[Firebob]

Any pics around showing how it's supposed to look? Anything not to do?

[steve25]

No room for even a 1/2" spacer?

Are you running a square bore or spread Bore Carb?

Even though you have 462 cid you may not be happy with the loss of bottom end with only 3.23 gears, but anyway the shape to make it is like a D with the center of the D going down to the floor of the short Plenum.

[Firebob]

I'll have to measure. I have just the thick gasket( 1/4", 3/16"?) under the qjet and with the biggest drop base and a 3" filter there is no room between the hood and air cleaner lid..

[Schurkey]

This is how Edelbrock did it:







I would NOT cut all the way down to the floor of the upper plenum, or you'll have liquid fuel sloshing into the lower plenum. The upper plenum will run lean. (It's also probably too large an opening.)

[STEELCITYFIREBIRD]

Originally Posted by Firebob

I'll have to measure. I have just the thick gasket( 1/4", 3/16"?) under the qjet and with the biggest drop base and a 3" filter there is no room between the hood and air cleaner lid..

An important question is how much room is there between your air filter lid and the carb!

[78w72]

not the best pic but this is how cliff & SD performance do it on a stock intake. its about 1" long & 1/2" deep & tapered to a "V" shape.

[Schurkey]

Originally Posted by 78w72

not the best pic but this is how cliff & SD performance do it on a stock intake. its about 1" long & 1/2" deep & tapered to a "V" shape.

Perhaps they'll explain the significance of the "v" shape. I don't understand it.

[Firebob]

Thanks fellas. That gives me something to go by. Easy enough done. Way easier than port matching another intake.

[Tom Vaught]

We did a bunch of testing years ago at Holley and found the 3/8" deep x 1.00" slot to be sufficient to equalize the pressure in both the upper and lower plenums. Less and the air is working too hard on the different pulses from side to side to balance properly.
Why a 1/8" gasket with that area removed does not do much.

Dave at SD with his 1/2" depth dimension is not hurting anything with the 1/2" x 1.0" slot. And the "Pyramid Cut at the top of the divider helps smooth the air/fuel vs a flat spot at that location.

Tom V.

[STEELCITYFIREBIRD]

Interesting, so will a thicker, say 1/4" gasket with no center accomplish the same?

[PAUL K]

Originally Posted by Schurkey

This is how Edelbrock did it:







I would NOT cut all the way down to the floor of the upper plenum, or you'll have liquid fuel sloshing into the lower plenum. The upper plenum will run lean. (It's also probably too large an opening.)

Edelbrock machined that notch in the plenum divider on the 4B series manifolds so the Holley 3bbl. carb would work with their manifolds.

[Firebob]

Sounds like any shape( square ,D, V ) 3/8-1/2" deep and 1" wide will work to equalize pressure on a dual plane. Any ideas if there is a change in performance?

[TCSGTO]

Originally Posted by Firebob

Sounds like any shape( square ,D, V ) 3/8-1/2" deep and 1" wide will work to equalize pressure on a dual plane. Any ideas if there is a change in performance?

Jim Hand,Cliff and others claim a gain at the mid to upper rpm range with no loss down low in power or throttle feel.

[gtofreek]

Originally Posted by PAUL K

Edelbrock machined that notch in the plenum divider on the 4B series manifolds so the Holley 3bbl. carb would work with their manifolds.

Exactly what I was going to say!

[Firebob]

So it looks like the placement of the notch is just forward of the center of the secondaries. Is the location critical? I'm just wondering if there is a reason for its placement in the divider?

[STEELCITYFIREBIRD]

The 3bbl, notch placed so the plate can open.

[Tom Vaught]

Nice picture of the notch location for the 3-BBL carb blade.

I have several of those 3-BBL carbs so in the future (on dual plane intakes) I will add that Notch to the intakes. Looks like about 7/8" from the back wall and the the notch needs to be 1" forward from that dimension and 1/2" deep.

The 3/8" deep notch might work of you had the thicker 1/8" Holley Gasket.
Not the thin paper one.

Tom V.

[STEELCITYFIREBIRD]

Originally Posted by STEELCITYFIREBIRD

Interesting, so will a thicker, say 1/4" gasket with no center accomplish the same?

Anyone? AS far as performance, not 3 bbl use.

[Formulajones]

Here are a couple of pictures of how I prefer to do it. I personally don't like cutting the intake itself because it's difficult to reverse if you don't like the results.
However I've never had a bad experience with these mods. I've done several and find a slight performance gain on every one I've done.

I don't do the "V" shape, mine is more of a contoured bowl with the sides smoothed so it doesn't disrupt flow or cause turbulence from side to side.

I like to do it with the spacer, whether that be plastic or wood it doesn't matter. I have a picture here using both types. A thin 1/2" spacer is just enough to get it done and have good results. The second pic is a 3/4 inch spacer, with a 1/2" spacer on top. The divider mod is only done on the top 1/2" spacer. I did this one in this manor so I could decrease or increase the plenum volume for some testing I'm doing, while at the same time, keeping the exact same divider mod in place by just adding or removing the lower 3/4" spacer.

The other idea to keep in mind is that at the base of the carb, you want the intake or the spacer (which ever you are using) to be the exact same size as the throat of the carburetor bores. So it helps to have the base plate handy of the carb you are using when making these spacers. Then as the spacer/intake goes down into the plenum it can slowly be opened up. Doing it in this manor gives excellent throttle response, idle, and off idle drivability.

[Formulajones]

Here's what I did on our 69Z. The intake being original and expensive, I'm not cutting or grinding on it. I opted to take a different approach.

On these intakes GM cast the center divider 1/4" below the carb flange all the way across. I don't really care for the primaries seeing each other, so I made a piece out of aluminum that sits tightly on top of the divider. I then run a 1/4" thick heat soak type of gasket for the carb, and just removed about a 1 inch long section between the secondaries so they can see each other. Basically what I accomplished here is I closed the primaries from seeing each other, left the secondaries seeing each other as they did before, and added 1/4" plenum volume in the process without modifying the intake in any way.

When I was done with this setup, it made a nice difference in off idle throttle response. Very snappy, and the idle seemed to be more consistent and smoother, more pleasant to drive (254 @ .050 in a little 302)