What's the difference between...

[Raoul Ross]

... a 4-hole base gasket and an 'open' one? What are the ramifications of using one over the other as far as performance and/or economy? Does some of the mixture get pulled into the secondary side of the intake using an open one when just on the primarys?

When I've ordered what shows as a 4-hole gasket from Cliff, I received an 'open' one and I'm just wondering what the differences are, if any.

[Raoul Ross]

So nobody has any facts or opinions on this? 

[Kenth]

I would use the gasket that matches the intake.
Open plenum 1-plane intake or 2-plane w/center divider, use an open gasket.
2-plane intake with 4 holes, use the 4 hole gasket.

The mixture will be pulled to all ports anyhow, there is no "primary" and "secondary" side in the 2-plane intakes as the are connected. The 2-plane intakes supports 4 cylinders each plane.

Hope this makes some sence.

[Cliff Ruggles]

GM went to an open gasket for their intakes starting in 1970 for a reason.  It allows both sides of the intake to see each other which provides a leak path the help smooth things out during engine operation.  I don't know the science behind it but like using an "X" or "H" pipe in the exhaust system ahead of the mufflers there are small positive benefits.....

[Raoul Ross]

Thanx Cliff, that's kinda what I figured. I ended-up using a 4-hole 'thin' gasket on top of a 4-hole metal heat shield plate on top of the thick 'open' gasket and it runs really well, very smooth. 17085292 on an '85 C10 suburban 5.7l 2-wheel drive. The customer is very pleased!

I put primary bushings in it also as I couldn't get the idle speed down, reference https://cliffshighperformance.com/simplemachinesforum/index.php?topic=5060.0