power valves tuning

75 Duster 360 said

You can also remove the power valve completely and block it off with a dummy. You will then have to increase your primary jet size, usually to the same size as the secondary jets, called "squaring it up". This will make your car more consistant, but use more fuel and be running slightly rich when street driving.

Click to expand...

You're pretty close. The power valve is like what Loco said. It's a spring-loaded-open valve that engine vacuum holds shut. Vacuum drops when the throttle's opened, and fuel gets pulled through it. But, it's not really metered, and it dumps unevenly into the intake. Race cars don't need part throttle transitions, so in order to get more accurate tuning, the power valves are pulled and the metered jetting is used to compensate for the fuel the power valve dumped only in a much better controlled fashion. The normal increase is 8 jet sizes when the PV is repalced with a plug. Squaring the jetting means having the same jets in primary and secondary.