Exhaust for Towing

[xjhauler87]

I have a 96 K2500 Burb with the 7.4 and am looking at upgrading the exhaust a little. The flange in the middle of the truck is leaking and after 170k I wonder about the cats. For low and mid range power, what would be the best route to go? I was originally planning to weld in two new cats and a high flow 2 into 1 muffler, retaining the stock manifolds, downpipes, and tailpipe. In other words, I'd like to do some upgrades without spending hundreds of dollars on an exhaust. I've got free time, a welder, and enjoy fabricating stuff so I'd rather go with something more along this route.

I came across a thread for TBI 7.4 upgrades and someone suggested using a single cat for a workhorse vehicle. Is this a good idea? It might make my installation a little easier. If so, how can I make sure it is appropriately sized for such a large engine (if one that large exhists)? Some list a max displacement size and others don't. What size should I be looking to run? Looks and sound aren't much of a consideration. I would prefer something on the quiet side as I will be towing for about 5hrs at a time, but it sort of breaks my heart that I can barely hear my big block at all as it is.

['05TJLWBRUBY]

Good question, looking forward to see what others think as well. I'm in the same boat and after thinking more about mine with 192K (it's a '97), I don't think headers and full exhaust is the most economical thing to do. I am thinking more along the lines of burning in a Spintech muffler to replace the bad stock one. I think I can get some pipe from the local exhaust guy for cheap to burn in depending on what I do with the cats... I haven't seen any that are rated for the single cat and it appears that buying two smaller ones may be the cheapest route to take here and still keep them rated for the engine size-that is unless I'm misunderstanding the ratings or just not finding one large enough. With the high mileage rigs, no real point in dumping a ton into a fancy setup as honestly-how much longer will it go in all reality??? I'll still do the tune and injector stuff but headers are out, keep this simple and economical doing the work myself. Interested to hear some thoughts.

Best of Luck,

Mike

[xjhauler87]

You and I are definitely on the same page Mike. I'm not trying to make a drag stip Burb, but I really wonder about how much the cats are holding things back. The only thing that really worries me is how weird things get in the area where the two downpipes start to come together and run parallel. They're not quite level, getting a welder in there will suck, and the pipes also lose their roundness for some reason. It looks to me like I would have to cut off both pipes in the same spot right before the first cat to be dealing with a round shape that I can weld something to. Possibly have to use a multi-slice and re-weld method to make some slight elbows. Run everything straight back ditching the flange, keeping both cats in relatively the same place and use a $40 3" dual in 3" single out muffler from Performance Curve.

Now that I'm typing all this, I guess I have a pretty solid plan and I'm not sure one cat would save me much effort besides maybe freeing up a little room to weld it all the way around. My other concern with one cat would be if both O2 sensors would be happy reading from the same pipe. Two cats is probably the way to go. Some input would still be great on what works best making these things breathe best though. If nothing else, Mike and I can just bounce ideas off each other until we come up with a good solution. :cool:

[1988 GMC 355]

I would go with two new high flow Magnaflow or Thunderbolt cats then do a 2 into 1 muffler of your choice, chambered will be best for torque and low end, or if you are going to keep stock manifolds but would be wanting to replace from them on back I would do a custom y pipe into a single high flow cat and SI/SO muffler,

[skills4lou]

unless the cats are restrictive, why replace them? A simple backpressure test will show you if they are plugged.

[1988 GMC 355]

With 170K on them they are more then likely plugged.

[skills4lou]

Unless the engine is running rich or puking oil, they should not be plugged. Miles means nothing to the cats. The reasoning that so many miles=plugged cats makes no sense. An analogy would be saying that the hood must be replaced at xx miles because it's had so much air passing over it. The catalyst's function is simply to increase the rate of the chemical reactions going on in the exhaust. By definition, the catalyst is NOT consumed in the reaction. The only reason a catalyst becomes plugged is if it is forced to ingest more unburned fuel or contaminants than it can react with. A brand new catalyst can be ruined in short order by an engine that's running way rich. Alternately, if an engine is running at an efficient level the catalyst can literally last forever.
I'm not trying to say that all cats are good at 170K. What I'm saying is that the only way to know for sure one way or another is to conduct a very simple backpressure test to see how much pressure is in the exhaust stream between the engine and catalyst. If you have too much pressure, then you'll not only need new cats but you also need to find out WHY they are plugged. And the answer is NOT 170K miles.

[devinwalterbmx]

A temp test pre and post cat can tell if clogged also

[xjhauler87]

To my knowledge, it is not running rich. No codes. It did have the typical bad injectors that I replaced after not owning for very long. Intake manifold gasket was also bad and losing coolant internally. Don't know if any of those things would be bad for the cats, but I figured for as cheap as they are these days, I would just replace them. That being said, if I can confirm they're good, I'm all for that. Looks like a backpressure tester is just a guage and hose with O2 bung threads on the end?

[skills4lou]

Or you can just drill a small hole in the pipe, use a low pressure fuel/vacuum gauge with a length of hose and a metal nipple that will seal inside the new hole in the exhaust pipe. Of course then you'll need to weld the hole back up, but that's very easy.
As for pressures to look for: I'm not sure what the cutoff would be so you'll need to do a little more research. IIRC, anything < 2psi is good. Between 2 and 5 is questionable and >5 is surely problem. That's just what I'm remembering though, and it's been a few years since I even thought about that. So please, don't take those pressures as gospel.

As Devin said, you can also do pre/post temps. What you are looking for is hotter AFTER the cats than before them. That indicates that the unburned fuel is in fact burning inside there. If the temp drops, then the reaction is not happening and you have a problem. Of course, this will require an IR gun or some type of K thermocoupler.

[1988 GMC 355]

I see what you are saying and am not saying you are wrong or that I am perfectly right, but I do not think they are high flow to begin with and if he wants good towing performance then stock cats is not an ideal setup, if he left them and towed alot I am sure getting everything really hot while towing would burn them up or cause them to fail faster.

[xjhauler87]

Or you can just drill a small hole in the pipe, use a low pressure fuel/vacuum gauge with a length of hose and a metal nipple that will seal inside the new hole in the exhaust pipe.

That's exactly what I would do. I don't like buying things like that that cost more than the sum of their parts. Haha

Anyone else have an opinion on the high-flowiness of the stock cats? Sorta seems to me that the pressure would tell it all.

[1988 GMC 355]

High flow cats are like $40 so its your call to test them and take that time and money to do it.

[KJMac]

I was told the factory cats rarely work efficiently last 100k and all cars from the factory run a little rich. I don't claim to know this for a fact
But most cars are running crappy before 100k do to lack of maintenance and tune up that I come across. I don't think there is a cat out there that will flow good enough for that big of engine, otherwise the factory may have done it. The TBI 454 had a single cat if I remember correctly but removal of it seemed to help that anemic beast.

[TigerEyz3]

Just to throw my $0.02 into the mix... 292k miles on factory converters and no problems. Stay on top of your regular maintenance and you're good to go.

[stevehayes01]

So my '90 truck has a single cat on it and the previous owner had it replaced 2 times in the 145K he put on it but that was due to a combo of factors.... overly rich running truck and towing 9,600LB trailer all over the country and back for a few years.

If you are taking the time to do the exhaust I would just spend the extra to pull it into a single hi-flow cat.... just my .02.