[Skip Fix]

So question for you all on intake carb to plenum openings-reacted to dual plane intakes. Most all aftermarket intakes had two openings -one large one for each planum -upper and lower. The Chevy Z28, L88 have similar common openings. Some of the other Chevy high perf intakes had 3 holes a large one on the upper plenum but 2 separate ones on the lower plenum entrance. Our Pontiacs , and some regular performance Chevy intakes have 4 separate holes for each carb hole. Cliff, SD,Jim Hand maybe also opened up the Pontiac to just two holes like the aftermarket and Hi perf Chevy intakes.

So why?

M/T crossrams I have seen one common opening or 4 holes. Not sure what variants the SD Bathtubs had.

[25stevem]

For me especially in terms of tweaking a Q-jet intake plenum it’s simply to gain a better shallower angle into the front runners to work them.

Also if your going to use a open hole spacer above the stock carb pad you want the open.

[Formulajones]

I have a bunch of those intakes here. SBC high performance was especially all over the place. Some of them have gotten pricey to the point I don't want to modify them.
The L79 is one of the odd ones that you mentioned. Completely open on one side and 2 holes on the other side. It's an odd duck.

I run the 69 Z intake as is, which just has a center divider and it's about 1/4 down from the carb base. GM made that change in 67 for the Z motors and stayed with it throughout LT1 production ending in 72. It's pretty much what the aftermarket has copied for decades.

BBC aluminum was pretty much a standard center divider unless you got into the L88 and ZL1 stuff that had that divider cut down.

That center divider design has shown on the dyno to work better with these Pontiac intakes, getting away from the 4 hole design. Since the iron pieces are still cheap and plentiful I don't feel bad modifying those.

Some possible reasons?
More plenum volume? Possibly better fuel distribution as more cylinders can share more of the carb? More air flow?

I say that and the Z intake still responds to stagger jetting though.

Interesting question though Skip when you look back at those other designs and wonder what they were thinking.

[Skip Fix]

The 409 intakes had a 90 degree turns in runners. I read one article where they talked to one of the engineers thought was to cause turbulence to keep fuel in suspension.

You wonder if keeping long individual "holes" into the bottom plenum like out Pontiac and 3 holed Chevy keep up velocity but then making the turn into the runners would be harder.

[Cliff R]

Just a warning for those considering modifying stock intakes. Be very careful how much of the divider that you remove. One very quickly reaches a point of no returns with that deal. I would NOT recommend cutting any of it down. Instead right between the secondaries simply grind a "notch" about the size of half a 50 cent piece. Grinding a bigger notch or all of it will gain nothing, and somewhere along that route you can start loosing power, or even worse induce some "turbulence" into the air stream that can cause stumble/hesitation/bog going quickly to full throttle.

There are no negatives anyplace taking the four holes on the stock intake and making two big openings out of them, or basically copying the Edelbrock Performer or RPM.

I've "modified" a good many of them here, and Dave at SD scanned my modified intake and for years offered a CNC modified/ported version of it. The intake is dyno and track tested and will run with and even outrun the RPM to at least 500hp. The margin of victory can even be quite a bit better with the stock intake as the RPM is over 1" taller and often requires modification to the factory air cleaner, Shaker, or Ram Air set-ups that can cost some power. For example having to use a drop-base air cleaner that moves the air cleaner lid less than 3" from the carbs air cleaner gasket mounting surface will cost you some power. Probably as much as or even more than you could have gained from the intake swap right to start with.

I attached a pic below of my modified intake next to a later version of the Edelbrock P4B Q-Jet intake. .....

[Formulajones]

Originally Posted by Skip Fix

The 409 intakes had a 90 degree turns in runners. I read one article where they talked to one of the engineers thought was to cause turbulence to keep fuel in suspension.

You wonder if keeping long individual "holes" into the bottom plenum like out Pontiac and 3 holed Chevy keep up velocity but then making the turn into the runners would be harder.

I haven't looked inside a 409 intake but I know what you mean about the 90 degree turns in the runners. They looks sharp and abrupt on the outside.

Are they the same on the inside??

[Half-Inch Stud]

Oh about 1990 i studied the intake inhales vs 21843657 with likely min-max performing cam profiles. Conclusion was the dual-plane was far-superior to Open Plenum for uniform inhales/CYL. I recall " 7 robd 5" and "2 robs 1", Yet the "flat-H" dual-plane implementation suffered from the bends, so i thought.

In terms of Street, Dual-plane would be best for FI, while open plenum best for carbs.

Well, to achieve a 7-5 & 2-1 full inhale solution, i formed& added a full-height sheet metal divider to lengthen, thus isolate port 7 runner from 5, and extend the sheet metal to the carb-middle and ending at the FRONT to isolate 1 from 2. when done the added shape was rather convincing. Yet the DS Pri-Sec served 3 CYLs and the PS served 5 CYL.

Note, interject that the OFFYL DUAL QUAD is also a fine solution for uniform inhales/CYL.

Well, i drove this divider thing around and convinced my self that no harm was done, and the performance was better (yet that is unproven). So a month or so later i pulled the carb to looksy and found the sheetmetal crumbled and rammed into CYL 8 mostly. Lesson learned is to have super thick, sturdy solutions to survive the radical flexures induced from such Pressure-vacuum cycling!!

Might redo the divider on a spare Torker-I while enjoying a ported mysterioso Dual-Plane.

[78w72]

Originally Posted by Cliff R

and Dave at SD scanned my modified intake and for years offered a CNC modified/ported version of it.

SD is still doing the CNC intake mods, but stopped doing the repro & OEM RA/HO intakes due to casting issues.

http://www.sdperformance.com/listPro...?categoryID=11

[Skip Fix]

Originally Posted by Formulajones

I haven't looked inside a 409 intake but I know what you mean about the 90 degree turns in the runners. They looks sharp and abrupt on the outside.

Are they the same on the inside??

Yep. The NASCAR 380 HP 409 intake had the plenum totally open.
Here is a picture of the production 380 HP '62 intake that was not open. HP bumped to 400 in 63 and 64 for the HP single 4 motor.s

[Skip Fix]

NASCAR intake

[Formulajones]

Originally Posted by Skip Fix

Yep. The NASCAR 380 HP 409 intake had the plenum totally open.
Here is a picture of the production 380 HP '62 intake that was not open. HP bumped to 400 in 63 and 64 for the HP single 4 motor.s

That design looks nearly identical to the 66 L79

[Skip Fix]

Yep some of the early 327 aluminum intakes shared the runner design.