Which Intake manifold gasket

12many said

Interesting. So when gasket matching the intake or not, refering to the transition as mentioned above (let’s just say you taper/blend it in, going as deep as possible) if the head port openings are smaller than the intake, the intake runner openings should either match the heads or they can be bigger? I’d have to say I always assumed, and likely read often elsewhere if you weren’t opening up the intake to match the heads, as long as the openings of the intake were smaller at least you wouldn’t have that step.

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It depends. Any time you do that you put an hourglass shape in the port. If you can picture it in your mind (I can’t draw worth a crap or I would draw something out) a port starting at the carb Mount and going to the intake valve where the dimensions never change. It’s the same shape from start to finish (which isn’t always a good thing either).

Now picture that same port at the manifold/head when you open them up right there. You now have a bulge in the port. And it’s a rather sharp bulge, especially when the pushrod pinch is right there.

At that bulge you will have pretty much a venturi. But it’s a bad venturi because you close down the port after it. Look at the shape of the venturi in a carb. You get a bigger opening, then it gets smaller and then it opens back up.

Doing all that grinding at the manifold face makes a sort of reverse venturi. And with that reverse venturi you get wacky pressure changes right there. So a local pressure change in which the air speed drops as the cross section increases and the pressure goes up. Then as the cross section gets smaller, the air speed goes up and the local pressure goes down (high air speed/low pressure and low air speed/high pressure).

So you always want to think through everything when you grab the grinder. Dimensional changes in port cross section should be kept to the minimum and if you can’t help it you want the changes to be as smooth and long as possible.