Running in a new diesel

[Greg Roles]

It's called compressor 'surge', and it's a major problem for turbo manufacture. Basically you're expecting the impeller to try and push air 'that isn't there'. The motor is runnung at low revs, there's a lower air flow, and you ask the inlet impeller ( cool side ) to boost. To my understanding, it starts to cavitate, not unlike an outboard motor that isn't in deep enough. The output oscillates, and puts tremendous stress on the turbo shaft, and it's in the surge area that impellers often shear off. Now most cars are tuned so that the potential to produce turbo destroying surge is small, but it's still possible.

Add to that the amount of soot and thus wasted diesel you're producing below the 1700 ish RPM boost range, and it's simply a good place to avoid. If I'm crusing in 6th and notice the revs drop below 2000rpm, I go up a gear. Bottom line is avoid high throttle / lagging the engine. Use the gears!!

Here's a technical explination that's far better than mine, and will give Obi Dobe a hard-on.

http://forums.tdiclub.com/showthread.php?p=417918

[gregozedobe]

Thanks for the link. I must admit that I have spent way, way too many hours snooping around the vast quantity of high quality info available on TDIClub. It is easy to get addicted to that forum if you are an info junkie.

[Oneofthegreats]

It's called compressor 'surge', and it's a major problem for turbo manufacture. Basically you're expecting the impeller to try and push air 'that isn't there'. The motor is runnung at low revs, there's a lower air flow, and you ask the inlet impeller ( cool side ) to boost. To my understanding, it starts to cavitate, not unlike an outboard motor that isn't in deep enough. The output oscillates, and puts tremendous stress on the turbo shaft, and it's in the surge area that impellers often shear off. Now most cars are tuned so that the potential to produce turbo destroying surge is small, but it's still possible.

Very close & nearly right on the money cogdoc

Actually compressor surge is (quoted directly from Garrett)

The surge region, located on the left-hand side of the compressor map (known as the surge line), is an area of flow instability typically caused by compressor inducer stall. The turbo should be sized so that the engine does not operate in the surge range. When turbochargers operate in surge for long periods of time, bearing failures may occur. When referencing a compressor map, the surge line is the line bordering the islands on their far left side.
Compressor surge is when the air pressure after the compressor is actually higher than what the compressor itself can physically maintain. This condition causes the airflow in the compressor wheel to back up, build pressure, and sometimes stall. In cases of extreme surge, the thrust bearings of the turbo can be destroyed, and will sometimes even lead to mechanical failure of the compressor wheel itself.
Common conditions that result in compressor surge on turbocharger gasoline engines are:
.A compressor bypass valve is not integrated into the intake plumbing between the compressor outlet and throttle body
.The outlet plumbing for the bypass valve is too small or restrictive
.The turbo is too big for the application

[DMS_Dan]

I work on the principle that if it sounds like it is doing mechanical damage it just might be (bearings, big ends, DMF etc). Plus I think I read somewhere some turbos can get a dynamic surge thing going at big loads/low revs, and I've heard it can damage impellers.

Some diesels can (and will) produce good torque at low revs, but that doesn't mean they are designed to do it or that it is good for them. What is low revs for a particular engine (eg 1,600 for a TDI) may be very high revs for a different engine (eg a 16L highway truck engine).

I don't mind low revs on a run-in TDI engine, just so long as I'm not expecting it to produce much power while at low revs (eg cruising, coasting to slow down, going downhill). If I'm expecting my 2.5 TDI to do significant work (pull a load, go up a steep hill or accelerate hard I prefer to keep my revs above 1,800 rpm so the engine can work smoothly without unnecessary stress. If I had a 1.9L or 2.0L I would probably aim for a minimum of 2,000 rpm. I apply the exact same principle to all my engines, regardless of whether they are petrol or diesel, NA or force-fed.

Diesels do thrive on hard work, but it should always be within the design envelope, not outside.

It is your engine, drive it exactly the way you want (but be prepared for the consequences of your choices).

It's all about peak torque though isnt it. Mine makes peak torque at 1900 RPM and due to the variable vein turbo holds it through to about 3000 RPM where it noticeably drops off