just saw in the "tragic rollover" thread that engine builders say high rpm, no load will hurt the rods, or is the most stressful condition to put the rods in.

why is that? I haven't heard that before. there's no load, so the only stress on the rod is the piston/pin/rings weight as it reciprocates.

how is high rpm, no load different than say, part throttle cruise when manifold vacuum is high? sure, during the compression and power stroke, the piston is being "pushed against the rod" so the rod doesn't see tension, but on the intake stroke, as the piston goes down it creates vacuum above it and puts the rod into tension.

is it just that under no load, the rod is in tension twice per 4 cycles vs under load/cruising, it's under tension only once per for cycles?

and bottom line, I love downshifting my engine with the manual trans and hearing the engine as it "engine brakes" while slowing down. is that adding additional stress to my parts and causing a higher chance of breaking parts?

The two big things for me having a clean neutral after a run are as follows. You are at max RPM and max ouput through the lights. You lift the throttle and instantly the engine goes from the pistons driving the crank, the crank rotation transfering to the the rear end driving the car. To the rear end turning the driveshaft and in turn rotating the crank and moving the pistons. In an instant the forces changed to direct opposites. Harmful? In a well built engine probably not really but sure does not help it any. The second and the biggie for my cars is RPM time. The less time I am at an elevated RPM the less wear my engines experience. When you have a limited ring and valve spring life less time is good time.

The no load condition to a loaded condition is just the
rate of the rpm's that the engine will see... no load
is easier to see high rpm(faster) if you missed a
shift... I down shift on my auto into second when
I'm low enough in rpm's to help slow my car and I
dont like hitting the brakes hard because of the
rotors(light weight 1/4" rotors).... a alum rod
engine has X cycles in them so in that case no you
dont want to down shift... thats why most guys running
alum rods also tend to use a clean neutral to kill
the engine

OK. I can certainly understand keeping the total number of revolutions down on an engine, when your engine has a limited life span for rods (aluminum) valve springs, etc.

so it's more an issue of engine life/wear, and not an issue of load? When I cross the line in my Dakota (last time at the track with the 5.2 anyway, haven't been with the 5.9 yet) I was crossing at about 4800 rpm in 4th gear, and I just leave it in gear, and barely use the brakes when slowing down.

big benefit this gives me is that keeping it in gear, the PCM sees the right conditions to cut off the fuel injectors, so I'm burning zero fuel and adding zero heat, yet I'm still at speed and RPM, so the radiator is cooling off and the water pump is pumping water through the engine still. I can be back down to my T-stat rating of 180 before I turn off the track

Cup engines are designed for that abuse. Heavier pistons, steel rods, ect. They are built to go 500 miles plus. A set to kill drag engine is lucky to see 20 miles. Apples to oranges there.

the cycle in the engine at wot that's always hard on a racing engine is the transition from the exhaust stroke to intake stroke. that's the cycle that destroys connecting rods by pulling the wrist pin/ piston from the eyelet or loading the big end of the rod and rod bolts.
when you let off the throttle at high rpm, you've effectively shut the valve (carburetor) that allows the normal compressive forces to take effect on the compression/power stroke as well. that effectively puts two times as much stress on the assembly. hence the invention of clean neutral valve bodies to help eliminate this product.
i never recommend downshifting a racing engine to slow the car for that very reason. who needs the extra wear and tear. if your brakes are insufficient to stop your car, upgrade the brakes. obviously the higher the rpm the engine see's, the worse the wear, and possibility of damage would be.
better components will take the abuse longer than others, but it's still abuse IMO.
think of your engine wear as engine cycles. the fewer the cycles, the less wear and tear. the less wear and tear, the longer you can go between freshen ups. seems like a no brainer to me.