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I never found anything explaining why some people say x% is better than y%. I am just copying/pasting as I find anything.
A good rule of thumb is that normally aspirated engines whose heads have exhaust-to-intake flow-ratios under 70 percent (i.e., the exhaust flow is 70 percent that of the intake) like a dual-pattern cam to crutch the weak exhaust port. Over 80 percent, you're in single-pattern territory. Blower, turbo, and nitrous motors generally prefer dual-pattern grinds because of the larger-than-normal exhaust volume they generate.
Then there's that 70-80-percent "gray" area in between. The only way to find out is to try both styles of cams, different ratio rockers, or (if the cam is mechanical) lash-loops. On a 9.25:1 383 small-block Chevy equipped with World Products Sportsman II heads, Ventura Motorsports' Ed Taylor tried both a Comp 268H High-Energy single-pattern cam and a Comp XE268H Xtreme-Energy dual-pattern grind. The XE represents more modern technology and has more aggressive lobes than the venerable 268H, but with the fairly good-flowing World Products heads, the 268H made more torque and power over 4,800 rpm on this engine.
A good rule of thumb is that normally aspirated engines whose heads have exhaust-to-intake flow-ratios under 70 percent (i.e., the exhaust flow is 70 percent that of the intake) like a dual-pattern cam to crutch the weak exhaust port. Over 80 percent, you're in single-pattern territory. Blower, turbo, and nitrous motors generally prefer dual-pattern grinds because of the larger-than-normal exhaust volume they generate.
Then there's that 70-80-percent "gray" area in between. The only way to find out is to try both styles of cams, different ratio rockers, or (if the cam is mechanical) lash-loops. On a 9.25:1 383 small-block Chevy equipped with World Products Sportsman II heads, Ventura Motorsports' Ed Taylor tried both a Comp 268H High-Energy single-pattern cam and a Comp XE268H Xtreme-Energy dual-pattern grind. The XE represents more modern technology and has more aggressive lobes than the venerable 268H, but with the fairly good-flowing World Products heads, the 268H made more torque and power over 4,800 rpm on this engine.