2000 454 MPI L29- with raw water cooling. I have 6 inch spacers between the stock manifolds and elbow. The online parts sources indicate that a 'restrictor' gasket was used between manifold and spacer (or elbow if not using a spacer)- this has the water passages on the sides open, but only a small hole on the fore-aft water passages. The Merc manual states that using restrictor OR full open gaskets is fine when replacing the manifolds. Obviously the full open gaskets would flow more water. When replacing the manifolds, I found that one side has the full open gaskets, while the other side has the restrictor gasket. wtf ?

Anyhow, what should I use? Full open, or restrictor gaskets ?

I went with the open gaskets on both sides of my 6" riser logs and notice no diff. If you want to run the restrictors, they go on the bottom side of the 6" risers at the manifold.

They do this to equalise the pressure between both risers..if one side is not restricted then the other side may flo more water then the other..then you may get steam..

thanks fixx, I was wondering on the why myself for a long time. but didn't bother me enough to ask.

Although in my case they were different left to right engine bank, the manual and the microfiche parts listing show that they should be identical, and the original parts were supposed to be restrictor gaskets. I know one side has been replaced before (the side with restrictor gasket) but the factory side showed open gasket. I guess I will go with open gaskets, but all I have on hand are restrictor gaskets, and they are the stainless sandwich style so they are hard to modify.