Any reason why I can't break in an engine with long tube headers only (no pipes/mufflers)? I mean beside it being LOUD?

No but its nice to hear what's going on. i adapt some old muffers to mine when I do it.

Agree. Also oil filters are cheap . After adequate break in time cut it open . Have a look. I do this after most days of racing. Has saved a disaster once so far.

Any reason?

Yes if something is going bad you won't hear it till it's too late , hang mufflers off the headers.

By all means, mufflers, as stated already.
Ditto the oil filter check.
Water only, no antifreeze, in case there's a leak.

And get a timing light on it and bring the advance up to 35* at 2000 rpm or your headers are going to glow cherry red and it WILL overheat ...

Good points guys. Thanks.

Everything above is true...

BUT

My first engine build was when I was 13 it was a 440hp with some decent improvements.

I broke it in with open headers. Engine gowling at 2k as I broke in the cam and the headers were glowing red. It was freakin awesome.


Haven't done a break in with open headers since but it was worth every second on my first build as a teenager.

Live and learn ....

I did it with open headers for cam break in back about 1980 , luckily nothing bad happened. Broke in a 440 about 2014 with headers and mufflers, cam started going flat about 5 mins in , if I had open headers we would have never heard the strange tick

Better with mufflers if possible. Loud noises don't do our ears any favors + if you live in certain types of neighborhoods a 20 minute cam break in with open exhaust may mean you get to explain yourself to the police.

amen to both of the above !