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Discussion Starter · #1 ·
'80 Bronco, 351M, near stock minus emissions equipment, no cat.

Exhaust stinks. Not a overly rich gas smell, just stinks. It will seep into your clothes in short order and makes it difficult to take the Bronco anywhere I may have to function with other humans for an extended period of time. Occasionally I will wear a sacrificial layer :rolleyes:.

I've seen high-flow cats; my local muffler shop quoted $250 to install one. Anyone have first-hand experience with their effectiveness, especially without the thermactor system/air pump functioning? I've read that they can get seriously hot if a carb is running rich. While I think my tune is reasonable, I do not have an A/F meter so I'm just guessing by smell.

I know I'm dealing with 40 year old equipment, but I got to believe I can do better in 2020. Just wanted some first-hand opinions from folks who have installed cats to clean up their exhaust smell before I drop some more money on this rig.
 

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You do not need the air pump working with a modern 3 way cat.
The highflows do work very well for what you are wanting to do. Though my experience with them has been on EFI vehicles, not carb applications. I do not see any reason why not be just as effective. As you mentioned the potential is there to make them run hot, if truck is not kept in tune.
Though I personally think this is minimized with the higher flow versions.
 
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85 Bronco, 309ci I6 w/4bbl, np435, 4" lift, 37" Irok NDs, 4.56 w/ Detroit Locker and tru trac
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In running a Magnaflow Spun Metallicat on my 85 with carbed 300. The smell is significantly reduced. And noise is not. Though it is a very short cat. The catalyst is only like 8" long and 4" in diameter.

This specific cat must be welded on. It has no clampable flanges.
 
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Discussion Starter · #5 · (Edited)
You do not need the air pump working with a modern 3 way cat.
In running a Magnaflow Spun Metallicat on my 85 with carbed 300.
Thanks guys. Gives me some hope and a recommendation to consider. I've got the 460 that will eventually make its way into the Bronco. I'm expecting to need to redo the entire exhaust for that exercise, so I may have to find a cat that will handle the 460 too - not sure I want to buy two cats if I can repurpose just one.

Stinky gas means improper fuel burn...do a sixliter tune. Adjust the carb and start there. The cat has little effect on exhaust smell if it’s not there.
I've read about the 6L tuneup but thought it was more for 302s. Regardless, I've done all the tenets previously. I do agree that, in a perfect world, I'd be spitting out just water and carbon dioxide. I've got a dumb fuel injection device, and it's probably not realistic to expect it to run 14.7 to the fullest extent possible. The big benefit could be to oxidize the hydrocarbons into water and CO2. Without supplemental air, I don't know if enough O2 will be freed in the reduction step to oxidize CO and NOx. (EDIT: Reduce the NOx to O2 and N2).
 

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Same I smell like I've been into a smokin bar if I have my windows rolled down, really need to figure out the best way of dialing down those smells. Not too sure if that's realistic with a carburator. Anyone have the link to the 6liter tune up?


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I went with an Eastern Universal 70318 catalytic converter for around $60. Seems to work just fine, although I don't go around sniffing tailpipes of running vehicles to do a proper A:B comparison.
 

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Discussion Starter · #8 ·
I went with an Eastern Universal 70318 catalytic converter for around $60. Seems to work just fine, although I don't go around sniffing tailpipes of running vehicles to do a proper A:B comparison.
Thanks for the recommendation. But you don't have to get your nose in the tailpipe to know I'm around.
 

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Thanks for the recommendation. But you don't have to get your nose in the tailpipe to know I'm around.
That's my point, it seems to do its job, but I'm not interested in comparing to other tailpipes to know if it's as effective as a much higher priced one.
 

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The "sixlitre" tuneup is for EFI only.

The process for a carb is to get a hot coil, big, low resistance plug wires, copper core plugs, and open the plug gaps up to .055"

Tailpipes have nothing to do with emissions. At least not discernible by the human nose. The longer the cat, the more fume reduction it will do.
 

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I love the smell of a carburetor equipped vehicle. Reminds me of good ole fashioned American made cars. :)
 

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Tailpipes have nothing to do with emissions. At least not discernible by the human nose. The longer the cat, the more fume reduction it will do.
Not sure I follow... You're saying the tailpipe would have no discernible difference with and without the cat? As in, exhaust fumes are the same regardless, and that's the only part we can smell?
 

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The sniper would help, but only because it is able to change the air/fuel ratio as per what is coming out of the manifold/header and get it closer to stoic (14.7 is usually used, or 1.0 lambda). You could pontentially get there with a carb too, however will take much longer and your going to have to sacrifce the mixture somewhere and it will be hard to maintain this with weather and temp changes (maintain the "perfect") This is where EFI in genreal shines in comparison, as it is able to compensate for these changes.

Personal experience with the Sniper Setup says that yes, it will clean up the tail pipe smell at idle especially. Though a wideband on a carb you can get it really close. Most people tend to set a carb up rich at idle from what I have seen. Add the timing controller to the sniper setup and you will really see a big differance.

Also thread has enough info now that I feel important to add that just because you fix the smell does not mean that your not getting CO into the cab still, just something to keep in mind is all.
 

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Not sure I follow... You're saying the tailpipe would have no discernible difference with and without the cat? As in, exhaust fumes are the same regardless, and that's the only part we can smell?
No no. The tailpipe is what comes after the muffler, and obviously has nothing to do with the cat or fume reduction. Thats all done in the cat itself. Your comment about trying pricier ones triggered that response. Though looking back at it, I kinda mis-read it.

Overall pipe diameter will affect fume reduction, but not discernible by us. Only a machine or maybe a dog could notice. A bigger pipe move gases at a slower velocity than the same amount of gasses in a smaller pipe. Which alters the time the gases spend in the cat
 

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You don't want a cat on that setup, personally I wouldn't try to run a cat on a carb unless everything was stock. The A/F ratio will never be exactly where it should be for efficient catalyst operation, and will frequently be rich, making the cat work harder, meaning more likely to clog or burn it out. The Thermactor system isn't just for older style cats. It is used to add oxygen to help the cat, many modern cars don't require it because the EEC programming is able to prevent the situations where additional oxygen would be needed. Fuel cutoff on decel is one of them. Still some more modern vehicles with 3 way catalysts will have smog pumps, My 05 trailblazer has an electric one.
 

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I have 1985 bronco with 300 I-6 and the exhaust smells. The engine was replaced not because of the smell just worn out. It still has the same smell just stinks from exhaust runs fine. It has the feedback carb that they had in 1985 non EFI from the smell it running rich. Never had O2 senor changed could that be the cause?
 

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Discussion Starter · #20 ·
Thanks for pitching in. Given me some facts to ponder, especially as I slowly gear up to make the conversion to the ‘91 EFI 460.
 
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