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Discussion Starter #1
What happens to duration between a higher compression motor and a lower compression motor? Example 400 inches both revving to 8000, same heads (400 cfm). One motor 7.5 to 1 and the other 10.5 to 1. Does duration change and how much?
 

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Duration is the ability to sustain power at a given rpm range. If you don't change the rpm than the engine will need the same amount of time (duration) to hold the valve open to make max power. The engine with more compression will just make more power.
So the lower compression motor is now max at 9000 rpm so the duration should generally increase? Is there a consideration in cam design for longer duration for cylinder fill on lower comp. engines? Thanks for responding to my question.
 

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Discussion Starter #5
I once changed a flat tappet cam to another in a 350chev 6/71 blown engine....1st cam was [email protected] on intake 2nd cam was [email protected] on intake......the second cam picked up almost 100HP with no other changes.....
This was a 7-8 to 1 deal? Just trying to get an idea of what longer duration will do in low comp. Supercharger deal. Thanks for showing your results on a cam change.
 

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It had 8.5:1...the bigger duration would kill off the engine a bit off boost at lower RPM but will allow more boost and more top end power....still need to consider stall and diff gear when choosing duration
 

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I mis read this. Super Chargers: You want to pack the cylinders, but since you are forcing air in shorter duration cams are used as you don't need to keep the valve open as long to draw air and fuel in. Duration is still the ability to sustain power at a given rpm. The higher the rpm the longer you need to hold the valve open.
 
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