Yes and no...Holley powervalves are marked ie: 65 for a 6.5 inches of vacuum to open and 95 for a 9.5 inches of vacuum to open and so on. Most aftermarket powervalve are not marked. If you have a big cam and an automatic trans you may be idling in gear at 4 inches of vacuum (like my old cuda with a 590 solid) so I used a 2.5 powervalve.Warm the engine up, put it in drive with parking brake on and observe vacuum reading. Then purchase a powervalve 1.5 - 2 inches below this number and you'll be fine.