Anyone run high-flow Cats to reduce hydrocarbon exhaust smell?

jbc426 said

Even at a 750 rpm idle the piston is going up and down 12 1/2 times a second. That's a high enough speed to introduce a bit of intake charge into the exhaust with a high overlap cam, but it's nothing compared to the amount of times the piston cycles up and down at 6700 rpm, which equals 111 2/3 times a second.

Of course the event we are talking about only occurs at half that rate. It is a four cycle after all. My point is that there are violent forces at work even at idle.

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I'm not sure I agree with that, the fact that the engine is throttled at idle and there is vacuum on the intake side causes reversion of the intake charge which most everyone is familiar with... it's a common occurrence with engines that have high-overlap cams to get carbon buildup in the intake ports from burned gases escaping back through the intake valve before it shuts. The absolute speed of the piston is only important with direct relation to the cam timing events; 12 1/2 times a second seems fast but not if you have an ICA of 70+ degrees past BDC, the gas charge is essentially bouncing back and forth instead of flowing completely from intake to exhaust.