If a ABC engine will be running on a pipe, should it be ran-in also on the pipe or should one use regular muffler first?

Hi!
Doesn't matter how you do as long as you run the engine a little rich. Simple as that!

Run it with castor oil in the fuel and a smaller than usual prop, then let it run fast as you monitor the exhaust spray and engine temperature. 1 minute or 2 minute short runs followed with checking the glow plug and bolt tightnesses between runs. When the exhaust residue is clear and the needle is steady, the engine is ready.
I usually use 25% oil for the initial break in.
Run it WOT only at a slightly rich scream. Do not play with part throttle setting until after the engine is broken in.

when i run in engines with a pipe i run them with the smaller prop very rich at the rpm i plane to run the larger prop at on the ground.

Clear oil is good and rich. once the engine is broke in and if you still have clear oil then you can increase compression or nitro. medium dark brown oil is best

You can run-in the engine on the pipe, as others noted.

Key is getting the engine up to operating temp, up to desired rpm (pipe rpm), and allowing it to run just slightly rich/cooler at that desired rpm.

The difficulty with this.is sometimes ...
You would want to have the pipe set up and know approximately what RPM your target is.

Generally not a problem. Starting with the pipe set a bit long is best. Get some time on the engine, prop for your target rpm (will likely get there with a light prop even if the pipe setup is little long). This will give you a broader needle valve range and that helps out a bit.

Thanks for the help guys