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This may be a crazy question but can you increase your HP if you adjust your valves (by changing your lifters or whatever other mechanism is used for shimming) to decrease the gap between the riding surface and the cams so that the cam pushes the valve more, thus increasing the opening through which the A/F mix gets in, increasing the intake ?

What would be the pros and cons of that approach ?

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The short answer, no. I can only speak for cars but the industry almost as a whole gone to hydraulic lifters. These lifters take all the slack out of the valve train and compensate for wear and temperature. In the cars that still don't have hydraulic lifters the valve train is already adjusted for maximum efficiency. The adjustment is compensated for temperature but it can't compensate for wear, the reason the valves need periodic adjustment.

When adjusting valves there needs to be a clearance in the mechanism that allows for thermal expansion plus a fudge factor for just in case. If they are adjusted too loose you will lose performance and they will make noise. If they are adjusted too tight, when the valve train gets hot the valves will get forced open when the should be closed, again diminishing performance.

The only way to achieve what you are speaking of is to change the cam shaft.

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    ...and if you change the cam, you'd better be really careful; if you add too much valve lift and/or alter valve timing too much, it's possible for a valve to collide (catastrophically) with the top of the piston. It's not like there's unlimited clearance in there. – TDHofstetter Sep 7 '15 at 1:37

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