i wanna know why a 318 prefers a single plane?, even though i am running a holley street dominator on one with a 1 in. open spacer. my butt says it ran better with an ld340.
They don't necessarily do. In fact nearly all the time they work better with a dual plane intake. Most single plane intakes favor upper rpm's and less torque due to their large open plenum. The streetmaster is the exception to the rule because of it's very small plenum. It also had small runners. Combine the two and it worked good on a small engine that wasn't prone to making a lot of torque yet it also opened up the top end power some. The street dominator was the next step up with a larger open plenum, but still much smaller than most single plane plenum intakes and it also had small runners which increase velocity. In your case you made a mistake by installing a 1" spacer. By doing that you increased plenum volume a lot which is contradictory to what most 318's respond to, unless their a high compression big cammed engine with a high stall converter and/or low rearend gears. My suggestion is to remove the spacer and see what it does.