Over the past few years I've been making changes to my car to hopefully get it up to Factory Appearing Stock Tire rules eventually.

I'm starting to mess with my stock intake and I'm questioning if I should leave an intake divider in it or just open the whole upper plenum up?

The intake is a 69 2806301 for a 383. In stock form it has a 4 hole opening for the carb which I've already started to remove but, I could leave a divider in it.

My personal opinion is that the extra plenum volume would most likely be more beneficial than the divider. I've seen some dyno results for other intakes where the divider was removed and as a result, hp and torque improved. This thing is really going to choke my 470 to begin with so; again, I think the extra volume would be more important.

I'm no pro so; I'd like to ask the more knowledgable individuals on this site what you would do?

If it were me, I'd remove part of it and test. I'd try a trimmed down divider first with a full radius on top of the divider. Taking the divider out entirely isn't going to gain you much in the way of volume. I'd look at adding spacers if the rules allow

Twice I have tested removing a divider on aftermarket aluminum performance dual plane manifolds and both times there was a decent increase in power. I think you would see the same, or maybe even more considering how restrictive the factory iron manifold is.

But with that, I think you should really do your own testing to see what works for your motor.

First off I would ask if you have another one
of these manifolds to keep stock.. if so then
I would start milling in steps.. get ready for
some dyno time.. I did a lot of this testing on
a 440 that I made about 10-12 tunnel rams for
just to see what happened.. I made all of them
out of sheet alum and pressed out my runners

I do have another of the same intake that will not be touched so; I could compare if I wanted.

If you test the car or motor with the std, unmodified intake, then do a back to back test by simply adding a 3/4" or 1" open hole spacer, you will generally know what you will get by milling the divider that amount. It won't be exact, but close enough for decision making before you actually cut into a manifold.

I agree.. he is looking for tendencies of what might
happen.. like I said before.. I did a lot of this
on a tunnel rams that I built just to see the
tendencies of shorter, taller, different shapes on
the runner

I have built several street stock pulling truck engines where the rules say you have to run a stock iron manifold. Removing the center divider and grinding a radius where the carb flange turns into the runner makes a big differance in power. Their rules allowed a spacer which also helped.
One of the engines ran so good they protested it and pulled the carb to see what we had done to it. It was all legal. If you have access to a flow bench you will find a factory dual plane has very differant flow to individual cylinders, a little grinding to try and get them more equal also helps. I have never gotten all the runners to flow the same. but did get them closer.

Vanke did a lot of experimenting carving up dual plane intakes and saw considerable gains in power. These were Hemi 2X4 but the results proved the benefit.

The 301 intake is a HUGE restriction to flow.

Making everything more or less "as big as possible" will still end up being way too small.

At about 500", in FAST type builds, the 4bbl motor with a reworked OE manifold(normal porting mods) is down 60+ hp to a 6bbl combo.

Jim Seiler had an extremely modified 301 manifold on his high effort FAST 511 build, which was still slightly down on power compared to a pretty "bread and butter" 500" 6bbl build.

I realize you're stuck running what you have to run in that class, but the 301 manifold is probably the single biggest hurdle to overcome for one of those builds...... Well, besides the exhaust manifolds.

If it's legal, you definitely want the OEM 1971-* ThermoQuad manifold, much bigger plenum. Adapt or modify it back to your bolt pattern as needed.
Removing the divider will reduce low speed torque by reducing manifold vacuum, but the extra volume from the other level generally more than makes up for it.
Expect some change in cylinder-to-cylinder distribution.

I'm almost positive that the low deck didn't get a Thermoquad intake and carb. until 1972, the 1970/71 383 Hp motors had Holley on the HP motors also, not AVS carbs.

Agree with Cab, again

For FAST, you are supposed to run the "correct" year/casting number intake manifold for the year of the car.

I can arrange that for you if it's really important.

It is and that is all they look for , it has to LOOK right .

A 383 based anything is NON COMPETITIVE so I bet they would allow a doctored up thermobog intake ...

I have a 301 intake with the plenum divider removed that was done by fast68plymouth

In a reasonably well set up 3650lb b body, in FAST legal trim, in decent(but far from mine shaft) air, a "normally" reworked 301 intake can get you into the 11.40-11.50 range.

With more exotic modifications, like what Jim Seiler did with his, the bottom of the 11's are doable, and probably some 10's with more time getting the car dialed in.

Much of it comes down to your goals, budget, and level of commitment.

I messed with the intake some last weekend and thought man, a vertical mill would make quick work of the upper portion of the intake. I had the day off yesterday so; I took the intake to a local machine shop to have them open the upper part of the intake up. I'm having the divider removed.

After I get it back, I'll decide from there if I want to invest in some better tools and port the thing myself or send it off to a professional.

Thank you Dwayne for all of your insight. I actually was not too familiar with Jim's dart. Most of what I've been thinking was based off of Harry's old Bee. I looked into Jim's car some more since you mentioned it. Do you specifically know what is done to his intake?

I started to think yesterday, could you weld the heat riser ports up, cut the bottom part of the intake off, open up the original runner and make a new larger runner utilizing the part of the intake that was originally intended for the heater riser and then weld the bottom of the intake back on? Does that make sense? Obviously, if I wanted to explore this option, I'd buy a third intake to perform exploratory surgery.

My end goal with this car is to run 11.50-11.70's on some drag radials and run whatever I run when I throw the original bias ply tires on it for FAST events. This car will remain primarily a street car so; I'm not going to get crazy with 14:1 compression or a huge solid roller cam and ext. I want the thing to run on pump gas and be streetable.

I think right now, with just some better flowing heads and some more tuning, I could get close to what my goals are as far as 1/4 mile times. With this said, I still have some parts on my car that are not FAST approved like a Performer RPM intake so; I know I'll still have to take some steps back.

Without getting into too many details, the gist of what Jim did was cut out all the sharp corners in the runners, and recast them in white bronze with a much larger radius, then had them furnace brazed into place.
It was a huge amount of work on his part, but I did some flow testing on it, and it was really a big step up from the normally reworked 301 piece.
Several runners were almost as good as an rpm.
After it was all done and painted, it looked like it just came that way.

Certainly more work than necessary to reach your goals.

AFAIK, Jim's Dart has the best ET's of any FAST legal single 4-bbl Mopar combo, and Harry's was #2.
Jim's motor was a much higher level build, so it should have been faster than Harry's.

I had the same idea and I actually went as far as removing the heat riser section .. I did it more to remove weight, I think it took about 4 lbs off the intake .... and in the process I broke thru in a couple of places so I put it in the bridgeport and started to open up the underside of the runners so I could access some of the reverse turns. I never went further than that and just bought the intake off Dwayne, I think I still have the butchered one laying under a bench somewhere?