Hi,

I heard some tuners tell me that a straight pipe replacing the DPF is bad because it has no backpressure and the turbo would overspool.

I'm not entirely convinced by this (I also read the same argument in a web page that explained how removing the dpf from your car is evil and makes little angles cry in heaven so you should always leave your car stock :roll: ) but I don't have enough mechanical knowledge to sort this out

Also, some people with extremely tuned TDIs prefer not to go the straight pipe way (from manifold to back end of the car) because they'd lose torque on the low end of the curve.

I get that, that's the reason why many engines have valves on the exhaust (to have both low-end torque and high-end power).

But let's stick to "oh no don't put that straight pipe and those sporty back boxes or your turbo will overspool!!"

1) Is there any truth to this statement?

2) If that's so, what needs to be adjusted on the map to avoid that?

Thank you for helping me clarify this subject

1) Yes.

2) Turbocharger actuator precontrol map (a.k.a N75 in VAG land)

Thanks for your answer!

How should that map be tuned with respect to the stock configuration?

My car is already mapped (not by me) and I'd like to see if the tuner did a reasonable job.

The car runs fine but the turbo spools and spikes very quickly. It is cool because that makes the car blast forward, but I'm afraid it's not very healthy for the turbo :oops:

post a boost log

On any turbocharged engine you want minimal backpressure. Backpressure in essence is the additional pressure required to overcome restrictions in the exhaust. By removing restrictions you increase the pressure differential between the turbine inlet and outlet driving a higher exhaust flow. This reduces turbo lag and increases power across the entire rev range. Exhaust velocity isn't overly important. The aim is to get the exhaust flow from the cylinder head to the turbo as quickly as possible to maintain the energy it has in the form of heat and flow. A straight pipe also aids with keeping EGTs down. Spikes are normal in diesels as they tend to reach peak torque just as the turbo spools up. 0.2 bar overshoot from the target pressure is acceptable. You will find all highly tuned diesels by reputable tuners like Darkside Developments always use straight through exhaust systems. The VNT system works in closed loop and can respond very quickly to regulate boost pressure. The vanes in the turbo at low rpm and low exhaust flow remain near fully closed. This narrows the inlet are and forces the exhaust to increase in velocity and spool the turbo. As rpm increases exhaust flow increases and the vanes open to account for this and to regulate the boost pressure.

https://www.youtube.com/watch?v=mMmIqOXxjQU

https://www.youtube.com/watch?v=t2GrVS-sGdQ

Experience: Honours degree in Mechanical Engineering and VAG TDI Tuner.