Does it do any harm to run and engine with only the manifolds. I know wear ear plugs.

I've done it

Won't hurt anything. Another myth saying that it will.

how many race cars out there with open headers?

how many "mud trucks" with nothing more than about a 2 foot length of pipe, bent to go straight up out of the hood?

you'll be fine, as long as there's nothing that the hot exhaust gasses can melt or burn in their path.

Manifolds are a little short. Too much ox. getting too close to the exhaust valves. I've seen some do it, but there's a big diff. between the extra collector length on headers and such and stock mani's. I wouldn't let it run very long, certainly not on a break in run up. I've started motors just to see if they do, but only fo 30 sec. or so, but's that's me.

Drove several cars this way to the exhaust shop when getting work done. Don't think I could handle it long term.

I am doing a 426ramcharger and plan to have the stock exhaust caps open most of the time. My wife says I can't hear anyway.

If you run it at operating temp with exhaust manifolds open, plan on a burnt valve or 2 unless it is rich as a pig (not likely since open exhaust will lean it out). Ask me how I know.

I worked in the exhaust lab at Chrysler for 35 years
and we would need to measure the stress on the exhaust,
the gauges couldnt take the temps so I would weld
a plug over the manifold exit then drill a hole in
the top of the manifold and weld a length of tubing
so it would exit over the roof line... we beat on
those cars ALL DAY LONG for days and never burnt a valve

i ran open headers on my demon. got pretty good at stabbing the clutch, and killing the ignition as i rolled past cops, then cranking it back up, after they had gone by. on the highway, the droning sound gets to be a bit monotonous. i wore ear plugs for any driving more than a quick trip somewhere local.

night driving was fun, as running WOT would make the coolest orange glow under the car. killing the ignition while in gear, then cutting it back on, made for awesome backfires, again that looked pretty cool in the dark.

Seems like everyone is saying approximately the same thing I have always heard. None of this is in play when the engine is running of course since cool air can't get back in when the exhaust is rushing out. So what happens when you shut it off? Supposedly manifolds are short enough that cool air can reach the valves before that air would be warmed by the manifold, so the cool air damages the valves. Headers or zoomies are (supposedly) long enough that the cool (ambient) air doesn't reach the valves to damage them. Are the tubes long enough that the air gets heated enough that when it arrives at the valves it isn't a problem, or does the air never really flow up a dead end long passageway at all? I don't have a clue, but I do know a couple people that had problems after running new engine installs (manifolds only) around town a day or two goofing off before taking the time to get exhaust installed. I have also seen my uncle and two buddies do break in runs and test drives on plenty of engines (open manifolds or headers only) and they never had a problem, but they always insisted on stuffing cloth shop rags in the manifold or collector hole as soon as the engine is shut off. Maybe they were being paranoid on the headers but that is what they did. You'd probably have to be an old timer to even remember when shops had red cloth "grease rags" that were picked up and laundered every week. The blue paper towels everyone uses now won't work so well. LOL
You be the judge, this is just what we did and it seemed to work. Even if it is hog dip, I will be following this practice with old towels since it makes me feel like I'm doing the right thing.

Thanks for the replys. I only want to run it short periods, like in and out of the garage and around the driveway. 5-10 min.

It's amazing what lengths people will go to to defend their cherished beliefs even when they aren't true.

Ever wonder why most of these beliefs make life more complicated?

R.

OK but make sure they are sealed to the head. This is where my problems comes from.
Broken bolts = burnt valves.
Those valves were burnt and not just a cherished belief.