I have done EXTENSIVE testing with spacers and before I go any further I'll simply say that most folks reading this are NOT going to like my results.
I've tried them on the street, dyno, and at the track, on several different engine/drivetrain/vehicle combo's and remarkably came up with the same results every single time.
The spacers tested were 4 hole, fully divided, fully open and semi-divided (open between the secondaries only.
First you have to divide the testing into two categories. Single plane intakes and dual plane intakes.
Single plane intakes absolutely LOVE at least a 1" spacer and often work very poorly without one leaving a LOT of power and vehicle performance on the table. They like fully open spacers the best and one that is nicely blended into the plenum area.
Dual plane intakes do NOT like spacers much. Fully open and four hole were the WORST when we tested them. We saw some power on the dyno with fully divided and semi-open spacers. However at the track we saw losses in 60' and short times that were NOT made up by a slightly stronger top end charge.
I'll also add here that anytime we had to use a drop base or air cleaner assembly that moved the lid closer to the carb we LOST power and vehicle performance, far more than was added by installing the spacer.
Anyhow, it would take many pages to cover all the testing, but for the most part they are a waste of time/funds on most set-ups they end up on. To this day I chuckle to myself when I walk thru a car show and see how many people have installed them on engines making a lot less power than the ones we tested them on, and think that somehow they are helping them out someplace.......FWIW.....
