How to properly break in an engine on a new car, with a focus on the TDI
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There are many opposing opinions on how to break in an engine.  Some people say to run it hard to get the engine piston rings to seat, some people say to keep turbo engine under boost to get the rings to seat, other people say to baby the engine when new.  The opinion you will find on this page is the same advice you will receive from automotive engineers and the same instructions that are in your owner's manual from the people who built your car.  It is ultimately your engine and your car, so like any other internet advice, take it with a grain of salt and figure it out for yourself.

"Run it hard from hour 0" is wrong - here's why

The "run it hard" opinion hit a resurgence with a website from an air cooled motorcycle mechanic who did no engineering study or large scale sampling.  Plainly put, roughly honed 2-stroke dirt bike engines that see a lot of expansion due to being air cooled are not the same as the micro finished water cooled car engines which see more regular heat cycles and expansion.  Short life race engine and motorcycle engines are not the same as Volkswagen TDI diesel engines designed to last 300,000 miles and more.

The overall engine break in goals are are proper heat cycling, mating of parts, and stress reliving accomplished by incremental increases in varying rpm and load.  You want to avoid excess engine wear, idling longer than you normally would, and high load on a cold engine.  Let's look at the various parts of an engine and drivetrain see how they are affected.

A component by component examination of engine break in

Clutch - Let's start with something that no one disputes.  Everyone will tell you that the clutch should be driven in moderate stop and go conditions for the first 500 miles with no excessive slipping.  This transfers a layer of bedding material on the clutch, flywheel, and pressure plate, and prevents glazing that is associated with excessive heat and slipping.  Holding the car on an incline using the clutch is really bad for a stock clutch and should not be done even after break in unless you want a glazed clutch.  This wears the clutch disc and creates excess heat and "hot spots".  Since the outer diameter of a clutch and flywheel disc travels more distance in one rotation than the inner diameter (because it is farther from the center and travels in a larger circle), the disc could, in extreme cases such as drag racing, promote warping and create an uneven friction surface.  See 1000q: clutch FAQ for details on the TDI clutch/flywheel.

Engine crankshaft and main bearings, cylinder head - these are precision machined and/or polished parts and require no special break in other than stress relieving through heat cycling associated with the rest of a normal engine break in (varying rpm/load, etc.)

Turbo seals and bearings - At no time should the journal bearing turbos in your TDI see metal-metal contact.  The bearings float on a layer of pressurized oil.  The turbo wheels can spin at speeds beyond 100,000 rpm and while it's spinning even at engine idle, it's much slower than at higher rpms. 

There are two types of TDI turbos.  In conventional fixed geometry turbos, the turbo speed is directly linked to engine load and rpm, so lower rpm/load while the engine is warming up will help ensure proper lubrication (engine oil is still cold).  Variable geometry turbos are brought up to speed at a lower engine rpm compared to a conventional geometry turbos because the VNT vanes keep the turbo "on boost", but the load and temperatures still increase with higher engine rpm/load.  It's possible that the vanes and machined actuators may need some minor break in but you can't control this because they are not lubricated and because the vanes reach full actuation pretty early in the RPM range.  It's true that one hard run before full warm up won't destroy the turbo, but consistent cold abuse will.  For an explanation of what VNT turbos are, please see 1000q: turbocharging and the TDI.  

Engine pistons and rings - this is the major source of controversy.  Engines in general see compressed air during the compression stroke.  During combustion, the pressures get much higher as the fuel burns.  Diesel engines see much higher pressures than gasoline engines.  For example, a diesel engine can see 2000 psi peak combustion pressure vs. 1000-1700 psi for a gasoline engine.  Running the engine hard when cold, even after engine break in, creates excessive blow-by because the combustion pressures are able to push past the piston rings and cylinders before they expand and seat properly.  Adding to the problem is that engine pistons are not perfect cylinders when cold because they are designed to expand into a more round shape when warm.  Turbo pistons are also designed to expand more to account for the higher pressures of turbocharging.  All engines benefit from a gradual warm up.  Before the engine is warm and before the engine is broken in, the pressures of combustion will push past the piston rings more than during a fully broken in and warmed up engine and contaminate the engine oil.  This reduces engine efficiency, power, and fuel mileage but will improve as the engine is broken in and seals better.  It's normal for a diesel to have more blow-by than a gas car even after the engine is broken in. 

From hour 0, the rings and freshly honed cylinder walls will scrape against each other and begin break in by wearing into each other.  The friction of the rings against the cylinder walls will also help heat cycle the rings.  Just remember that the car you buy off the showroom floor has already had it's engine tested, installed and driven around the factory, on and off a truck or ship, and around the dealership.  Some high performance cars are even driven hard on a track at the factory for quality control.  Sometimes, a bad transporter or joyriding dealer will "wear down the tires" on the dealer prep test drive.  The basic engine break in principles still apply but on a rebuilt engine, take care in initial running and start up.  

From hour 0 you want to begin incremental increases in heat cycling, load, and rpm.  The reason why you want to avoid moderate and high load/rpm operation from hour 0 is because this sudden increase can generates excessive blow by and oil glazing.  This excessive blow by also contaminates the oil, causing unnecessary wear.

Remember that the engine is lubricated by oil.  It is normal for a light coat of oil to be present on the cylinder walls, even more so during engine break in.  Excess heat generated by high rpm/load can cause this oil to flash burn and glaze.  During engine break in, this is the worst thing that can happen because it happens unevenly and generates hot spots where the rings are not yet seated evenly.  This glaze is very smooth and prevents proper ring and cylinder break in.  Remember the section above about the clutch break in and hot spots?  This is even worse.  

Excessive idling and "babying" the engine consistently also creates the same problem.  If the engine does not consistently heat up and expand to close the tolerances, the rings can "hydroplane" over the coat of oil on the cylinder wall, leaving deposits that will eventually build up an oil glaze and create this same problem.  Remember the section above where I wrote "the truth is somewhere in the middle"?  The best engine break in is a moderate break in - not hard and not easy.  Just follow the owner's manual written by the people who made your car.  

In the end, remember that anecdotal evidence or small sample sizes do not make a break in method right or wrong.  We would all like to believe that individuals hold some special power over their cars that will result in a perfect break in and the best car.  The truth is that different cars of the same model will have some engine and powertrain variation, even if everything else is equal.  Some cars are lemons, some come off the assembly line perfectly.  If you still believe in the "hard break in" method, read this MotorTrend article which gets the advice of multiple automotive engineers from Ford, Honda, and GM who all say that the hard break in method is wrong.

http://www.motortrend.com/features/editorial/112_0603_technologue_race_performance

Proper new engine break in procedure

On a brand new engine (rebuilt or replaced engine), let it idle for 20 seconds to let the oil circulate.  You should also be checking coolant levels and other things on a new engine.  Then drive it with normal load below 50-60% until the engine is warm.  Try to keep it below 50-60% (about 2500 rpm on a VW TDI) for most of the driving.  You won't kill the engine if you go above this briefly, so if conditions warrant slightly higher rpm to avoid higher load at lower rpm (like going up a hill), go for it.  You want to drive with medium rpm and load in general.  Drive around peak torque, about 2000-2100 rpm on a VW diesel when on a flat road.  After you've put some miles on it, once the engine is warm, rev it at varying throttle up to but not exceeding 75% with both low and medium load conditions while driving in stop and go conditions.  Do not go to redline, try to avoid going above 75%, and avoid steady rpm driving and excessive idling.  The best thing you can do is to drive up to a maximum of 75% load and rpm then coast down in gear under engine braking (foot completely off the accelerator pedal).  This heats up the engine and then lets it cool down.  Remember, there are oil spray jets in your turbodiesel (and most turbocharged cars) that spray the underside of the engine pistons to cool them down.  Driving with moderate load and then letting it coast down in gear will heat cycle the engine.  This is much easier on a manual transmission car.

If it is a rebuilt engine, change the oil after 10 miles, then again at 50 miles.  Of course, you can decrease this or increase this interval at your discretion.  If this is on a new car from the dealer, just follow the recommended service/oil change in your owner's manual.

Repeat the above for the first 500 miles, gradually increasing the load/rpm cycles of the engine.  Between 500-1000 miles, drive more normally and try to not exceed 80% throttle/load.  A brief, smooth, rpm cycling to 100% throttle with medium load is fine after 1000 miles.  Avoid the use of cruise control or steady rpm driving.  If you have a rebuilt engine, change the oil at your discretion, I recommend at about 1000 miles.

Once you exceed well over 1000 miles, you can drive normally after warm up.  You even want to rev it to redline under moderate load a few times to finish relaxing and heat cycling the engine.  This is not the "hard break in" method that encourages high load and high revving from hour 0 on a cold engine.  Always let the engine warm up with low load driving before higher rpm operation.  This is the difference between the "hard" break in and proper break in: varying the rpm between low and high rpm and load on a warm engine is not the same as the hard break in method of high load and rpm on a cold engine - NEVER do that, even after engine break in.   Avoid the use of cruise control or steady rpm driving, although at this point it is not as critical as from hour 0.  

Once you exceed 5000 miles, change the oil again on your rebuilt engine and/or follow the recommended service in your owner's manual.  Most of the initial break in has occurred.  You will notice engine oil consumption or lower fuel mileage up to this point.  You still want to vary the speed when you can, but it is not as important as before.  

Past 10,000 miles, most of the engine break in has occurred on a diesel but your compression, oil consumption, and fuel mileage may still improve slightly until 25,000-50,000 miles.  Oil analysis shows that even gasoline engines are still slightly breaking in over 10,000 miles.  From now on, avoid lugging the engine, try to regularly drive at moderate rpm and load, and occasionally rev the engine under sustained high load/rpm.  This helps to clear soot out of the turbo by raising exhaust gas temperatures and burning the soot up and blowing it out, especially for cars with VNT turbos.  Even with a gasoline car, going to occasional sustained high load/rpm will help burn up engine deposits.

Final notes

Remember, engine lifespan and condition is more about consistency, maintenance, and long term habits rather than a secret technique that will give you some competitive edge.  Anecdotal evidence or small sample sizes do not make a break in method right or wrong.  People like to believe in a story or that their car is special and in a way, it is special.  Variations in engine build tolerances, overall build quality, if the guy on the assembly line wasn't paying attention, these all make your car unique and produce variances in power, fuel economy, and engine compression.  If a secret engine break in technique gave extra power, why don't hand built Corvette Z06 engines or Ferrari engines get this treatment?  Surely Ferrari can add it to the cost of a $250,000 (before dealer markup) car!  Every Nissan GTR is put on a track and driven hard by a factory test driver so any "hour 0" break in technique is moot.  Every dealer demo car is driven very hard very early, if break in was the single most important factor in car life then all their motors would fail early.  Even if you didn't follow the optimum engine break in techniques, proper engine warm up and consistent maintenance all contribute to a long engine life span.  In the end, almost all original owners of passenger cars get rid of the car before they need a new engine due to wear, so does it make a difference anyways?

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