Thinking of adding catalytic converters

[CraigFulton]

Just finished some serious work on my ‘76 K5 with a 400. I added a Holley EFI and repainted with all new seals. Now I want to tackle making it not smell so bad. The fumes are awful. I start it up in the garage and pull out, but the 15 seconds in the garage stinks up the whole house. It’s a stilt house and the garage is the ground floor. So the fumes rise through the next 2 floors. I don’t mind taking away some horsepower. It’s only 175hp. Won’t even notice. But will I really eliminate the fumes?

 

[K5wrench]

Are they just paint fumes?
Takes a long time for engine enamel to stop stinking.
Coming from tailpipe?
Are you sure you aren't running too rich?
If you are, the last thing you want to do is subject catalytic converters to is an overly rich mixture.
I'd hook up a fuel/air meter and dial that EFI in better before spending money trying to clean the exhaust.
My 400 with a quadrajet did not stink, and it had a lot of miles.

 

[CraigFulton]

It’s not paint fumes. It’s from the exhaust. If you stand near it for a minute you’ll be ready to pass out and your eyes burning. Can I adjust the fuel on the Holley EFI? I really haven’t spent much time with it but I guess it must be possible. Even with the quadrajet it was plowing through gas. 7mpg. So maybe too rich?

 

[1-ton]

If this truck did not come with CAT's stock...don't bother. Like already said if it is running too rich, then the new EFI needs a tune. These things are supposed to be self-tuning, but that has been proven wrong by some members of this web site.

 

[Bent77]

Ive had several fuel injected vehicles without cats. There is a distinct smell to it, but I second what @K5wrench said. Get it tuned in first

 

[tenderfoot]

I just eliminated my cats. It stunk all to high heaven before and still does of course. I'll get mine figured out sooner or later. Cats are not the solution.

 

[CraigFulton]

Now I’m thinking I have an exhaust manifold leak leading to the EFi running rich. Lovely.

 

[Bent77]

Yeah that is a concern

is the O2 sensor back from the merge point 8-10" and 15* above the center of the pipe?


Also, it takes a bit of driving to get these Snipers to tune. It takes patience

 

[CraigFulton]

I know hard to see but hopefully you can make this out. It’s definitey 10” past the union on the stock exhaust manifold. This is a dual exhaust and they don’t come together. The sensor is mounted from the bottom up. Maybe I am impatient. I’ve only driven it a couple hundred miles since installation. The O2 sensor reading on the EFi display has been good according to instructions.

 

[Bent77]

So a concern with that location is that the sensor is in the "bottom of the bowl" so to speak. Here it is more likely to collect condensation

The unit is designed to be quite rich when cold weather is about on purpose. There is a certain degree of that that cannot be avoided. When I was using the Sniper, I kept the idle air fuel ratio as was set by the basic programming. This number was in the 12-13 range below 20*, and 14:1 once the engine is warmed up


How is the "smell" or fumes once it has been warmed up?

 

[CraigFulton]

I figured since it was on the down pipe it didn’t matter where it was mounted. If it was where the pipe goes horizontal than I understand.

Still smells like a gas chamber after warmed up and 20 miles of driving.

I’m going to hook the display up permanently and see what the ratios are regularly. It’s at 13-14:1 now.

 

[TuckerK5]

My LS3 crate engine smoked up the garage like that with the factory tune. It also sucked gas like an Ate point one pig.

I took it to Ryan Trussel in Sardis, Alabama. He has a chassis dyno and works on a lot of fuel injected drag cars.

He changed my gearing from 3.73 to 4.56 (I know that’s not relevant to fumes) and leaned up the mixture. No more fumes at all. Can’t tell it from a new Yukon that starts up in the same garage. Also doubled the fuel economy from 10 ish to 20 ish mpg.

You might look for someone of the sort in your area or I can check with Ryan and see if he knows anyone down there.

 

[fordsucks!]

Cats do not work when cold..... if your problem is cold starts, then don't bother!!

 

[ekajkrats]

The clamp on style bungs will also tend to leak, and if oxygen gets in through that, the sniper will over fuel. If you can weld on a bung for it that could help if that is the scenario. I think you'd see black exhaust marks if that were the case though.

I've also heard of the O2 sensors getting killed faster when on the lower part of the pipe from condensation, so I had a bung welded up on the top/side part of the pipe.

 

[1980Kfive]

You may need to go with a heated O2 sensor. I won’t claim to know the science behind it, but TBI trucks with headers, high flow exhaust, or the O2 sensor mounted downstream from the exhaust manifold often don’t work correctly without the heated sensors. You will often see them referred to as three wire O2 sensors, and I believe they were used in early to mid ‘90s trucks.

 

[89 jimmy]

Ok, I know this is not an apples to apples comparison but should be sort of the same principle. On my Harley Davidson street glide I had to get a custom tune or map to flash my ecm after an air filter mod and an open head pipe with no cat. There is only so much range a stock O2 sensor can compensate for. When running an auto tune if I remember correctly a wide band 02 sensor is called for. My point is, you may need a custom tune for your setup to get the AFR correct, or a dyno shop tune/map. I have no idea if this is possible with what you have and I have not read any of the sniper manuals . Just thought I’d share my experience with the bike. Sounds like they may be similar in a way And by the way, nice truck

 

[JoshHefnerX]

If the A/F ratio is very far off the standard O2 wont be able to do much for correction. The std sensors are only sensitive around stoichiometric. If it gets more than half a point or more either way outside of it's sensitivity range it's useless. Which is where wideband O2's work. Also why computers go open loop when you go full throttle. Most engines make more power down around 12:1 which is well outside of where the o2 is worth anything.

O2's also need to be up to temp, can't remember how high maybe 400deg or more or so or they wont meter properly. Which is why they really need to be as close to the collector point as possible. Hot gasses. A foot or 2 back and they wont get hot enough.

Get one of those temp guns and take a look at the pipe temps.

 

[dyeager535]

How much exhaust smell should one get on a non-cat vehicle, with properly tuned injection?

I haven't been around another converter free vehicle in a long time, so I have no idea what "normal" is. Since mine isn't running one (yet) I don't know if what I smell is normal. It smells a bit at idle, in a garage I can imagine it wouldn't take long to stink it up. Not horrible, but noticeable. I figure a converter wouldn't be a big deal to run, but I know that placement is pretty particular.

 

[ekajkrats]

The sniper kits come with heated wideband O2 sensor, the Bosch LSU 4.9 oxygen sensor.

Holley's install instructions say to mount it 6-10 inches after collector with at least 18" after it to get good readings from all cylinders on that side.

 

[JoshHefnerX]

The sniper kits come with heated wideband O2 sensor, the Bosch LSU 4.9 oxygen sensor.

Holley's install instructions say to mount it 6-10 inches after collector with at least 18" after it to get good readings from all cylinders on that side.

Good info there! Never really understood why wideband wasn't used on all systems out there. Unless they're really needing that ultimate resolution in the one point area for emissions/fuel mileage. Heated is good too, more tolerant to various mounting possibilities.