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Hi Rob,

I read with interest your article about new engine break-in. It answered most of my questions except this one: If I drive the new engine locally for about 300 to 400 km, can I then take a long road trip where I will be driving at average speed of 110 km/h for about eight to 10 hours each day?

Thanks, Sugrim

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Here's the thing Sugrim, the key to engine break-in is NOT operating the engine at a constant speed, or RPM.

The sole purpose to the break-in procedure is to seal the piston rings to the cylinder walls. This can only happen with high combustion pressures, and these pressures happen when the engine is forced to accelerate under load.

In a perfect world after a new engine is installed in a given vehicle, the car or truck should be driven on a vacant airport runway. The drive should consist of full throttle acceleration from a stop to approximately 60 km/h then brought down to about 15 km/h – with the throttle closed. Followed by another acceleration run to at least 60 km/h, followed by another deceleration to 15 km/h. This accelerate/decelerate process should go on for about 50 kilometres.

During the acceleration period, the piston rings are forced squarely against the cylinder walls, which trues them to each respective cylinder. In effect, the rings will conform to the shape of the cylinder.

The deceleration period creates a very low pressure in the cylinders (relative to the full power combustion pressures) and in the intake system. This low pressure is exposed to the cylinder valves and the oil that gathers in and around the valve guides. This low pressure (essentially a vacuum), draws this oil past the guides and into the engine cylinders. This helps lubricate the cylinder rings in excess of normal operation. The rings need extra lubrication because of the excess friction created by the break-in process.

Now Sugrim, this is perfect world stuff – none of which can be normally expected in the real world. The manufacturers realize this and have been building engines for many years with construction tolerances that have almost eliminated the need for engine break-in. What most owner's manuals stipulate is normal driving, but to avoid constant driving speeds and not to tow or haul heavy loads for the first 500 kilometres. Your manual may outline a different time frame as each manufacturer has their own specifications for engine break-in.

The easy answer is to avoid driving at constant speed. In your case, you have likely broken in the engine already so you will not have any problems with the long journey. For the next 100 km, just try to raise and lower your speeds when the opportunity allows – no cruise-control this trip.

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Send your automotive maintenance and repair questions to Globe Drive experts at globedrive@globeandmail.com

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