Plenum volume, open hole is supposedly better for a dual plane intake since one half of the floor is higher than the other.
an open spacer provides more plenum volume and helps the fuel dropplets to make the turn into the ports smoother/less sharp into each port. It can...in effect make the engine "think"...there is a larger carburetor on it. It slows the air/fuel speed some. Much of how the engine "see's" the spacer, depends on the engine combination itself and the size of the current carburetor. A 1/2", 1", 2", 3" thick spacer etc., to get the most from what you do, will take some extra tuning time. If you just slap something on and don't take the extra time to properly tune for the change, you might not see any change at all...! The four hole basically is just a riser, maybe helps the fuel turn the corner at the bottom of the spacer into the ports (slightly slower thAn no spacer). Normally it doesn't do a whole lot for actual performance. While it does inlarge the volume a little, its effect is very small. Best thing t do is to buy a couple, then find a chassis dyno shop with an exh. gas meter...and go to work. Otherwise, you need a really sensitive butt dyno..! Mike
have used a 1" aluminum spacer with the four holes that had swirl lines in them (Trans-Dapt & others) and have noticed better bottom end performance with dual plane manifolds on Chevy & Ford. used same basic style on a Chevy with a TBI set up with same positive results.
I am using a wiend steath and it has the plenum divider and heard to put a 1 inch riser as to make it more of an open plenum so i was just trying to figure which one to use so sounds like the open one is the one to use?
I experimented a bunch with spacers on the weiand stealth intake on my motor. I tuned the jetting with each spacer as it DID make a difference. I ended up using a 4 hole 2 inch spacer AND changing to a larger carb since it is a street truck and I liked the bottom end it gave and the throttle response. With the 4 hole I couldn't get the 600 jetted rich enough for my motor. I was way off the tuning chart so I put a 750 on. With the open spacer I was able to use less jetting and a 600 carb but the bottom end was lacking. If I were tuning it for the drag strip only I would have probally kept the open spacer as it seemed to really pull at higher rpms.
Two or three effects cooking here... 1. Open plenum allows each cylinder to "see" all four barrels, 4 holes keeps each cylinder pretty much fed by its own half of carb. Thus open has some of the effect of more carburetion, good if engine wants more but quite often a low-end problem on mild engines. 2. Spacer lengthens intake passage, usually longer improves low-end. 3. Turn in from vertical to horizontal flow in the manifold...this can affect flow and also tendency for fuel to separate out from air as it turns. There is a great Dave Vizard article on reworking a stock manifold in this area, mostly in the radius of the turn, that helps to explain what's happening...but bottom line is that a larger radius in the turn from vertical to sideways is good.
wow, we were discussing the same thing in shop class today. One of my students has the 1 inch spacer with the removable 4 hole insert and was wondering what the performance difference was between running it with or without the insert on his single plane intake. I got close on the answer, will share tomorrow ...Thanks HAMB
A nice thing about this is that cost of experimentation is low...you can find out what your engine likes.