Kacyc3

I see more posts coming up about using them, I thought that was old technology and using dual pattern was better for making power.

Solid LT1

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.......

Kacyc3

http://www.airflowresearch.com/index.php?cPath=24_25

based that they are at the very minimum, wouldn't they benefit from a smaller dual pattern?

63mako

If you have 80% I/E ratio, free flowing exhaust with headers single pattern is best. Single pattern reduces overlap for better manners. Stock heads and exhaust might need 12 - 14 degree split. That is why AFR is what I usually recommend. No one has a better I/E ratio and less overlap helps bottom end torque, streetability, vacuum and power where you use it with little to no sacrifice in peak power or RPM. Win, Win.

7t9l82

in the "old days" guys would use a 1.6 rocker on the exhaust to crutch a lousy exhaust port.

gcusmano74

Alright then, it kind of begs this question:
Why didn't Chevrolet make the exhaust port/valve bigger?

7t9l82

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.

427Hotrod

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

StraubTech

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.