Staggered jetting with Rebel Machine intake

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67 Marlin

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Topic: Staggered jetting with Rebel Machine intake
Posted: Oct/17/2019 at 10:21pm

I read that David Vizard used staggered jetting on the RPM air gap intake. I think two sizes bigger jets on the primary driver side and secondary passenger side. Was this something AMC tuners did with the Rebel Machine intake, which is also an open plenum dual plane setup?

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Posted: Oct/17/2019 at 10:45pm

Have only seen this in official Chevrolet documents.

Holley carbs for Chevys have a couple small "distribution blocks" on the booster sides.

In the GM 'Heavy Duty SS Parts catalog view-able on GMPartsWiki it is documented.

Have yet to see similar in reference to the AMC rebel Machine.

Have you seen the 12-page special Rebel Machine Performance Package Owners manual Supplement ?...

Tom Benvie 10 years ago posted images of it here... very significant document imho.

http://theamcforum.com/forum/1970-rebel-machine_topic763.html

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Posted: Oct/18/2019 at 6:56am

I remember you mentioning that AMC document, pretty cool. I think Vizard is a Chevy nut, are the Chevy RPM Air Gap intakes designed such that staggered jetting is advantageous, but this isn't applicable to other makes?

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Posted: Oct/18/2019 at 7:24am

If you have O2 sensors or high end exhaust analyzer then I can see fine tuning the jets on a dual plane intake manifold.

But most of us don't have the equipment, so likely only useful for big budget builds.

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Posted: Oct/18/2019 at 1:44pm

The O2 or high end gas analyzer will not get all the results that one needs to utilize the stagger jet.

The SBC has the same 5-7 firing as an AMC, and as such tends to have a potential “robbing” in the plenum and resulting lean condition which then causes detonation.

Stagger jetting therefore can be very helpful. But one cannot make assumptions on what or how the distribution works and the net effect, without careful plug reading and or individual cyl monitoring. And assuming that a bigger jet is the solution may prove wrong. The carb just sees airflow.

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Posted: Oct/18/2019 at 2:26pm

Holley Cross Jet info from the 1971 GM Super Sport Equipment Catalog:

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Posted: Oct/18/2019 at 6:47pm

If this forum was a carburetor, it would be the Dominator Cool

I asked the question about the staggered jetting on the Machine intake because it was intriguing to me, don't think I'll be tuning my carb to the extent of staggered jetting myself. As always, posts on this forum reveal cool info!

Jmerican wrote:

The SBC has the same 5-7 firing as an AMC, and as such tends to have a potential “robbing” in the plenum and resulting lean condition which then causes detonation.

Can you explain this in more detail? Interested to know more. Why does the robbing occur? Is this characteristic of just the dual plane intakes with an open plenum?

Geez Phat, if you start posting up Chevy documentation that's as good as the AMC stuff you contribute, you'll inadvertently crash the server due to all the bowtie people who'll start visiting the site.

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Posted: Oct/18/2019 at 6:56pm

67 Marlin wrote:

If this forum was a carburetor, it would be the Dominator Cool

Jmerican wrote:

The SBC has the same 5-7 firing as an AMC, and as such tends to have a potential “robbing” in the plenum and resulting lean condition which then causes detonation.

Can you explain this in more detail? Interested to know more. Why does the robbing occur? Is this characteristic of just the dual plane intakes with an open plenum?

I would assume this is because the firing order is 18436572, and cylinders 5 and 7 fire sequentially, which causes some pull from the adjacent cylinder, since 5 and 7 are next to each other. I guess that on a dual plane with a full length divider, it wouldn't matter.

Edited by Sonic Silver - Oct/18/2019 at 7:03pm

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Posted: Oct/18/2019 at 7:27pm

Shucks, forgot, the 5-7 thing...

2 pulses 1 after the other side by side, in one corner, disrupts "equal distribution" ?

Look at the floor of the GM CrossRam, well there's a story there too...

The last stages of developement of the GM Crossram was the addition of "distribution dams"

on the crossram plenum floor, and they were non-symmetrical, and there were at least 2 dams

in that rear drivers side corner, the objective being to equalize "plenum floor wet flow",

and therefore exhaust gas temperatures, and max power output through equalization.

This goes along with the discussion of the 5-7 cylinders firing 1 after the other, a dual pulse.

BUT... the pictures of the claimed prototype GM Xram with solder/epoxy "distribution dams"

show that the GM Mold Maker made a boo-boo and incorrectly rotated the dam layout 180 degrees !

Research over the past 18-20 months sure seems to suggest there was quite a bit of

"cross pollinization" between AMC and GM, Traco built Trans Am and other motors for both

and at the same time, for example. In the "AMC Power Book" there is a blip where the author

says on a build up 390 Javelin they used a big Holley as was used on a "425 HP Corvette"...

The Holley List 4584 Hurst S/S AMX CrossRam carburetors sure do appear to be

re-numbered and re-applied 1968 GM Trans Am Z28 302 CrossRam "1st Design" List 4210 Carbs,

which GM quickly superseded with what they called the "2nd Design" List 4295 Holley

which sure does appear to be the 1st ever Holley Double Pumper 600-ish CFM,

and unfortunately, imho, AMC got "robbed" of using on the Hurst AMX CrossRam.

Although Hurst and AMC did use the brand new at the time Holley List 4224 660 Center Squirter

on the Trans Am Javelins and the Nascar GT Javelins, but it had 4x 1-11/16" Butterflies,

whereas the Xram 302 4295 Double Pumpers, and the 4210, and AMX 4584 had 4x 1-9/16" Butterflies.

I too am surprised that recent research has lead to the GM community and the surprising amount of

same period AMC relative information that has been posted, shared, and found there.

All the manufacturers were chasing after yearly changing sanctioning body rules then

by Nascar, FIA / SCCA / Trans Am, NHRA, etc. relative to # of carbs and throttle blade sizes.

Edited by PHAT69AMX - Oct/18/2019 at 7:41pm

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Posted: Oct/18/2019 at 11:51pm

I’m concerned that the terminology being used here is not understood to be the same across the “board” here, incl. the papers.

Open plane manifold. Is this a single plane 4bbl., a Crossram, or a dual plane with divider altered?

In the case of the Machine manifold and poster, we have a dual plane with divider slot, yes? The divider slot vs an open spacer on a dual plane V8 intake can be two different animals, though they are sometimes thought to be the same. The slot is to get

A little shared cfm between planes... a little crosstalk. Usually when there is improvement in performance with the slot being bigger or opened up, its because the carb was too small. But maybe, just maybe, it’s also the velocity change - that allows for better distribution or a little less pulsing when reversion is happening too. Usually a dual plane situation like this would be better as a proper single plane. Single plane has a more pronounced 5-7 issue on engines that have that order. All v8s have a ba bump bump.

All will present different issues with a staggered jetting setup. All will need a different method.

In regard to the Machine poster’s original thought, I’ve found that AMCs can use particular care in the 5-7 cylinders. They are the first to show stress. And crossfire can be a thing too. They are a bit hotter. They are the tuning reference.