[alfa] Re: hogging out exhaust ports

[RonHorowitz <RonHorowitz@xxxxxxxxxxxxxx>]

>majority of vintage British motorcycles have pretty good intake ports and
>pretty horrible exhaust ports. A lot of power is to be had in these
engines
>by flooring up the exhaust ports to increase exhaust gas velocity. This
>improves scavenging and therefore cylinder filling, resulting in more
power.

More power, yes, decreasing emissions, probably not. Unless you change the
exhaust valve events, increased scavenging means more unburned hydrocarbons
sucked out the exhaust during overlap and consequently higher hydrocarbon
emissions. I think the original poster mentioned that he had some sort of
"hot" cams in his engine. The better you can get the exhaust to breath, the
more "mild" you'd want the exhaust cam to be. That wouldn't be a bad thing,
performance-wise, it's just not quite what I was getting at in the last
post. I suspect a hogged-out exhaust might reduce emissions because it
would breath worse, not better. Lower velocity yeilds more reversion, i.e.,
poorer scavenging, i.e., lower hydrocarbon emissions, i.e., cleaner
exhaust. Capiche?
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