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OEM LA intake anatomy

John Drake

John Drake

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I am beginning a refurbishment of a cast iron intake from a 1973 340. It needs to be media blasted. I would like to remove the baffle plate from the bottom of the manifold. It's held on by just two blind rivets and was clearly never meant to be removed. It has to come off, however, because there's a ton of carbon chunks coming out from under it and I also don't want any blast media to get caught underneath it. If I grind those rivets off will there be anything worthwhile remaining to connect the plate to upon reassembly?

Also, what are the two nuts doing at the base of the plenum below the primary ports? Mine are pretty rusty and I don't want to hack them up trying to remove them. They don't align with the rivets in the baffle plate. So I'm baffled.
20210130_231940.jpg
20210130_232051.jpg
 
The underside plate held on by the rivets can be removed. The shank of the rivet has a twist in it so try and use a screw driver blade to counter clockwise chisel it out. They may “POP” out as well when doing this.

Those plenum items are EGR ports. They should screw out. Some are just simply a hole drilled in the floor. It is model year dependent.
 
The baffle plate under the manifold should go back on. The hot exhaust cross over under the manifold will fry any oil that hits it without the baffle and the chunks will then fall into your engine.
Not perfect but it does sort of work. I have mostly found a lot of chunks/carbon under baffle plates were oil has not been frequently changed.
I drilled them out and tap the holes and use little bolts with Loctite on them.
 
rumblefish360 said:
The underside plate held on by the rivets can be removed. The shank of the rivet has a twist in it so try and use a screw driver blade to counter clockwise chisel it out. They may “POP” out as well when doing this.

Those plenum items are EGR ports. They should screw out. Some are just simply a hole drilled in the floor. It is model year dependent.
Click to expand...

I AGREE .....these are the EGR ports that connect to the exhaust crossover passage. These were calibrated orifices designed to introduce a percentage of exhaust gas to the fuel charge to control NOx emissions. IMO...one of Mopar's worst decisions....supplying uncontrolled EGR all the time, resulting in P-P operation especially when cold. These orifices are a stainless steel alloy that can be removed. I've removed them, welded or brazed them closed and reinstalled or install flush fitted plugs to not interfere with the fuel charge distribution. Either way.....but GET RID OF THEM.
BOB RENTON
 
Gentlemen, thank you for your responses. I feel better now about what I'm going to have to do. I'll add follow-on pictures on this thread as I proceed. Perhaps others may want to see what lurks underneath as well.
 
Last edited:
I was able to get the 2 bottom rivets out of mine with a pair of vise grips. Locked them on and twisted, they turned right out
 
RJRENTON said:
I AGREE .....these are the EGR ports that connect to the exhaust crossover passage. These were calibrated orifices designed to introduce a percentage of exhaust gas to the fuel charge to control NOx emissions. IMO...one of Mopar's worst decisions....supplying uncontrolled EGR all the time, resulting in P-P operation especially when cold.
BOB RENTON
Click to expand...
Now I’m not so sure about a ‘72 intake manifold. I do think that one was EGR-less. The EGR intakes are not as you say because the EGR valve on the driver side is a vacuum operated valve. So running one isn’t hurtful to W.O.T. Performance.

As far as a “MoPar decision” is concerned, it was how they met the new standard set up by the government. Along with the other various equipment items.

The lack of performance was multi fold IMO. Not just the EGR valve alone. Lower compression, leaner running carbs, charcoal canister, that fire wall mounted timing delay thing. (I can’t remember what it is called right now..) All of these things (and more) combined sucked for performance.
 
rumblefish360 said:
Now I’m not so sure about a ‘72 intake manifold. I do think that one was EGR-less. The EGR intakes are not as you say because the EGR valve on the driver side is a vacuum operated valve. So running one isn’t hurtful to W.O.T. Performance.

As far as a “MoPar decision” is concerned, it was how they met the new standard set up by the government. Along with the other various equipment items.

The lack of performance was multi fold IMO. Not just the EGR valve alone. Lower compression, leaner running carbs, charcoal canister, that fire wall mounted timing delay thing. (I can’t remember what it is called right now..) All of these things (and more) combined sucked for performance.
Click to expand...

Not entirely accurate....the intake manifold/exhaust crossover EGR orifices were unique to the 340 engine and TQ carb combination. The vacuum operated EGR valve mounted EXTERNALLY to the manifold on the cross over passage was on the base engines (318, 400, 440 low performance). There was a vacuum delay/thermal operated valve mounted on the coolant passsge or air cleaner body or radiator top tank, sensing coolant temp, that inhibited EGR operation until the engine reached a temperature to be able to tolerate a percentage of EGR introduced in the fuel charge. The sole purpose of EGR is to reduce the combustion chamber temps to below 1600°F, to inhibit the formation of NOx (oxides of nitrogen) reputed to be the chief component of photo chemical "smog", especially in California, innitially, and later, country wide.
The rest of the emission control devices, low compression ratios, retarded and lazy spark advances, and lean fuel mixtures, were an attempt to control CO (Carbon monoxide and carbon dioxide) emissions and HC (Hydro Carbons), both burned and un burned, by combustion "modifications". Just my opinion of course.
BOB RENTON
 
Nice thing about those orifice's is that they could plug up quick.
 
I a. Sure there is an externally mounted EGR valve on a ‘73-340 intake manifold. I could take a picture of mine if you wish.
Not entirely accurate....the intake manifold/exhaust crossover EGR orifices were unique to the 340 engine and TQ carb combination. The vacuum operated EGR valve mounted EXTERNALLY to the manifold on the cross over passage was on the base engines (318, 400, 440 low performance
Click to expand...
 
rumblefish360 said:
I a. Sure there is an externally mounted EGR valve on a ‘73-340 intake manifold. I could take a picture of mine if you wish.
Click to expand...

Thank you, but there's no need to do so on my account (although I cannot speak for anyone else that's interested).

@ckessel Yes, you're correct. Not only were the orifices plugged but the entire "hollow bolt" was solid carbon up to and flush with the top of the nut.
 
If your wanting more performance and can live with rougher cold start... block the exhaust passage on both intake gaskets and then you don't need to put that plate back and the cooked oil smagma is not a issue.. Go with a electric choke or pump the heck out of it to start it , mechanical choke won't work w the heat risers blocked obviously. Seems like the gas boils bad out of the carbs now anyway so we are cranking on them more nowaday with a mechanical pump. Plug the egr crap.
 
Just for reference in case anyone has a similar question in the future, here's as example of one of the two "nozzles" at the base of the plenum. Also, the two twist rivets which held the baffle in place are also shown. I was unable to grab them with vise grips because they'd walk up the crown and pop off. So I simply cut a slot in the head of each one and unscrewed them with a screwdriver. They came right out.

20210206_203758(1).jpg
 
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