Single pattern cams
Le Mans Master
Joined: Dec 2003
Posts: 5,727
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From: Fremont CA
The practice of using dual pattern cams comes from the fact that most cylinder heads have a lousy exhaust port flow compared to intake flow. If the exhaust port flow in in the 80% region of the intake flow, you can cut back on exhaust duration and build more power. There aren't many heads out there with good precentages, the AFR stuff is pretty good.......
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Drifting
Joined: Jun 2012
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From: Port St. Lucie Fl
The practice of using dual pattern cams comes from the fact that most cylinder heads have a lousy exhaust port flow compared to intake flow. If the exhaust port flow in in the 80% region of the intake flow, you can cut back on exhaust duration and build more power. There aren't many heads out there with good precentages, the AFR stuff is pretty good.......
based that they are at the very minimum, wouldn't they benefit from a smaller dual pattern?
Race Director
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From: Millington Illinois
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based that they are at the very minimum, wouldn't they benefit from a smaller dual pattern?
based that they are at the very minimum, wouldn't they benefit from a smaller dual pattern?
Last edited by 63mako; Dec 20, 2014 at 10:41 PM.
Le Mans Master
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2023 C3 of the Year Finalist - Modified
some factory heads were better than others, but guys ported heads back then more commonly than today. chevrolet because of input from racers like Smokey Yunick and Bill Jenkins got into hard racing parts more seriously. what we always called the Turbo casting was the first attempt at giving us a decent foundation. plus the layout of the small block sort of dictated to a degree the port size. And remember it was originally only 265 cu,in.
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This all depends on the RPM range you want to play in also. If you want it to hang on past peak HP...you're going to need some added exhaust duration in most cases.
JIM
JIM
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Drifting
Joined: Jul 2013
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From: Tri-Cities TN
At 75% I/E ratio with a head sized to the CID and rpm application a cam will see the same duration at .050" for both intake and exhaust. Above this the duration will favor the intake. If you look at import cams and cylinder head flow the import stuff is in the high 80's on I/E ratio. Most import cams have more duration on the intake.
As CID grows and rpm grows this rule will change as these demands will change what the engine needs.
As CID grows and rpm grows this rule will change as these demands will change what the engine needs.