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Old 03-04-2006, 10:13 PM
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Rocker-Arm ratio questions

Exactly what all do rocker-arm ratios change about valve timing? I know that they change lift, but what about duration at .050 lift? How do I determin the best ratios for any application?

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Old 03-05-2006, 03:41 AM
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Rocker arm lift will change duration at the valve a little bit. If you are going from 1.5:1 to 1.6:1, its not a huge change and doesn't really affect overall engine RPM range too much. Since the ratio is faster, the point at which the cam reaches lift at the valve is sooner, so it adds a bit more duration.

The best way to choose ratios is by the head flow numbers. The heads will peak their flow at a certain lift. For instance, if you have a head that peaks its flow at .500" lift, and a cam that lifts .350, a 1.5:1 rocker will get you .525 lift. Changing to 1.6 rockers won't net you much power if any at all.

Sometimes the extra lift you get from higher ratio rockers will pay off with the added duration. Often times, heads will flow the same amount at lifts above their peak. Many times a head that flows 250 cfms at .500 lift will flow something like 252 cfms at .600 lift. The added 2 cfms doesn't account for any measurable power, but the added valve duration may add power. Basically the added lift doesn't add any power since it doesn't add flow, but the fact that its open longer by a couple degrees may give it more power.

In a carefully planned build, everything is set up properly for a good match. You would choose a cam with a duration that suits the head flow and power levels you wish to achieve. In a factory-stock setup often times extra duration can be simulated with higher ratio rockers.
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Old 03-05-2006, 04:41 AM
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Go to ChevyHiPerf they have loads of cam and head info.
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Old 03-05-2006, 09:12 AM
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great link malc!!!!!!!
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