Running engine without heater hoses attached

[elflamo]

I'm doing engine running in without the heater hoses to the flapper boxes attached to the heat exchangers

Can it do any harm? would it negatively affect de mfi thermostat? I know the thermostat respons to the heat provided, but maybe the hot air is not delivered in sufficient quantities.

Any drawbacks that people know about?

[bkreigsr]

The MFI thermostat needs a forced hot air supply to function.
The MFI race cars use a cockpit hand lever to manually move the rack in/out in lieu of the forced air. Your car will run very poorly (rich) without an operable thermostat. Not the best conditions to be breaking an engine in.
Bill K

[elflamo]

OK, but would there be a relevant air pressure difference if the hoses are off vs connected + flapper boxes set to just dump into open air?

[bkreigsr]

yes

[will hung]

Additionally, having the heater hoses attached helps evacuate the hot air that sets in the heat exchangers, allowing the engine to run cooler. If you run the engine this way for a prolonged amount of time, it will be like running it with the old thermal reactors.