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Fresh 12a rebuild from 1979. Strange exhaust sound lead me to pulling off the intake manifold to discover that a rectangular port isn’t covered by intake or exhaust manifold, thus exhaust gasses are exiting through here, seemingly after transiting the thermal reactor.
What is supposed to cover this hole? Where does it lead to? Intake manifold covers part of this port.
In the '79-80 engine, air from the air pump entered the exhaust ports through ports in the exhaust manifold face. (The air went through a preheater in the exhaust, then a double walled downpipe, then through a passage in the manifold to the ports)
In the '81-85 engine, which did not have a thermal reactor, air from the air pump entered the exhaust ports via that passage, through the intake manifold.
Thank you. I wondered about that, and referred back to my photos from when I removed the engine and that center plate is there with the rectangular port. This car was bought new off the truck in 1979, by my uncle, so I’m certain that’s the original center plate that came with the car.
That center port did not get covered by the 1979 intake. There is a specific intake gasket for 1979/1980 that does not include that part of the gasket. I'll dig around because this has been discussed before.
On the 1979 and 1980, that port is open to the atmosphere.
Last edited by KansasCityREPU; Nov 25, 2024 at 08:02 PM.
I have new rotor housings but original plates. Crazy that it’s not supposed to have that port. I’m told I could put an intake manifold from 81-85, so maybe I’ll do that… if I can find one.
Ah ha, that may explain it with the rotor housings. The '79-80 rotor housings had the air injection come in from a small port next to the exhaust ports. (Front and rear housings were different)
The '81-85 engines had the air come in through inside the engine, in a passage under the exhaust ports that was open to the intermediate housings.
Did your rotor housings have the passages on the exhaust faces? You can just barely see them in the picture I uploaded. If they don't, those are '81-85 rotor housings and your emissions system will be nonfunctional. If they DO, then they may have manufactured your rotor housings incorrectly.
I wouldn't use an '81-85 intake manifold, they're pretty crappy. It is fairly easy to add some epoxy to the bottom of the intake manifold to cover that port, and then use an '81-85 intake gasket. This is what I am doing right now with a '79 manifold on an '81 engine.
It's a peripheral port engine. The as-produced intake ports are filled with epoxy to blank them off, and new intake ports are bored in through the rotor housings. One can make a lot of power this way (all of the Mazda Factory Race engines had similar porting) at the expense of low load drivability.
@peejay Did your rotor housings have the passages on the exhaust faces? You can just barely see them in the picture I uploaded.
Yes! My original rotor housings did have those diagonal oblong exhaust ventilation ports. Since the new rotor housings didn’t have those, I didn’t bother replacing the rusted out emissions tube that led from the thermal reactor to the exhaust pipe…that pipe no longer served a purpose.
That center post did not get covered by the 1979 intake. There is a specific intake gasket for 1979/1980 that does not include that part of the gasket. I'll dig around because this has been discussed before.
On the 1979 and 1980, that port is open to the atmosphere.
So that rectangular port pushing exhaust gas out under the hood and making exhaust-leak noise is normal? I didn't have that sound before but your explanation makes sense otherwise I guess. Maybe I just have a loose exhaust bolt.
If you no longer have '79-80 rotor housings, then you have '81-85 and that is why you have the exhaust leak. You'll need to add a little bit of material to the intake manifold as I detailed in photos and that link, and use an '81-85 intake gasket.
Since I generally build non emissions engines, I usually plug the exhaust port passages in the rotor housing with a piece of, I think, 5/16" dowel. I chew it up a little in a vise to make it a press fit then insert it coated with muffler cement. It's a bit late for that now, so you have to modify the intake manifold.
I found an intake from an 81-85, which extends down far enough (1/2 inch) to cover that port. I'm going to pick that up and hope that quiets things down. I don't know where that port leads to but I suspect it's connected somewhere to the tube I no longer have, and blocking it off won't cause any harm. the newer manifold seems like a simple way to do that. LMK if you know otherwise, I feel like there's another surprise around the corner .
Thanks much for your advice.
The newer manifold will require a bit of carburetor recalibration to work because it's missing a few key components like the idle compensator valve (the engine will be uncontrollably rich without that valve, especially at idle), and will not be any kind of plug and play part. It also has a throttle plate in the rear primary port that is meant to connect to a shutter valve, and a whole host of other issues that will make it a lot more difficult to use than simply adding epoxy to your existing intake manifold.
If your Cleveland is the same one as mine, I have a box with five or six of them that I'd been going to use for experimenting with casting... they're mostly worth scrap value.
If you used 81-85 rotor housing, I'm not sure you'll want to use the thermo-reactor. It needs the air ports to help burn the excess fuel in the exhaust.
It's not emissions, it's exhaust heat. That exhaust manifold can start to glow red.
That should only happen if the thermal reactor is functioning. If no air is being pumped into the exhaust ports, then there shouldn't be any fire (literally) in the thermal reactor to make it super hot.
If the air control valve and air pump are still in place, then the ACV should still direct air pump air to the cooling jacket around the thermal reactor, too.
(This is another reason to not use an '81-85 manifold, incidentally. You won't be able to make this work)
I believe the original concern for overheating might be that normally those extra ports in the housings on the 79-80 engines are actually for air injection, not exhaust gasses. However if I recall correctly that air is preheated by the heat exchanger and is actually intended to complete combustion in the thermal reactor itself, not cool it. You definitely want the air pump and other cooling features if you keep the thermal reactor though (it has separate internal passages for cooling that are separate from anything on the engine).
The difficulty mostly comes from the fact that the early style emissions system (79-80) doesn't play with the later style (81+). The ACVs are different, as are the intake and exhaust manifolds, carburetors, and more. If you have a fully emissions stripped car this isn't really a problem, but if you don't it will not be fun. I had to do the whole swap on my SA years ago because I too had the wrong year engine and nothing worked correctly, and even then the computer won't know what to do with the solenoids so it's only a partial "swap".
The most simple option is finding a way to block the port as suggested by others and just disabling any smog equipment related to the exhaust. I would personally get rid of the thermal reactor as they are prone to internal failure anyway, but you wouldn't necessarily have to. Just make sure the air pump and ACV are functional and the "air injection mode" is disabled if you do keep it. If it were to try and switch to the air injection mode it could pop hoses off or worse since those passages are now blocked. It still needs to be able to cool the thermal reactor normally as well as actuate the bypass for high rpm.
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