Engine idling and running in

[TwoWheelTerror]

I know that when running in an engine it's bad to do this by idling.

Could someone explain what causes the damage at idle? - is it oil related? .. and perhaps this will also explain why its not recommended to warm up engines by idling also.

[ian_w]

The load on the nose of the camshaft is highest at low speeds. So if you have a very agressive cam and ultra high load springs then you cam wipe out the cam during cranking and idling. However, for most engines this is not really an issue and idling puts very low loads on the rest of the engine so will not cause any problems.

[phoenix]

Piston rings and bores have to 'bed in' to each other. This often will not happen at low, constant engine speeds - all that happens is that the bores glaze and the optimum seal will never happen after that. As this bedding in creates more heat than usual, extreme high revs should be avoided - moderately high engine speeds are better.

On modern engines machined and built with close tolerances, bearings, I am told, do not need the same treatment as they bed in almost immediately.

[Jeff 8]

For the same reason do people still recommend mineral oil for running in, I notice some manufacturers recommend synthetic from new.

Jeff

[AU N EGL]

If your running an accu or dry sump oilling system, you set it up to switch ON the sump and pre-lub the block before cranking over the motor.

Most engine parts are made with such close or good tolorances that synthtic oil all the time, even for break in periods.

[SidewaysFeltham]

When an engine idles, it is the most destructive in many ways.

The engine is not reaching correct working temp. This allows the creation of acids and other corrosive elements in the post-combustion product (which has not been totally consumed) to "wash" down past the rings into the sump.

It also allows the creation of more water vapour: which again washes down into the sump.

This water can emulsify with the lubricant, reducing its effectiveness.

An idling engine does not consume all of its fuel: additionally, the fuel:air mixture tends to be richer than normal (for various reasons including overcoming the tendency to stall.), fuel washes oil from the bores and is carried down into the sump; diluting and changing the lubricant's composition.

At idling speed, the lubrication system is not delivering sufficient lubricant to ensure a film coats all wearing surfaces. With reduced lubricant, high contact presssure surfaces (as has already been stated, e.g. cam lobes to followers) become metal-to-metal contact.

Most race engines are designed to function, optimally, within a narrow rev range centred above and beneath maximum power: additionally they tend to have a very narrow torque band which is also centred around the optimal power curve points.

Pistoon ring gaps are normally designed to function best at this narrow rev window. At low speeds, pistons "Blow By" significantly, thus allowing a whole series of nasty stuff into the crankcase and thus sump. (Since the ring gaps are set to reduce until full race temp is achieved. If set lower, to seal at low temps, they would "Pick Up" and smear at race tmp).

Go to an historic meeting. Watch the techs warm up (e.g.) A Maserati 250 F.

It is up on rear stands, in gear, with "soft" plugs in, set at 4,000 RPM ish.

Think why!

[MGDavid]

TWT, can you put your question in context and tell us what engine the advice will be used on ?(the numbers game tells me it probably isn't a Maser 250F!)

The most common engine in motorsport is probably the A series, what advice would be relevant to that?