Leaving an engine running?

[JimNiki]

Originally Posted by Auturbo6

Has to do with the size of a given piston when cold, or not at operating temperture. Certain pistons tend to shrink more when cold, requiring more time to get to correct size within the bore. However much more able to handle combustion processes from that point on. Some highly worked engines can sound horrible on cold start up.....

the hypereutectic pistons have a higher silicone content as opposed to normal
Silicone resists the shrinking/enlarging with temperature changes.

But too much makes them brittle...
I think they're about 16% instead of the normal 10-12%

[Franco Cozzo]

I turn on the car, chuck the heater on and walk away for 5 or so minutes, but thats to defrost the windscreen and get the interior nice and warm before I drive off.

[4.0i_SiX]

Modern fuel injected cars can be started and pretty much driven straight away. Always take it easy on a cold engine until it reached operating temp. However warming the engine up before taking off will not cause any harm either. Diesel or not. 5 minutes is excessive and simply a waste of fuel. They have such thin oil now that they should get oil to the top of the engine almost immediately

Older carby engines need to be warmed up for a few reasons, they generally have thicker oil, larger piston to bore tolerances etc. But most importantly, some wont run properly til warm. It can be dangerous when the engine dies whilst pulling into traffic