More likely what DAVE said... air blowing across the top enters the turbo and also exhausts thru the muffler also.... just like driving down the road with one window open..

Best demonstration ever, was while sitting in a railyard with a spot tamper parked alongside the building.  Little Cummins that was ALWAYS shelling turbos, the shop guys blamed Halsett for the failures.  Prevailing wind blowing off the ocean by that maintenance building basically 24/7, the rain cap would bounce up and down, and while standing beside the manifold, you could hear that turbine whirl. 

It doesn't matter which way ANYTHING is pointing, any difference in pressure can cause the turbine to spin, and if there's no oil pressure, the bearings will run dry 'till they die.

Like I said, what does a sock or shop rag cost?

tbran wrote: In 1976 AirResearch had some bad turbos. They blamed it on hauling the tractors with uncovered exhausts and thus spinning the turbo w/o oil. We were working in La. with carpenter tractor east of Alexandria.  We pulled the intake hose off the turbo  - put a paint drop on the compressor wheel and drove the unit on a truck at 70 MPH - never got the turbo to spin. We even left the intake hose off - made sure some valves were at overlap and took off the muffler of the 7080. Nere a spin hauling forward or backwards. The amount of air that could go thru an engine is so small in CFM that the air will go around the fins and not spin the spool. The amount of venturi effect on the exhaust is equalled on the intake.  AirResearch had to recant and paid for the turbos.  Having said that we try to cover the exhaust when hauling to prevent customer blame if a turbo fails..

Thanks, just could not fathom enough flow to hurt the bearings.

Had a boat dealer tell me to put the engine in gear for road travel so the prop wouldn't spin. I've watched this on other boats going down the road, if the prop spins, it's very slowly and the amount of air going over it will keep it cool.

Mike- it's not the bearings, etc that are in a problem situation with a whirling boat prop... the lower unit of outboards and I/0s, and the V-drive transmissions, are full of oil, and under basically no load, so there's not a really serious problem here... the clutch dogs will get a little more wear, and if they're rattling against the dog faces as you go, they WILL start to chip the facing off and destroy the dogs...

...but the biggest problem is the propshaft seal.  The seal's lip is designed to be submerged and wet, which prevents the rubber from cutting into the shaft.  Spin it dry, and two things will happen-  first, the lip seal will start to burn up, and second, the shaft will develop an abrasion groove.  Run it in the lake for a bit, and your drive lube will weep out, eventually leaving no lubrication.  Especially bad in I/O drives, because the upper gearcase is the first portion to become unlubricated, and cooling in the I/O depends on that level being proper, and oil circulation (flung off the tapered vertical shaft) stops when the level is too low... hence, those heavily loaded (and now uncooled) upper pinions melt together catastrophically.

In inboards and V-drives, the shaft log seal is a graphite or wax-impregnated rope, and it, like propshaft lip seal, is lubricated by lake water.  Spin that shaft dry for long, and the stuffing box will fail to seal, and in the case of a boat that'll be stored in the lake, it will leak, swamp, and sink.

So it's better to drop it in gear, so that just doesn't happen... or in the case of my runabout, I have a heavily varnished plywood cover that drops over the end of the prop, larger diameter than the prop blades, and a bungee cord that goes around the front of the drive leg.  Not only keeps it from spinning, if someone walks up and bumps into the prop, it won't cut a big gash in their leg.  It's a surfacing cleaver, and it certainly lives up to it's name...  I got a few scars bearing witness ;-)

I, personally, have never seen a turbo damaged from spinning dry, but I can't imagine that it would do any good by having the turbo spin with no lubrication. The "experts" say it should be covered, so why take a chance?

When I was younger I was employed by a dealer that sold red equipment. I did all the hauling of the red things, the rule was to in some way or other block the air intake on turbo engines.

Dave