I was talking to someone about exhaust recently and he said something that alarmed me a little. He said when you have a straight pipe, or straight through muffler, and no cat converter, cold air can rush up the exhaust pipe and hit the hot turbo, after you have shut the motor down, causing damage to the turbo.

Is this really a concern? It sounds a little funny to me because I haven't read about any 305/555 HO Cummins owners (no cat from the factory, right?) with after market exhaust saying this happened to them.

BTW, he said you need a "heat sink" (like a cat converter) to stop the cold air from rushing in and damaging the turbo.

What do you guys think?

 

Sounds like a load of crap. I bet somebody heard about running motors w/o an exhaust manifold (which CAN damage the exhaust valves because of cool air right there) and "extrapolated" this to mean that air is somehow going to:

a) find a reason to rush down a considerable length of hot exhaust pipe when there's nothing at the other end drawing it in

b) still somehow be cold after traveling through a hot pipe


Has anyone EVER heard of this happening?

 

As mentioned above, I want someone to explain to me how cold air is going to rush through a hot 4" exhaust pipe from one end of the truck to the other.

 

Ask him to explain how all the Semi's with VERTICAL Exhaust and straight pipes have run like this for years and years without a problem.

If he's dumb enough to believe that nonsense, he dumb enough to believe that the cold air will rush faster down vertical pipes (because hot air rises, cold air mist fall at an extemely fast rate, right ) and actually spins their turbos backwards

 

He probably got confused with facts... If an 18 wheeler is towed backwards, the stacks can injest air and drive the turbo backwards causing it to run with no lube. When transporting an engine the exhaust and intake need to be plugged.

 

it doesn't matter if you tow it backwards [wind pressure creating a positive pressure in the stack] or forwards [possable negitive pressure in the stack]... and the direction of the turbo turning doesn't matter either [no oil pressure there, it will toast it in both directions]. it should not be allowed to spin at all while being towed [like you said, cap the intake/exhaust]

 

horse pucky

 

wish i new this sooner ...

i had a straight pipe on my first gen for 275k ...

ran perfect untill the day i sold it

 

Riddle me this....how will the semi turbo spin at all if the air has nowhere to go? The air "coming in" the backward facing exhaust stack would have to pass through the engine....how would it do that when the engine isn't rotating (i.e. valves closed, etc)?

Same for the original question....the air has nowhere to go so how would it rush up the pipe?

 

well, if you had some valve overlap, I suppose it could happen, but big diesel engines probably have no overlap based on the RPM band they operate in...

I see brand new double stacked 18 wheelers being pulled down the highway all the time... no caps or anything over the stacks.

Forrest

 

Thanks for all the responses! I was thinking it was a bunch of rubbish but this guy did have some background with driving 18 wheelers so that's what made me consider it... why did I even consider it in the first place?

 

Believe it or not, when I worked for a heavy equipment dealer all of our asphalt rollers came in on flatbeds. All were Cummins powered. We switched shippers and all of the sudden we had a rash of turbo's going bad within the first hour of run time. Everyone of them was a bushing failure. It took the factory about 4 months to figure out that the new shipper wasn't taping over the exhaust stacks. The day they figured it out the turbo failures stopped. I've never seen so many turbo cores in our shop at one time in my entire life!

The cold air coming up the pipe comes from the racing world and the short pipes that they run (aka top fuel). Cold air coming in the extremely short pipes they have after a run was thought to be causing exhaust valve failures during the subsequent runs. I don't recall if it was ever proven but some of the competitors put caps on the shorties after a run while the engine cools down.

Just my experiences, YMMV.

Richard

 

If you look at heavy equipment being trailered, you should notice that the exhaust is blocked. Mostly done with a free coffee can. This keeps the turbo from spinning as said above.

 

As my good friend Forest Gump once said, "I may not be a smart man".......where does the air go allowing the turbo to spin?

 

Through the overlapped valves and out the intake tract. We pulled a new one off before we ran it and it was galled from the cross country trip on the flatbed. I had to see it for myself to believe it even though it made sense to me.

Richard