Remove intake divider or not?

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?

It's a bit of a moral conundrum, yes?
The more specific and detailed they make the rules, the greater the advantage to cheating and the higher the skill level needed to do so.

From another source: "Dual-plane (180º) intake manifolds (only) should have their upper (shallow) plenum dividers cut away between the primary and secondary bores, and a large radius added to the lower edges of the revised opening (where it joins the plenum). Mixture velocity is higher as it hits the floor in the upper plenum due to the shorter distance, which makes the radius more critical. The upper plenum has much smaller volume than the lower, so this closes the gap somewhat." YRMV

As I recall, when Russ(BSB67) was dyno testing his exhaust manifold equipped 505 on the dyno, he tested several intake and carb combos. One of those was the Indy dual plane.
He cut the divider down incrementally, testing each step..... And it kept making more power the more the divider was cut down. In the end I think he ended up removing it completely, which made the best power of all the combos he tested, and that's what went in the car.

The performance numbers in his sig speak for themselves.



I'm not sure if you're referring the Jim's intake or not, but the way I look at it, his approach to improving the induction system are exactly what the spirit of the rules are about.
Reworking a "correct" original part, and have it "appear" unmodified(externally) when done.
He applied the same thought process to the carb and heads as well, and his motor made very good power for one of those combos....... Especially considering it also had the added handicap of the pinched down BB Dart drivers side ex manifold.

Sorry to simply state the obvious, but Just thinking out loud......, The more I read through this thread, the more it makes sense to me. We here at Moparts have all seen many guys with lots of experience say that Mopar Wedge engines seem to always make more power with more carburetor. The more you throw at them, the more they make. Heck, even Ma threw 1250 cfm at the 440, in the form of the six-pack. It seems to me that opening up a dual plain exposes all cylinders to the whole carb instead of isolating half of it. So in my head it seems that by cutting down the divider, you are not only adding plenum area, you are also effectively adding more carburetion that each cylinder draws from.

I run a box-stock RPM manifold and have long contemplated cutting further into the "slot" that is cast into it. I also just switched out my 830 for a new annular 1000 HP, and to my surprise, throttle response and idle quality seems to not only be better.... it's WAY better. Haven't made any passes yet, but initial feedback is good. Just further evidence of the "more is better" theory. Maybe someday I'll nut up, and go after the plenum of my RPM (???).

Do it! Add a 1" open spacer if you can fit it under your hood. Worked great on my old "509" cammed 440 that ran 11.40s at 3700 lbs. w/ the Edelbrock Performer RPM manifold.

Remove and blend or make it as thin and low as possible.....Works awesome....

Mines a 340 but the same idea. The funny thing is with my combination I cut the divider down leaving about 1/4" of it, because that's what everyone does. Then as a test made a divider. Picked up a tenth in the 60' and didn't lose any mph. I cross the stripe at about 6200-6300 rpm so not really spinning it. We'll see how two points of compression changes whether it wants a divider or not. It

Years ago, I completely removed the divider and cut 2 notches in both dual and single plane manifolds to where I could put in a slide in divider. Gave me the option of trying infinite divider sizes. Application, carb size, engine size, torque /HP band and range all played in to it - and you get to throw in class rules / limitations. Working on Chevy stuff back then. You can definitely make the factory stuff better. Might cost you $$$$$$$$$$$$$$ in dyno time though. Party down Garth!!

Removing the divider can definitely numb up the motor down low, but with the bigger cube, high TQ combos......... And the skinny bias ply tires....... That's actually part of the plan.

A lot of what it takes to make a combo run well in FAST is forgetting all the stuff that used to work when you had headers and sticky tires.