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In general the previous response is quite true. There is another ramification
though.
If you have a cam with a lot of overlap (both valves open for some time)
it will get the best power at speeds where the exhaust valve closes just
before any intake blows out with it. Below that speed some of your intake
charge is blown out the exhaust, resulting is less torque at lower speeds,
poorer fuel economy (not usually very significant in the overall scheme of
things) and perhaps driveability problems around town. It is a lot quicker
and easier to add a little back pressure to your system than to change cams,
and it will result in better low-speed performance and poorer high-speed
performance. Back when I was a teenager (when dinosaurs roamed the earth)
they had "Lakes Pipes" which were branches off the forward part of the exhaust
coming out under the sides of the car with blind flanges bolted over them, and
when you wanted high speed performance you just unbolted the flanges and
let 'er blast. In my dad's day they had exhaust cutouts that you could install
but they became illegal in a lot of places (not long after they became
popular). They would accomplish the same thing.
--
George Downs, Bartlesville, Oklahoma, Central US
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