Which Intake manifold gasket

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GREEN DART 5

GREEN DART 5

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I want to install a cast iron manifold with big ports on to a 1984 318 with small ports. Should I use the small port gaskets or big port gaskets? And what’s the best gasket to use? The 318 is all stock. Thanks FABO.
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I believe the gasket for the manifold, I personally would not think it would be worth the work//massive reversion issues==no throttle response/no mid range... better off finding a better carb..... or port match/polish a post 66 4bl 273 intake..
 
jazak5 said:
I believe the gasket for the manifold, I personally would not think it would be worth the work//massive reversion issues==no throttle response/no mid range... better off finding a better carb..... or port match/polish a post 66 4bl 273 intake..
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As long as the port in the manifold is the same size or bigger than the port in the head it’s fine. When the intake manifold port is smaller than the port in the head I whe it’s very bad.
 
I believe that is backwards. Ok for manifold to be slightly smaller. You just don’t want a wall at the transition. A small jump ok. Running into a small wall bad.
 
Also you need a spread bore carb or adapter. The manifold came with a thermoquad.
 
furrystump said:
I believe that is backwards. Ok for manifold to be slightly smaller. You just don’t want a wall at the transition. A small jump ok. Running into a small wall bad.
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No, that’s exactly right. You never want the intake smaller than the head. You can test that is 5 minutes on a flowbench to see it.

You never want a dimensional change in the port unless you can’t avoid it or you are talking about taper in the runner.
 
yellow rose said:
No, that’s exactly right. You never want the intake smaller than the head. You can test that is 5 minutes on a flowbench to see it.

You never want a dimensional change in the port unless you can’t avoid it or you are talking about taper in the runner.
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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 mixture hitting that step.
 
I’m just discussing the transition from intake to head. Not the runner size/shape etc. It is my understanding you never want the flow to impact a right angle surface. So an intake port ever so slightly smaller than the head port is the better way to go. It also helps with reversion because those pulses hit the right angle as the flow tries to reverse up the intake tract. Perfect being exactly the same.
 
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.
 
furrystump said:
I’m just discussing the transition from intake to head. Not the runner size/shape etc. It is my understanding you never want the flow to impact a right angle surface. So an intake port ever so slightly smaller than the head port is the better way to go. It also helps with reversion because those pulses hit the right angle as the flow tries to reverse up the intake tract. Perfect being exactly the same.
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Yup. Certainly YR is talking “big picture”, the entire port/runner from intake plenum to bowl. Not just widening at the gasket surfaces with only short minor tapering/blending creating a shape he is referring to. Going deep, working a smooth radius or contour is part of the gasket matching step, which many here are aware of. Others maybe not, but are now:thumbsup:
 
:lol:
jazak5 said:
I believe the gasket for the manifold, I personally would not think it would be worth the work//massive reversion issues==no throttle response/no mid range... better off finding a better carb..... or port match/polish a post 66 4bl 273 intake..
Click to expand...
like I said (reversion)
 
furrystump said:
I’m just discussing the transition from intake to head. Not the runner size/shape etc. It is my understanding you never want the flow to impact a right angle surface. So an intake port ever so slightly smaller than the head port is the better way to go. It also helps with reversion because those pulses hit the right angle as the flow tries to reverse up the intake tract. Perfect being exactly the same.
Click to expand...


No, you need to think in terms of air flow (fluid really) and what it does when it encounters transitional changes.

If you make the port in the manifold smaller that the port in head you WILL induce turbulence right there as the air will try and get to the low pressure area made by that mismatch. You NEVER want to induce turbulence or swirl or any of that junk in a port, and that’s exactly what you are doing. Air is always looking for low pressure areas to get to, and that mismatch does just that.

If the manifold is bigger than the head, the air will pull away from the mismatch and form its own smooth transition. It’s not optimal, but it’s better than the other way around.

It is 100% a power killer and it makes tuning a PITA.

And I’m not sure how you look at a port and isolate out one part. It is a system. In fact, the valve job has a direct influence on the carb and how it behaves. So you can’t take a part out of the system and isolate it.

You never want the port in the manifold smaller than the port in the head. Ever. And if matching the ports makes an hourglass shape don’t do that. Better of leaving it alone.
 
yellow rose said:
Reversion? That smaller port in the manifold will create enough turbulence that you’d wish you had some reversion.
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My reference is the high pressure created by the "wall" of the heads mating area//is there enough turbulence and/ or port volume of the intake "dimples on a golf ball" effect on a "stock" 318 to make this a MOOT issue or will it do like I said and kill everything besides WOT ???? due to the increased port volume in the intake VS the volume of the head port/killing velocity ??
 
Absolutely, the exact same size is ideal. In practice with tolerances/gasket/intake shifting and with the flow hitting a right angle being very bad. I leave a very little margin. My guess is I’m running exactly that hour glass situation. The stock intake is gasket matched, but the runner tapers at the final bend in the intake. My cylinder head is ported and I have to assume the port ends up being bigger than the taper in the stock intake even with being gasket matched. I’m “building” an intake currently, that is such a shot in the dark due to me being a ham fisted monkey, no idea what I’m doing and the space constraints I have to live with. What could go wrong!
 
I have been there with a 360 intake on a 318, i bought the truck that way, and it ran like crap, switched to a 318 intake, and it fixed the problem, i have seen a smaller port intake work with bigger head port, but not big intake on small head ports.
 
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