There's some talk about doing it with an Air Gap in this article:
https://www.hotrod.com/articles/0406phr-edelbrocks-air-gap-performer-intake-manifold-tech/

It is intended to account for intake flow differences to each cylinder.

 

Where's Grumpy or Smokey when you need 'em? :crying:

 

Subscribing, as I am interested in seeing the discussion.

 

The only way you'll know is to test. The original stagger jetting stuff came from the L-88 carbs with the tabs on the boosters to help with the cut down plenum. Different carbs, different intakes, different cams, headers, heads etc all impact what you're contemplating.

I would start here and read through the "496 project" and the intake/carb testing he did. Great stuff!

Vintage Muscle Car Parts


JIM

 

A IR heat gun on the header/manifold exits will tell you a lot about mixtures per cylinder I would think also.

 

I personally played with staggered jetting back in the early 90's, my conclusion came to this, unless your flirting with boarder line lean air fuel ratio for max power ( lean is mean) it has some merit, otherwise if your marginally running on the fat side of things ( being a little safer on tune) it's not worth the hassle and no real e.t. or trap speed is expected.

I have not worried about it since then, i let driveability and the trap speed dictate jetting, keeping the front two jets the same and the rear two the same, I set the front jets up for driveability, drop the jets drastically for lean condition to cause stumble and surges and increase jetting til it cleans up- never getting into the secondry's during testing and then head to the race track and jet the rears up or down til the trap speed falls off- done, it's about as good as it's going to get in all driving conditions. Any more than this I felt it was a waste of time. but thats me, spend my energy elsewhere.

 

There are a few paragraphs discussed in the "how to hot rod big block chevy's" book. It is specifically for the plenum removed dual plane configuration.

 

Staggered jetting is usually a waste of time with a dual plane intake.

 

69ZL1, all I can add factually is that I bought a Weiand plenum divider and put it atop the "L88" manifold we physically milled out. It made a HUGE difference across the band in a street car. It may work for a narrow band road racer, but not for street or drag imho. The car never "zinged" to 7000 rpms faster.It may have been that the jetting preferred the stronger signals of a true 180, I dont know. I determined it to be useless, maybe for a can am style road race bbc vette maybe. Even then, I'd think the now typical dual plane with .500" to 1.00" spacer is more ideal coming out of corners.

 

There are a few of us that race in the Supercar class that I know for sure have no plenum divider with either a 163 or 569 intake. Myself included. I had the machine shop that milled mine down leave the .090 lip like the factory recommended. I haven't really experimented with jetting but I did go down to 2 jet sizes on the secondary side and plan on testing that out in the fall at the track. My fellow racers are a little cagey will not reveal any info so I have to find out on my own.

 

all you guys are trying to limit the "hit" due to polyglas tires, correct? It would make sense then, favoring the upper end hp.

 

I had it removed to get all the HP out of it, for what it is (low rise LS6 intake)

There are other ways to limit the hit on those tires. I personally do not use anything.

 

My dad went through this back in the mid 80's reading whatever he could. And like Jim said, there was a GM Holley 850 with tabs on the boosters, one without, plenum with the divider, and without etc etc. We wound up with a "aftermarket" 850 holley on a chevy manifold with the plenum divider cut out, and never saw a difference stagger or straight jetted. We even tried different carburetor spacers on it too. The one picture I have of it, shows a 2" spacer on the manifold and that was going 9.90's in a 2800 lbs nova in 1985. I think it went a best of 9.87 before we swapped to a strip dominator with the same carburetor.

 

Do you remember how it did after the swap to the Strip Dominator?
That's a great intake.


Years ago we did some at the track testing between the "trick of the week" Weiand Team G single plane and an Edelbrock C454 dual plane on a 10 sec Camaro. Couldn't tell the difference in ET or MPH really.


JIM

 

Been tied up with weight reduction activities on the car for the past couple years, and finally getting back to working on the engine (swapping out the intake manifold). The present antique C396 manifold I'm using (for the stock/sleeper look) has a divided plenum, but I've got a second similar manifold that I cut down the plenum divider height. (I'm also currently working on cutting out the under-manifold exhaust crossover casting material to take another bit of weight off the top of the engine.) Once I get the manifold swapped I'll need to play with the jetting, of course, but I'd like to be a bit more familiar with the actual physics of why BBCs need this stagger jetting when used with an open plenum. I'm familiar with the unequal port lengths in the heads, but what exactly is happening there that requires the stagger jetting "bandaid"? And given that I only have one WBO2 sensor installed at a time on the exhaust, I'm trying to make sure I can make sense of the average A/F readings that the sensor is reading.

All I've ever seen in any manual or carb book is a recommendation to change the jet sizes on the cross locations, but not any actual reason why. Any constructive information I can use from the collective would be appreciated.

Thanks.

It should be real easy to find the actual why as its my understanding that the 4 BBC intake ports are different shaped.Due to the way the head bolts and pushrod area's get in the way there are two good ports and two bad ports on each cylinder head.Meaning 2 ports have a straighter shot at the intake valve than the other 2 ports do.If you look at a BBC cylinder head and try to see the intake valve through the port you'll see it for yourself.It's more complicated than just that but that is the basic why.
Stagger jetting tries to help even out the flow pattern with a slightly richer or leaner wet air flow which would flow faster or slower.

 

Years ago we did some at the track testing between the "trick of the week" Weiand Team G single plane and an Edelbrock C454 dual plane on a 10 sec Camaro. Couldn't tell the difference in ET or MPH really.


JIM[/QUOTE]

Jim,

After all done with the testing which manifold did you leave on that combo?

 

Years ago we did some at the track testing between the "trick of the week" Weiand Team G single plane and an Edelbrock C454 dual plane on a 10 sec Camaro. Couldn't tell the difference in ET or MPH really.


JIM

Jim,

After all done with the testing which manifold did you leave on that combo?[/QUOTE]

The C454.

JIM

 

From my ancient how to Hotrod Big Block Chevys book.
From what was found is that 4 of the cylinders on the BBC do not breathe the same as the other 4. More on that in the second attachment.
A bit on Edelbrock in the 3rd attachment, I once talked to a guy that picked up a Weiand manifold and told me that he was going to fix the mis-matched ports. I told him I wouldn't do that, not sure what he did with it.

If you want a really good but dated (1971) book with tons of info on the BBC I would highly recommend this one.

 

Thanks for the files. I appreciate it. I've had the How to hot rod BBC book for a long time, but it's MIA from the last move.

I was hoping the book mentioned a few details on how much of the plenum divider height to retain (1/8", 1/4", 1/2", etc), but I didn't see anything in those pages.

 

Staggered jetting is done in an attempt to equalize distribution to all the cylinders. Every intake will be different, different cams will change it, you can even change it with different header sizes. Factory Holley's had tabs on the DL boosters to help with more extreme distribution issues that the factory dual plane manifolds had. Chrysler had some fairly extreme jetting differences to equalize distribution, and there ae cases where booster leg sizes are different to change the curve, Pro Stock had different annular inserts for different cylinders in some cases. Some engines won't see much difference if they already distribute well, or when the carb does a real good job of atomizing fuel. Some can see noticeable gains.

 

While I'm waiting for my intake gaskets to arrive I'm cleaning up the manifold and getting rid of excess casting weight. In addition to the plenum divider, I cut out the exhaust crossover material under the center of the manifold (gotta weld up the flange ports yet), cut off the left front alternator bracket casting piece, the Edelbrock label casting at the front of the carb area (looks a little bit more like a stock Chevy part with the Edelbrock name removed), and gotta finish grinding and welding the last piece of casting that contained one side of the steel tubing that was part of a heated choke system. I also lightly polished the underside of the manifold to reduce the surface area for less heat conduction if hot oil splashes on the underside surface. I did retain a short tubing section from the choke setup. It's just an external nipple that is connected via a passage to the underside of the manifold. I've always been curious what the pressure is in the crankcase/valley area at WOT, and I think I'll just use this port to hook up to a pressure gauge.

I'll have 1 1/2-2 pounds off the manifold from this bit of low skill manual labor. May not seem like much, but it's up front and up high, and I'm just about out of ideas on how to pull more weight off the car while retaining a stock/sleeper look (the car is 2750# at the moment, which is helpful when running a "small" displacement big block).

OBTW, the Fel-Pro gasket kit comes without end gaskets. What sealant are you guys using on the front and rear with decent success?

Thanks
Mike

 

69ZL1
Here ya go.

 

Skier and 4 Speed, I greatly appreciate the info and files. One more request. I'm still striking out finding my misplaced copy of HTHBBC. Would either of you have a picture/file of the page that recommends the ninety thousandths height for the remaining divider? I'd like to see if there's any additional information or context with that number before I grind out the remaining bit of height.

Thanks again, and I'm off to digging through boxes again to see it I can find my copy (I did find my copy of HTHSBC, but fat lotta good that does me).