Just wanted to share some personal experience, as I have seen this question come up many times. Take with as many grains of salt as you wish....
I've been running a 1" 4-hole phenolic spacer on my 440. It's 9.5ish/10 to 1 compression, auto trans, RPM heads, 850 speed demon mech. carb, XE274 cam(230° @.050 and .488 lift on the intake) with 1-7/8 headers and 3" pipes. 2500 stall converter and 3.73s to 28" rear tires. I originally installed the 4-hole spacer to alleviate fuel bowl heat problems and it worked very well. So this morning I decided to swap in a phenolic open spacer that I had stashed....and it works well, contrary to some people's fears that it would not function well with the dual-plane intake. The first thing I noticed, is that the car would not idle at the present settings so I needed to adjust the four corner mixture screws(no big deal).. I also noticed that I lost one inch of vacuum at idle in gear(no big deal). Once readjusted, as I've driven it throughout the day I have noticed a very slight loss of some torque down in the 1500 to 2000 range. That is pretty much my cruising RPM though, so there isn't much throttle movement at that point anyway. Coming off the line with some throttle, it moves right past that point and as it approaches 3000 rpm(which happens very quickly) I feel a definite increase in power, no question. Before, this damn car would begin to spin the tires if I throttled it too quickly at even 25 or 30 miles an hour so I decided that I could afford to lose a couple lbs/ft way down low, and it seems the trade-off was a good one. One caveat: I don't have track times to back this up, it's strictly seat-of-the-pants feel...but I can definitely feel the difference in the rpm range where the car can use the extra power. Another caveat: the Performer RPM, as you're well aware, it's not a full dual-plane intake, as it has the divider cut down some. So this may not be an apples to apples comparison on a regular dual-plane. The point of my rambling is, using the open spacer didn't "kill" the throttle response, it didn't "ruin" the vacuum signal or "flatten" the bottom end, or any of the other maladies I'd read about. With no changes other than about 1/8 of a turn out on the mixture screws (ha that's a lot for a Demon LOL) it's feeling stronger than before.. Next step will be to hook the A/F meter back up and go through and fine-tune it, I have a feeling there's a little more to be wrung out of this baby!
I've been running a 1" 4-hole phenolic spacer on my 440. It's 9.5ish/10 to 1 compression, auto trans, RPM heads, 850 speed demon mech. carb, XE274 cam(230° @.050 and .488 lift on the intake) with 1-7/8 headers and 3" pipes. 2500 stall converter and 3.73s to 28" rear tires. I originally installed the 4-hole spacer to alleviate fuel bowl heat problems and it worked very well. So this morning I decided to swap in a phenolic open spacer that I had stashed....and it works well, contrary to some people's fears that it would not function well with the dual-plane intake. The first thing I noticed, is that the car would not idle at the present settings so I needed to adjust the four corner mixture screws(no big deal).. I also noticed that I lost one inch of vacuum at idle in gear(no big deal). Once readjusted, as I've driven it throughout the day I have noticed a very slight loss of some torque down in the 1500 to 2000 range. That is pretty much my cruising RPM though, so there isn't much throttle movement at that point anyway. Coming off the line with some throttle, it moves right past that point and as it approaches 3000 rpm(which happens very quickly) I feel a definite increase in power, no question. Before, this damn car would begin to spin the tires if I throttled it too quickly at even 25 or 30 miles an hour so I decided that I could afford to lose a couple lbs/ft way down low, and it seems the trade-off was a good one. One caveat: I don't have track times to back this up, it's strictly seat-of-the-pants feel...but I can definitely feel the difference in the rpm range where the car can use the extra power. Another caveat: the Performer RPM, as you're well aware, it's not a full dual-plane intake, as it has the divider cut down some. So this may not be an apples to apples comparison on a regular dual-plane. The point of my rambling is, using the open spacer didn't "kill" the throttle response, it didn't "ruin" the vacuum signal or "flatten" the bottom end, or any of the other maladies I'd read about. With no changes other than about 1/8 of a turn out on the mixture screws (ha that's a lot for a Demon LOL) it's feeling stronger than before.. Next step will be to hook the A/F meter back up and go through and fine-tune it, I have a feeling there's a little more to be wrung out of this baby!