I'm not an engineer...

[RustySho]

I've been reading post after post after post and I've been wondering-

How do particles of the catalytic converter get sucked back into the engine? I have seen a few posts with people talking about this problem, or listing it as a fear they have, but...
I don't understand how this could occur. I've never heard of this problem outside of the SHOtimes and SHOforum.

Even if backing off the throttle at 7200 rpm significantly reduces the pressure of the exhaust, the pistons are still pushing the spent gases out through the valves, right?

For particulate matter to get past the exhaust valves i'd think that you'd need a significant reduction (to below atmospheric pressure, or vacuum) on the exhaust side combined with valve timing that allows exhaust valves to stay open during part of the intake stroke.

Am I right in my thinking?

Thanks,
Steve

 

[Rob94]

Opinions vary, but it is pretty much an "old wives tale". Common sense says it is impossible to happen.

 

[smcallis]

Actually there is some physics behind it. When the engine is producing exhaust, at the engine there is high pressure due to the pistons pushing the exhaust out of the combustion chamber. At the end of the pipe atmospheric pressure is lower than at the engine so the exhaust achieves some velocity. When a gas is given a velocity it's static pressure goes down...this is the key. If you're in a WOT situation and the exhaust gas velocity is high and pressure is below atmospheric. When you let off the throttle, the engine is no longer providing that high pressure zone to keep the exhaust flowing in the right direction. So at that instant, you still have low pressure in the pipe and atmospheric outside (higher pressure than in the pipe) and the engine high pressure source has been removed. What happens next is a back pressure wave propagates backwards through the pipe and can conceivably blow cat pieces upstream. If the timing is right an exhaust valve is still open and the wave takes the chunk right into the combustion chamber.

I once had a suzuki GS750 with a Vance and Hines header that would do this at a specific RPM with moderate throttle. It would make this characteristic bobble noise where the freqency and magnitude of the pressure waves caused by the pistons allowed pressure waves from the atmosphere to make their way back up the pipe. Kind of irritating actually. But it's a common phenomea on motorcycles with headers.

Hope this answers your question.

 

[twr]

smcallis:
Actually there is some physics behind it. When the engine is producing exhaust, at the engine there is high pressure due to the pistons pushing the exhaust out of the combustion chamber. At the end of the pipe atmospheric pressure is lower than at the engine so the exhaust achieves some velocity. When a gas is given a velocity it's static pressure goes down...this is the key. If you're in a WOT situation and the exhaust gas velocity is high and pressure is below atmospheric. When you let off the throttle, the engine is no longer providing that high pressure zone to keep the exhaust flowing in the right direction. So at that instant, you still have low pressure in the pipe and atmospheric outside (higher pressure than in the pipe) and the engine high pressure source has been removed. What happens next is a back pressure wave propagates backwards through the pipe and can conceivably blow cat pieces upstream. If the timing is right an exhaust valve is still open and the wave takes the chunk right into the combustion chamber.

I once had a suzuki GS750 with a Vance and Hines header that would do this at a specific RPM with moderate throttle. It would make this characteristic bobble noise where the freqency and magnitude of the pressure waves caused by the pistons allowed pressure waves from the atmosphere to make their way back up the pipe. Kind of irritating actually. But it's a common phenomea on motorcycles with headers.

Hope this answers your question.

What he said thumbs_u

 

[pjtoledo]

Overlap.

Perry Toledo,Ohio

 

[DavidT]

so, what would be some symptoms of an engine that had this happen?