With a 454 and setup that has transom pickups (no pickups through the outdrive) and garden hose connection for flushing. Why do you need to run the engine while flushing? I noticed when you first hood up the garden hose before you run the engine water comes out of the tail pipe. Why?

Two things i can think of off hand....
1st your seawater pump will act as a block inthe line due to the rubber impellor and not let water run through properly if at all sometimes.
2nd most head gaskets from what i hear dont like more then 18-20 psi on them....and most municipal water supplys are 35psi or more so you run the risk of blowing a head gasket and filling engine with water.....

Thats my guess anyway i always run the engine anytime i have water hooked up and running

You are only flushing water through the exhaust. If the engine is not running, then the T-stat is closed and no fresh water will enter the engine.

Griff is correct. The cooling system is equipped with a bypass so that there is ALWAYS water flowing through the exhaust manifolds to keep them cool.

So in your example you're only flushing the exhaust system unless you fire up the motor and let it come up to temperature so that the thermostat opens and circulates water thoughout the entire system.

Ok, so is there anything wrong with just flushing the exhaust system with out firing up the engines? Some times I can't flush the engines because I come back to late a night, so maybe doing a flush with out the engines running would be better than nothing???

If you are running in salt water, I would flush the motors also. By flushing the exhaust only you will be leaving salt water in the engine causing problems down the road.

I realize that, but I just want to know if there is any harm in flushing with out running the engine besides the circumvention water running through the block.

You want to flush the engine, not just the exhaust.

To complicate this, lets say you have a crossover w/ no thermostat.
Will the flush run through the engine if not running?

How does it work with a closed cooling system like a 496 or a 525
is it safe to hook up the water hose to the hookup on the swim platform and run the engine to heat up for a oil change ?
Or should I put on outdrive ears or put out drive in a tank of water?

Any harm? No. Any good? That depends on what you're trying to accomplish. What they are telling you is by doing that, you are not flushing the engine, only the exhaust. You're not harming anything but you aren't helping the engine, but it's not hurting it. If you have salt water in it, THAT could hurt it obviously.

Kind of like washing my car, but only the trunk and doors. Doesn't hurt anything, but doesn't help the rest of the car either.