[greggj]

What if you ran the car in just the lower 3 gears, so a 50mph cruise in 2nd or 3rd would have steady high revs? (slight thread drift here)

as far as the engine goes, it doesn't care about a gear box. It cares about the rpm, and that's what this is all about ;p

[GrahamB]

Actually, as far as Lycomings/Continentals are concerned, It's not about rpm, it's more about keeping sufficient combustion chamber pressure to force the piston rings out hard enough to wear down the high spots and seat them snugly to the cylinder walls.

[Keef]

Exactly so - effectively, it's about percent power.

When we fitted the new engine in G-UTSY a couple of years ago, there was a bedding-in instruction that was strict enough and warned enough about the consequences of not following it, that we did a "bedding in" flight following the instructions exactly. It involved so many minutes at this power setting, then so many at that, and so on.

[johnm]

My donkey had a 2 hour bench run followed by a two hour bedding flight and I followed it to the letter!

[Charles Hunt]

/it's more about keeping sufficient combustion chamber pressure to force the piston rings out hard enough to wear down the high spots and seat them snugly to the cylinder walls.

OK, I'll display my ignorance here.

What difference does the rpm make to the pressure inside the combustion chamber?

Surely as the mixture burns, it develops a particular pressure inside the cylinder irrespective of the rpm.

(What I have in my mind here is a piston somewhere about TDC, and as the spark fires the mixture it creates some specific pressure.)

Having said that, I can see that at higher rpm this maximum pressure is achieved more often. Is that what BMEP is all about?

[GrahamB]

OK, I'll display my ignorance here.

What difference does the rpm make to the pressure inside the combustion chamber?

It doesn't. It's just that in a fixed pitch prop installation, you can't decouple the two, all other things being equal (i.e. in straight and level flight). Slightly more complex on a variable pitch machine, but the principle remains the same - you need high cylinder pressure to do the bedding-in job, and this is best done at high revs/MP, i.e. high power.

To run a typical piston aero engine in, usually requires something like 2 to 5 hours at as close to maximum power as possible, followed by the remainder of the first 50 hours being carried out at a minimum of 75%. Some or all of the 100% power bit may be done on the bench by the overhauler.