the stock ECU) around 1750 RPM. It revved smoothly up to about 5000 RPM, then there was a noticeable drop in torque and some visible black smoke – but no "black walls." Without the box, the engine felt a bit sluggish at higher RPMs, it seemed less responsive to the throttle, but still revved smoothly up to 5000 RPM. The "turbo lag" was more pronounced without the box: initially, there was nothing, and you felt like you were pressing on empty. Then, after exceeding 1750 RPM, it suddenly kicked in and pulled hard, spinning the wheels again. The drop in torque then reappeared around 5000 RPM. Here again, we can see some soot, although slightly less than before.

Acoustically, the warm engine with the exhaust sounds slightly less harsh in terms of combustion noise. The fact is that the small diesel engine has no trouble with the Punto body and can be driven agilely even without the exhaust. The car showed no errors, either with or without the exhaust. It's amazing that the "tuning" of the engine control unit apparently went completely unnoticed.

Ahoy...

"I definitely wouldn't drive a car like that. Things like that significantly decrease the resale value. The harsh sound of the engine also doesn't suggest anything good. Generally, I don't have a problem with increasing the fuel rail pressure, but only within the correct range, depending on the engine speed and load." With extremely long injection times, a higher rail pressure can actually be beneficial, but only the rail pressure itself.

If the device is already fully pressurized with rail pressure when it's supposed to be stationary, and the vehicle has been severely abused, I would be very cautious. Especially at low engine speeds and with excessive pre-injection, excessively high rail pressure can damage the engine --> leading to issues like piston knocking and more.

The engine was making a loud knocking noise only when it was cold. When the engine was warm, the sound was "with" a bit softer.

The thing will probably just generally increase the rail pressure. Well, it's the usual cliché: a salesperson in their mid-30s, of German-Turkish descent... Otherwise, the car is completely unmodified. So, you mean taking some space?!

Hutfahrer wrote:

The engine only made that loud knocking noise when it was cold. When the engine was warm, the sound was "with" a bit softer. The thing will probably just generally increase the rail pressure. Well, it's the usual cliché: a salesperson in their mid-30s, of German-Turkish descent... Otherwise, the car is completely unmodified. So, you mean taking some distance?!

Yes, I would definitely avoid doing that, especially at low RPMs and with a cold engine, because the knocking is quite bad --> the cold oil and the high peak pressures can cause problems.

*Maldummfrag:* Do you think he'll keep using it without the box for a while longer?

Hutfahrer wrote:

*Maldummfrag* Do you think he'll keep doing it without the box for a while longer?!

Most rail pressure boxes are comparable to the 10-cent tuning solutions, just with a few more connectors, possibly a Z-diode, and you're ready to go. You can't really influence the characteristic curve properly with something like that, so they usually increase the pressure even at idle. By the way, a common rail system typically has about 300 bar of pressure at idle. The JTD machines I'm familiar with are quite noisy, and I think it's unhealthy and unusual to nail directly into them.

If the engine is already making such extreme noises without the exhaust system, that's not normal. If you know anyone who has the equipment, I'd suggest using a borescope to inspect the cylinders. Otherwise, don't touch anything if you want to keep the vehicle running for a long time.

Hmmm... Do you have any idea why that thing sounded better (softer, rounder) "with" for my feeling?

Yes, it should still run for a while longer...

If the thing is actually being lifted to full rail pressure while stationary, and the vehicle has been abused accordingly, I would be very careful. Especially at low RPMs and with excessive pre-injection, excessively high rail pressure can damage the engine --> leading to piston knocking and so on.

Okay, here we go, things are really getting wild now:
Aron, it's not as wild as it sounds regarding the rail pressure.
In the low-load and low-speed conditions, the fuel pressure sensor (MSG) attempts to maximize the rail pressure and controls the fuel pump to its maximum capacity. At low RPMs, I don't even reach the maximum rail pressure because the pump simply can't deliver that much yet.
It would be an ideal scenario to have 2000 bar of pressure at just 800 rpm. In that case, no exhaust aftertreatment would be necessary to meet EURO5 standards. Therefore, in the LL system, it doesn't matter whether a pressure accumulator is used or not; the potential rail pressure is determined by the pump and is not regulated.

Therefore, a massive pre-injection will not occur! Furthermore, the pre-injection is so short that you won't get enough diesel into the combustion chambers, which could lead to problems. If that were the case, one wouldn't even be able to start the engine, because the main injection system provides significantly more power and is responsible for the actual performance gain.

What is interesting for hat enthusiasts:
When the rail pressure is increased, the additional power comes from the larger amount of fuel that is injected into the cylinders within the same time frame due to the higher pressure.
The pump is generally subjected to higher stress because it has to generate a higher pressure than in normal operation. The same applies to the injectors, which have to operate under increased pressure.
It's now important to know how much the rail pressure has been increased, because the additional load increases proportionally with the pressure.

However, what is far more important is to know how the driver handled the vehicle and how often the potential performance was utilized. This is more critical than the fact that the fuel rail pressure was increased.
A production engine that is constantly run at full load and subjected to cold starts is likely to be less durable than a modified engine that is properly warmed up and where the maximum power output is rarely used. The maximum rail pressure only occurs when maximum power is required, and not during partial or low-load conditions.

With an extremely long injection time, a higher rail pressure can actually be beneficial, but only the rail pressure itself.

It's not possible to make such a general statement, especially when dealing with extremely long injection times.
There are load points near OT (Otto cycle) where the rail pressure is reduced and the injection duration is extended. This is done to prevent the diesel spray from hitting the piston, which would cause excessive soot formation and material damage.
If you want to achieve the same performance here with a shorter injection time, you would need to increase the rail pressure. However, at higher pressures, the diesel jet penetrates deeper into the combustion chamber and would once again hit the piston.

Therefore, the problem of simply increasing rail pressure is not only related to the stress on the pumps and injectors, but also to a spray pattern that is not always optimal for the engine's crankshaft angle.

Best regards, Jochen.

Hmmm... Do you have any idea why that thing sounded better (softer, rounder) 'with' for my feeling?


At idle, a common rail system typically has around 300 bar of pressure (for your information). The JTD machines that I know are quite noisy, and nailing directly is, in my opinion, unhealthy and atypical!

No, it doesn't have to.
It depends on how much you value the smooth engine running that a diesel engine provides.
It's important to remember that the PD model and the VP model were developed and produced without any form of direct injection.
Pre-injection is a convenience feature, but it can negatively impact fuel consumption. If one wants to reduce fuel consumption, the pre-injection duration can be shortened, which leads to less pre-heating of the combustion chamber. The subsequent main injection is then correspondingly more forceful, which again leads to a 'pecking' or 'hammering' sound.
If the 1.3 CDTI engine has been optimized in this regard, the increased fuel rail pressure now injects a slightly larger amount of fuel, which preheats the combustion chamber further and results in a smoother combustion process.

'By the way, 300 bar of fuel pressure at idle is not much.' It's so low that at 220 bar, the injection is completely stopped because the injectors no longer open properly, and the spray pattern doesn't allow for proper combustion.

Best regards, Jochen.

Hmmm... So, it seems that my observation about the smoother engine running at idle when the engine is warm was indeed correct. And if I understand Jochen_145 correctly, it's not as bad as Aron and I initially thought. Is this correct now?


Regarding the car's usage: The seller worked from Monday to Friday in Bramsche, near Osnabrück. During the week, the car was probably driven only once, and for shopping trips, it was likely used for short distances. On Friday afternoons or early evenings, he would drive back to Berlin. On weekends, he might have made a few trips to visit friends, go to the cinema, or to a nightclub. On Sunday evening, I returned to Bramsche.

I don't know how fast he was driving, or whether he was driving at full speed the entire time. However, I know from my own experience that it's possible to drive a bit faster on the A2 and A30, especially on Sunday evenings. I was in a similar situation back in 2005, commuting between Berlin and Münster. On the other hand, on Friday afternoons, you should be happy if you manage to drive at all!

This job is now over for the seller, as of the end of 2006. He is now unemployed and needs to get rid of the car. It's too new, and it will still be considered an asset for him. Furthermore, Berlin-based "agencies" apparently have a cold-hearted approach when it comes to cars: A jobless person in Berlin doesn't need a car, especially not one that's relatively new and still has some resale value.

Jochen_145 wrote:

Aron, it's not as wild as it sounds regarding the rail pressure. In the low-load and low-speed conditions, the fuel pressure sensor (MSG) attempts to maximize the rail pressure and controls the fuel pump to its maximum capacity. At low RPMs, I don't even reach the maximum rail pressure because the pump simply can't deliver that much yet. It would be an ideal scenario to have 2000 bar of pressure at just 800 rpm. In that case, no exhaust aftertreatment would be necessary to meet EURO5 standards. Therefore, in the LL system, it doesn't matter whether a pressure accumulator is used or not; the potential rail pressure is determined by the pump and is not regulated. Therefore, a massive pre-injection will not occur! Furthermore, the pre-injection is so short that you won't get enough diesel into the combustion chambers, which could lead to problems. If that were the case, one wouldn't even be able to start the engine, because the main injection system provides significantly more power and is responsible for the actual performance gain.

I can't say for sure, but the pressure regulator's duty cycle is proportional to the rail pressure. At idle, I'm seeing a control signal of around 10 or 20% (I haven't looked at the oscilloscope in a while). A common rail pump has roughly 4 times the flow capacity needed for injection, which helps maintain a relatively constant rail pressure even during injection. My CP1 has a 7mm pump plunger with a stroke of 8mm. If the pressure regulation were fully open, something would burst. "In my opinion, with CR (Common Rail) injection, the rail pressure is kept lower at low engine speeds because a slower combustion process is desired in that range, similar to other injection systems. Lower engine speed means lower pressure, and the injection duration should also control the combustion pressure."

Jochen_145 wrote:

It's not possible to make such a general statement, especially when dealing with extremely long injection times. There are load points near OT (Otto cycle) where the rail pressure is reduced and the injection duration is extended. This is done to prevent the diesel spray from hitting the piston, which would cause excessive soot formation and material damage. If you want to achieve the same performance here with a shorter injection time, you would need to increase the rail pressure. However, at higher pressures, the diesel jet penetrates deeper into the combustion chamber and would once again hit the piston. Therefore, the problem of simply increasing rail pressure is not only related to the stress on the pumps and injectors, but also to a spray pattern that is not always optimal for the engine's crankshaft angle. Hi Jochen

I completely agree with you about the load. However, I only have experience with MB engines and PSA diesel engines (with Bosch systems operating at 1350 bar, electro-hydraulic, and since 2001, Siemens systems with pre-injection using piezo injectors and higher pressure). Nevertheless, all these engines have a similar characteristic: the rail pressure is 300 bar at idle. Depending on the load, this pressure is increased according to a map. At 3000 rpm, the coil of the rail pressure regulator receives its maximum PWM signal (approximately 70%). As mentioned, this is only at full throttle. If you give less throttle, the pressure is reduced, and the injection duration remains relatively constant within a certain range. From approximately 60% throttle position, the injection duration is also reduced. However, during an injection, the rail pressure is not regulated, as it naturally decreases (the instantaneous delivery rate of the pump is achieved). The rail pressure control is deliberately made relatively slow, as sudden pressure fluctuations in the injector control units can cause unwanted opening.

Regarding our hat driver. It's best if you go to an Alfa dealer and take a test drive, then you'll have a comparison. It's difficult to say exactly what a motor is like, as is often the case --> you can't be inside it (the motor is meant). [/quote]

"@ Jochen_145: However, the engine felt more powerful in the partial load range (noticeable torque). The soot that you described occurs with or without the DPF, starting at 5000 RPM, but it's only slightly more pronounced "with" the DPF. As a small-displacement 16-valve diesel engine, it's quite rev-happy, but you can tell that it doesn't really enjoy going above 5000 RPM." That's not what I would call "species-appropriate" keeping.

Quote:

[...] This is done so that the diesel jet does not hit the piston, which would lead to excessive soot and material damage. [...] However, at higher pressures, the diesel jet penetrates deeper into the combustion chamber and would hit the piston again. [...] Therefore, the problem with simply increasing rail pressure is not only the stress on the pumps and injectors, but also a spray pattern that is not always suitable for the crankshaft angle. [...]

If I understand correctly, injecting excess fuel might not necessarily damage the engine if it's only done with a warm engine and only for short periods, for example, when overtaking or merging onto the highway. However, we definitely agree that it puts extra strain on the pump and injectors. So, using the extra fuel to increase the maximum speed would likely be the worst thing you could do to the engine? ... Which brings me back to the seller's highway driving, and I'm not sure if he was driving at full throttle the entire time. You can imagine how much truth there is in his answer.

In your opinion, is it generally reasonable to assume that the box has already caused damage, or...? Have the lifespan of the pump and nozzles been significantly reduced?

@ Jochen_145: The engine felt more powerful in the partial load range (noticeable torque). The soot that you described occurs with or without the DPF, starting at 5000 RPM, but 'with' the DPF, it's only slightly more pronounced. As a small-displacement 16-valve diesel engine, it's quite rev-happy - but you can tell that 5000 RPM or more is not really something it enjoys anymore. That's not what I would call 'species-appropriate' keeping.

I hope that the kind of situation I described never happens to you, as that would be a disastrous application. A diesel spray should, under no circumstances, ever hit the piston.
At 5000 rpm, the turbocharger on these small engines reaches its limit, and the airflow is no longer sufficient for verschandeln combustion. As a result, it will start producing excessive smoke and soot. Since the multi-rail pressure is not known to the MSG (Motor System Generator), and the smoke limitation is calculated, it naturally produces more smoke when using a 'box' (likely referring to a specific configuration or setting).


If I understand correctly, injecting the excess fuel might not necessarily damage the engine if it's only done with a warm engine and only for a short period, for example, to overtake or merge onto the highway. However, we all agree that there is an increased load on the pump and injectors. So, exploiting the excess fuel injected to increase the maximum speed would then be the worst thing you could do to the engine?

Yes, that's one way to put it.
Generally speaking, this is a very modern motor. Features like protective mechanisms prevent it from overheating, so it's unlikely to fail due to thermal overload.
However, the increased workload cannot be ignored.
However, one can try to 'spin' it in the following way:
- Currently, a motor is designed to last for approximately 200,000 kilometers.
- If the engine is now tuned to, say, increase its power by 25%, it will also wear out exactly 25% faster.
-Are you now driving 75% of the time with only 75% of the power, and 25% of the time with 125% of the power? Is the load on the motor roughly the same?

Of course, the stress or overload of individual components is not taken into account in this case. But roughly speaking, it fits.
According to what you've already said, continuous full load is, therefore, the biggest enemy of a tuned engine.
However, the engine control unit (ECU) is his friend: when things get tight, it reduces the performance to such an extent that tuners can confidently offer a warranty even on engines that have been modified.


In your opinion, is it generally reasonable to assume that the box has already caused damage, or...? Has the lifespan of the pump and nozzles been significantly reduced?

It depends solely on the maximum rail pressure that was actually present or is present.
You can apply the approximate calculation method described above here as well.
However, again, the driving style is crucial. If only the upper part of the load range was used on the highway, and the actual rail pressure was mostly below the maximum standard pressure, the pump was not damaged. There are plenty of Boxster drivers who are perfectly happy with engines that have exceeded 250,000 kilometers.
If the pressure was excessively increased, causing it to consistently exceed the maximum series pressure, premature failure of the pump or injectors is likely.

A crucial factor here is the constructively maximum rail pressure for which the pump and injectors are designed. There are plenty of engines where the maximum rail pressure is significantly below the design limits of the pump and the injectors.
If you increase the pressure here, it won't really affect the pump.

The average fuel consumption reported by the driver can be a useful indicator. If it consistently consumed 7 liters per 100 kilometers, it probably wasn't a car that spent much time operating under partial load.

Best regards, Jochen.

Fiat campervan owners are installing the RD sensor from the 2.3 JTD engine into their 2.8 engines to gain more power. Just a small tip...

I called the seller again this morning. I inquired about various things, including his fuel consumption, under the pretext of wanting to estimate the costs for fuel, insurance, etc. I was told that for the Punto, a consumption of around 5.5 liters was typical in city traffic. When asked about highway driving, I received the answer that the car "started to guzzle fuel from 160 km/h onwards, and then easily consumed 8.5 liters." So, it had seen a few full-throttle runs... However, he added that he "stayed below 155 km/h to be able to cover the distance with one tank of fuel." The car wouldn't go above 170 km/h, whether it was equipped with the limiter or not.

Then I also looked at a Fiat dealer in Berlin. The fact is that I can get the almost 2-year-old Punto for about 1600 EUR less than a similarly equipped one from a dealer with comparable mileage. The used diesel Puntos currently available have all had annual mileage ranging between 25,000 and 40,000 kilometers.

After careful consideration and encouragement from my partner, I bought the car. Price difference compared to dealer offers: 1700 EUR.